Strategies and Sensors for Detection of Nuclear Weapons
description
Transcript of Strategies and Sensors for Detection of Nuclear Weapons
Strategies and Sensors for Strategies and Sensors for Detection of Nuclear WeaponsDetection of Nuclear Weapons
Gary W. PhillipsGary W. PhillipsGeorgetown UniversityGeorgetown University
February 23, 2006February 23, 2006
A Primer on the Detection ofA Primer on the Detection ofNuclear and RadiologicalNuclear and Radiological
WeaponsWeapons
AuthorsGary W. Phillips, Georgetown University
David J. Nagel, George Washington Universityand Timothy Coffey, National Defense University
Published byCenter for Technology and National Security Policy
National Defense University
Based OnBased On
http://www.ndu.edu/ctnsp/Defense_Tech_Papers.htmPaper Number 13
OutlineOutline• Nuclear Weapons• Detection at a distance• Gamma-Ray Detectors• Neutron Detectors• Portals, Search Systems, Active Imaging Systems• Summary and Conclusions
Nuclear WeaponsNuclear WeaponsThe True WMDThe True WMD
• “Nuclear weapons are the only weapons that could kill millions of people almost instantly and destroy the infrastructure and social fabric of the United States. – Frederick Lamb, in APS News, Aug/Sep 2005
Aftermath of Nuclear Bombing of Aftermath of Nuclear Bombing of HiroshimaHiroshima
Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm
Terrorist WeaponsTerrorist Weapons
• To date have used conventional or improvised weapons– 9/11 most destructive single act
• Nuclear weapons have not been used– Nuclear weapons difficult to steal– Nuclear materials difficult to obtain
• Radiological weapons – could contaminate many city blocks, no immediate casualties– material highly radioactive, difficult to handle and transport
safely• Chemical weapons have been used in conventional warfare
– Terrorist attack could kill thousands• Biological weapons – dangerous to make and handle,
anthrax not contagious, smallpox could start a worldwide epidemic, kill friends as well enemies
The primary observables from nuclear The primary observables from nuclear weapons are gamma rays and neutronsweapons are gamma rays and neutrons
• Emissions from nuclear materials– Charge particles (alphas and betas)
• Short range, easily shielded will not get out of weapon
– Neutral particles – Neutrons and high energy photons (x-rays and gamma rays)
• More difficult to shield, no fixed range, continuously attenuated by matter
• Mean free path: distance attenuated by factor of e (2.7)
energy (keV) air water aluminum lead
alpha particles 5000 0.04 4x10-5 2x10-5 1x10-5
beta particles 1000 4 0.004 0.002 7x10-4
x-rays (mfp) 10 1.9 0.002 1.4x10-4 7x10-6
30 30 0.03 0.004 3x10-5
gamma rays 100 50 0.06 0.02 1.7x10-4
(mfp) 400 80 0.09 0.04 0.0041000 120 0.14 0.06 0.013
neutrons (mfp) 1000 200 0.1 0.1 0.08
range (m)
Ranges of Nuclear Particles
Radiation from nuclear weapons cannot Radiation from nuclear weapons cannot be detected by satellite or high flying be detected by satellite or high flying
aircraftaircraft
• Factors which limit the distance at which nuclear weapons and materials can be detected– Inverse mean square law
• Intensity decreases as the square of the distance
– Air attenuation• Gamma and neutron mfp’s in air are ~ 100-200 m
– Shielding• Can greatly reduce emissions
– Interference from natural and manmade background– Counting errors due to random statistical noise in the relatively weak
signals
Radiation from Nuclear MaterialsRadiation from Nuclear Materials
• Natural uranium– Primarily gamma emitter
– 99.3% 238U, not fissionable by low energy neutrons
– 0.7% 235U, fissionable isotope, need >20% enrichment to make a usable fission weapon
• Weapons grade uranium – typically > 90% 235U– Emits very few neutrons
– Primary observables – gammas, mostly low energy
• Weapons grade plutonium – 239Pu– Primary observables – both gammas and neutrons
– WGPu contains about 6% 240Pu• 240Pu has a relatively high neutron activity
CriticalityCriticality
• Subcritical masses of 235U and 239Pu have a small probability of decay by spontaneous fission emitting 2 to 3 energetic neutrons– These can be captured by neighboring nuclei inducing
additional fissions, leading to a chain reaction• A critical mass is that just necessary for a self-sustaining
nuclear chain reaction– Nuclear reactors adjust the neutron flux using control rods to
sustain criticality• Rapid assembly of a supercritical mass can result in a
nuclear explosion– Rapid release of energy in the form of radiation, heat and blast
Neutron Induced Nuclear FissionNeutron Induced Nuclear Fission
The Oxford Encyclopediahttp://www.oup.co.uk/oxed/children/oise/pictures/atoms/fission/
How to Build a Nuclear WeaponHow to Build a Nuclear Weapon
Glasstone and Dolan, “The Effects of Nuclear Weapons,” 3rd edition US DoD and ERDA, 1977http://www.princeton.edu/~globsec/publications/effects/effects.shtml
Gun AssemblyGun Assembly
• A (probably) more realistic design is shown here
• The target is a subcritical sphere with a cylindrical hole
• The projectile is a cylindrical plug that is propelled into the hole to create a supercritical mass
• The fuel is WGU– WGPu has too high a neutron activity– Weapon would pre-ignite
From: “The Los Alamos Primer”, Robert Serber, Univ. of California Press
Schematic of Implosion Weapon DesignSchematic of Implosion Weapon Design
• The fuel can be WGU, WGPu or a combination
• Ignition of the explosive lens compresses the spherical core increasing the density to a supercritical state
• The tritium gas serves as a source of additional neutrons
• The 238U tamper serves to contain the blast and reflect neutrons back into the core
• The Beryllium serves as an additional reflector
http://nuclearweaponarchive.org/Library/Brown/Hbomb.gif
Implosion Critical MassesImplosion Critical MassesWith and Without a TamperWith and Without a Tamper
Uranium Sphere Plutonium SphereBare Sphere 56 11Thick Tamper 15 5
Critical Masses (kg)
http://www.fas.org/nuke/intro/nuke/design.htm
Models of Little Boy and Fat ManModels of Little Boy and Fat Man
National Atomic Museum, Albuquerque, NMhttp://www.atomicmuseum.com/
Little Boy Bomb Dropped on HiroshimaLittle Boy Bomb Dropped on Hiroshima
Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm
Fat Man Bomb Dropped on NagasakiFat Man Bomb Dropped on Nagasaki
Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm
Mushroom Cloud over HiroshimaMushroom Cloud over Hiroshima
Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm
Structural Damage at HiroshimaStructural Damage at Hiroshima
• On closer inspection even concrete reinforced buildings suffered significant damage
Glasstone and Nolan, “Effects of Nuclear Weapons”, 3rd edition (1977)http://www.princeton.edu/~globsec/publications/effects/effects.shtml
Aftermath of NagasakiAftermath of Nagasaki
Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm
Energy Released by FissionEnergy Released by Fission
Effects of Nuclear WeaponsEffects of Nuclear Weapons
• Most of destruction comes from the blast or shock wave– Due to rapid conversion of materials in the weapon to hot
compressed gases– Followed by rapid expansion generating shock wave
• High temperatures result in intense thermal radiation– Capable of starting fires at considerable distances
• Radioactivity– Initial radiation is highly penetrating gamma-rays and neutrons
• Fallout comes from slowly decaying fission products – Mostly delayed beta particles and gamma rays
• The greatest fallout from a ground level terrorist explosion would come from activation of debris sucked into the fireball
Requirements for Gamma-Ray DetectorsRequirements for Gamma-Ray Detectors
• High atomic number (Z)– For good peak efficiency
• Reasonable Size– Depth for stopping the gamma rays
– Area for solid angle
• High Resolution – For detection of gamma ray peaks above background
– For separation of close-lying peaks
• Ease of operation– Room temperature preferred
– Simple electronics
Common Gamma-Ray DetectorsCommon Gamma-Ray Detectors
Characteristics of Gamma-Ray Detectors
detector atomic size peak room temp
type number resolution operation
plastic scintillators low sq. m. none yes
crystal scintillators high 1000 cm3 moderate yes
Ge semiconductor high 250 cm3 very high no (77 K)
CdZnTe semiconductor high 1 cm3 good yes
Requirements for Neutron DetectorsRequirements for Neutron Detectors• Thermal (low energy) neutrons
– Gas filled cylindrical proportional counters– Plastic or glass scintillator– Require moderator to reduce fast neutron energies– Characteristic requirements
• Low atomic number• Reasonable Size• High thermal neutron reaction efficiency
– Maximum a few percent• Ease of operation
• Fast neutron detectors– Plastic or glass scintillator– No moderator needed– Similar requirements
• Efficiencies < 0.1%
Ge Detector Spectrum WGUGe Detector Spectrum WGU
Depleted Uranium SpectrumDepleted Uranium Spectrum
WGPu SpectrumWGPu Spectrum
Gamma-Ray BackgroundGamma-Ray Background
Natural gamma-ray backgrounds can be divided into three sources1. Terrestrial background
– Natural radioactivity primarily due to decay of 232Th, 238U and 40K– Known collectively as KUT gamma rays– 232Th and 238U have long decay chains ending in lead– 40K decays by one of two branches either to
40Ar (10.7%) or 40Ca (89.3%)• Atmospheric background from radon gas
– member of 238U decay chain– released from decay of radium in soil
• Cosmic-ray background – Primarily from muon interactions with environment– Increases rapidly with altitude
Gamma Ray Background SpectrumGamma Ray Background Spectrum
212Pb
e+e-208Tl
214Bi228Ac
214Bi
214Bi208Tl
40K
Neutron BackgroundNeutron Background
• Primarily from cosmic rays– At ground level, cosmic rays consist primarily of high
energy muons– Interactions with matter produces neutrons
• Ground, buildings, ships, any massive object• Broad spectrum (no characteristic peaks)
Factors Influencing Detection CapabilitiesFactors Influencing Detection Capabilities• Configuration of the weapon or material
– Outer layers shield the inner layers• Depends on material and thickness of outer layers
– Self-shielding• Thick layers shield radiation from inside the layer
• Characteristics of the emitted gamma-ray spectrum– Low energy gamma rays are attenuated more than high– Continuum from higher energy gamma rays obscures lower energy
gamma rays
• Interaction with the environment– Attenuation and scattering by intervening materials
• Interference from the environmental background• Interaction with the detector
– Detector may not be thick enough to completely absorb the gamma ray– Detector resolution may not be high enough
Case Study: Hypothetical Weapon DesignCase Study: Hypothetical Weapon Design
Steve Fetter et al. “Detecting Nuclear Warheads” http://www.princeton.edu/~globsec/publications/pdf/1_3-4FetterB.pdf
Gamma-Ray EmissionsGamma-Ray Emissions
One 100% Relative Efficiency Ge DetectorOne 100% Relative Efficiency Ge Detector1000 Second Counting Time1000 Second Counting Time
Peak Gamma-Ray Counts from a Hypothetical Nuclear Weapon
1
10
100
1000
10000
100000
0 10 20 30 40
distance (m)
counts/1000 s (100% Ge)
peak countsbackground3 sigma
Peak Gamma-Ray Counts from a Hypothetical Nuclear Weapon
10
100
1000
10000
100000
1000000
0 10 20 30 40
distance (m)
counts/1000 s (ten 100% Ge)
peak countsbackground3 sigma
Ten 100% Relative Efficiency Ge DetectorsTen 100% Relative Efficiency Ge Detectors 1000 Second Counting Time 1000 Second Counting Time
Neutron Emissions Neutron Emissions
1 Square Meter Neutron Detector1 Square Meter Neutron Detector1000 Second Counting Time1000 Second Counting Time
Neutron Counts from a Hypothetical Plutonium Weapon
0.1
1
10
100
1000
10000
0 10 20 30 40
range (m)
counts/1000 s (1 m
2 detector)
source countsbackground3 sigma
Neutron Counts from a Hypothetical Plutonium Weapon
1
10
100
1000
10000
100000
0 10 20 30 40
distance (m)
counts/1000 s (10 m2 detector)
source countsbackground3 sigma
10 Square Meter Neutron Detector10 Square Meter Neutron Detector 1000 Second Counting Time 1000 Second Counting Time
Principles of Gamma-Ray DetectionPrinciples of Gamma-Ray DetectionSize MattersSize Matters
• Gamma rays are long range neutral particles– Do not produce an electrical signal when they pass through a
detector
– For detection, energy must be transferred to a short range charged particle (typically an electron)
• Gamma rays interact with detector in one of three ways– Photoabsorption – full energy transfer to atomic electron
– Compton scattering – partial energy transfer to atomic electron
– Pair production – electron/positron pair creation• Requires energy > twice electron/positron mass (1.022 MeV)
• Probability of detection increases with– Thickness of detector, area of detector, density of detector
Gamma Ray Interactions with LeadGamma Ray Interactions with Lead
NaI(Tl) ScintillatorsNaI(Tl) Scintillators
• Thallium activated sodium iodide has become the standard crystal scintillator for gamma-ray spectroscopy– Common configuration of 3” diameter cylinder by 3” deep – Often used as standard of comparison for efficiency of
gamma-ray detectors• High fluorescent output compared to plastic scintillators• Moderate photopeak resolution
– Typically ~ 8% at 662 keV• Large ingots can be grown from high purity materials• Polycrystalline detectors can be made in almost any size
and shape– By pressing together small crystal fragments
New Halide New Halide Scintillator Scintillator CrystalsCrystals
• Resolution better than half that of NaI
– LaBr3:Ce (top) < 3% at 662 keV
– LaCl3:Ce (bottom) < 4% at 662 keV
Bicron – St. Gobain
Germanium is the Gold Standard for Germanium is the Gold Standard for Gamma-Ray DetectorsGamma-Ray Detectors
• Germanium semiconductor detectors were developed to overcome limitations of low resolution scintillator detectors– Resolutions typically 0.2% or less at 662 keV
• Roughly a factor of 40 better than NaI
– Easily separate peaks close in energy
– Easily observe small peaks on high background
Resolution Resolution MattersMatters
Multiplet peaks unresolved in NaI spectrum (top) are easily seen in Ge spectrum at bottom
Effect of Effect of Resolution on Resolution on
Signal to NoiseSignal to Noise
The peak is lost in the statistical noise as the resolution worsens (top to bottom)
Neutron DetectorsNeutron Detectors• Neutron Detectors rely on neutron scattering or nuclear
reactions to produce an energetic charged particle
• Typical reaction cross sections are much greater at thermal energies– This requires moderating the fast neutrons by multiple
elastic scattering– All spectral information is lost by moderation
• The physics of moderation and detection means useful detectors cannot be too small or lightweight– Several cm of moderator required to slow neutrons to
thermal energies– Detection at a distance requires large enough areas to
give reasonable solid angles
Thermal Neutron DetectorsThermal Neutron Detectors
• Thermal neutrons usually defined as energies less than 0.025 eV– Approximate kinetic energy of gas molecules at room
temperature
• Thermal neutron detectors make use of neutron reactions which produce one or more heavy charged particles (HCP) – e.g. 3He(n,p)3H, 6Li(n,)3H, 10B(n,)7Li
– HCP reaction products highlighted in green
– One or both reaction products are detected
• The most common neutron detectors are gas proportional counters
• Others include lithium doped plastic or glass scintillators
Cross Section versus Neutron EnergyCross Section versus Neutron Energy
Fast Neutron DetectorsFast Neutron Detectors
• Use fast neutron reactions which produce charged particles that can be measured directly– Efficiencies relatively small
– No moderation so some spectral information possible
• Fast detectors typically make use of one of two reactions– 3He(n,p)3H and 6LI(n,)3H
Fast Neutron Reaction Cross Fast Neutron Reaction Cross SectionsSections
Lithium Lithium Doped Doped
Glass Fiber Glass Fiber ScintillatorsScintillators
NUCSAFE Inc.
Oak Ridge, TN
PortalsPortals• Portals are used to detect gamma-rays or
neutron sources on pedestrians or vehicles• Pedestrian portals similar in concept to
airport metal detectors– Except use nuclear detectors instead of
ferromagnetic • Contain plastic or NaI gamma ray detectors• May be combined with 3He neutron
detectors
Search SystemsSearch Systems
• Vehicle or helicopter mounted arrays of gamma ray and/or neutron detectors– Usually contain large NaI(Tl) scintillator
crystals and large 3He or BF3 neutron proportional counters
– May be combined with GPS and mapping software
Active ImagingActive Imaging• Active imaging
– Not limited by natural emissions from the target
– Can give a much improved signal to background ratio
– Useful for finding a weapon hidden inside other cargo
• Transmission imaging – Takes an “x-ray” image of the target
– However uses much higher energy x-rays or gammas than traditional medical x-ray machines
– Most sensitive to high Z materials
– Can penetrate low density materials and image high density uranium or plutonium
Other Active Imaging Technologies Other Active Imaging Technologies
• Backscatter imaging – Complementary to transmission imaging
– Looks at backscattered gamma rays from the source
– Most sensitive to low Z materials such as explosives
• Stimulated emission imaging– Source of high energy x-rays, gammas or neutrons can be
used to induce emissions from the target
– Can look for induced gammas or neutrons or both
– Source can be pulsed to look for delayed emissions
Transmission ImagesTransmission Images
Rapiscan Corporation
Backscatter ImagesBackscatter Images
AS&E Corporation
Combination Combination ImagingImaging
• Transmission image at top reveals heavy shielding
• Bar shows approximate location of radioactivity detected by passive array
• Backscatter image at bottom shows organic explosive material in bright white
AS&E Corporation
Summary and ConclusionsSummary and Conclusions• Gammas and neutrons are the only detectable emissions from nuclear
weapons– Both have limited penetration in air or solids– Cannot be detected from satellites or high flying airplanes
• Emissions from weapons are weak and difficult to detect– Size Matters– Resolution Matters – Background Matters
• Germanium is the Gold Standard for gamma-ray detectors– Has very high resolution, good efficiency, requires cooling
• Thermal neutron gas proportional counters are the standard for neutrons– Moderate efficiency, requires moderation
• Active imaging has the best chance of detecting a weapon hidden inside a container – Systems are large and complex– Require experienced operator to interpret