Synchrotron Notes

8/11/2019 Synchrotron Notes

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Synchrotron Notes Year 12 Physics Unit 4

Detailed Study

Unit 4

Physics

SYNCHROTRON

1 Electron Gun

2 inac

! "ooster Rin#

4 Stora#e Rin#

$ "ea% ine

& E'(eri%ental )or* Station

Images courtesy of Victorian government www.synchrotron.vic.gov.au - 1 -

http://www.synchrotron.vic.gov.au/






Synchrotrons

• -are particle accelerators

• -produce high intensity (electromagnetic radiation) light

• -have been around since the 1930’s

What is a synchrotron?

A synchrotron is a device that uses very high energy electrons to create very bright,

pinpoint beams of light !hese beams of synchrotron light have become essential

tools for science and industry for investigating the molecular structure of things

Who will use it?

"ynchrotrons around the #orld are being used in almost every field of science and

technology - both for research and for product development Australia$s synchrotron

#ill be used by the biomedical community, manufacturing community and many

others

A synchrotron consists of si% ma&or components An 'lectron un, A inac (inear

Accelerator), A *ooster +ing, A "torage +ing, many *eam ines and at the end of

each *eam ine are the '%perimental or "tations

The Electron Gun $+,c• !he electron gun is the device #hich produces electrons for the synchrotron

!his is done by thermionic emission #hich means that a metal plate is heated to

release valence electrons 'ffectively the electrons are boiled off

• *y maing this metal plate negative it #ill repel any e&ected electrons once they

leave the surface !he negative plate is called the cathode• !o accelerate the electrons a positive plate (the anode) is placed a distance d

a#ay from the cathode !he anode attracts the electrons and they accelerate to#ard

it

• !he anode has a small hole in it to allo# the electrons to pass through

• An electric field is set up bet#een the anode and cathode .ts direction is the

same as that a positive charge #ould travel in

EECTR-C .-EDS

• A charged particle inside an electric field #ill act in a similar #ay to a mass in a

gravitational field As it moves closer to the body attracting it #ill gain inetic

energy !he sign of the charge on the particle #ill determine #hether it is attracted

to or repelled from a negative charge 'lectric field flo#s from positive to negative,

so a negative particle such as an electron #ill move in the opposite direction to the

electric field lines


/ -

As the field

lines move

closer together,

the 'lectric

field strength

increases







• 'lectric field strength ' 2 (for a point charge in space)

• 4agnitude of a uniform 'lectric field E / 0d (measured in 5m-1)

• #here 5 is the potential difference bet#een the charged plates and d is the plate separation in metres

• "ince the net force on a charged particle is . / E then the acceleration that the

particle e%periences is a 2m '2m

• !he #or done in a uniform electric field is d 5 'd• #here d is the distance the charge has moved parallel to the electric field lines

• Also ∆' -∆' p 'd 0 / 12%32

• "ince ' 126mv6 then v √(6' 2m) √(6∆52m) √(6'd2m)

• 'nergy can be measured in electron 5olts (e5) #here 1 e5 17 % 10-19 8

• !his is a more convenient unit #hen dealing #ith electrons and similarly

charged particles !he charge on an electron is 17 % 10-19

• !he potential difference 5 (voltage) bet#een the cathode and anode determines

the e%tent of the acceleration and the final energy of the electrons once they reach

the anode in electron volts (e5)

• 'lectrons guns are limited to potential differences in the 5 range• e can’t go higher than about 10005 due to difficulty in preventing leaage

through insulation and euipment, danger to scientists and from 'lectric :o#er

remember that above ;005 ionisation of air occurs !o prevent ionisation the

synchrotron operates in a very lo# pressure vacuum

• A !5 has an electron gun #ith 5 ≈ ;5 (light from a !5 is not synchrotron light

though)

• or the Australian synchrotron the electron gun #ill accelerate the electrons to a

speed of ;9< of the speed of light (0;9c)

• !o accelerate electrons to higher speeds a linear accelerator is reuired

• rom relativity #e no# that as speed increases the mass of a particle increases

hen approaching the speed of light the mass approaches infinity so huge amountsof energy are reuired for even small increases in speed

• *elo# 1=<c #e can ignore relativistic effects


/ / / / / / /

- - - - - - -

*et#een parallel

plates the electricfield is uniform

v

"chematic of an

'lectron un'lectrons that collide #ith

the anode help completethe circuit

'lectrons that do not pass

through the hole in the

anode complete the circuit

Heater element

Cathode (-) Anode (+)

Potentialdifference V

d

Electric field E

Acceleratedelectrons







inac inear 5ccelerator6 ++7++$,c• >se drift tubes in a long straight (linear) line

• 'ach drift tube can change its polarity

• !ubes become longer as electrons accelerate to eep time in tubes constant

• :olarity changes are timed to occur #hen electrons are inside tubes :rvided by

high voltage A sources• ?o acceleration occurs inside tubes as there is no electric field 'lectrons drift

inside tubes at constant velocity

• Acceleration occurs bet#een tubes due to the presence of an electric field

"ooster Rin# ++7+++++4,c• .s used to accelerate electrons even further

• rom circular motion if a particle changes direction it #ill accelerate

• ma so a force #ill act

• @irection of force is determined by the right hand slap rule +emember that the

electron travels in the opposite direction to conventional current used in the right

hand slap rule

• *ending magnets are used to deflect the beam• . / 3" / e3" #here is the charge ( e for the synchrotron 17 × 10-19),

3 is the velocity of the electron and " is the magnetic field strength in !esla (!)

• . / %a / %32r from circular motion r is the radius of the curve

• . / %32r / e3"

• r / %3" / (" #here % is the relativistic mass of the electron, and ( is

the momentum of the electron (including relativistic mass)

• ?o gain in speed #hen electron passes through a magnetic field, in fact it loses

some energy due to #or done in changing direction

• !o boost speed radio freuency R.6 ca3ities (or chambers) are located in the

straight sections of the ring to in&ect energy into the electrons (ie to boost the energyof the electrons)

• !his radiation is micro#ave radiation and as in the energy levels of electrons in

atomic orbits the absorbed photons #ill increase the electrons energy levels

(absorption spectra)

• +adiation in&ection is timed to coincide #ith the passing of a bunch electrons

!he radiation is in phase (coherent) so all electrons receive the same amount of

energy at the same time !his eeps the electrons at the same energy levels

• .n the Australian synchrotron the electrons go from 004e5 to 3e5 in the

booster ring

• !o avoid collisions and reabsorption of electrons the synchrotron is operated at

e%tremely lo# pressure in a vacuum (; × 10-7 :a compared to normal air pressureof 1 × 107 :a)


e-

1 2

1 2e-

+ve

+ve-ve

Electron from electron gun approaches

1st drift tube hich has a positi!epolarit" to attract and accelerate it#

Electron is pushed from 1st drift tube andpulled toard 2nd drift tube due to diferentpolarities of the drift tubes creating anelectric field hich accelerates the electron

across the gap#







• 'nergy is produced by a *lystron !hese are also used to pulse the original beam

of electrons from the electron gun into small bunches

Stora#e Rin#

• 'lectrons are in&ected into the storage ring every = - 60 hours appro% in theAustralian synchrotron

• 'lectrons #ill circulate for bet#een ; and ;0 hours before losing too much

energy and being absorbed by shielding or involved in collisions

• !he ring is not circular but a many sided polygon Australia’s synchrotron #ill

have 16 straight sides

• urved sections contain the 8endin# %a#nets !his is #here the synchrotron

light is produced

• *ending magnets are dipole (t#o pole) magnets

• !he bunched electrons produced pulses of synchrotron light

• !he + cavities are tuneable so they can determine the period of the light pulses

(Aust "ynch normally operates at ;004hB)

• !he beam typically loses half its energy every ; - ;0 hours

• .nsertion devices are placed in the straight sections #hich can modulate the light

produced

• -nsertion de3ices

• R. ca3ities C in&ect energy into electrons to replace energy lost as synchrotron

light hen the + cavity is positive the electrons are accelerated to#ards it, #hen

negative they are pushed a#ay !his changing electromagnetic field results in the

electrons bunching up again

• .ocussin# 9a#nets C help realign the electron beam (uadrupole D= polesE) and

rebunch the electrons (se%tupole D7 polesE)

• !he arrangement of the magnets and their strengths is referred to as the lattice

• Aust synchrotron #ill have 1= cells in its lattice each having 6 dipole, 7

uadrupole and se%tupole magnets to bend and guide the beam


$he %l"stron is used to

bunch (modulate) theelectrons from the electrongun and also to produce thecoherent high energ"microa!es hich boost theelectrons in the &' ca!itiesinside the booster ring#

Electron beam

"nchrotron light







)i##lers

• Are multiple pole magnets designed to mae the electron beam rapidly change

direction (#iggle) along a straight section ?o net deflection of beam

• 'ach bend produces synchrotron light #hich results in the intensity of the light

produced being much higher (up to ×1000 brighter than a bending magnet)

• +oughly 8ri#htness increase :actor / 2×

No7 o: (oles used ie F poles in the

#iggler results in G17× brighter light

• !he synchrotron radiation is still produced across a broad spectrum

• ight produced is of higher energy and is shifted up the '4+ spectrum

• !his light is no# incoherent and is in a #ide beam as the light is produced at

different times during each #iggle

Undulators• >se more magnets than a #iggler #hich are much smaller (up to 100)

• ?arro#er beam produced (collimation)

•"ince poles are very close together interference effects #ill produce highlycoherent light at specific freuencies

• "pacing of poles determines #avelength of ma%ima

• At these freuencies constructive interference produces e%tremely bright,

coherent, collimated light (up to ×107 brighter than a bending magnet)

• @oes not produce a continuous spectrum


road incoherent bright light

Wiggler *a series of alternate magnetic fields

arro collimated , intensel"bright and coherent light produced

Undulator *an" small poles ith !ariable spacingto produce specific a!elengths of light#







"ea% ines• :ath of light from storage ring to e%perimental #or station

• 4irrors are used to direct light

• A monochromator (single freuency filter) is used to select #anted freuencies

of light before the #or station

)or* Stations• @epending on the e%periment or end use of the light the set up of #or

stations can vary significantly

• !his is #here the synchrotron light is used to analyse materials

Synchrotron i#ht

• .s produced in the range of #avelengths (λ) from 10-11 (hard H-rays) to10-1 m

(micro#aves) !hese photons have energies respectively from 10; e5 (hard H-rays)

do#n to 10-; e5 (micro#aves)

• .t is of high intensity (brilliance)

• 'mitted in short pulses

• Arrives in parallel rays (collimated)

• .s coherent (all photons are in phase C #ave property)

• Iighly polarised (limited to a single plane for direction of #ave vibration)

• "pecific #avelengths can be isolated using di::raction #ratin#s or

%onochro%ators and used for e%amining ob&ects #hose structural dimensions is

similar to the single #avelength

• ight is released as a very narro# cone from the bending magnets and this isthen focussed to provide a very narro# beam at the #or station

Images courtesy of Victorian government www.synchrotron.vic.gov.au - -







• "ri#htness is a measure of ho# many photons per mm6 per second there are .f

the originating electron beam is #idened the brightness #ill be less !he narro#er

the cone of light the greater the brightness

;<rays = Synchrotron ;<rays• A conventional H-ray is a useful diagnostic tool as it #ill allo# us to detect

fractures is bones due to the H-ray being able to penetrate the fracture site

• !here is no coherence or collimation #ith conventional H-rays and they are lo#

intensity

• "ynchrotron H-rays are high intensity, collimated and coherent !his allo#s

them to e%tract more detail than a conventional H-ray

• !he tuneability of synchrotron radiation allo#s us to interact #ith lighter atoms

than conventional H-rays, this means #e can see clearly flesh as #ell as bones,

resulting in a much more detailed image of the fracture site and the surrounding

tissues

"ra## Di::raction• .s used in H-ray crystallography #hich is the study of crystalline structure of

comple% molecules

• !his reuires the use of the interference patterns from #ave theory (Joung’s

double slit e%periment) to determine atomic spacing

• onstructive interference #ill occur #hen coherent light is diffracted off

different layers in a crystal 2dsin / n

• here d is the spacing bet#een layers in the crystal, is the angle bet#een the

H-ray beam and the crystal surface, is the #avelength of the H-ray n is the

number of ma%ima (1, 6, 3, )

• *y rotating the crystal the smallest angle at #hich a ma%ima is detected #ill

correspond to #hen n 1

Po>der Di::raction• .s used #hen a substance does not have a regular crystalline structure that is

large enough to aim H-rays at

• A po#der is a collection of e%tremely small crystals #ith no regular

arrangement

• hen %-rays are fired at a po#der the beam is diffracted at different angles due

to the individual crystals having differing orientations to the incoming beam

• !he scattered photons no# mae rings on a screen instead of discrete spots

(.magine that you put a pen on a piece of paper to mae a dot .f you no# rotate the paper about some other point on the page you #ill produce a ring on the paper

Images courtesy of Victorian government www.synchrotron.vic.gov.au - ! -

θ

d

E.tra distance tra!elled b" photon

in second la"er is tice dsinθ /

2dsinθ #

0f this euals a hole number of a!elengths constructi!einterference occurs# $he reflectedbeam ill be of higher intensit"#







instead of a dot .f you used t#o pens originally you #ould get t#o concentric rings

about your a%is of rotation and so forth)

• !he position of the rings determines the structure of the po#der

• !he scattered photons spread out as hollo# cones (funnels)

• *y surrounding the specimen in a circular photographic film the rings leave

concentric bands on the film !his is called a @ebye-"herer camera

• Alternatively a moving detector such as a eiger counter can orbit the specimen

and determine the direction of the scattered photons and their intensity

• *ecause the beam is pro&ected at an angle θ to the crystal surface the angle the

cone maes #ith the scattered beam is eual to 6θ

• @etectors produce graphs #hich sho# the relationship bet#een the intensity of

the scattered beams and their angle to either the undeflected beam (6θ) or the

crystal surface (θ)

Images courtesy of Victorian government www.synchrotron.vic.gov.au - " -

$he podered sample is in the centre# $hecones of scattered photons strie the circularphotographic film

3hen the film is laid flat the concentric rings aree.posed# $he" centre about the to holes in the film

here the 4-ra"s enter and e.it#

θθ

θ

urface of small cr"stal

small cr"stal

"nchrotron 4-ra"

Photographic film

5ndeflected beam

ragg diffraction beam#Constructi!e interference ith la"erbeneath surface maes the beambrighter than of deflection fromsurface la"er onl"#

2







Scatterin#Kccurs #hen a photon transfers some of its energy to electrons !his is different to

photoelectric e%periments #here all of a photon’s energy is absorbed by an electron

• !here are t#o ma&or types of scatteringL

• Tho%(son (elastic) scattering #here no inetic energy is lost

• Co%(ton (inelastic) scattering #here inetic energy is lostAn e%ample of Tho%(son scatterin# is *ragg diffraction #here the scattering

is elastic ?o energy is imparted to the interacting atoms !his is usually the case for

lo# energy (M100 e5) H-rays

• !he H-ray photon before the collision effectively transfers no inetic energy to

the electron or atom and retains virtually all its inetic energy after the collision

• !he #avelength of the scattered photon is eual to that of the incoming photon

.magine a billiard ball bouncing of another stationary ball)

• hen the H-ray photon energy is above 100 e5 the photon is liely to be

absorbed and the energy transferred to an electron is enough to ionise the atom

and the electron is e&ectedCo%(ton Scatterin# results from the H-ray photon colliding #ith a relatively

stationary electron #hich is not tightly bound to an atom (compared to the speed of

light) and the photon transfers some of its momentum to the electron

• !he :hoton has less energy and so changes its #avelength and freuency

∆' h∆f h(f in C f out), and the electron gains an euivalent amount of inetic energy

(∆' Nmv6 h∆f)

• At all times the total energy is conserved

• 'nergy of photon before interaction energy of photon after interaction plus

energy gain of electron hf in hf out / Nmv6

• !he scattered photon has a longer #avelength (shorter freuency) than the pre-

collision photon as it has less energy and ' hc2λ hf

• !he reduced momentum of the photon ∆ p h∆f2c h2c(f in C f out), must eual

the gain in momentum of the electron (∆ p m∆v) +emember momentum is a

vector uantity ?ote classical mechanics can be used due to the relatively slo#

speed of the electron

The Photoelectric e::ect• Kccurs #hen a photon is completely absorbed by an electron

• !he energy from the photon enables the electron to brea free from the metal

surface and thus produces a current

• +efer to your notes from the topic .nteractions of ight and 4atter

Ris*s -n3ol3ed >ith Synchrotronshilst producing more intense light and higher resolution images there are still

dangers involved in the use of "ynchrotrons

• Iigh energy photons and electrons are contained #ithin a concrete tunnel #hich

is thic and dense enough to absorb any errant radiation that escapes from the

storage ring

• !he high voltages used in the electron gun and linac are also haBardous and

reuire heavy insulation to prevent electrical shoc

• !he lo# pressure vacuum is a ris as any breach of the synchrotron apparatus

#ould cause a dangerous implosion as the atmosphere rushes in to eualise the air

pressure (!he reverse of a pressurised plane losing a door or #indo# at highaltitude)

Images courtesy of Victorian government www.synchrotron.vic.gov.au - 1# -




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