Particle Accelerators and Detectors

7/23/2019 Particle Accelerators and Detectors

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Particle acceleratorsJ.2.1 Explain the need for high energies in

order to produce particles of large mass.

J.2.2 Explain the need for high energies in

order to resolve particles of small size.

J.2.3 Outline the structure and operation of alinear accelerator and of a cyclotron.

J.2.4 Outline the structure and explain the

operation of the synchrotron.

J.2.5 tate !hat is meant "y "remsstrahlung

#"ra$ing% radiation.

J.2.& 'ompare the advantages and disadvantages

of linear accelerators( cyclotrons and

synchrotrons.

J.2.) olve pro"lems related to the production

of particles in accelerators.

Option J: Particle physics

J2 *article accelerators and detectors


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Particle acceleratorsExplain the need for high energies in order to

produce particles of large mass.

+,ecalling the mass-energy relationship !e see

that from E mc2!e o"tain

+/ote that the "igger the mass of the particle !e

desire to produce( the larger Emust "e.



mass-energy equivalencem E/c2

E0*E uppose you !ant to create a proton from

energy. o! much energy must you provide6

O789O/ *article creation must adhere to

conservation la!s.

+ince the "aryon num"er of a proton is :1( and

the "aryon num"er of a photon is zero( an anti-

proton must also "e created.

+8hus !e need at least E 2#;3< e=% 1


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*,'89'E

>ind the energy needed to create an electron.

8hen sho! that conservation of charge andconservation of lepton num"er are "oth satisfied.

O789O/

+8he rest energy of an electron is ?.511 e=.

+8hus !e need E 2#?.511 e=% 1.?22 e= to

create an electron( anti-electron pair.

FYI

8he minimum energy of any accelerator must "e

t!ice the rest energy of the heaviest particle itis desi ned to detect@create.



2e-: e:

CHARGE

LEPTON NUMBER

? -1 :1

? :1 -1


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+Aou may "e as$ing yourself BHow do you get high

energy photons6C

+Aou can BharvestC high energy photons from t!osources

1% 8he energy that is released "y the

annihilation of matter@anti-matter particles

#that !ere created else!here%.

2% 8he D9/E89' E/E,A of the particles Fust"efore they collide.



FYI

8o maximize the amount of EKconverted to the

photonsG energy the particle and anti-particleare made to travel in opposite directions.


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*,'89'E ccelerators use E-fields and H-fields

to control the speeds and paths of the charged

particle "eams they accelerate. Explain ho! thisis advantageous !hen using matter( anti-matter

"eams traveling in opposite directions.

O789O/

+atter and antimatter have opposite charges.

+,ecall FB qvBsin( and FE qE.+8hus the directions of the magnetic force #given

"y the right hand rule% and the electric force

depend on the signs of the charges the forces are

acting on.

+ince matter and antimatter have opposite signs(

the samefields !ill control "oth "eams at onceI


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resolve particles of small size.

+,ecall that the !avelength of a photon ofenergy Eis given "y E hf hc/.

+'learly( the "igger the energy Ethe smaller the

!avelength .

+,ecall also that the smaller the !avelength( the

"etter the resolution.

+8hus( the "igger the energy( the smaller the

particle that can "e resolved.



wavelength energy relationship hc/E


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Particle acceleratorsOutline the structure and operation of a linear

acceleratorand of a cyclotron.

+s the name implies( a

linear acceleratoror

linac is a straight tu"e.+8he longer it is( the more

energy the accelerated particle

!ill have !hen it reaches the

end of the linac( at !hich

point it smashes into a targetat the end of the tu"e.

FYI

8he accelerated particle is

charged. 8he alternating p.d. must "e timed so

that the charge is repelled from the B"ehindCtu"e and attracted to the Bin-frontC tu"e.




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acceleratorand of a cyclotron.

*,'89'E Explain !hy the successive tu"es

increase in length in the linac.

O789O/+ince the particle is

accelerating( the distance

covered in a fixed time

interval increases.

+8he tu"es are designed so

that the freKuency of the

alternating p.d. can remain

fixed.

FYI

n alternate design might vary the freKuency and

$eep all of the tu"e lengths eKual.




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accelerator and of a cyclotron.

+8he cyclotronaccelerates

particles !ithin t!o L-shaped

hollo! containers( li$e theones sho!n here.

+Each L is connected to opposite

terminals of an alternating p.d.(

!hich accelerates the charged

particle from the center.+ perpendicular magnetic field causes the

particles to follo!ed a curved spiral until

reaching the outside circumference of the LGs.

+s the follo!ing slide !ill sho!( the period T

of the alternating p.d. is independent of theradius of the particleGs traFectoryM



+

B


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+9n order for the particle of

mass m and charge qto

experience a centripetal force(it must satisfy F mac.

+8he force Fis caused "y the

magnetic force F qvB.

+Hut the centripetal acceleration ac v2/r.

+8herefore F qvB macso thatqvB mv2/r( or

qB mv/r m#2r@T%/r 2m/T.



+

B

FYI

/ote that f# 1@T% does not depend on theradius of the path. 9t depends onlyon m,qand B.

period of a cyclotronT 2m/#qB%


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+8o get an idea of the energy

capa"ilities of a cyclotron

consider the intermediate stepfrom the previous slide

+>rom qB mv/r !e get v qBr/m

so that

EK #1@2%mv2 #1@2%m#qBr/m%2.



+

B

FYI

8he energy capa"ilities of the cyclotron are

proportional to the sKuare of its radius.

ince creating large dis$s of vacuum !ithin the

LGs is very difficult( the largest cyclotrons areonly a"le to produce energies in the e= range.

energy of a cyclotronEK q2B2r2@#2m%


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Particle acceleratorsOutline the structure and explain the operation

of the synchrotron.

+8he synchrotronis similar to a linac in that

acceleration occurs "et!een tu"es.

+8he tu"es in a synchrotron(ho!ever( are all the same

length( and they are arranged

in a circle.

+8he charges are accelerated

"et!een the tu"es "y anelectric field applied across

t!o plates.

+8he tu"es are surrounded "y a

magnetic field that causes the

charged particles to follo! a curve.



!a"netic #iel$%en$s %ea!

electric #iel$

accelerates

%ea!


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of the synchrotron.

+8he alternating p.d. units are complex. 8hey

must "e synchroniedprecisely to the

accelerating particles #hencethe name synchrotron%.

+8he magnetic field strength

must also vary as the

particles pic$ up speed.

+8he advantage of asynchrotron over a linear

accelerator is that the

particle can ma$e as many

circuits as needed to

accelerate it to any energy.



!a"netic #iel$%en$s %ea!

electric #iel$

accelerates

%ea!


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of the synchrotron.

+nother advantage of the synchrotron over either

of the previous accelerators is that you can have

a matter "eam( andan antimatter "eam(

circulating at the

same time in

opposite

directions.

+s !e have

already discussed(

head-on collisions

maximize the energy

output of the

collisions.



&er!ila%

Bata'ia()llinois.

protonantiproton


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of the synchrotron.

+8he synchrotronGs maximum energy capa"ility is

limited "y its curvature. 8he faster a particle

travels( the larger the centripetal forces themagnetic field needs to provide. *7 due to

relativistic effects( the faster a particle goes(

the more massive it

"ecomes-thus adding to

the difficulty of ma$ing

it turn in a circle.

+8he largest synchrotron

in the !orld is at 'E,/(

in eneva( and has a

circumference of 2) $m.




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Particle acceleratorstate !hat is meant "y "remsstrahlung #"ra$ing%

radiation.

+Nhen a charge is accelerated( it produces

electromagnetic radiation called%re!sstrahl*n"

ra$iation.+iven that an

acceleration is a

change in velocity

and that velocity

can change in itsmagnitude #across the plates% or its

direction #the magnetic field tu"es% !e get

radiation at "oth locations in the synchrotron.

+ince the radius of curvature is so large( the

radiation from the tu"es is relatively smallcompared to that of the plates.




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Particle acceleratorstate !hat is meant "y "remsstrahlung #"ra$ing%

radiation.

+8he radiation at the plates is in the 0-ray

region of the spectrum( and is highly polarized.

+Hecause the 0-rayradiation is very

intense( very

parallel( and very

polarized( it is

!ell-suited for0-ray diffraction and other experiments

investigating the properties of materials.



FYI

9n fact( there are some synchrotrons that have

"een "uilt solely for 0-ray diffraction andmaterials physics.


18/37

Particle accelerators'ompare the advantages and disadvantages of

linear accelerators( cyclotrons and synchrotrons.

+ince there are no curves in the linear

accelerator( there is no radiation loss due to

direction change as there is in the synchrotron.+9n the linac and the cyclotron( if you miss the

collision at the end of the run( the proFectiles

are lost( !hereas in the synchrotron the

particles can continue to go around as many times

as is needed to effect a collision.+8he difficulty in constructing large enough

evacuated LGs and large-area magnetic fields

prevents the cyclotron from "eing a serious

contender in very high energy physics research.

+8he cyclotron is the simplest to construct forlo!-energy applications.




19/37

Particle acceleratorsolve pro"lems related to the production of

particles in accelerators.

+9f a particle and an antiparticle collide head-

on then !e not only get the energy of

annihilation #the rest mass energy of the t!oparticles% "ut !e harvest all of the original

$inetic energy of the particles "efore their

collision.

+9f a particle collides !ith a stationary target

not all of the energy can "e converted to ne!particles. 8his is "ecause to conserve momentum

the ne! particles must move in the same direction

as the original "eam.

+8hus the energy needed to produce particles

stri$ing a stationary target must exceed theactual rest mass energy of the particles created.




20/37



+8he minimum availa"le energy Eafor particle

creation o"tained "y the collision of a particle-

proFectile of rest mass m and total energyE!itha stationary target of mass !is sho!n here



energy available in

collision of a

moving mass mwith

stationary mass M

Ea2 2!c2E :#!c2%2:#mc2%2

M+ tar"et rest !ass.m+ pro,ectile rest !ass.

E+ pro,ectile total ener"y.

FYI

/otice that each factor has the units of

energyP2. LonGt miss a sKuareI ny!here.

9n general( the num"ers are more managea"le if!e $eep the energies in e= #or even e=%M


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energy available in

collision of a

moving mass mwithstationary mass M

Ea2 2!c2E :#!c2%2:#mc2%2

M+ tar"et rest !ass.

m+ pro,ectile rest !ass.E+ pro,ectile total ener"y.

E0*E o! much energy !ould "e availa"le if a

15 e= proton collided !ith a stationary proton6

O789O/

+m ! ;3< e= c-2 ?.;3< e=c-2. E 15 e=.

Ea2 2!c2E :#!c2%2:#mc2%2

Ea2 2#?.;3


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olve pro"lems related to the production ofparticles in accelerators.



+The E#iel$ accelerates the char"e$

particles.

+The B#iel$ !a/es the char"e$

particles t*rn an$ #ollo0 a circ*lar

tra,ectory.


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+As vincreases so $oesm( an$ thecentripetal #orce Fc+mac +mv

1/r!*st also

increase to /eep the particles !o'in" in the

synchrotron2s circ*lar path.

+3ince FB+ qvB + Fc(mv1

/r+ qvBqB+mv/r.+Th*s r+mv/qB.

+3ince r+ CON3T #or a synchrotron( B!*st

increase 0ithmvas the particle ener"y

increases.


24/37




+Each %ea! sho*l$ s*pply E = 4415 Me-

6hal# o# 1175 Me-8 in /inetic ener"y.

+EK+ Em5c1+ 4415 9 ;< + 4 EK%eca*se the

annihilation pro'i$es so!e ener"y.


25/37




+&or a stationary tar"et*seEa

1+ 1Mc1E> 6Mc181> 6mc181( 0hereEa+ 1175(M+ ;


26/37




@EK+ 4


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Particle accelerators



linac

cyclotron

synchrotron

detail


28/37

Particle detectorsJ.2.< Outline the structure and operation of a

"u""le cham"er( the photomultiplier and the

!ire cham"er.

J.2.; Outline international aspects of research

into high-energy particle physics.J.2.1? Liscuss the economic and ethical

implications of high-energy particle physics

research.




29/37

Particle detectorsOutline the structure and operation of a%*%%le

cha!%er( a photomultiplier and a !ire cham"er.

+8he particles created in collisions must

someho! "e detected. 8he%*%%le cha!%er!as

one of the first detectors for such particles.+Nhen you pop the top of a soda( pressure is

suddenly released !ith a fizzing sound.

+9f pressure is released the liKuid reaches

its "oiling point( and "u""les form. 9n the

case of soda( the "u""les are 'O2.

+Each "u""le forms at the site of an

impurity - if there !ere no impurities( "u""les

!ould not form automatically.

+9nstead of !ater and 'O2( a "u""le cham"er uses

hydrogenthat has "een cooled to the liKuid stateFust "elo! its "oiling point.




30/37

Particle detectorsOutline the structure and operation of a%*%%le

cha!%er( a photomultiplier and a !ire cham"er.

+8he "u""le cham"er consists

of a liKuid-hydrogen-filled

cylinder having one of itsfaces made of glass.

+9ncoming particles stri$e the

target and produce ne! particles.

+8he piston moves out!ard(

lo!ering the liKuid pressure.

+Hu""les form in the depres-

surized liKuid hydrogen along

the path of the particles.

+ magnetic field passing through the "u""le

cham"er ensures that the particles !ill have

curved traFectories if they are charged.



Stationary Target

Piston

Camera

Light


31/37

Particle detectorsOutline the structure and operation of a "u""le

cham"er( aphoto!*ltiplierand a !ire cham"er.

+ome of the products of collision experiments

are gamma particles( !hich are of course high-

energy photons.+amma particles are very adept at ionizing

matter( "ut the act of ionization a"sor"s

the photon( removing it from the picture.

+Letection of a single photon "eing

a"sor"ed is difficult( "ut it is made

possi"le "y a device called a

photo!*ltiplier.

+ single photon enters a photomultiplier

through a small !indo! and is a"sor"ed "y

a photosensitive plate( releasing an

electron according to the photoelectric effect.



50V

100V

150V

200V

250V

dynode

photo

!*ltiplier

photo

sensiti'e!aterial


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cham"er( aphoto!*ltiplierand a !ire cham"er.

+9n the photomultiplier are a cascade of dynodes(

each at a higher potential than the previous.

+Hecause of the acceleration caused "ythe p.d.( more electrons are released

during each step in the cascade.

+8he end of the cascade has enough

electrons to create measura"le current.

FYI

8he eiger counter uses a photomultiplier.

>or very high-energy photons a photomul-

tiplier doesnGt !or$. 9nstead a scintillator

is used. scintillator has a screen of

phosphorescent material that glo!s !hen

struc$ "y a very high energy gamma photon.



50V

100V

150V

200V

250V

dynode

photo

!*ltiplier

photo

sensiti'e!aterial


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cham"er( a photomultiplier and a0ire cha!%er.

+9f high-energy particles pass through a

gas( the gas particles "ecome ionized.

8hin$ of the high-energy particle asB$noc$ingC electrons off of the gas

atoms as it passes "y.

+9f the gas is "et!een t!o !ires !hich

have a p.d. applied to them( these

freed electrons travel to the

positive !ire and a!ay from the

negative !ire( setting up a current.

+8he voltage across a resistor !hich

the current is made to pass through

can then "e digitally recorded.

+8his device is called a0ire cha!%er.



A

V R

0ire

cha!%er


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cham"er( a photomultiplier and a0ire cha!%er.

+o ho! does one get a 3L picture

of particles6 imply set up an

array of !ire cham"ers( and recordnot only the place( "ut the time

the cham"ers detect a particle.

+8he%l*earray tellsus the left-

right coordinate of the particle.

+8he re$array tellsus the up-do!n

coordinate of the particle.

+8he ti!in" from one dou"le-grid to

the next tells us the for!ard-"ac$!ard

coordinate of the particle.



$o*%le"r

i$

$o*%le"ri$

12

3

4

1

2 3

4

12

3

4

1

2 3

4

FYI

3L images can then "e computer generatedI

64(4(t48 6;(1(t18


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Particle detectors



Lar"e Ha$ron Colli$er

%*%%le cha!%er

Gei"er

co*nter


36/37

Particle detectorsOutline international aspects of research into

high-energy particle physics.

+Hecause of the extreme expense in "uilding and

operating high energy particle physics

installations( the larger ones have to "e aninternational colla"oration to help share the

costs.

+Hecause of the international aspect of particle

research( there is a tendency of the academic

edifices of the !orld to come together( even intimes of !ar.

+8he research is also transparent-no country can

"e the BclearinghouseC of the information

generated "y any international facility.




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Particle detectorsLiscuss the economic and ethical implications of

high-energy particle physics research.

+*eople as$ !hether the cost of E* is !orth it.

+8heoretical physics stagnates !ithout

experimental verification.+'uriosity is a fundamental part of the human

mind.

+haring large research costs among many

countries encourages cooperation "et!een

different cultures.+ynchrotron radiation has a large range of

applications( including "iology( medicine( and

technology.



Particle Accelerators and Detectors

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