Linear Accelerators and Detectors

8/8/2019 Linear Accelerators and Detectors

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2.

1 for the relativistic case Kinetic energyNote: If is the charge of the ion, is the peak value of the ac frequency applied to the drifttubes, then in each gap ions acquire energy .If there are such gaps then, kinetic energyacquired in the gaps For electrons, whose speed very rapidly approaches the speed of light, the accelerator consists of

a straight tube in the form of a series of cylindrical metal cavities. Power is fed to the accelerator

from a series of devices called klystrons, which produce electromagnetic radiation in the form ofmicrowave pulses that are transported via waveguides to the accelerator. There they generate an

oscillating electric field pointing along the direction of the metal tube and a magnetic field in a

circle around the interior of the accelerating tube. The magnetic field helps to keep the beam

focused, and the frequency of the microwaves is adjusted so that the electrons arrive at each

cavity of the accelerator at the optimal time to receive the maximum energy boost from the

electric field. As long as this phase relationship can be maintained, the particles will be

continuously accelerated. The largest electron linac at Stanford has a maximum energy of50 .In linear accelerator we face two types of focusing problems.

1. Phase focusing2. Radial focusing

Phase Focusing

It will enable an accelerator to produce more energetic particles. It is possible to obtain phase

focusing, if the particle beam made to cross the accelerating gap or cavity during the raising part

of accelerating potential. This is because particles with low velocities arrive at the cavity later

than the particles with higher velocities. Therefore, they will receive more acceleration and

equalize their velocity. This is not possible or will have opposite effect, if the particles made tocross the cavity during falling part of the accelerating potential.

Radial Focusing


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This will enable to the accelerator to produce a well-pointed beam of particle. As can be seen

from the figure, an ion off the axis is acted upon by an axially directed field component,which accelerates the ion and by the radial component of the field. The radial componentacts inwards in the first half of the gap, tending to push the ion towards the axis. In the secondhalf of the gap, the radial component acts outwards, tending to push the ion away from the axis.

Thus, the radial component has a focusing action in the first half and a defocusing effect in the

second half. However, since the ions moves faster than in the second half than in the first, they

spend less time in the second half of the region, so that there is a net focusing effect due to the

radial field. It can be shown that the radial focusing is effective if the ions cross the gap during

falling part of the accelerating potential, in which the accelerating potential decreases from the

maximum towards to zero.

These requirements for phase focusing and radial focusing will be opposing one another. Toovercome these problems we can use either,

1. a third electrode (grid)2. quadruple magnets.

A metallic gird has been placed across the entrance end of each drift tube. This makes the lines

of force of the electric field converging all the way from the exit-end of a tube to the entrance

end of the next tube. Radial focusing now occurs at any time during the accelerating half cycle of

the r.f field, i.e., for the phase of the field between 0 to . Hence, this covers the range of phasefocusing.

A second method of achieving radial focusing is to use a quadruple magnet within each drift-

tube, which compensates the defocusing of the ion beams during the gap crossing.

The light arrows indicate field directions; the heavy arrows, the force on the positive particle

travelling into the paper.


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Cyclotron

Cyclotron is based on the principle that charged particles acquire energy when they repeatedlymove through an alternating electric field along a spiral path. They move in a spiral path by a

perpendicular magnetic field. A cyclotron consists of two D- shaped hollow semicircular

chambers and called Dees because of their shapes. These dees are connected to theterminals of an alternating high frequency and high voltage peak value.

This arrangement makes one dee is positive and other is negative during one-half cycle and vice

versa in the next half cycle. An ion source S of positive ions, such as protons, deuterons, particles, etc, is placed in the central region of the gap between the dees

and

.

A uniform magnetic field is applied perpendicular to the cross sectional area of the dees byplacing them between the pole faces of a large electromagnet.

For an instance (say) is positive and is neagative.These positive will be acceleratedforwards dee and enter the dee . As there is no electric field inside the dees, the ions aresubjected to perpendicular magnetic field. The perpendicular magnetic field acts on the ions and

positive ions move in a circular path.

Once inside the dee, they experience no acceleration and move with a constant velocity. After

traversing the semicircular path inside the dee , they return to the gap between the dees. Thefrequency of the oscillator is adjusted in such a way that when ions reach the gap, the dee

becomes positive and becomes negative. Now the positive ions are accelerated towards thedee , thus gaining in kinetic energy. The ions enter the dee with higher kinetic energycompared to the value it was in dee . In the dee , they move again in a circular path withlarger radius but with a constant velocity. After covering the semicircle inside the dee , theions reach the gap between the dees. At this instance gain the polarity of the dees is reversed,

positive ions again experience acceleration and this process is repeated many times. Finally,

when the positive ions reach the periphery of the dee after gaining a maximum energy, it is

extracted out of the dees by means of high voltage deflecting plates.


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Let be the mass of the positive ions to be accelerated is the charge on the ion. The ion movesin a semicircular of radius with velocity in a perpendicular magnetic field .Magnetic force acting on the ion As positive ions gain energy, i.e. increases, the ions move in a semicircle of larger and largerradius. Therefore, the path of the ions is spiral in the dees.

If is the time taken by the ions to complete a semicircular path, then

12 2

Where is the angular frequency and Time for completing a full circular path is given by

2 2

is independent of the velocity of the ion, radius of the semicircular path and radius of the dees.Hence, the frequency of the oscillations required to keep the ion phase is given by 1 2

where cyclotron frequency and it is the frequency of ac oscillator also.If is the radius of the orbit from which the ions are extracted out of the dees and is the velocity of the ions in the last orbit, then,

Correspondingly, the maximum kinetic energy of the ions is 12

12

12 2


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The above equation shows that the final energy depends on the radius of the dees and thestrength of the magnetic induction field (or the frequency f of the rf voltage which depends on).So to achieve higher energy, a magnet of larger radius is to be used.But Where -total number of circles completed by the ions before it is extracted out of the dees, peak value of the ac voltage12

1

2

It can be shown that the radius of the orbit after the dee crossing is given by

1 2 Plane Focusing

Due to the curvature of the magnetic field lines near the periphery (outer edge) of the magnet the

particles moving in orbits with large radius are focused towards the central plane. In other words

due to the end effect the plane focusing is possible.

It is also possible to obtain plane focusing using specially constructing the outer edge of the

magnet as shown below.

In this diagram magnetic field lines are made progressively curved from the centre.


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The electrons are initially accelerated in an electron gun and are released into circular orbit due

to the magnetic field (of strength ) which is at right angles to the electron velocity.

Or A flux cuts this orbit and is changed at the rate of .Due to the change of flux, an induced .. is produced causing more speedy motion of theelectrons.

Let the induced field component tangential to the orbit be and the orbit radius be , then theinduced

..is given by

. Induced .. in the electron orbit is equal to work one by the unit charge in going around theorbit or radius .i.e. 2and the force acing on the electron will be equal to the rate of change of momentum.i.e.

But

Hence, 2

If we start from a zero flux and zero field at the orbits, we get 2 This relation between the magnetic flux and field at the orbit involves only the radius of the orbit

and is independent of energy or the mass of the particle.

A large central flux is created initially by the doughnut shaped magnet pole face. In order to

produce flux changes, a sinusoidally varying current is introduced into the coils of the

electromagnet during the first quarter of the cycle.


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Limitation Due to Relativistic Effect

The maximum energies of particles attainable in a cyclotron cannot be increased indefinitely byincreasing the size of the apparatus because of relativistic increase in mass of the particles to

1

Where is the mass and , the mass at velocity . As the speed of the particle increases tobecome comparable with the speed of the light, its mass m increases appreciably.

Therefore, the frequency goes on decreasing and the particle traverses each dee too slowly andbecomes more and more out of step with the applied alternating potential difference, disturbing

the synchronization. The frequency of circular motion of the particle.

1 2 There are two methods possible to compensate for the relativistic increase in mass.

a. By keeping the factor 1 constant to keep the orbital frequency of the ionsunchanged. It is achieved by using such an electromagnet for which the magnet field

increases with an increase in the velocity of the ions during their motion along orbits of

increasing radius. Such an accelerator is known as synchrotron

b. The decrease in ion frequency due to relativistic effect can be compensated by reducingthe frequency of the applied alternating electric field with increase of velocity such that

the two frequencies always equal each other. Such an accelerator is synchrocyclotron.

Synchrocyclotron.

In this machine, the limitation introduced by the relativistic effect was removed by introducing a

periodic step-by-step increase of the period of the r.f. field. Initially the particles are accelerated

like in cyclotron machine. Due to the relativistic effect, it is not possible to accelerate beyond

certain radius of the particle orbit. At this orbit, all particles move with the same phase butwithout any acceleration, when they cross the gap.

This orbit is called phase stable orbit.

In synchrocyclotron machine, this radius is made small by using strong magnetic field. After

reaching this radius or phase stable orbit, the frequencies of the r.f. field is reduced by small step

and accelerate the ions until, they get again another phase stable orbit. In this way, it is possible

to accelerate particles to higher energies by reducing the frequency of the r.f. field in steps.


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The radius of the machine limits the maximum energy attainable in the machine. i.e. Radius of

the dees.

In the relativistic limit 1

And momentum

1

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1

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1 1

2 2

2

For each value of particle energy in the magnetic field there is a particular orbit radius The specific frequency of revolution is given by

2 2Where

11

Let be the angular frequency of the phase stable orbit near the centre of the machine and be the angular frequency of the accelerating field for phase stable orbit near the periphery of the

magnets.

Then

magnetic ield at the centre


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magnetic ield near the peripheryThe kinetic energy attainable

.

.

Where . . .

The output of the machine is the form of pulse, if is the pulse repeation time, then 2

Where

- is the number of revolution and

is the average angular frequency.

Synchrotron

In this machine the charged particle are accelerated in fixed radius path by increasing the

strength of the magnetic field.In this machine, the particles are maintained at constant radius in a

ring shaped vacuum chamber contained in a magnetic field.


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In relativistic

Hence, to increase the momentum and energy of a particle in a fixed radius path the magnetic

field has to increase.Maximum momentum attainable is therefore limited by both themaximum strength of available magnetic field and the size of the ring.

The circulating angular frequency of the particle at any momentum is given by Acceleration is achieved by having the particles pass through suitable phased r.f. cavities in the

ring.

To keep particles well contained inside the beam pipe and achieve the stable orbit, particles are

accelerated in bunches, synchronization with the radio frequency field. Analogously to linacs, all

particles in a bunch have to move in phase with the radiofrequency field. In high-energy

machine, the particles are accelerated in a linac before injection to the synchrotron.

For electrons, which become relativistic at very low energy, the velocity, and thus circulating

frequency is essentially constant. In the case of protons, we need to reduce the frequency of theaccelerating potential as they accelerated.

The machine is usually constructed by using large number of annular tubes and straight tubes.

The annular tubes are covered with c shaped magnets in order to give circular path for protons.

The straight tubes consists accelerating electrodes, electrostatic deflectors and quadrupole

magnets. Charged particle, which travels in a circular orbit with relativistic speeds emit

synchrotron radiation. Amount of energy radiated per turn is

4

3

For relativistic particles, the energy loss increases as , becoming verysignificant for high energy light particles ( electrons)

For relativistic electrons and protons of same momentum the ratio of energy loss electronproton 10

The radius of an electron synchrotron must be large to compensate for the synchrotron radiation

loss.


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Example: The LEP (large electron positron) acceleration at CERN was ~27 8.6 incircumference. The CERN SPS (super proton synchrotron) has a radius 1.1 .. Muchhigher energies are achieved for protons compared to electrons, due to smaller losses caused bysynchrotron radiation.

Depending on whether the beam is shooting into a stationary (fixed) target or is colliding with

another beam, both linear accelerator and cyclic accelerator are divided into two types.

1. Fixed target machine2. Colliders

In fixed target machine, accelerated particles are extracted from the accelerator and directed onto

an external target. This can be the source of secondary particles , , , , , , , , thatneed to be stable or long-lived but need not to be charged.

Some Fixed Target Accelerators

Machine Type Particles Tevatron II

Fermi lab, USA

Synchrotron P 1000

SPS

CERN, Geneva, Switzerland

Synchrotron P 450

SLAC

Stanford, California

linac 25Note: For collisions between two particles

and

, the total four momentum squared of the

system in the laboratory frame is 2 2. . 2 2. . This is a Lorentz invariant.i.e. the same in all the frames. The centre of mass systemhas 0 by definition. If the total energy in the frame is then, 2 2. . If the target at rest then

Secondary Beams

Accelerator

Extracted Beam

Target


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2Example: For a 450

proton a stationary proton target (

0.938

)

2 450 0.938 29 . Only 29 is available for interesting physics, i.e. to createnew particles. The rest goes into propelling the centre of mass forward.In colliders machines two beams of particles traveling in almost opposite directions are made to

collide at a small or zero crossing angle. In most of these machines, currently in operation the

colliding particles have the same mass.

Some colliders

Machine Particles E-beam KEKB, KEK,Tokyo, Japan , 8,3.5PEP-II, SLAC, California, USA , 9,3.1LEP, CERN, Geneva, Switzerland , 105HERA, Hamburg, Germany , 30,920Tevatron II, Fermi lab, Illinois, USA , 1000LHC, CERN, Geneva, Switzerland , 7000

If incident particle and target collide head on with and and they both highlyrelativistic

and

then,

2. 2 2and 2 2 4 2 i.e. a 450 proton hitting a 450 proton gives 900 available.i.e. Great advantage of colliding beam machines over fixed target machines. For or machines one ring is sufficient since particles with opposite charges and samemass can go in opposite directions using the same magnets. For or two rings arerequired with different magnets. The disadvantage of colliding beams rate is much lower because

the target is much smaller.

Luminosity

The reaction rate is given by Where cross section units of area, Luminosity units of area,


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-number of bunches of particles in each beam (typically4 8), revolution frequency 450 40 . , are the number of particles in each bunch ~10 the area of each beam ~Particle Detectors

The nuclear particle detectors are broadly divided into following two categories.

a. Electronic DetectorsThe passage of a particle through such detectors produces charged particles which can be

collected by applying an electric field to produce a transient electrical pulse.

These pulses are then amplified and analyzed to find the intensity or energy spectrum of the

incident radiation. The following types of electronic detectors are commonly used.

i. Ionization chamberii. Proportional counteriii. Geiger-Muller counteriv. Scintillation counterv. Cerenkov countervi. Semiconductor detectorb. Track Detectors

Such detectors produce charged particles due to ionisation loss along the trajectory of the

particle. These charged particles produce a particular physical phenomenon which leaves a trackalong the trajectory of the particle. The following track detectors are commonly used.

i. Cloud chamberii. Bubble chamberiii. Nuclear emulsion detectorsiv. Spark chamber

Gas Filled Ionization

When nuclear radiation passes through a gas contained between two electrodes, it ionizes the gas

molecules. The ions may be collected by applying an electric field (to the electrodes) to produce

an ionization current pulse across a resistor the magnitude of which depends upon the nature and

velocity of the incident particles and properties of the detector. Gas filled detector consists of a

cylindrical metal container filled with a readily ionizable gas such as argon or krypton at a low

pressure and carrying (along its axis) a thin tungsten wire, which is well insulated from the

container. The (outer) wall of the container is connected to the negative terminal and the central

tungsten electrode to the positive terminal of a d.c. power supply. The ions collected by the

electrodes produce an output pulse across the resistor which can be amplified by a linear

amplifier and analysed by a counter.


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18

The above curve can be divided into six distinct regions. In region I , the voltage applied is so low

that recombination of ions takes place. This is known as ion recombination region.

In region II, the voltage is sufficiently high so that only a negligible amount of recombinationtakes place and the ion pairs move to the electrodes so rapidly that virtually every ion pair

reaches the electrodes and all ion pairs are collected in the region II. The gas filled detector

operating in region II is called an ionization chamber.

When the potential difference is further increased, the ion pairs get accelerated by the electric

field and produce secondary ion pairs in successive collisions. This phenomenon is called

Townsend avalanche. The ionization current is directly proportional to the energy of the incident

particle in this region III. The gas filled detector operating in this region is called the

proportional counter. Such a proportional behaviour is lost in the region IV for which the applied

potential is further increased. The pulse heights are still related to the applied voltage but nolonger proportional to initial ionizing intensity. This region is not useful for measurement and

limitations may be because:

(i) ultraviolet photons may be formed;

(ii) new electrons may be formed when positive ions reach the cathode; or

(iii) the space charge may distort the electric field

In a typical Townsend avalanche created by a single original electron, many excited gas

molecules are formed by electron in addition to secondary ions. Within usually a few

nanoseconds, these exited molecules return to their ground sate through the emissions of

ultraviolet (UV).These photons may be reabsorbed in the gas by photoelectric absorption less

tightly bound electrons creating new free electrons. Alternatively the photons may reach the

cathode wall where it could release a free electron upon absorption. In both cases, the newly

created free electrons move towards the anode and trigger another avalanche.

In region V, the multiplication of secondary ions increases by a large factor and the curves for and particles merge showing that the ionization current is independent of the initial ionization.A minimum ionizing particle produces a pulse of large height and the detector operating in

region V is called a Geiger Muller Counter.

At the end of the Geiger region, the counter goes into continuous discharge region and the

discharge consists of a large number of multiple pulses. The region is of no interest because the

discharge is indifferent to the presence of incident charged particles.


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Ionization Chamber

When the detector operates in region II, it collects actual number of ions produced because therecombination is negligible. The count-rate records actual ionization current produced without

any multiplication of ions. The pulse height produced is 10 for an or particle of1 energy. As small number of ions are produced which are collected instantaneously, thecounter remains dead (inoperative) for a time of about 1 between successive counts. Itsenergy resolution is about 0.5%. The ionisation chamber does not give a count of individualparticles, but an average effect of a large number of particles.

Proportional Counter

When a particle of low specific ionization passes through an ionization chamber, the pulse

produced is too small to detect. Potential difference is increased so that it works in proportional

region, in which the size of the output signal is proportional to the number of ions formed by the

primary ionization process.

The proportional counter consists of a cylindrical gas filled tube with a very thin central wire,

which is insulated from the tube. The central wire is always made positive with respect to the

metallic cylindrical tube and serves as a collecting electrode which is connected to a pulse

amplifier. The tube is filled with methane and approximately

20%argon, the first to improve

stability and the second to raise the amplification factor (The number of ion pairs formed bycollision as each electron travels toward the central wire is known as the multiplication factor or

gas amplification factor). Multiplication factor depends on the anode radius, radius of the

cathode, voltage and the nature and pressure of the gas. If a potential difference is appliedacross anode wire of radius a and cylindrical cathode of radius , the electric field , at a radialdistance is given by

When a charged particle is incident on such a detector, it produces ion pairs along the trajectory

of the particle. For 1000 volts, 1.0 and a = 1.0 mm the field strength at thecentral wire is 6.7 10.The ion pairs produced are accelerated through such a high electric field which causes gas

multiplication (production of secondary ions). The electrodes collect respective negative and

positive ions and the total ionization current produces a pulse through the resistor. As the height

of the pulse is proportional to the energy of the incident particle, it can be used to estimate the

energy of the particle. The number of particles incident per unit time or intensity can be

estimated from the number of pulses produced per unit time


20/30

Geige

G. M.length

cylinde

coated

usually

is gove

A thick

and

are tran

still thi

density

A high

differe

C and

across

Since t

ionizin

differeonly at

wire. T

ionizin

After t

behind

in its o

Muller

ubes haveand from

r may be s

with a thi

of tungste

ned by the

ness corres

articles. W

smitted, bu

ner wall,

of

resistance

ce develop

resistance

in going t

e potentia

particle p

ce betweeone point

he amplifi

particle.

e avalanch

a positive

iginal state

Counter

been madeto

upported in

layer of

with a thi

nature of p

ponding to

ith a glass

t the part

nd be equi

.

R is conne

ed at the en

R'. The co

o the count

l differenc

roduces a l

proportiohereas in t

ation doe

e, the elect

heath of io

after all th

in a greatin di

side a glas

n electric

kness of

articles to

all of

icles are e

ped with a

cted in seri

ds ofR is s

ndenser C

er.

across th

arge ioniz

al and G.e latter th

not there

rons get c

ns which d

e charges h

variety ofmeter. Th

s tube, or t

l conducto

to

e detected.

of wall

, al

cluded. To

window o

es between

ent to a co

obstructs t

electrode

tion pulse

. counteravalanche

ore depen

llected at t

rift slowly

ave been cl

sizes andwalls can

he interior

r (for exa

area readil

l the gam

receive

thin mica,

the axial

nter throu

e d.c. com

correspon

because o

is that in thspreads al

on the i

he anode i

toward the

eared from

hapes, frobe of met

surface of

ple, silver

. The wa

passes ga

a rays and

particles, t

stainless st

anode and

h a circuit

ponent of t

ds to regio

Townsend

e former, tng the who

itial ioniz

a short ti

cathode. T

the active

toal (copper)

the glass t

). The cen

ll thickness

ma rays b

most of the

e counter

eel or plio

cathode. T

ontaining

he potentia

n V, even

avalanche

e avalanchle length o

tion produ

e

e counter

space. This

2

i, a metalli

be may b

tral wire i

of the tub

ut blocks

particle

must have

fllm, with

e potentia

a condense

develope

a minimu

. The mai

e is formethe centra

ced by th

and leav

omes bac

situation i


21/30

created

second

Scinti

When

pheno

fluores

of phot

the pho

Scintill

fluores

of a ph

The sel

sulphid

naphth

scintill

A suita

is shiel

produc

de-exci

photo-

photoel

potenti

when the

ry quenchi

lation C

n electrom

enon of f

ent materi

ons in the

tons produ

ation count

ence. The

tomultipli

ection of t

e in the f

lene is co

tion count

ble scintill

ded from

s a tiny fla

te in a sho

ultiplier t

ectrons. T

ls are appl

positive io

ng process.

unter

agnetic rad

lourescenc

l which ar

visible or u

ed are call

er is a dev

scintillation

r tube and

e scintillat

rm of a t

monly em

r is shown

tor crystal

all stray li

sh of light,

rt time

be and fa

e tube has

ied. The ph

s are eith

iation is in

. The inci

then de-e

ltra-violet

d scintillat

ice used fo

s produced

are then rec

or depends

hin crystal

ployed, wh

below.

is fitted to

ght by an

due to fluo

to

ll on a tra

many ele

oto-electro

r collected

cident on c

dent radia

cited in a

adiation. S

ions.

r detecting

are conver

orded elect

upon the r

is used.

ile for y ra

he end of

aluminium

rescence. (

s). The

nsparent p

trodes call

s are pulle

by the ca

ertain mat

ion (like

very short

uch materi

radiations

ted into a

ronically.

adiation to

or part

ys a crysta

photomul

casing. T

he atoms

hotons tra

hotosensiti

ed 'dynod

d to the dy

thode or g

rials, it ca

rays) ex

ime and it

als are kno

like a

plified ele

be detecte

icles, a cr

of NaI (T

iplier tube

he radiatio

nd molecul

vel throug

e layer (p

s' to whic

ode 1 whe

t neutraliz

emit ligh

ite the at

results in t

wn as scint

d rays b

trical puls

. For pa

stal of an

) is used.

through a l

n entering

es are first

the light

hotocathod

progressi

re a numbe

21

ed in som

due to th

ms of th

e emissio

illators an

y means o

s by mean

rticles, zin

hracene o

ne type o

ight pipe. I

the crysta

excited an

pipe to th

e), ejectin

vely highe

r (4 to 5) o


22/30

22

secondary electrons are emitted for each primary photoelectron. These in turn are pulled to

dynode 2, 3, 6 in succession where electrons get multiplied due to secondary emission. Finally a

highly amplified electrical pulse is delivered at the anode where it is amplified and afterdiscrimination from the noise level pulses it is given to the recording scalars and counters.

Semiconductor Detectors

These are based on the principle that if an energetic particle is incident on a reverse biased junction, it releases electron hole pairs from the depletion layer and an electric potential

difference applied across the junction collects these electrons and holes to produce an electricpulse. The electrons and holes collected by the respective electrodes send an electric pulse of

about

10

with a short rise time of about

10 to

10

s across the external resistance

.

These pulses are linearly amplified and recorded by counters for detecting the incoming radiation

and measure its intensity. The energy spectrum of the incoming radiation can be obtained by

using a single channel or multichannel pulse height analyzer. Most semiconductor junction

detectors fall into three categories according to the formation of junction.

1. Diffused junction detector2. Surface barrier detector3. Lithium ion drifted junction detector

Track Detectors

The Wilson Cloud Chamber

The Wilson cloud chamber can be used for photographing the tracks of-particles, particlesand secondary ionization effects due to the passage of-rays or -rays.It is based on the principle that when a charged particle passes through a supersaturated vapour,

it produces ionization along the trajectory of the particle and droplets are formed on the ions due

to condensation of vapour. A gas (air) mixed with saturated vapour, either of water, or alcohol or

ether is contained in a cylindrical vessel. When gas is allowed to expand adiabatically, a

considerable fall in temperature takes place and the space becomes supersaturated with vapour.

The excess of vapour will condense on the ions formed due to passage of the particle through the

gas. Due to condensation, a cloud is formed and if it is properly illuminated, the track appears as

a white line on a dark background which can be photographed by means of a camera.


23/30

A simp

chamb

an opti

is illust

Air sat

G. The

moving

increaswith a

photog

provide

avoid t

The th

disting

pressur

only

pair pr

magnet

studied

the disc

Bubbl

Wilson'

lified form

r CC in w

ally-flat gl

rated from

rated with

pressure i

the pisto

s. The adipearance

aphic arra

d within th

e formatio

ickness of

ished fro

produce

ion pairs

duced, t

ic field to t

. (Wilson cl

overy of p

e Chamb

's cloud ch

of a cloud

ich a pisto

ass plate G

a side with

given liqui

nside the c

down su

batic expaof cloud i

gement is

cloud cha

n of a diffu

the track

those m

bout

er cm. Sin

racks are s

e chamber

oud chamb

sitron, whi

er

mber suffe

chamber i

P is fitted

and a cam

the help of

is taken i

hamber is

ddenly, wi

sion resultchamber

made usin

mber to sw

se fog. Thi

is charac

de by ele

ion pair

e both a

ort and th

the curved

er had bee

ch had a cu

red from th

s shown b

at the bott

ra from th

a strong lig

the space

kept high.

th the res

s in coolin. The trac

g intense l

eep out any

sweep fiel

teristic of

trons. On

s per cm i

nd particl

ick, where

tracks of t

earlier use

rvature op

e following

low. It co

om. On th

top views

ht source.

between th

The pressu

ult the vol

g and conss can be

ight. A s

stray ions

d is cut off

the partic

an averag

air. On t

es lose ene

s tracks

e incident

d for the st

osite to th

drawback

nsists of a

top of the

inside the c

movable

re in the c

ume of th

quent supeseen by t

all electric

which are n

just before

le. Alpha

, -particl

e other ha

rgy by givi

are long a

particles ca

udy of cos

t of an elec

:

transparent

cylindrical

hamber. T

iston P an

hamber is

e expansi

r saturatiohe eye, b

field of f

ot of intere

expansion.

tracks can

es under

d -partic

ng up

d thin. By

n be photo

ic radiatio

tron).

2

cylindrica

chamber i

is chambe

glass plat

lowered b

n chambe

of vapourt normall

ew volts i

st just to

be easil

tmospheri

les produc

per io

applying

raphed an

n and led t

,


24/30

24

low stopping power due to low density of vapour and its large dead-time, as. a result very

energetic particles could not be detected. This problem can be avoided if somehow stopping

power can be increased, say, by filling the chamber with a particular liquid of necessary thermalproperties.

A liquid boils at a higher temperature under high pressure. If the applied pressure is more than

the saturation vapour- pressure at that (high) temperature, the liquid will boil without formation

of vapours, that is it will be a superheated liquid. If now, the pressure is suddenly decreased,

instead of vapours, the bubbles will be formed.

Now suppose the particles to be detected are allowed to pass through the chamber so, they will

ionize the liquid bubble-atoms in the vicinity of their paths. As a consequence, those nearby

atoms will be ionized (nascent atoms), liberating few electrons with opposite momentum (may

be so called recoil-electrons).These recoil electrons cause local heating of the bubble's-surfacescausing them to grow within a few milliseconds. At this moment, a light is flashed and the

bubbles are photographed simultaneously from many different angles and afterwards, their

spatial distribution can be obtained by stereographic reconstruction.

Generally bubble chamber is subjected to a strong magnetic field on order to distinguish the sign

of the charge on the ionizing particles and to measure their momenta from the radius of curvature

of the bubble tracks. Through most commonly used liquid in bubble chamber is liquid hydrogen,

other liquids such as deuterium, helium, xenon, etc, are used in some cases.

The schematic diagram of the bubble chamber is shown below. The main body of the chamber is

made up of stainless steel with thick glass ports at the top for viewing camera. A box of thick

walled glass is filled with the liquid hydrogen and is connected to the expansion pressure system.

In order to maintain the chamber at the constant temperature, it is surrounded by liquid nitrogen.

High energy particles are allowed to enter the chamber from a side window .


25/30

Note:

The B

Nucle

Charge

emulsi

through

crystals

issing trac

ubble C

i. Dupre

yiel

inte

ii. Thdiff

iii. Thiv. Thv. Thr Emuls

d particles

ns can be

a photogr

producing

ks in the ph

amber h

to high d

ent in a gi

ds of even

ractions to

tracks ar

usion withi

chamber i

re is an abs

recycling t

ion Tech

interact wi

used for re

phic emul

a line of la

otograph r

s followi

nsity of li

en path to

ts occur w

be examine

very cle

n the liquid

sensitive t

ence of bac

ime is only

ique

h photogra

cording th

ion of a ph

ent images

present ne

ng Adva

uid (as co

interact wi

ithin the b

d in a great

arly define

is very sm

o particles

kground d

a few seco

phic emuls

tracks of

otographic

along their

tral particl

tages

mpared to

h the inco

ubble cha

detail.

d, becaus

all.

of low ioni

e to old tra

nds.

ions in the

charged pa

plate they

path.

es.

air), a larg

ing particl

ber, whic

the bubb

ing ability.

cks

same way

rticles. As

interact wit

number o

e. Consequ

enables

les grow

as photon

charged p

h the tiny s

2

f nuclei ar

ently, larg

igh energ

apidly an

; hence th

rticles pas

ilver halid


26/30

26

The optical photographic emulsion is not suitable for quantitative work with nuclear radiations.

The sensitivity is low and the tracks due to charged particles have non-clear range because the

developed crystals grains are large and widely spaced. The composition of the emulsion has to bechanged to make it suitable for study of various ionizing particles such particles, protons,mesons and electrons etc. The table shows a comparison of nuclear and optical emulsions.

Because of larger density and large Z number the stopping power of emulsions for high energy

particles is much better than an ordinary cloud chamber. When a particle passes through an

emulsion plate or a stack of plates it reduces the silver and after developing, the path of the

particles is made visible under a microscope. With a binocular microscope a three dimensional

view of a particle and its associated reactions or interactions can be obtained. Different

commercially available emulsions differing chiefly in grain size, are used to discriminate

between different particles.

In cosmic ray research, the nuclear emulsion plate offers the advantage of simplicity. These

plates can be readily taken to mountain tops or sent up to great altitudes in balloons or satellites

for recording primary cosmic ray phenomena. For the study of machine produced reactions, the

energetic particles are passed through a stack of emulsion and the tracks thus produced are

analysed for identification of the mass and charge of the resulting events.

Advantages of Nuclear Emulsion Detectors

i. These detectors have a large stopping power for highly energetic particles andthese provide a continuously sensitive medium for permanent record of events

involving low, medium and high energy.

ii. These detectors can measure a large number of parameters of most of the particlesincluding estimation of charge, mass, energy, life time etc.

iii. Nuclear emulsion detectors have excellent spatial resolution ~ 0.5 m and highangular resolution ~10 radian which is invariably required for accuratemeasurements inhigh energy interactions involving multiple particle production.

iv. The emulsion is relatively light and cheap. This makes it better for high altitudecosmic ray experiments.

v. They were widely employed in cosmic ray studies and led to the discovery of the and -mesons


27/30

The sp

counterspark c

high sp

The wo

an inert

particle

the par

the hel

The ins

first, th

all eve

order o

The pl

neutron

helium

sufficie

Spark C

rk chambe

and to givhamber de

atial resolu

rking princ

gas but th

enters the

icle. Thus

of automa

trument co

ird, fifth a

number p

is

tes are m

s and neutr

neon mixt

nt to prod

amber

r utilizes a

e track geoector is ca

ion.

iple is as f

electric fi

gas space,

the path of

tic operatio

nsists of a

d all other

lates are c

aintained

de of bra

inos which

re at about

ce a disc

ll incipient

metry infoable of co

llows. A

ld is insuf

spark pas

the particl

n of the ins

eries of thi

odd numb

nnected to

etween ad

s, lead or

interact in

an atmosp

arge betw

electrical

mation, likunting part

igh voltag

icient to p

es and ten

is marked

trument.

n parallel

r plates ar

a high vol

acent pairs

other hea

the plates

heric press

en a pair

ischarge i

e that proicles with

is maintai

rmit the p

s to follow

by the sp

lates space

grounded

age dc pul

of plates.

y conduct

o produce

re. The hi

of adjacen

a gas like

ided by aa high cou

ned betwe

ssage of s

the path o

rk and can

d about

and the se

se generato

ing materi

charged pa

h voltage i

t plates. A

that used

ubble chat rate as

n two plat

ark. When

ion pairs

be photog

to

ond, fourt

r so that a

ls to dete

ticles. The

s almost, b

trail of i

2

in a Geige

ber. Thusell as wit

s placed i

an ionizin

roduced b

aphed wit

apart. Th

, sixth an

field of th

t gammas

gas used i

ut not quit

nization i

,

,


28/30

produc

micros

molecuelectro

success

The po

produc

opposit

particle

chamb

very sh

which f

Ceren

Ceren

A char

velocit

Cheren

atomsleads to

the dip

For a p

wave fr

and pr

resultin

d by an i

cond

les), a sms produce

ive multipl

wer suppl

d. A swee

e to that o

. The delay

r and the e

ort in com

ollow the t

kov Cou

kov Rad

ged particl

of light

kov radiati

long its trathe emissi

le where it

rticle trav

ont of Che

duce a co

g from the

cident cha

du

ll localizeare accel

ication due

is cut of

ping field

the high v

time is ab

ergy of th

arison to t

acks of the

ter

ation

, traversin

in that

on. Cheren

ck so that ton of electr

originated.

lling with

renkov radi

nstructive

interaction

rged partic

ing which

spark disrated to s

to seconda

in about

f about 20

oltage spar

ut .

spark, and

hat of clou

particles a

a medium

medium,

kov radiat

hey becomomagnetic

elocity les

ation origi

interferenc

are not inte

le. If the (

electrons i

charge occch energie

y collision

by

0 volts per

k field to

The recov

it is usuall

d and bub

d the track

with refra

mits a ch

ion is emi

e electric dradiation. T

than the v

ating from

. i.e. The

rrelated an

pulse) volt

n this trail

urs alongs in the el

s).

dischargin

cm is con

repare the

ry time de

y between

le chambe

s are photo

tive index

racteristic

ted becau

ipoles. Thehis radiati

elocity

different d

wave fron

are entirel

age is appl

are free (

he particlectric field

g a conde

tinuously a

detector fo

ends on th

to .

s). Series

graphed by

with a v

electroma

e the char

time varian spreads o

of li

ipoles are

ts of the

y independ

ied in a fr

efore attac

's path (Tthat they c

ser after t

pplied in t

r the detec

gas, the d

(This reco

f sparks a

stereoscop

locity ex

netic radia

ged particl

tion of theut in the m

ght in the

nable to gi

lectromag

ent.

2

action of

hing to ga

e ions anan produc

he spark i

e directio

ion of nex

esign of th

very time i

e produce

ic cameras.

ceeding th

tion, calle

e polarize

dipole fieledium fro

edium, th

ve interfer

etic wave


29/30

But, if

constru

The wis same

Above

at the t

wave f

observ

The pa

ABC. T

is thus

charge

The val

Photo

particle

The Ch

detecto

of the d

These

i.ii.iii.

the veloc

ctive interf

ve front othe directi

figure sho

ime of thir

ont can b

d at a parti

ticle move

he time in

the wave

particle is

ue of ca

ultiplier (

is known i

erenkov d

rs. Also Ch

etectors m

odes are

Thresh

Differe

Ring i

ity is larg

rence of th

the radiatn of the pa

s the inter

interactio

drawn tan

ular angle

s along

hich parti

ront. The

given by

be estima

) tube

t is possibl

tectors are

erenkov de

dium is ca

ld Cheren

ntial Chere

aging Che

r than the

e wave fro

ion that prrticle.

actions wh

n. The Hu

gent to th

with respe

. It radiat

le travels

angle whic

ed by mea

can be use

to estimat

commonl

tectors can

efully sele

ov detecto

kov detect

enkov dete

velocity

ts is possi

duce a co

n

gen's cons

spherical

t to the dir

s energy i

distance

h the Cere

uring the v

to detect

the value

used for i

be used in

ted.

rs

ors

ctors

le. It make

nstructive i

. The wa

ruction of

surfaces a

ction of m

all directi

, the radi

nkov radia

alue of b

radiation. I

of .

ndenting p

three diffe

of light

s Cherenko

nterference

e fronts fr

wave optic

d the Cere

otion of the

ons from th

ation will

tion makes

y optical m

f the mom

articles in

ent modes,

in the me

v radiation

is conical

m 1 and 2

s shows th

nkov radia

particle.

e points al

ove a dista

with the

thods.

ntum or e

onjunction

if the refr

2

dium, the

detectable.

with axis

lie inside

at a conica

tion can b

ng its pat

nce

irection o

ergy of th

with trac

ctive inde


30/30

30

Threshold Cherenkov Detectors

This type consists simply a radiator and a light detector (usually P.M.T). In this particles. In thiscase, particles with velocity greater than the threshold velocity for the generation of the

Cherenkov radiation are detected.

For a counter filled with material of refractive index , the threshold momentum, for a particlewith mass is given by

1If a gas is used as radiator then it is possible to tune the detector to match the velocity of the

particles with by adjusting the pressure of the gas.Note: The refractive index of the gas depend on the pressure

,

1 1 Where and refer to an atmosphere measures.Differential Cherenkov Detectors

This accept Cherenkov radiation only in a narrow range of angles, . In other word, in anarrow velocity interval.

Resolution

of the order

10

can be obtained by using this detector. A chromatic dispersion

is the major source of the error at high momenta. A special type of achromatic counters called

Directional isochronous self collimating ( DISC) counters has been developed. This design is

capable to give the resolution ~10 10.

Documents

Linear Accelerators and Detectors