linear accelerators and detectors
TRANSCRIPT
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2
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
Hence
1
Or
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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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
-
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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
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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
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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.
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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
,
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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 .
-
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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
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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
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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
,
,
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8/8/2019 Linear Accelerators and Detectors
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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
-
8/8/2019 Linear Accelerators and Detectors
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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
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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.