How the Zinc/Air Battery Is Refueling the Competitiveness of Electric … · 2019-05-29 · 14...

October 1995October 1995

Lawrence

Livermore

National

Laboratory

Lawrence

Livermore

National

Laboratory

How the Zinc/Air Battery IsRefueling the Competitivenessof Electric Vehicles

How the Zinc/Air Battery IsRefueling the Competitivenessof Electric Vehicles

Science and Technology R

eviewLaw

rence Livermore N

ational LaboratoryP.O

.Box 808, L-664

Livermore, C

alifornia 94551

Printed on recycled paper.

Nonprofit O

rg.U

. S. P

ostage

PAID

Livermore, C

AP

ermit N

o. 154

SCIENTIFIC EDITOR

Becky Failor

PUBLICATION EDITORS

Sue Stull and Dean Wheatcraft

WRITERS

Bob Berlo, Kevin Gleason, Arnie Heller, Robert D. Kirvel, andDale Sprouse

ART DIRECTOR

Ray Marazzi

DESIGNERS

Ray Marazzi and George Kitrinos

GRAPHIC ARTIST

Treva Carey

CONCEPTUAL ILLUSTRATOR

John Maduell

COMPOSITOR

Louisa Cardoza

PROOFREADER

Catherine M. Williams

2 The Laboratory in the News

4 Patents and Awards

5 Commentary on Energy Research

Features6 Powering Future Vehicles with the Refuelable Zinc/Air Battery

The zinc/air battery developed at the Laboratory can make large fleet electric vehicles competitive with gasoline-powered vehicles in terms of driving range,speed of refueling, and highway-safe acceleration potential.

14 Gamma-Ray Imaging SpectrometryThe Laboratory’s gamma-ray imaging spectrometer is being used as a peace-keeping and environmental safety tool. It uses the gamma rays emitted by nuclear materials to locate and identify those materials in weapons and processing equipment and at storage facilities and contaminated sites.

Research Highlights27 Positioning Health Care Technologies for the Needs of 21st Century

29 The Short-Pulse Laser: A Safe, Painless Surgical Tool

32 Abstracts

S&TR Staff October 1995

LawrenceLivermoreNationalLaboratory

Printed in the United States of America

Available fromNational Technical Information ServiceU.S. Department of Commerce5285 Port Royal RoadSpringfield, Virginia 22161

UCRL-52000-95-10Distribution Category UC-700October 1995

This publication is a continuation ofEnergy and Technology Review.

9

Page 6

Page 14

About S&TR

About the Cover

The Laboratory is making noteworthyprogress in the development of innovative energysources for alternative transportation vehicles.This month, S&TR features a report about thezinc/air battery developed by Laboratory scientistJohn Cooper. Cooper and his colleagues recentlyroad tested this refuelable battery in an electricshuttle bus on loan from the Santa BarbaraMetropolitan Transit District in southernCalifornia. The bus, pictured on the front cover,was powered in part by the light-weight,environmentally clean zinc/air battery, whichgenerates power from small (1-mm) pellets likethose spilling across the back cover. The zincpellets and electrolyte that fuel the battery can berecycled quickly and inexpensively. Our reporton this important advance in alternativetransportation, especially electric fleet vehicles,begins on p. 6.

The Lawrence Livermore National Laboratory, operated by the University of California for theUnited States Department of Energy, was established in 1952 to do research on nuclear weapons andmagnetic fusion energy. Science and Technology Review (formerly Energy and Technology Review) is published monthly to communicate, to a broad audience, the Laboratory’s scientific and technologicalaccomplishments, particularly in the Laboratory’s core mission areas—global security, energy and the environment, and bioscience and biotechnology. The publication’s goal is to help readers understandthe accomplishments and appreciate their value to the individual citizen, the nation, and the world.

Please address any correspondence concerning S&TR (including name and address changes) to Mail Stop L-664, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551,or telephone (510) 422-8961. S&TR is also available on the Internet at http://www.llnl.gov/str/str.html,and our electronic mail address is [email protected].

Prepared by LLNL under contractNo. W-7405-Eng-48

• •

S&TR is available on the Internet athttp://www.llnl.gov/str/str.html. As referencesbecome available on the Internet, they will beinteractively linked to the footnote references at the end of each article. If you desire moredetailed information about an article, click on anyreference that is in color at the end of the article,and you will connect automatically with thereference.

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3

Science & Technology Review October 1995

Lab joins study of childhood thyroid diseaseMembers of the Laboratory’s Environmental Programs

directorate are on a U.S.–Ukrainian team that is undertakingwhat has been described as the largest and most detailed studyof its kind of the effects of radiation exposure on the incidenceof thyroid disease in children.

The work focuses on 70,000 children who lived in areas ofUkraine that were heavily contaminated in the 1986 accidentat the Chernobyl nuclear power plant. A primary objective ofthe study, estimated to last 15 years, is to determine to whatextent exposure to radioiodine, especially iodine-131, resultsin thyroid disease in children.

The scientific protocol that lays out a plan for the study wassigned in May by the U.S. Ambassador to Ukraine and theacting Ukrainian Minister of Health. Work under the protocolis expected to cost roughly $1 million annually, which will befunded primarily through Department of Energy funds.

As a designated “Center of Excellence” for theepidemiological study, the Laboratory will play a lead role inthe dose-reconstruction efforts and will purchase and delivermedical and other supplies needed to keep the studyfunctioning over its 15-year lifetime.

Other U.S. participants in the study are the National CancerInstitute and the Nuclear Regulatory Agency. Ukrainianparticipants include the Ministry of Health of Ukraine, theUkraine Academy of Medical Sciences, the UkrainianResearch Institute of Endocrinology and Metabolism, theUkrainian Scientific Center for Radiation Medicine, and theUkraine Radiation Protection Institute.Contact: Lynn Anspaugh (510) 424-6409 ([email protected]).

Climate models may have ignored sulfate aerosolsConventional climate-modeling studies may have omitted

an important factor in predictions of greenhouse warming—theeffects of sulfate aerosol pollutants, which result fromphotochemical reactions of sulfur dioxide emitted by fossil-fuel combustion.

That is the assessment of Laboratory researchers whodeveloped a computer model to track the effects on theatmosphere of both sulfate aerosol pollutants and greenhouse gases, such as carbon dioxide emissions, fromburning fossil fuels.

During the past 100 years, average temperatures haveincreased 0.5°C, instead of the 1°C predicted by conventionalcomputer climate models. The predictions, however,considered the effects of greenhouse gases only, not sulfateaerosols pollutants, which tend to lower temperatures.Predictions of climate change in response to both greenhousegases and sulfate aerosols are a closer match to actual observedtemperatures, the new Livermore studies indicate. The work byphysicists Joyce Penner and Karl Taylor and climatologist BenSanter is discussed in the June 16, 1995, issue of Science. Contact: Joyce Penner (510) 422-4140 ([email protected]).

MIR crosses $1-million royalty, licensing fee markTeleflex of Plymouth Meeting, Pennsylvania, has become

the tenth company to license the Laboratory’s new radartechnology—micropower impulse radar, or MIR. Originallydeveloped by engineer Tom McEwan as part of a diagnosticsystem for our Nova laser, MIR has earned more than $1 million in royalties and fees.

All companies that receive a license for the radartechnology are required to have their MIR-based productssubstantially manufactured in the U.S., thus creating jobsinside the country. Teleflex, which specializes in fourindustries (aerospace, marine, automotive, and medical), said itplans to construct a new manufacturing facility in Florida,partly to turn out products based on our radar technology.

In recent months, the technology has generated more than2700 calls to the Laboratory from businesses and individuals.Because of continuing interest from business, we held the firstMIR trade show in August. At press time, a second one isplanned for October.

Michael Odza, publisher of Technology Access Report, anindustry newsletter, said he believes “the Livermore radar isthe fastest growing technology license in the entire federallaboratory system.”Contact: Tom McEwan (510) 422-1621 ([email protected]).

2 The Laboratory in the News


Terawatt laser tests are prelude to petawattLaboratory researchers have been putting their new

100-terawatt, high-power, ultrashort-pulse laser through itspaces, testing some of the basic concepts underlying apetawatt laser they plan to debut in January. Ramped up to full power for the first time on July 31, the terawatt systemproduced 125 trillion watts of power in an extremely shortpulse—less than 0.5 trillionths of a second.

The laser was made possible by a revolution over the lastfew years in the ability of scientists to produce extremely shorthigh-power and high-intensity pulses. “The regime of physicsaccessible with the 100-terawatt laser is only now beginning tobe explored,” said Mike Campbell, Associate Director forLasers at LLNL.

The system will have applications in basic laser–plasmaphysics, x-ray lasers, fast-ignitor research, and many otherareas, while simultaneously serving as an engineeringprototype for the Laboratory’s petawatt (1-quadrillion-watt)system. Initial testing of the petawatt is slated for December,when it will produce 10 to 20 times the power of the 100-terawatt laser.Contact: Michael Perry (510) 423-4915 ([email protected]).

Upgraded FXR back in business this monthShut down since June to complete a $3.5-million

performance upgrade, the Laboratory’s Flash X-RayRadiography (FXR) facility was scheduled, at S&TR presstime, to be back in business in mid-October.

In the era of science-based stockpile stewardship, FXR isviewed as a key tool to help assure the safety and reliability ofthe nation’s nuclear weapons stockpile. In addition to itsstewardship role, FXR is used for experiments on advancedconventional weapons and on shaped charges, and it is atestbed for future flash x-ray technology.

In operation since 1982, FXR was designed to provide testdata for verifying computer-simulated predictions of howimploding objects behave. The 40-meter-long linear inductionaccelerator shoots an electron beam at a tantalum target. Thetarget emits an intense cone of x rays aimed at a test objectthat is detonated on the firing table. The high-speed x-rayimage produced is similar to a picture taken in 60 billionths ofa second.

Prior to shutdown, the FXR was composed of 54 cells—6 injector cells that generate the electron beam and 48 acceleratorcells. Modifications this summer and early fall involve adding four

cells to the injector to produce a 2.5-megavolt, 3-kiloampereelectron beam, a significant increase from the previous 1.2-megavolt, 2.2-kiloampere beam system. A more powerful,higher quality beam will reduce the beam spot size from 2.4 millimeters to 1.6 millimeters, resulting in a higher resolution x-ray image.

Upgrading of the FXR will permit development of aprocess called “double pulsing,” in which two snapshots aretaken in close succession. Work on double pulsing will beginin 1996.Contact: Charles F. “Joe” Baker (510) 422-9536 ([email protected]).

Lab scientists aid in discovery of dwarfism geneLaboratory scientists were among members of two national

biomedical science teams that earlier this year identified thegenetic cause of two forms of dwarfism—pseudo-achondroplasia, or PSACH, and multiple epiphyseal dysplasia,or MED. Both conditions are marked by shortened limbs,loose joints, and the early onset of osteoarthritis.

Working over a two-year period, the research teams found that the conditions are caused by mutations of a genelocated on human chromosome 19. The gene—called thecartilage oligomeric matrix protein, or COMP, gene—isimportant for normal bone development and joint function.The scientists’ discovery is discussed in the July 1, 1995, issue of Nature Genetics.

One of the Livermore scientists, Greg Lennon, says that inthe long run the discovery may lead “to a better understandingof joint and bone disorders—including osteoarthritis—andways to treat them.”

Lennon and his colleagues Harvey Mohrenweiser, AnneOlsen, and Susan Hoffman provided the research teams withexpertise that helped localize the region of chromosome 19, withwhich both diseases and the COMP gene were associated. Theywere also resources for detailed analysis of the normal gene.

Lennon worked with scientists from the University ofTexas—Houston Health Science Center, the University ofTexas—Houston Medical School, the National Center forHuman Genome Research at the National Institutes of Health,and Harvard Medical School. Mohrenweiser, Olsen, andHoffman collaborated with scientists from the Cedars–SinaiMedical Center in Los Angeles and the Jefferson MedicalCollege in Philadelphia.Contact: Greg Lennon (510) 422-5711 ([email protected]).

The Laboratory in the News

4 5


Each month in this space we report on the patents issued to and/orthe awards received by Laboratory employees. Our goal is toshowcase the distinguished scientific and technical achievements ofour employees as well as to indicate the scale and scope of thework done at the Laboratory.

Patents and Awards

Patent issued to

Howard W. HaydenJames A. HortonGuy R. B. Elliott

Thomas E. McEwan

Joseph Farmer

Joel B. TruherJames L. KaschmitterJesse B. ThompsonThomas W. Sigmon

John C. Whitehead

Patent title, number, and date of issue

Process for Continuous Production ofMetallic Uranium and Uranium Alloys

U.S. Patent 5,421,855; June 6, 1995

Linear Phase Compressive Filter

U.S. Patent 5,422,607; June 6, 1995

Method and Apparatus for CapacitiveDeionization, Electrochemical Purification, and Regeneration of Electrodes

U.S. Patent 5,425,858; June 20, 1995

Pulsed Energy Synthesis and Doping of Silicon Carbide

U.S. Patent 5,425,860; June 20, 1995

Valving for Controlling a Fluid-Driven Reciprocating Apparatus

U.S. Patent 5,427,507; June 27, 1995

Summary of disclosure

A process for the conversion of uranium oxide and the production of metallic uranium and/or uranium-containing alloys that would beeconomical and result in the production of little, if any, uranium-contaminated waste products, eliminating the problem of theirdisposal as waste material.

A filter circuit and process consisting of a ladder or series of stages oflow-pass filters in which each stage has an inductor with a voltage-dependent variable capacitor, varactor or PIN diode, to ground. Theinductors and varactors are noncommensurate such that the values areaccurately and independently established and made to correspond to aconventional phase-linear filter.

An electrochemical separation process and apparatus for removingions, contaminants, and impurities from water and other aqueousprocess streams and for subsequently electrically placing theremoved ions back into a solution during the regeneration process.Fluid flows through a continuous open serpentine channel betweenelectrodes of carbon–aerogel composite.

A process for selective synthesis of polycrystalline beta silicon carbidefrom sputtered, amorphous, silicon carbide thin films using excimerlaser radiation by simultaneously depositing (sputtering) a film ofamorphous silicon and carbon on a fused silica substrate and directingpulses of excimer laser radiation onto the formed film to selectivelycrystallize areas in the film.

An improved valve control system for a pair of free-piston pumps thatuse valve assemblies operatively connected to each pump andinterconnected so as to have fluid pressure communication betweenthem. The valve arrangement permits rapid switching between thepump’s intake and exhaust and provides large cross-sectional areas forthe flows with a short stroke as well as minimal hardware mass and size.

Patents

AwardsIn mid-August, 16 Laboratory employees received WeaponsRecognition of Excellence awards from the Department of Energy.Rear Admiral Charles Beers, Jr., DOE Deputy Assistant Secretary forMilitary Applications and Stockpile Support, presented the awards in aceremony attended by over 300 people. The Awards, which representachievements over the past three years, are as follows:

1995Technical Excellence in Weapons Design: David Stanfel, Leon Keller, and Gary Carlson

Technical Excellence in Radiochemical Diagnostics: Ronald Lougheed

Technical Excellence in Weapons Materials: Carlos Colmenares

Technical Excellence in Weapons Science: Neil Holmes

1994Technical Excellence in Experimental Diagnostics: Charles McMillan

Technical Excellence in Nuclear Design: Jon Bryan, Dan Davis,George Kramer, Joseph Sefcik, and Peter Stry

Technical Excellence in Computing: Randy Christensen

Technical Excellence in Opacities: William Goldstein, CarlosIglesias, Theodore Perry, Forrest Rogers, Paul Springer, andBrian Wilson

Technical Excellence in Material Sciences: Jagannadh Akella, John Moriarty, and Choong-Shlk Yoo

1993Technical Excellence in Nonnuclear Weapons Design: Dennis Baum, Leonard Haselman, and Larry Shaw

Technical Leadership in Weapons Design: Thomas Thomson

Technical Excellence in Weapons Disassembly: Peter Baylacq,Patricia Billy, Lee MacLean, Albert Celoni, Thomas Devlin, Gus Grogan, William Hubbel, James Lewis, David Prokosch,James Raymond, Joseph Schmitz, and Stephen Thompson

Technical Excellence in Weapons Design: James Ferguson, Richard Sharp, and Jim Slone

Technical Excellence in Weapons Codes: Eugene Canfield, William Chandler, Lila Chase, E. Gary Corman, Ned Dairike, Gary Henderson, Samuel Stone, and Roy Swiger


AWRENCE Livermore National Laboratory’sdistinguished history in energy R&D is rooted in its

beginnings, when work began on harnessing nuclear fusion forenergy production. This early work made our Laboratory oneof the first to apply its technologies for civilian energypurposes. This work has since broadened to many aspects ofenergy and has had a deep impact on the energy industry. Asan example, Laboratory-developed models of petroleumformation are now used routinely in the search for oilworldwide.

Reasons for emphasizing energy research in our earlyyears were straightforward: deep convictions that energy isvitally important to society and that there is a role forgovernment in achieving long-term goals that are partly ortotally external to the marketplace. Energy is the fuel that hasdriven all economies since the beginnings of civilization,providing mobility, communication, comfort, and relaxation.A good example of goals external to the marketplace isproving the scientific and technical feasibility of fusion.

In the U.S., gasoline is readily available and is cheaperthan it has ever been, adjusting for inflation. However,significant changes are now taking place that will affect all of us in the developed world. The economies of thedeveloping countries are growing so rapidly that their energyuse will soon tax the capacity of known energy sources. Thedeveloping countries have four-fifths of the world’spopulation and are rapidly developing their economies. TheChinese have increased their energy use more than 50% in 10 years and have recently become the second largest energymarket in the world, after the U.S. The trends mentionedabove must make us seriously question whether the currentsituation is sustainable and what the role of the U.S. will bein future energy markets.

The Laboratory has led in advancing technologies to helpsustain world economies while protecting the environment.One example is research on hydrogen as a future fuel,

particularly for use in vehicles (see Science and TechnologyReview, July 1995). Conservative economic analyses indicatethat hydrogen can be used as a fuel at a cost comparable to thecost of fueling today’s vehicles, with essentially none of theassociated detriments to air quality. Hydrogen burns veryefficiently, and the product of its combustion is water.

The advanced flywheels that we are developing will helpstore energy for advanced vehicles and allow superior powerconditioning and storage in our homes and workplaces. We will no longer need to suffer power interruptions and surges.Another vehicle technology is the zinc/air power systemdescribed in this issue. Our research into the technologies usedfor manufacturing will result in energy savings by reducing themachining time and materials needed. These technologies willreduce our dependence on foreign oil, along with associatedvulnerabilities to interruptions and price shocks.

Nuclear fission power has real potential for greatly reducingthe amount of carbon dioxide we put into the atmosphere as aresult of electricity generation. No one can say for sure whatwill be the consequences of greenhouse-gas-induced climatechange, but evidence grows relentlessly that the temperaturerise over the past half century is due to such emissions. We areworking to resolve the technical issues inhibiting publicacceptance of nuclear power as well as those relating to theprudent and safe disposition of nuclear waste.

At present there is a crisis in energy R&D, particularly overthe role and size of the government’s funding. Can and shouldthe private sector do it all? Energy now accounts directly for$500 billion of the $6-trillion U.S. economy. Yet we arespending only $2 billion (less than 0.5%) on governmentenergy R&D, and the budgets appear to be headingsignificantly lower. That $2 billion represents about 2¢ perevery gallon of gasoline we use. By comparison, our totalgovernment R&D budget is about $60 billion; so we spend 3% of the government’s R&D money on energy, whichcontributes 8% to the economy, clearly an underinvestment.Industry spends an amount similar to what government spends,on both energy R&D and on all research. Can we as a nationafford to spend so little on something so important to ourlivelihood and productivity now and in the future? If we needadditional impetus, it comes from world markets for energytechnologies, currently $1.5 trillion a year and growing rapidly.

For these reasons, energy R&D must continue to be animportant facet of the Laboratory’s mission of fulfilling thestrategic objectives of the nation.

Commentary on Energy Research

L

Robert SchockActing Associate Director, Energy

ABORATORY employees mayhave done a double take one day

last February when they saw a blue-and-white bus emblazoned with the words“Downtown Waterfront ElectricShuttle” coursing around the site. Thebus (Figure 1), on loan from the SantaBarbara Municipal Transit District, wasbeing powered in part by a revolutionarynew type of battery called the refuelablezinc/air battery. The battery wasoperated as part of the bus’s power trainto verify the road-worthiness of a fullyengineered prototype.

Developed at Lawrence LivermoreNational Laboratory, the battery weighsonly one-sixth as much as standardlead/acid batteries and occupies one-third the space, yet costs less per mileto operate. What’s more, because thebattery is easily refuelable, it promisestrouble-free, nearly 24-hour-a-dayoperation for numerous kinds ofelectric vehicles, from forklifts todelivery vans and possibly, one day,personal automobiles.

The battery’s inventor, LLNLelectrochemical engineer John Cooper,was one of a handful of bus riders thatday, monitoring the battery’sperformance and occasionally drivingthe remarkably powerful vehicle. Theroad test underscored the potential ofthe battery to give electric vehiclessome of the attractive features of gas-driven cars: a 400-km range (250 miles)between refueling, rapid refueling (10 minutes), and highway-safeacceleration. The positive test resultsalso cleared the way for discussionswith a host of interested commercialpartners about further development.

For all of its advancements, thechemistry of the zinc/air battery isrelatively simple (Figure 2). Thedevice combines atmospheric oxygenand pellets of zinc metal in a liquidalkaline electrolyte to generateelectricity with byproducts of zincoxide and potassium zincate. Inoperation, the battery consumes all of the zinc. Refueling is easily

accomplished by replacing spentelectrolyte with fresh electrolytecontaining recycled zinc pellets.

Such a refuelable battery has clearadvantages over rechargeable andreconstructible batteries. For example, it can be “topped off” and even refueledon the roadway in an emergency.Except for the alkaline electrolyte(which contains the same hydroxidefound in popular liquid drain cleaners),all of the materials making up thebattery are relatively safe and do notpose the environmental dangers foundin other battery types containing lead,concentrated acids, flammable metals,and other toxic or hazardous materials.

Because new zinc fuel can begenerated from spent zinc oxide in theelectrolyte by using relatively small and simple equipment designed byLaboratory researchers, the batteryneeds only a modest investment tosupport it. Refueling would be done at a company’s home base using existingmaintenance personnel. Quick, easy

7


Zinc/Air Battery6


A recent road test at

LLNL underscored the

zinc/air battery’s

capacity to give electric

vehicles some of the

attractive features of

gas-driven cars: a

400-km range between

refueling, 10-minute

refueling, and highway-

safe acceleration.

L

Powering Future Vehicleswith the RefuelableZinc/Air Battery

Powering Future Vehicleswith the RefuelableZinc/Air Battery Figure 1. Santa Barbara Municipal Transit

District bus at Lawrence LivermoreNational Laboratory.

Figure 2. Nearly continuous use of fleetvehicles is possible with 10-minuterefueling at 4- to 6-hour intervals usingthe process illustrated here.

same principle could be used toadvantage in designing a particleelectrode in a small gap, where wewant an open matrix, allowingelectrolyte to flow through freely.”

Some months later, he hadsuccessfully tested a self-feeding designfor a zinc/air battery, with a hopperfrom which zinc particles fell through arestricted opening into a reaction cell(Figure 3). This novel design, the basisfor Cooper’s 1993 patent on the battery(see box p. 10), solved a persistentproblem with previous zinc/air batterydesigns, in which the zinc particles andreaction products would eventually clogthe cell, preventing complete oxidationand reducing power generation.

Cooper is no stranger to batterydesign. In the early 1980s, he was theDepartment of Energy’s nationalprogram leader for an aluminum/airbattery development effortheadquartered at the Laboratory andconducted in cooperation with fivecorporate partners. That programadvanced to the stage where it wastransferred to industry, but the battery

did not see commercial development in autos because of ample andrelatively cheap worldwide petroleumsupplies and the battery’s chiefdrawback—aluminum corrosion. Thetechnology instead evolved intobatteries for emergency power reserveunits, submarine propulsion, andforklift trucks.

With Livermore’s LaboratoryDirected Research and Developmentfunding in hand, Cooper worked for twoyears beginning in mid-1992 with a smallteam of mechanical and chemicalengineers and technicians as a part of theLaboratory’s Energy, Manufacturing, andTransportation Technologies (EMATT)program. The project’s goals were todevelop an engineering zinc/air prototypebattery based on Cooper’s design,optimize its energy storage and powercharacteristics, and show that the self-feeding cells could operate under realisticroad vibrations and accelerations.

Finally, to be successful in themarketplace, the zinc/air battery had tobe able to be refueled quickly, easily,and safely without the need for a new

infrastructure of expensive, complexequipment. Energy researchers havelong recognized that if a way could befound to refuel a battery simply andrapidly, it could provide an electricvehicle acceptable to consumers. Quickand easy refueling is important in fleetelectric vehicles such as shuttle busesand delivery vans, which often mustoperate more than 8 hours daily.

The alternatives to refueling posesignificant disadvantages. For example,exchanging batteries doubles theinvestment in batteries and specializedequipment; adding a gasoline or dieselengine produces air emissions; andelectrical recharging takes hours tocomplete. (The traditional lead/acidbatteries used in Santa Barbara’s 12electrically powered city buses require4 to 8 hours of recharging after 4 ormore hours of use.)

During the development phase, thebattery was tested in variousconfigurations of cells and air electrodes.Cells were tested with air electrodes of80, 250, 600, and 1000 cm2, with the250-cm2 size judged the most

9


Zinc/Air Battery

refueling is a particular advantage forcompanies needing rapid refueling orextended use throughout the day fortheir fleets of shuttle buses, taxis,delivery vans, passenger vans, forklifts,or aircraft support vehicles.

Fast Development

The successful bus test capped aremarkably short development periodfor the battery. It started in 1991,when Cooper

noticed a Lab energy program posterdisplay of a proposed oil shale retort.One display pictured crushed shalerock falling by gravity through anarrow channel before it was heatedfor oil extraction.

“The rock formed an open matrix bybridging small gaps, which slowed thefeed rate,” Cooper recalls. “Ireasoned that the

8 Zinc/Air Battery

Zincrecovery

unit

Fresh electrolyte,zinc pellets

Spent electrolyte

Spentelectrolyte

storage

Discharge cells

Electrolysis cells

Pelletizerand

electrolytestorage

Figure 3. Zinc pelletsare gravity fed fromhoppers to cells.


Hopper Bipolar stack+ –

Aircathode

Electrolyte flow

Zincanode

Particlebridges

and voids

The LLNL zinc/air battery is constructed in modular form ofunit cells (see the drawing below), each of which is made up of ahopper, a self-feeding galvanic cell, and refueling ports. The cellsare joined together in a battery module that is connected to anelectrolyte storage tank containing electrolyte and dischargeproducts. The hoppers in each cell act as buffers, helping to protectthe fragile air electrodes from damage during refueling, handling,or road shocks.

Each cell contains a lightweight plastic frame, an electroniccircuit board, and a paper-thin air electrode. The electrode is themost expensive component in the battery and accounts for half thebattery cost. As many as 12 cell stacks can be combined with oneelectrolyte storage tank to form a battery, and Cooper envisionsboth a 6-cell and 12-cell basic unit, as with lead/acid batteries.

In operation, 1-mm zinc particles are pushed along the base of ahorizontal fill tube by the flowing electrolyte. The zinc particlesflow through slots leading into individual hoppers above thereaction cell. (Cooper likens the process to “hosing gravel down adriveway.”) The very narrow (less than 3-mm-wide) cell openingallows the particles to feed uniformly into the cell to form an open,loosely packed structure, which permits the oxidation of all of thezinc and easy flow of electrolyte. At the same time, electrolyte flowseasily upward through the cell and hopper to remove heat andreaction products. Electric power to drive both air and electrolytepumping is negligible,consuming less than 0.5% of thebattery’s gross power output.

The battery’s unusualmodular design permits theindependent choice of thebattery’s power (measured inkilowatts) and energy capacity(measured in kilowatt-hours).The very lightweight cellsprovide the power, and astorage tank holding the reserveof heavy electrolyte determines(along with the zinc) the totalenergy capacity of the batteryand most of the weight.“Within broad limits, we cancombine any number of cellswith any amount of electrolyteand zinc, depending on theratio of power to energy weneed,” Cooper says.

Given this uniquecharacteristic, each kilowatt of

power contributes weight of 2.8 kg and volume of 3 L to thebattery, while each kilowatt-hour of energy contributes 6.5 kg and4 L. Thus, a battery with 30 kW of power and 60 kWh of energyweighs 474 kg (1040 lb) and occupies 330 L (12 ft3). Becausepower and energy are independent, the traditional terms of“specific power” and “energy density” have no unique meaning inthis case. Still, Cooper speaks loosely of the battery yielding 140 Wh/kg in vehicle sizes, or about five times greater than thatdelivered by lead/acid batteries.

During refueling, spent electrolyte containing oxidized zincparticles is drained and fresh electrolyte is added. Stacks of 12 full-size cells are refilled at a rate of 15 seconds per cell in thelaboratory. A battery pack consisting of three parallel branches,each with three modules of 12 cells—108 cells in all—would befilled in parallel from a common flow in about 10 minutes.Because the batteries of any vehicle would be filled in parallel, therefueling target of 10 minutes is a reasonable goal for anycombination of three-by-three battery packs.

Existing bus or delivery van maintenance facilities wouldoperate as ready-made refueling and recycling stations. Crewswould replenish the electrolyte mixture with fresh material theyhad recycled on site. Exhausted electrolyte (an alkaline liquidcontaining zinc compounds) removed at the time of refuelingwould be recycled to produce new zinc fuel entrained in fresh

electrolyte. Recycling would bedone with small-scaleelectrolysis equipment the sizeof a vending machine, whichLivermore researchers designed.

The recovery equipmentwould be operated by the fleet’svehicle maintenance crew andlocated at the fleet’s home base.The recovery unit’s cost wouldbe about one-fifth that of atypical zinc/air battery becauseit would operate continuously(hence, be smaller) and wouldnot require an expensive airelectrode or heavy, expensivehigh-amperage power sources.The electrodeposited zinc platewould be shredded into smallparticles and then pressed intouniform, 1-mm pellets by asmall machine.

appropriate for bus-size vehicles. Cellswere operated for as long as 16 hours,with intermittent refueling. Stacks of 12 cell modules were refueled in only 4 minutes. Batteries were discharged inunits of 1, 3, and 6 cells. To simulateroad conditions, the research team useda vibration table for some tests.

Road Testing the Prototype

With laboratory tests complete bythe end of 1994, the team prepared fora vehicle test sponsored by the U.S.Department of Transportation’s FederalTransit Administration. Early this yearthe Santa Barbara Metropolitan TransitDistrict provided the Laboratory with a6.6-m (22-ft), 5.7-metric-ton electricshuttle bus.

For the bus test, one six-cell, 7-Vengineering prototype zinc/air batterywas cabled in electrical parallel with athree-cell, 6-V lead/acid battery. Thishybrid unit then was placed in serieswith the standard 216-V lead/acidbattery power plant of the bus, usingdiodes to prevent reverse polarization ofthe zinc battery (Figure 4).

The bus was driven on a 1.2-km looparound the Lab for about 5 hours for atotal of 120 km (75 miles), at whichpoint the lead/acid batteries were 80%discharged. The road test validated thedesign of the self-feeding zinc/air cellsunder typical driving vibrations fromstarts, stops, turns, and accelerations.The self-feeding cells functionedcorrectly without becoming cloggedwith zinc particles or starved for fuel,and they operated well in the hybridconfiguration. In addition, the testvalidated the integrity of the electrolyticair seal, the weak link in all metal/airbatteries. Fatigue of this crucial sealcaused by variations in full particle flowand stresses from the road surface couldlimit a battery’s lifespan; long-term

tests will be necessary beforecommercialization of the battery.

The test run also indicated thezinc/air battery’s potential savings invehicle weight, battery weight andvolume, and cost to operate over alead/acid battery. For example,replacing the lead/acid batteries in theSanta Barbara bus with zinc/air unitswould reduce vehicle weight from 5.7 to 4.0 metric tons, the batteryweight from 2.0 to 0.35 metric tons,battery volume from 0.79 to 0.25 m3,and electricity cost from 5.6 cents permile to 4.7 cents per mile (Figure 5).Yet power remains the same.

The zinc/air battery is less efficientthan the lead/acid battery (60% versus

70% conversion efficiency, respectively).Nevertheless, the lighter weight of thezinc/air battery cut a total of 1.7 metrictons from a 5.7-ton bus, so the totalenergy use is 17% less than that of alead/acid-powered bus because of thelower vehicle mass. Additionally, themarked reduction in vehicle weightmeans reduced tire and brake wear,which are major cost factors in largevehicles.

The Savings Are Long-Term

When commercialized, zinc/airbatteries will probably be the leastexpensive advanced battery on themarket. Cooper estimates a unit sized

11


Zinc/Air Battery10


Zinc/Air Battery

��

��

Battery Design and Operation

Details of a zinc/air battery unit cell (end view)

Refuelingports

Electrolyteoverflow

Electrolyte storage tank

Battery module

Zincpellets

Electrolytereturn

Electrolyte in Electrolytedistribution

Air out

Air in

Air cathode

Separator

Screen

Transferslots

Hopper

Galvaniccell

Figure 4. Developer John Cooper, left, and electronictechnician Douglas Hargrove, inside the SantaBarbara bus during its test run at the Laboratory.

the battery can easily be tailored toaccommodate varying needs, fromsmall forklifts to large urban buses tosilent military vehicles.

Down the road, passenger cars mightalso be powered by zinc/air batteries. Asthe cars proliferated, they could berefueled at service stations. Cooper sayssome oil companies are interested in theidea because it is a potential new marketfor their stations. In the long run,commercial zinc recovery plants couldtake advantage of large-scale productionusing electrolysis cells combiningelectrical and hydrogen energy to reducezinc oxide to zinc. This process wouldoperate with a substantial savings overthe purely electrical route to recoveryand would give the zinc/air battery analmost unbeatable total energyefficiency. The zinc/air battery’s totalenergy use would be comparable to afuel cell at one-tenth the cost.

Another factor favoring electricvehicles is energy flexibility. The U.S.transportation sector depends almostexclusively on crude oil, most of itimported. Electricity, by contrast, can beproduced by power plants using a widevariety of energy sources, includingcoal, natural gas, hydroelectric power,solar energy, and nuclear energy.

A final argument for electric vehiclesis that they require less maintenance andare easier to repair because the vehicle’s“power train” is simpler than that foundin gasoline-powered vehicles. In atypical electric vehicle, an electric motorpowered by batteries provides torque tothe front wheels. An electronic motorcontroller regulates current through themotor and transforms the battery’s dcvoltage to ac.

The switch from gasoline powerplants to zinc/air batteries for passengercars would make the most economic andenvironmental sense in urban areas,where gasoline-powered cars burnexcessive fuel and generate significantpollution in start-and-stop driving. Sucha fundamental shift to electric propulsionis no longer just an environmentalist’sdream. In 1990, the state of Californiaenacted regulations requiring 2% of allcars sold in the state to run without anypolluting emissions by the year 1998.The exact benefit to the environment,however, depends on the emissionsgenerated by local power plants thatproduce the electricity to build, recharge,or refuel the batteries.

Cooper says that the new technologyis likely to have its greatest impact onfleets of electric vehicles. “Buses, vans,and industrial vehicles have a uniquecombination of high daily usage, lowpower requirements, and in-place serviceinfrastructure. This combination makestheir owners and operators an idealmarket for refuelable zinc/air batteries.”

Key Words: alternative fuel; electricvehicle; refuelable; zinc/air battery

For further information contact John Cooper (510) 423-6649([email protected])

13


Zinc/Air Battery

for a bus would cost around $2000. Thedominating cost is that of the airelectrode, about $120 per square meterin large quantities. Since the batterydelivers more than 4 kW/m2 of airelectrode, the cost of this unit is onlyabout $30 per kilowatt. When allcomponents are considered, the totalproduction cost is about $50 perkilowatt for peak-power production plus$2 per kilowatt-hour capacity.Lead/acid units cost about $75 perkilowatt-hour.

The cost of the recycling equipmentdepends on how much fuel is neededeach day. For buses running 12 hours aday, the recycling unit would cost about 25% of the battery it serves. Therecycling unit has no expensivecomponents and uses metal sheets insteadof the more expensive air electrode.

Although the zinc/air battery hassufficient power by itself to power large

electric vehicles, combining it withanother power source (lead/acid battery,supercapacitor, or flywheel) isrecommended. The hybrid power plantallows better acceleration and greaterflexibility in selecting routes withvaring grades. Finally, with few peaksin the current, the life of the electrodesin the battery is expected to be as greatas 12,000 hours. Regenerative braking,in which the considerable brakingenergy turns magnets to recharge aflywheel or capacitor, will decreasetotal energy use by another 10%.

Cooper’s interest in zinc/air batterieshas been shared by many energyresearchers. Zinc/air batteries have beendeveloped for both mobile andstationary power applications becauseof their low cost and high energydensity. The batteries have found otheruses in hearing aids, military fieldelectronics, and laptop computers. Inthese applications, however, zinc/air

batteries are very expensivefor the low power theyprovide, and they cannot berefueled.

Commercial interest in zinc/airbatteries for powering electric vehicleshas grown significantly during the1990s. An Israeli company in 1994demonstrated a zinc/air-battery-powered van that had a range of morethan 420 km with highway accelerationand sustained speeds. However, unlikethe LLNL design, that battery is notrefuelable; the battery’s spent electrodesmust be delivered to a plant forreconstruction.

Commercial Development

The LLNL refuelable zinc/air batteryis now ready for advanced developmentas part of a commercialization effortwith one or more industrial partners.Cooper has received scores of requestsfor more information from batterymanufacturers and potential users.Many inquiries have been in response to news articles about the battery thatappeared earlier this year in the LondonTimes newspaper and Design Newsmagazine. “There is an enormous worldmarket, with literally millions of unitsthat could take advantage of the newbattery,” he says. “More than 90% ofnew battery concepts fail, so no onetalks to you until you’ve demonstrated aprototype unit that works on the benchand in a vehicle.”

Cooper cautions that beforecommercial units find their way intovehicles, more work needs to be done,particularly in determining thelongevity of the air electrode underactual or simulated road use and howmany times the zinc reaction productscan be recycled. Cooper believes thesechallenges are not insurmountable andthat full-scale production can begin asearly as 2000. One strong selling pointis that, because of its modular design,

12


Zinc/Air Battery

JOHN COOPER holds a Ph.D. in physical chemistry with specialemphasis on electrochemical engineering. For 10 years at theLaboratory, he has worked on electrochemical batteries as powersources for alternative, environmentally clean vehicles. From 1978to 1983, he was the national program leader for the Department ofEnergy’s efforts to develop aluminum/air batteries. Hisprofessional career has focused on active research in

electrochemical process development for environmental remediation and wasteprocessing, textile manufacturing, elecroplating, and metals recovery. Other researchinterests include optical crystals and processes, transport phenomena in aqueoussystems, and molten salt processes. Cooper is the author or coauthor of more than 80publications in the area of electrochemical processes and devices.

About the Engineer

The following organizations and agencies contributed to the development and testing ofthe zinc/air battery described in this article:Lawrence Livermore National Laboratory Directed Research and Development Program,International Lead-Zinc Research Organization, Santa Barbara Metropolitan Transit District, Santa Barbara, California,U.S. Department of Transportation through CALSTART, Inc., Burbank, California.

Veh

icle

wei

ght

Bat

tery

wei

ght

Ele

ctric

ity c

ost

5.7 metric tons(12,500 lb)

4.0 metric tons(8900 lb)


Bat

tery

vol

ume

0.79 m3

(28 ft3)

0.25 m3

(9 ft3)

5.6¢/mi

4.7¢/mi


Lead/acid battery

Zinc/air battery

Figure 5. Projectedsavings, comparinglead/acid andzinc/air batteries invehicles of fixedrange and power.

advances in position-sensitive detectortechnology, coupled with advances fromgamma-ray astronomy, have allowedresearchers to design and build agamma-ray camera capable of takinggamma-ray “photographs” that quicklycharacterize radiating materials. Whenthese images made with invisibleradiation are combined with visible-lightimages, they clearly show the exactlocation of the gamma-ray emittingmaterials.

Looking at Gamma Rays

The gamma-ray imagingspectrometer (GRIS) we haveassembled comprises four coaligned,independent imagers, each with its owndetector and coded-aperture mask(Figure 1). Each detector “sees”incoming gamma rays only through itsmask, which serves as the imaging opticfor the gamma rays (see box, pp. 18-19). This mask is mounted on amovable mask plate in front of thedetector plane; moving the plateprovides different levels of zoom for thegamma-ray images.

At the back of the housing are theelectronics that take the relativelyweak signals from the detectors andamplify them before they are sent tothe data-acquisition system, which canbe located remotely. Our systemcurrently consists of a commercialelectronics module, whose data areread out by a notebook computer(Figure 1). Coaligned with the gamma-ray imagers is a video camera. Imagesfrom this provide both a visual aimpoint and visible light images that canbe overlaid with the gamma-rayimages to pinpoint the location of theradioactive material.

Applications and Results

Although the spectrometer wasdeveloped to control the specialnuclear material (SNM) associatedwith nuclear weapons, there are amyriad of applications in other areas,including environmental cleanup,astronomy, medicine, the nuclearpower industry, and any otherenterprise where radioactive sourcesare used.

Arms InspectionGRIS was initially designed for use

in arms inspections called for by theStrategic Arms Reduction Treaty(START)—specifically, to count thenumber of warheads on board a missilewithout requiring either close access tothe missile or its disassembly.Inspections would be conductedremotely, based on the premise that thegamma-ray signature from the on-boardwarheads, although weak, is strongenough to be detected through the top of the missile. GRIS was constructedwith four detectors to decrease the timeit takes to obtain a good imageapproximately 10 m from the source.Figure 2 shows GRIS being used toinspect a Peacekeeper missile in its silo;the missile’s ten warheads in the GRISimage are easily seen in Figure 3.

Confidence through TransparencyAs the U.S. and Russia strive to

reduce their respective nuclearstockpiles, each must have the ability toidentify and verify the location of theother’s weapons componentsthroughout the demolition process. Each

15


Gamma-Ray Imaging Spectrometry

NE of the challenges facingtoday’s world is to keep track of

the nuclear material generated duringthe Cold War. Some of the materialsare radioactive isotopes that fuelnuclear weapons; others are used inthe nuclear power industry. AtLawrence Livermore NationalLaboratory, we have developed aninstrument that can help locate andidentify these materials.

One of the characteristics of manynuclear materials, including those usedin weapons, is that they emit gammaradiation. Each isotope emits a uniquespectrum of gamma rays that canpenetrate substantial amounts ofordinary matter without being scatteredor absorbed like visible light. Thisradiation is imagable and can be used toindicate the presence and specific typeof nuclear material.

Although nonimaging, nondirectionalgamma-ray radiation detectors have longbeen used to monitor the presence andgeneral location of nuclear materials,gamma rays have been poorly exploitedto provide information about the preciselocation of the nuclear material. Recent

14



Gamma-Ray ImagingSpectrometry

O

Figure 1. A Gamma-ray imaging spectrometer (GRIS) configured for work ingaseous diffusion plants. On the left, the GRIS imager head has four independentgamma-ray imagers. On the right is its data-acquisition system.

Laboratory scientists have developed an imaging instrument for locating andidentifying nuclear materials by taking “photographs” of the gamma rays emittedby these materials. This instrument, the gamma-ray imaging spectrometer, hasmany potential applications as wide ranging as treaty verification, environmentalcleanup investigations, gamma-ray astronomy, and nuclear medicine.

rods inside a storage drum. To simulateshielding, we placed a depleted uraniumplate about 3 mm thick outside thedrum. The uranium serves as shielding,as a source of confusing radiation, andas a different radioactive isotope.

Figure 5 indicates both the energyresolution of the system and howimages using data from differentspectral regions can show the locationsof different materials. The imageobtained using only the data in theregion of the spectrum shaded blue is onthe left. This image represents emissionfrom uranium and shows only the largeuranium plate. On the right is the imageobtained using data in the region of thespectrum shaded pink. These data arecharacteristic of plutonium and revealthe rectangular figure behind theuranium inside the container.

Safeguarding WeaponsWhen nuclear arms and their

components are secured and stored, theprimary concern is to verify that nomaterial is removed from a storage area. In addition to armed guards, aninventory control system that constantly

17



must have confidence that the SNM inthe other’s storage vessels is associatedwith nuclear weapons components butmust be able to develop that confidencewithout performing an inspection thatis sufficiently detailed to raiseclassification issues. This ability, orconfidence, is called transparency.

In a recent joint U.S.–Russiandemonstration at LLNL, we obtaineddata with a conventional, nonimaginggamma-ray detector and with GRIS. Thedata were collected from a radioactivesource hidden inside a typical weaponscomponent storage container. Bothdetectors possessed similar energyresolutions and could identify the typeof material present. However, in a singlemeasurement, the non-imaging detectorcould not verify the quantity of SNMpresent or the likelihood that thematerial was a weapons component.Such information could only be obtainedfrom the nonimaging detector byscanning it across the storage vessel insmall steps. Although this generated acrude image of the object that allowedidentification, it also required most of amorning to complete. By comparison,the inspection with GRIS took half anhour—a time which could be easilyreduced to a few minutes. The GRISimages taken from two directions 90 degrees apart (Figure 4) clearly show that a disk of plutonium and not a weapons component is in thestorage container.

Related applications that takeadvantage of GRIS’s ability to “see”behind shielding occur in nuclear wastedisposal and in the characterization ofnuclear weapons. Figure 5 illustratessuch an application. Here, we placed arectangular shape made from plutonium

16



Figure 2. Rendering of the configuration used forgamma-ray imaging of a Peacekeeper missile. TheGRIS imaging module is suspended above the open silodoor and generates an image from the radiation given offby the warheads at the top of the missile.

Figure 3. This enhancedgamma-ray image is froman emplaced Peacekeepermissile. The warheads areshown in a ring of nine,with the tenth inside thering at the 10 o’clockposition. The colorsrepresent radiationintensity contours.

Figure 4. Overlaid on a video picture, a color gamma-ray imageshows the difference between a face-on (left) and an edge-on(right) image of a plutonium disk. The images were obtained ata distance of 1.8 m and a position resolution at the source of3.8 cm. Black represents the highest radiation intensity.

Figure 5. Demonstration of gamma-rayimaging and energy discrimination inapplications for arms control transparency,contaminated waste identification, andweapons forensics. That the plutoniumsource is distributed inside a storage drumcan be clearly seen, even through 3 mm ofdepleted uranium. The image at left isgenerated from 100-keV gamma radiationof the depleted uranium; the image on theright is generated from the plutoniumenergy band at about 400 keV. With theappropriate energy selection, the plutoniumcan be seen through the uranium.

6000

4000

2000

00 100 200 300

Energy, keV400 500 600

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Depleted uranium source Plutonium source

19


Gamma-Ray Imaging Spectrometry18



Astronomers have worked on the problem of imaginggamma rays for about 30 years. Although cosmic sources ofgamma rays are extremely bright, they are also exceedingly faraway, so the problem is how to image dim sources in arelatively large background. In principle, a pinhole cameracould be used, but only a small fraction of the availableradiation would reach the film or detector. In the late 1960s, itwas recognized that one could improve the pinhole camera bypunching more holes in the blocking sheet. Each hole projectsits own image on the detector, and the different images overlap.If the hole pattern is known, one can mathematically recreate afaithful reproduction of the scene.

Although initial attempts showed that the technique worked,they also showed that the pattern had to be selected carefully, orfalse sources would appear in the image. The research on patterneffects was largely completed in the 1970s when a class of patternscalled uniformly redundant arrays was created. These patternspossess a unique property: the information present in the shadow

pattern from any one source in the image is not affected by thepresence of gamma-ray sources in other parts of the image.

In the schematic of the imager (see the illustration below), weassume that radiation is coming from a very distant source. Thelight rays from this source are parallel, so a shadow of the maskis projected on the detector much the way it would be projectedby the sun. Each pixel (the smallest picture element) in the imageis represented by parallel gamma rays incident from onedirection that project a detector-sized portion of the mask patternonto the detector. The pattern is selected such that eachprojection is unique and independent of all other projections.

The image is recreated by a cross-correlation technique: thecomplete detector pattern is summed against each unique maskposition by adding counts to the sum if the mask is open at thisposition and subtracting them if it is closed. Physically, countsare added if they could have come from that direction andsubtracted if they could not. If no source is present, any detector-sized portion of the mask pattern has the same fraction of open

and closed area relative to all other portions of the mask of thatsame size, so the sum is zero (except for statistical fluctuations).If a source exists at the particular location being summed, thenevery time there is an opening there will be counts, and the sumwill recreate the true flux (amount of signal per unit time) fromthe source.

The advantage of this technique is that half the detector areais exposed to each of the sources in the field of view. The rest isbehind closed mask elements. Compare this with a pinholecamera, in which the open area is only one pixel’s worth. For apoint source, the signal-to-noise ratio increases as the square rootof N, where N is the number of open holes. For our system, N isapproximately 200, meaning a 14-times-greater signal strengthand significantly reduced data-acquisition time.

Unfortunately, because all the counts in the detector are usedat each image location, the more sources there are in the field ofview, the less one gains from this technique. It reverts to onewith the same sensitivity as a pinhole camera if the whole fieldof view glows at the same intensity.

The resolution of a coded-aperture camera is just what itwould be for a pinhole camera. For each pixel, the angularoffset in incoming radiation is the basic hole size divided bythe focal length (detector-to-mask spacing). To obtain theresolution at the source, one must multiply this angle by thedistance to the source.

Position-Sensitive DetectorConverting the signal to a visual image requires

a position-sensitive detector. Moreover, the positionresolution must be comparable to the mask holesize; otherwise the pattern washes out. Becausetypical position-sensitive detectors (known as Anger cameras) for gamma rays of energies from 20 kiloelectron volts to greater than 1 megaelectronvolt have position resolutions of the order of 1 cm,an imager must be quite large to have a reasonablenumber of pixels across the detector. An imagermade with such a detector must also have a longfocal length to achieve even modest positionresolutions at the source

Our development of a gamma-ray detector with aposition resolution of about 1 mm allowed the fullexploitation of the coded-aperture technique in a

compact system. In the schematic of the detector at the left, aposition-sensitive photomultiplier tube is combined with athin cesium–iodide crystal. When a gamma ray hits thecrystal, it causes a brief flash of light, which is converted toan electronic signal by the photomultiplier tube. The tube isunique in that it allows the position of the light flash to bedetermined from its four output signals. The amount of lightis proportional to the energy of the gamma ray and is alsomeasured by the photomultiplier tube. The 4- ¥ 5-cm activearea of the detector yields about 40 pixels across its face,allowing for a mask pattern about 20 ¥ 20 pixels (ideally, oneoversamples by a factor of two.)

How the Gamma-Ray Imaging Detector Works

Detector

Mask

Incoming gammaradiation

Mask

Incoming gammaradiation

Detector

Uniformly redundant-array coded apertures produce an image by having each source pixel cast a unique mask shadow patternon the detector. The mask is four times the area of the detector. On the left is the system response for a source in the center ofthe field of view. On the right, is a response for a source near the left edge of the field of view.

This schematic of the GRIS detector shows how itlocates gamma radiation. A sodium-dopedcesium–iodide crystal emits a flash of light when struckby a gamma ray. This light is converted to electrons andamplified by the photomultiplier tube on which the crystalis mounted. The tube uses a unique mesh dynodestructure and a crossed-wire anode to determine thelocation of each event over the face of the tube.

Input window

Photocathode

Dynodeamplifiers

Crossed-wiredanode Resistor

divider network

Photomultipliertube

Cesium–iodide crystal

Gamma ray

gas at elevated temperatures. Separationtakes advantage of the fact that the gas,composed of the lighter uranium-235isotope, diffuses at a slightly higher ratethan the gas containing heavieruranium-238. The UF6 is enriched inheated equipment and piping containedwithin insulated housings.

Occasionally, because of leakage ofwet air or environmental changes in thehousing, solid UF6 deposits develop.Such deposits routinely occur in anoperational plant and must be locatedand identified. This task is not trivial.Many different pipes share the sameheat shielding in the miles of pipegalleries. To enter these enclosures,workers must don protective gear toavoid radioactive contamination frompossible residual leaks from more than30 years of operation. In addition, somefacilities—including those goingthrough decontamination anddecommissioning—contain highlyenriched uranium, which could cause acriticality accident if a deposit ofuranium-235 becomes too large.

Current characterization of theuranium deposits in these plants isperformed primarily using sodium-iodide-based radiation detectors. Theseare carried through the plant, andreadings are taken at fixed intervals tomap the radiation fields. If a “hot”region is found, workers must eitherenter the heat-shield-enclosed area ortake many measurements with acollimated version of the detectors totry to locate the deposit. Both are time-consuming, expensive, and potentiallyhazardous tasks. GRIS avoids theseproblems by generating images fromoutside the heat shielding thatdefinitively locate the hot material.

Our first use of GRIS in thisenvironment was at the idled K-25 plant.GRIS was mounted on a cart to look up

some 4 m at the pipe galleries overheadthat range in width from a few meters tomore than 12 m across. Each gallery,enclosed in heat shielding, containspipes ranging in size from a fewcentimeters to more than a meter indiameter. The building had been entirelyscanned by K-25 personnel walkingunder and on top of the galleries usingan uncollimated radiation detector; theresults from this survey were used toselect sites of interest for application ofthe GRIS imager. The first image was apipe used to exhaust the building’s manyvacuum pumps. We selected this pipebecause the lack of heat shieldingallowed us to verify that the gamma-rayand video images identified the hot pipe(Figure 6).

A second exposure was taken of amore representative location where anisolated deposit of material was known

to exist. After an initial wide-fieldimage was taken to see the completedeposit, we moved the imager under thehot spot and zoomed in on this region.Figure 7 shows a deposit in a 1.2-m-diameter pipe, where an expansion jointexists. The deposit is probably uraniumoxide, formed when a leak developed inthe expansion joint.

The images from the next location,although they are nearly featureless,clearly demonstrate the power of thetechnique. We took GRIS to a locationwhere we expected to find a series ofradioactive pipes running the length ofthe area covered in the image. Twoexposures were needed to cover the fullwidth of the 12-m-wide pipe gallery.The resulting images (Figure 8)revealed only a few hot spots, not thecontamination expected from thestandard analysis.

21



monitors the radiation from eachradioactive component is desirable.However, such a level of security is notalways possible. Particularly inestablishing an interim storage area, thecosts and time required to makeindividual security monitors for eachlocation can be prohibitive. However,the need for such facilities will beparticularly important as U.S. and statesof the former Soviet Union dismantlenuclear warheads. In this case, a GRIS-type imager can be a relativelyinexpensive and very rapid way toestablish inventory control.

Although we have not fielded suchan application, the implementation isstraightforward. The gamma-ray imageris installed so that it can “see” allsources, and a baseline image is taken.Then, the imager is set on a timer totake that image over and over again. A mathematical comparison of eachsuccessive image to the original can be used to sound an alarm shouldsomething be moved; we developed

suitable algorithms to do this in thecourse of analyzing the Peacekeeperdata. The advantages of using an imagerin this case are that it can be set up veryquickly, personnel need not leave theroom, and visible light is not required.

Locating SNM in Process PlantsGRIS has been demonstrated at two

U.S. gaseous diffusion, uranium-enrichment plants—K-25 at Oak Ridge,Tennessee, and the Portsmouth plantnear Portsmouth, Ohio. The images weobtained from these plants demonstratethe utility of gamma-ray imaging in anumber of complex situations.

Gaseous diffusion is used to separatethe useful uranium-235 isotope from thepredominant uranium-238 isotopepresent in natural uranium. Uranium-235 is the fissionable material used both as nuclear fuel in reactors and asweapons components. In the gaseousdiffusion process, uranium metal iscombined with fluorine to makeuranium hexafluoride (UF6), which is a

20



Figure 6. Video (right) andcomposite gamma-ray/videooverlay (left) of a contaminatedpipe at the K-25 gaseousdiffusion plant at Oak Ridge.The gamma-ray image clearlyshows which of the pipesoverhead is contaminated.

Figure 7. Overlay of gamma-ray intensity as a function of position for wide-field(left) and zoom views (right). The gamma-ray image on the right, which is overlaidon a video image, was taken after the imager was moved under the hot spot initiallyidentified from the image on the left. The radiation is emitted by a uranium depositinside a 1.2-m-diameter steel pipe hidden behind heat shielding.

Video imageGamma ray image

5.2- ¥ 4.6-m field of view

1.9- ¥ 1.7-m field of view

areas of particularly intense radiation.In a similar application, GRIS could

be used to find “lost” radioactivesources. Intense radioactive sources are sometimes used for materialscharacterization in construction andmaintenance. If these sources are lostfrom their holders, they present asignificant radiation hazard.

Finally, nuclear medicine couldpotentially benefit from application of agamma-ray imager with capabilitiessimilar to those of GRIS. The gammaemissions of several well-knownradionuclides used in medicine fall withinthe range of energies GRIS exploits.

Spectrometry and the Stars

In addition to the programmaticimaging work described so far, we havecollaborated with the University ofCalifornia at Berkeley and at SantaBarbara to combine our uniquedetectors with a novel implementationof coded-aperture imaging to build theworld’s highest angular-resolution,gamma-ray telescope (Figure 11).Constructed with Laboratory DirectedResearch and Development funding,GRATIS (gamma-ray arc-minutetelescope imaging spectrometer)comprises 36 individual imagersspecifically tailored to work in theastronomical energy band from 20 to200 keV. Our high-position-resolutiondetectors combined with a 4-m focallength allow GRATIS to achieve anunprecedented angular resolution of 2 arc-minutes (arc-min). By providingeach of the 36 detectors with its ownone-dimensional coded-aperture mask(Figure 12), we provide better overallperformance at lower manufacturingcost than a more conventional telescopeof similar size. Every one of these

23



Following the K-25 visit, we tookGRIS to the diffusion plant atPortsmouth. There we made twomeasurements of note. The first wastaken to determine the exact locationof a known deposit of highly enricheduranium. There were concerns that acriticality accident was possible if thedeposit was in the main 20- or 30-cm-diameter pipes of the gallery. Oneimage (Figure 9) shows that this wasnot the case and that the deposit was inmuch smaller instrumentation pipes.The second image (Figure 10) shows adeposit in a diffuser cell, a large heat-shield-enshrouded area about 25 m ¥ 6 m. The image, overlaid onto a plantblueprint, clearly shows plantpersonnel where the deposit is locatedbefore someone enters a cell.

In addition to its usefulness topersonnel who operate and clean upthese facilities, gamma-ray imaging alsopromises to be very useful to theInternational Atomic Energy Agency’ssafeguards programs for monitoringreactor fuel production facilities aroundthe world. One of the major uncertaintiesin inspecting such plants is the nuclearmaterial remaining in the processequipment. The ability to take images ofboth deposits and gas in the equipmentcan significantly increase the accuracy ofthe estimates of the quantity of materialpresent. In addition, the settings ofvalves and the flow of gas through aplant can be independently verified.

Other Applications

Other GRIS applications are beingconsidered. For example, a privatecompany working for the nuclear powerindustry is studying the feasibility ofusing the gamma-ray/video overlayimagery to direct workers away from

22



Figure 8. A powerful example of the advantages ofgamma-ray imaging, this image shows little contaminationwithin the heat shield. Instead, the image shows that thecontamination is in a nearby area.

Figure 9. These imageswere obtained at thePortsmouth diffusion plant.The overlaid engineeringdrawing shows that onlysmall pipes used forprocess monitoring arecontaminated, and thus thedeposit does not pose acriticality hazard.

Figure 10. Overhead view of process equipment at thePortsmouth, Ohio, facility overlaid on engineering drawings of thearea. The gamma-ray image clearly localizes the deposit to onelength of pipe. The cylindrical diffusers are spaced about 2 m apart.

Figure 11. GRATIS isheld by the launch vehicleas it is transported to thelaunch site at Palestine,Texas. Althoughsignificantly larger in size,the telescope isoperationally very similarto the GRIS systemdeveloped for LLNLprogrammatic work.

Zoom view (3.8-cm resolution) identifies pipe

Mid view (10.2-cm resolution) localizes deposit

Video outline of heat shield

Heatshield Compressor

Diffuser

telescopes produces a one-dimensionalpicture of the sky; the images arecombined mathematically to give a fulltwo-dimensional image.

GRATIS provided a specialchallenge because viewing radiationfrom the cosmos requires that thetelescope be above all but the mosttenuous portions of the atmosphere.Thus, GRATIS is hung from a heliumballoon, and the pointing system isoperated by remote control. To keep asource in the center of the field of viewrequires that the pointing system bestable to 1 arc-min. To reconstruct theimages properly requires that we knowwhere the telescope is pointing to aneven higher accuracy, which is obtainedby using a coaligned star camera and a

gyroscope system that allow us toreconstruct the pointing after the flightto approximately 20 arc-seconds.

GRATIS was first flown successfullyin spring 1994 from Palestine, Texas.During its 11-hour flight, we observedthree scientific targets: Cygnus X-1,Cygnus X-3, and Her X-1; we are in theprocess of analyzing the data.Meanwhile, GRATIS is on the groundin Alice Springs, Australia, ready for itsnext flight this fall, when we willobserve the center of our galaxy.

Continuing Development

Our ongoing efforts in gamma-rayimaging include improvements in thedetectors and in image-generationtechniques. We are building a newdetector that takes advantage of therotated one-dimensional imaging usedin GRATIS to extend the useful energyrange of this work and to significantlylower the cost per unit area of detector.Called the Gamma-Ray Bar ImagingTelescope (Figure 13), GRABITachieves these advances by separatingthe energy- and position-resolvingfunctions of the detector.

A series of scintillator bars is mountedon a nonimaging photomultiplier tube.Most of the scintillation light from agamma-ray event is collected by thistube, the signal from which is used todetermine the energy of the gamma ray.To determine where the gamma ray hits,we pick off a small fraction of the lightwith a fiber-optic bundle and transmit itto an imaging device such as thephotomultiplier tube used in GRIS. Byobserving which fiber end glows andknowing its arrangement on the imager,we can determine which bar is hit by thegamma ray.

To understand how this featureimproves the system performance, notethat the GRIS detectors determine an

24



event’s position by finding the center ofthe light footprint at the input to thephotomultiplier tube. However, as onemakes the crystal thicker, the averageevent size will increase because thelight spreads out more before it reachesthe tube, thus decreasing the ability tofind the flash location. By dividing thecrystal into bars, we remove thisproblem: the position resolution islimited only by the width of the bar.The costs are lower because the unitarea of nonimaging tubes is only aboutone-tenth that of imaging tubes. Byreading out a bar with a fiber optic, weeffectively increase the expensiveimager area some 40 times. We arecurrently assembling a laboratoryprototype of this detector system.

Our previous imaging work clearlydemonstrates the advantage ofgenerating images using different partsof the energy spectrum. Unfortunately,the energy resolution of the cesiumiodide currently used is only about 10%,not enough to distinguish commercial(reactor-grade) plutonium fromweapons-grade plutonium. Higherenergy resolution makes this distinctionpossible because it separates thedifferent gamma-ray energy lines of thevarious plutonium isotopes.

Another advantage of improvedenergy resolution is the ability to obtaininformation from a strong source thatlies behind a significant thickness ofother material. In such a case, theoverlying material acts much like thediffuser in front of a light, scattering theradiation and blurring the image.However, unlike visible light, thescattered radiation at these higherenergies is also shifted to a lowerenergy. By restricting the image tophotons, which are in a known spectralline from the source, one can removethis type of blurring. With theseadvantages in mind, we plan to developposition-sensitive, solid-state detectors

such as germanium- or zinc-dopedcadmium telluride, both of whichprovide much better energy resolution.

Because it was developed forgamma-ray astronomy, the coded-aperture imaging technique as it hasbeen applied by others assumes that thesource is very far away. In the closeimaging work we have described, thisassumption does not hold. We haveapplied several techniques tocompensate for this difference and arecontinuing to make improvements to theimaging techniques.

We are investigating the applicationof more advanced imaging algorithmsto the coded-aperture data. Thesetechniques rely on iterative approaches,based on Bayesian logic, that seek the

25



Figure 12. Close-up view of theGRATIS mask plane. There are 36individual one-dimensional masks,each rotated with respect to all theothers. The resulting rotated individualimages are combined mathematicallyto give a two-dimensional image.

Gamma ray

Imaging PMT

Nonimaging PMT

Figure 13. This schematic of the GRABIT detector shows howthe position- and energy-resolving functions are separated. Thelight collected from the bottom of the bar arrays provides theenergy information for an event. The small amount of lighttransported to the image tube by fiber optics allows one todetermine which bar was struck.

Fiber optics

27


AST year, expenditures for health care reached about 14% of the U.S. gross domestic product, or a staggering

$1 trillion. Many experts agree that the annual bill for healthcare will grow even larger in the next few years. Moreover,the effects of escalating costs extend beyond the domain ofhealth care per se; they are reflected in added costs of U.S.manufactured products, in labor–management relations, and inmany other ways that are not always obvious.

Can the trend be reversed? In some industrial fields, suchas electronics, technological innovation is part of an effectivestrategy to reduce costs without decreasing quality. In markedcontrast, investment in technology development accounts foronly a tiny fraction of national health care spending, and evenmedical research and development represents only about 3%of its overall spending. LLNL is marshaling its world-classtechnology base to help the nation to contain escalating costsfor health care.

Over the last decade, a broad spectrum of Livermoreresearch projects has explored new or improved health caretechnologies that can potentially reduce health care costs. Weare developing better imaging systems, such as pulsed x-raylasers, improved instrumentation and information systems, andadvanced sensor and detection systems, such as acceleratormass spectrometry. Other efforts around the Laboratory—ofteninterdisciplinary and involving external collaborators—arealready having an impact on the frontiers of research ortreatment in maladies such as cancer, heart disease, stroke,diabetes, osteoporosis, and repetitive strain injury as well as inspecialties such as ophthalmology, dentistry, and prosthesisdesign and manufacture.

To coordinate these activities, we established the Center forHealthcare Technologies at LLNL. Its goals are to:• Continue to pursue the high-quality science and technologyefforts that are already directed toward improved health care.• Become better known in the health care community.• Propose LLNL initiatives in health care that are moreintegrated than others’.• Promote a national focus for federal activities in health caretechnology.

The Center has an external advisory committee of seniorhealth-care professionals and an internal coordinating and

advisory committee.Perhaps most importantly,the Center represents asingle point of contactthrough which interestedorganizations outside the Laboratory can gain access to theLLNL individuals or groups that are most appropriate foraddressing specific health-care needs.

Our current strategy entails three phases of activities, whichwe have launched in parallel.

In Phase I, we are delivering results on current projects andgaining recognition for our accomplishments in health-caretechnologies. More than two dozen projects at the Laboratoryare currently funded at about $6 million per year. The boxillustrates developments from one of our most recent andexciting initiatives—the prevention of hemorrhaging in stroke-damaged blood vessels.

During our first year, we contacted more than 80 medical,industrial, and governmental organizations. We are identifyingand coordinating projects that extend LLNL core competencyin the multidisciplinary focus of biotechnology, helping tomeet future DOE Defense Program requirements andproviding cost-effective medical technology at the same time.

In Phase II, we are initiating and participating in largerhealth-care projects through multidisciplinary teams ofcollaborators. For example:• Digital Mammography Systems is a proposed team ofmilitary, government, and industrial partners led by an Armymedical center. Livermore would be responsible for systemintegration at 14 sites, for data integrity and archiving, and—with Sandia National Laboratories, Livermore—for newalgorithms for computer-assisted diagnosis.• This year, we have been asked to define and coordinatepotential roles for the DOE laboratories in Testbed’95, whichwill set up a telemedicine system. The Center is a partner inthe health-care working group of the National InformationInfrastructure Testbed (NIIT), a consortium oftelecommunications, computer, and other companies. InSeptember 1994, we participated in a successful, one-dayNIIT telemedicine demonstration, Testbed’94, held at theCongressional Office Building in Washington, D.C.

Research Highlights

L

Positioning Health Care Technologies for the Needs of the 21st CenturyPositioning Health Care Technologies for the Needs of the 21st Century

26



KLAUS-PETER ZIOCK came to Lawrence Livermore National Laboratory 10 years ago as a post-doctoral scientists in V Division. Since 1988, he has been a staff scientist in V Division’s Laboratory for Experimental Astrophysics. Hereceived his Ph.D. in Physics from Stanford University in 1985and his B.A. from the University of Virginia in Physics andChemistry in 1978. His primary area of scientific research is low-

energy gamma-ray astrophysics. He has been involved in the development of GRIS,GRABIT, GRATIS, GRB (a gamma-ray burst detector), and SXRP (an x-raypolarimeter).

His numerous publications to date (about 40) are in the area of atomic physics,including high-atomic-number systems, positronium spectroscopy, andinstrumentation development for astrophysical research.

About the Scientistbest image on the basis of priorknowledge of the source andinstrument. We are already applyingone such technique, known asmaximum entropy, to obtain the two-dimensional image from our set of one-dimensional images in GRATIS data.This technique selects the “flattest”image (the one with the least structure)commensurate with a statisticalgoodness-of-fit indicator based on theknown instrument properties. In thiscase, we assume that the scene naturesupplies will not have a lot of rapidvariations in counts versus position.

Key Words: gamma rays—gamma-ray arc-minute telescope imaging spectrometer(GRATIS), gamma-ray astronomy, gamma-ray bar imaging telescope (GRABIT),gamma-ray camera, gamma-ray imagingspectrometer (GRIS); special nuclearmaterial (SNM); Strategic Arms ReductionTreaty (START).

For further information contact Klaus-Peter Ziock (510) 423-4082([email protected]).

HE very best surgeons possess exceptional skill anddexterity, but most have not had access to what sounds

like the best of high technology tools—a medical laser.Medical lasers have been used successfully in someprocedures such as repairing detached retinas, breaking upkidney stones, and removing port wine stain birth marks.However, their remarkable ability to simultaneously cut andcauterize has not caused them to eclipse steel orelectrosurgical tools because they have lacked predictableprecision with body tissue and have been too large andexpensive to be a practical part of medical offices andoperating rooms.

Recent advances by Livermore researchers are changingthe capabilities of surgical lasers so that they may fulfill theirpromise to become the “blade of choice,” especially insurgical procedures on organs and tissues that bleed profuselywhen cut. By applying chirped pulse amplification to solid-state lasers, researchers have built systems that may increasethe use of laser surgical techniques in operating rooms. Thetechniques are expected to revolutionize dentistry and areparticularly well-suited to precision microsurgery on corneas,delicate ear bones, and the brain.

Limitations of Earlier Lasers for SurgerySince their advent about 30 years ago, lasers have been

observed to induce damage in transparent solids, such asdefect-free dielectrics (electrical insulating materials usedduring laser beam amplification and as mirrors to directbeams). For pulses longer than a few picoseconds,* thegenerally accepted theory of bulk damage to these materials isthat the incident radiation heats electrons (those in an atom’sconduction band, to be exact), and these electrons transferthermal energy to other electrons and atoms in the lattice. Inother words, the dielectric material is damaged by the melting,boiling, and thermal shock that result.

Similarly, a fundamental limitation of past surgical lasersystems was the heat caused by the beams—either acontinuous beam or one made up of multiple, 1000-ps pulses,

or “bursts.” As a result, laser surgery was soon found to beunexpectedly complicated. Lasers affect irradiated tissuethrough absorption of the light energy, and the absorption isdetermined largely by the properties of the material beingirradiated, such as its color. Consequently, a surgeon might becutting tissue quite effectively with the laser at the properspecific energy when the beam suddenly hits a different kindof tissue. If the tissue has different absorption properties, thecutting rate and heat generation can suddenly increase ordecrease dramatically—by orders of magnitude. Surgeonshave had to worry about how much energy the beam depositedand the degree to which the irradiated tissue absorbed orreflected the light. They might irradiate tissue with noapparent effect, only to discover that the light was beingreflected or transmitted and absorbed by other tissues andcausing undesired burning at these sites.

Indeed, this collateral damage has been a real concern withconventional laser-surgery pulsed systems. For example, lasercutting can significantly heat material some distance from thearea being irradiated, causing either desiccation of the materialor, if there is water below the surface, explosive vaporizationresulting in torn tissue.

There has been a clear need for a tool better than thescalpel, but one that does not require continuous adjustment orpresent the possibility of catastrophic mishap. Introducedabout the same time as lasers, electrosurgical tools, which useradiofrequency waves to interact with tissues, have becomethe tools of choice for many applications in which lasersproved disappointing. Electrosurgical tools, for example, cansimultaneously cut and cauterize; they are also less expensivethan lasers, are easier to use, and cut much faster. They are,however, not suited to microsurgery, and they too can causecollateral tissue damage.

Ultrashort Pulses Provide the AnswerRecently, however, laser technology has cleared some

hurdles. By applying chirped-pulse amplification to solid-statelasers, Livermore researchers have built systems producing

28


Research Highlights

• In a concept paper, we proposed development of minimallyinvasive medical technology, an area in which the medicalindustry has great interest and which is now being consideredas a government focus area.

Our Phase III activities seek to establish a national strategyfor health care technology programs. The focus is on reducingcosts without reducing access or quality of service. We haveproposed a government organization operating under theNational Institutes of Health (NIH). We envision this newagency as a forward-looking, expert organization that pursuesthe best technical solutions from the best sources combined

with the medical expertise of the NIH. We have presented ourconcepts to the largest health care, medical research, andindustrial organizations in the U.S., to Congressional staffmembers, and to members of the Clinton Administration.Science and Technology Review will report on developmentsand feature important technological advances as they occur.

Center for Healthcare

For further information contact J. Patrick Fitch or any member of the Center for HealthcareTechnologies (510) 424-4806 ([email protected]).

In the first six months of this project, we developed a modeling code,LATIS, to control and localize temperature when heating tissue. Thiscode (similar to those used in inertial confinement fusion research)allows us to select the right combination of laser pulse duration,repetition rate, and number of pulses to obtain the ideal temperature(see red zone) for patch welding a blood vessel. At the wavelengthwe use, about 65 pulses every 200 microseconds are optimal forrepairing a vessel without damaging the surrounding soft tissue.

Damage to the vascular wallcan occur after physicaltrauma, during attempts toclean out occluded vessels,during angioplasty, from ananeurysm (a bubble on thevessel wall), or as aconsequence of otherconditions.

UnweldedSurrounding tissue

Vessel wall

Gel + dye

Gel

Balloon

Welded

Saline-filled balloon

Optical fiber

Vessel wall

Catheter

Patch Welding for Damaged Blood VesselsEvery year in the U.S., over three million Americans are victims of stroke. The annual cost of care for stroke patients is about$30 billion. When the brain is deprived of oxygen, cell death can begin in a matter of minutes. Now, a change in treatment iscoming. To promptly restore blood flow and oxygen supply, we are using a multidisciplinary approach to characterize vessels and occlusions, to do “clot busting,” and to “weld” damaged vessels, as illustrated here.

A tiny saline-filled balloon is inflated in the vessel until abiocompatible gel (tan) and the patch or “solder” material(green dye) make contact with the damaged vessel wall.Within the balloon is an optical fiber. Pulsed laser light fromthe tip of the fiber is absorbed by the dye until the idealpatch-welding temperature is obtained.

29


Research Highlights

The Short-Pulse Laser: A Safe, Painless Surgical Tool

The Short-Pulse Laser: A Safe, Painless Surgical Tool

T

* One picosecond (ps) = 1 trillionth of a second or 10–12 s.

this difference decreases to only afactor of six. Thus, the ablationthresholds for transparent andnontransparent tissues converge atshorter pulse widths.

Short pulses cause highly localized,shallow energy deposits, and each oneremoves only a thin layer of material(less than 1 micrometer). Varying thenumber of pulses controls how muchmaterial is removed. High repetitionrates (greater than 100 Hz) achievehigh average removal rates (greaterthan 1 mm during 10 seconds). This method of tissue ablationhas several advantages:• It is efficient. With decreasing pulse width, the energydensity needed to ablate material decreases.• Minimal collateral damage occurs because ablation is efficientwith the short pulse and because the ablated tissue carries awaya large fraction of the deposited energy from the laser.• The ablation threshold and ablation rate vary only slightly astissue type and state change.• Ablation depth can be controlled with extreme precisionbecause a small amount of tissue is ablated per pulse andbecause the number of pulses can be controlled.

With ultrashort laser pulses, the absorption threshold andablation rate (depth of material removed per pulse) arerelatively insensitive to factors such as laser wavelength andtissue state (e.g., structure, hydration, and oxygenation).Because an ultrashort laser pulse (0.005 ps to more than 20 ps)is medically useful by virtue of its duration rather than itswavelength, a variety of lasers operating over a wide range ofwavelengths can be used, such as lasers based on dyes,excimers, or solid-state materials. However, solid-stateinfrared lasers such as those used at the Laboratory offer theadvantages of safety and convenience.

Our experimental results are still preliminary; furtherresearch is needed to understand what happens to the energyafter it is deposited by the laser. How the energy couples tothe atoms of the tissue, how the tissue is ablated, how thestress wave is generated, how the stress wave propagates intothe tissue, and how the collateral damage occurs all warrantfurther investigation.

Practical for SurgeryUltrashort-pulse lasers have an important niche in surgery.

The precision ablation of these lasers allows microsurgeons to

perform intricate operations, such asdrilling holes through small middleear bones, for which they usemicrodrills. Likewise in brainsurgery, which also puts a premiumon accuracy and control and onminimizing thermal collateraldamage, a system with a high pulse-repetition rate could allow a surgeonenough speed and control to virtually“sculpt” the tissue.

An added benefit of lasers tosurgery is the fact that the pulses can

be transmitted by fiber optics. Laser energy can be delivered tomany remote locations within the body through fiber-carryingcatheters. In the future, instead of making large incisions to gainaccess to certain organs or cavities, surgeons will use moreendoscopic and laproscopic techniques. Fiber-delivered lasersare already being used to break up kidney stones, for example,and are performing better than ultrasound for this purpose.

Ultrashort-pulse laser systems also have great potential indentistry to replace the dentist’s drill for cavity and root-canalwork. The drill produces pain chiefly by heat—the drill getsthe tooth very hot—and secondarily by mechanical vibration.We have demonstrated that the ultrashort-pulse lasers produceminimal heat and vibration and virtually no collateral damageto surrounding enamel.

The Laboratory is not alone in developing ultrashort-pulselaser systems. However, the critical step, once their medicalutility has been conclusively proven, will be to make themcompact and inexpensive. We have been developing thetechnology to make comparatively compact, easy-to-use high-power optical systems that produce pulses with flexibledurations and wavelengths. Essentials for practical use are veryhigh beam quality and reproducibility, in which the Laboratoryhas long-standing expertise. Using recent advances, we candesign high-power, ultrashort-pulse systems small enough tobe practical—and affordable—for surgeons and dentists.

terawatt (1012 W) pulses with ultrashort durations—well undera picosecond. (Chirped-pulse amplification is described insome detail in the article on multilayer dielectric gratings inthe September 1995 issue of Science and Technology Review.)According to theory, subpicosecond durations are far too shortfor appreciable electron energy to be transferred tosurrounding material. As a result, less laser energy is absorbedby the tissue, so material should be able to be removed bysubpicosecond pulses with essentially no collateral damage.Theory also predicts that the energy absorption mechanism forultrashort pulses makes material removal by laser much lesssensitive to tissue type than is the case with longer pulses.

To confirm the theoretical promise of ultrashort pulses, weperformed a series of experiments on various materials. Theseextensive experiments, with materials such as fused silica andcalcium fluoride, yielded results that agreed well with theory.We then produced “phantom tissues”—materials whoseproperties mimic the densities and effective atomic numbersof living tissue. We mixed gelatin and water to make collagengels (collagen is a fibrous protein found in all multicellularanimals), and we mixed aqueous solutions of cupric chloridein different concentrations to produce a range of light-absorption properties.

We then performed ablation measurements—measurements ofhow much material was removed by being “blown off,” or

ablated. We used a chirped-pulse amplification laser to obtainpulses of continuously adjustable duration from 0.3 ps to 1000 ps, with a laser spot size of 0.5 mm in diameter (about thesize of a period). After irradiating the sample, we inspected it forformation of an ablation crater. The smallest craters we couldobserve were approximately 1 micrometer in diameter, 500 timessmaller than the area the laser had ablated. We determined theablation threshold for a given pulse width. Then, to determine theablation efficiency, we measured ablated crater depth anddiameter after 10 to 100 laser pulses, recording the energy ofeach pulse. Now we could predict how the laser was “cutting.”

The figures below show typical craters produced by 1000-ps pulses and by pulses of less than 1 ps in a clearcollagen gel (a and b) and in tooth enamel (c and d). Muchmore thermal damage appears with the longer pulses (a andc). For ultrashort pulses, collateral damage is practicallyabsent, and a clean, smooth crater is produced (b and d).

The figure on page 31 presents the results of ablationthreshold measurements for collagen gels dyed to havedifferent linear absorptions. The ablation thresholds forcollagen gels follow the trends that we predicted. Theablation threshold for clear gels, which are similar to corneas,in the 1000-ps pulse width range (75 joules/cm2) is about1000 times higher than the ablation threshold for black gel(0.074 joules/cm2). In the subpicosecond pulse width range,

31


Laser Surgery30


For further information contact Luiz da Silva (510) 423-9867 ([email protected]), Mike Perry (510) 423-4915 ([email protected]), Michael Feit (510) 422-4128 ([email protected]), or Brent Stuart (510) 423-0479 ([email protected]).

100

100010010Pulsewidth, ps

Transparent gelSemi-transparent gelOpaque gel

10.1

Abl

atio

n th

resh

old,

joul

es/c

m2

10

1

0.1

0.01

Laser Surgery

a and b: Typical cratersproduced when clear collagenis struck by laser pulses (a)1000 picoseconds in durationand (b) less than 1 picosecondin duration. The shorter laserpulses (b) ablate the tissuelike collagen cleanly and precisely andproduce less collateral damage than the longer pulses (a).

c and d: Enamel tooth materialhit with (c) 1000-picosecondlaser pulses and (d) pulses ofless than a picosecond. Theultrashort pulses (d) produceless collateral damage than thelonger pulses (c).

a

b

c

d

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At Lawrence Livermore National Laboratory, we havedeveloped an instrument that can help locate and identifyspecial nuclear material (SNM) and other radioactive materials.Recent advances in position-sensitive detector technology,coupled with advances from gamma-ray astronomy, haveallowed researchers to build the gamma-ray imagingspectrometer (GRIS), capable of collecting the radiation andgenerating an image of its source. Such images of invisibleradiation can be combined with visible-light images to clearlyshow the location of the materials. The gamma-ray energyinformation from different parts of the image can uniquelyidentify the type of material present. Although GRIS wasdeveloped to control the SNM associated with nuclearweapons, we have tested the instrument in several of a myriadof applications in other areas, including environmental cleanup,astronomy, medicine, the nuclear power industry, and any otherenterprise where radioactive sources are used.■ Contact:Klaus-Peter Ziock (510) 423-4082 ([email protected]).

Powering Future Vehicles with theRefuelable Zinc/Air Battery

Developed at Lawrence Livermore National Laboratory,the zinc/air battery weighs only one-sixth as much as standardlead/acid batteries and occupies one-third the space, yet costsless per mile to operate. Further, because the battery is easilyrefuelable, it promises trouble-free, nearly 24-hour-a-dayoperation for numerous kinds of electric vehicles, fromforklifts to delivery vans and possibly, one day, personalautomobiles. The test of a Santa Barbara Municipal Transitbus with a hybrid of zinc/air and lead/acid batteries capped ashort development period for the zinc/air battery. The test runindicated the zinc/air battery’s potential savings in vehicleweight from 5.7 to 4.0 metric tons, in battery weight from 2.0to 0.3 metric tons, in battery volume from 0.79 to 0.25 m3,and in electricity cost from 5.6 cents per mile to 4.7 cents per mile. The power, however, remains the same. ■ Contact:John Cooper (510) 423-6649 ([email protected]).

Abstracts

This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of Californianor any of their employees makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information,apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service bytrade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the Universityof California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California and shall not beused for advertising or product endorsement purposes.

U.S. Government Printing Office: 1995/683-074-30001

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