As per RTU Syllabus

1. Nuclear Radiation Detector
Unit-V (B.Tech-II sem)
Dr. Vishal Jain
Assistant Professor
Department of Physics

2. Radiation
Radiation is the kind of energy, that comes from a source & travels
through space & may be able to penetrate various materials
Or
Is the emission of energy as electromagnetic waves or as moving
subatomic particles, especially high-energy particles which cause
ionization.
In this chapter we discuss about the “Ionization radiation” These type of
radiation can be produce charged particles in matter. Ionization radiation
is produced by unstable atoms. They have access of energy and mass or
both.
Types of the radiation
(1) CHARGED PARTICLES (2) UNCHARGED PARTICLES
(a) Alpha Radiation (a) Gamma Radiation
(b) Beta Radiation (b) Neutrons

8. The three basic types of gaseous ionization detectors are
1) Ionization chambers,
2) Proportional counters, and
3) Geiger-Müller tubes

9. • The most common type of instrument is a gas filled radiation
detector.
• This instrument works on the principle that as radiation passes
through air or a specific gas, ionization of the molecules in the air
occur.
• When a high voltage is placed between two areas of the gas filled
space, the positive ions will be attracted to the negative side of the
detector (the cathode) and the free electrons will travel to the
positive side (the anode).
• These charges are collected by the anode and cathode which then
form a very small current in the wires going to the detector. By
placing a very sensitive current measuring device between the
wires from the cathode and anode, the small current measured and
displayed as a signal. The more radiation which enters the
chamber, the more current displayed by the instrument.

10. Detection influence of anode voltage
RECOMBINATION REGION:- The recombination region is the region of
lowest applied voltage (less than 20Volts). Detectors are not operated in this region
because many of the ions produced in the detectors never reach the electrodes. They will
recombine before move towards cathode only few ions will reach at cathodes and anode .
The Curve is divided into six region

11. IONIZATION CHEMBER REGION:- In this region applied voltage is greater
than the recombination region. This portion of the six-region curve is flat because there is no
change in the number of ion pairs collected as the voltage increases. The voltage is high
enough so that there is no recombination, but it is not high enough to cause gas amplification
(ions moving towards the electrodes so fast that they cause additional ionization).
It is important to remember that in the ionization chamber region, the number of ion pairs
collected to the electrode are equal to the number of ions produced by the radiation in the
detector. The number of ion pairs collected does not vary with the voltage. This is one region
why some detection instruments are used in the ionization chamber region. Even if the
voltage varies a little, the same reading is achieved
PROPORTIONAL REGION:- When the applied voltage V is further increased
beyond (200V) V2 the electrons and ions produced by ionizing particles gain sufficient
kinetic energy to generate secondary electron-ion pairs by colliding with the atoms of the
gas. These electron further ionize the atoms of gas. Thus the electron-ion pair increase
exponentially. This process is cumulative and primary electron produce avalanche of small
magnitude. In this region the number of ions collected at the electrode is proportional to the
energy of ionizing particle if its path length in the counter is sufficient so that it can lose all
its energy. The proportionality constant called multiplication factor M. In the region from
200 volt to 800 volt the multiplication factor remains constant so this region is called
proportional region. The detector working in this region is called proportional counter.

12. LIMITED PROPERTIONALITY REGION:- Now if the applied voltage is further increases
upto 1000 volt, the height of voltage pulse also increase but the proportionality behavior of
the counter is lost. The reason is that the positive ion start interacting with other atoms and
produce more electrons. This region is not suitable for detection and measurement.
GEIGER REGION:-
The pulse height in the Geiger-Müller region ( Region V) is independent of the type of
radiation causing the initial ionizations. The pulse height obtained is on the order of
several volts. The field strength is so great that the discharge, once ignited, continues to
spread until amplification cannot occur, due to a dense positive ion sheath surrounding the
central wire (anode). V4 is termed the threshold voltage. This is where the number
of ion pairs level off and remain relatively independent of the applied voltage. This
leveling off is called the Geiger plateau which extends over a region of 200 to 300 volts.
The threshold is normally about 1000 volts. In the G-
M region, the gas amplification factor (A) depends on the specific ionization of the
radiation to be detected.
Continuous Discharge Region.
In the continuous discharge region (region VI), a steady discharge current flows. The applied
voltage is so high that, once ionization takes place in the gas, there is a
continuous discharge of electricity, so that the detector cannot be used for radiation
detection

13. IONIZATION CHAMBER
The most widely used radiation detectors are devices that respond to ionizing
radiation by producing electrical pulses. Is work on the principle of that chage
particles can ionize a gas. The number of ion pairs formed gives information
regarding the nature of the incident particles as well as its enrgy.
The ionization chamber is the
simplest of all gas-filled radiation
detectors, and is widely used for
the detection and measurement of
certain types of ionizing
radiation; X-rays, alpha particles.
Conventionally, the term
"ionization chamber" is used
exclusively to describe those
detectors which collect all the
charges created by direct
ionization within the gas through
the application of an electric field.

14. Construction
The ionization chamber consisting of a cylindrical geometry having a thin metal
wire along the axis, which is used as anode and outer cylinder as cathode. In the
chamber a number of gases can be used, but in general argon with carbon dioxide
(Co2) or argon with methane (CH4) are used. Air is the most common filling gas.
The operating voltage more than 20V.

15. Working
The ionizing particle is admitted to the chamber through a side window W (made of very
thin sheet of mica or aluminum) ionizes some gas molecules. The ion thus formed, drift
along the lines of force, thus producing ionization current. When an ion chamber operated
in direct current mode, the negative charges can be collected either as free electron or are
negative ions.
Typical ionization currents in most applications are very small. The magnitude of the
ionization current is too small to be measured using standard galvanometer techniques. An
electrometer highly sensitive electronic voltmeter indirectly measures the current by
sensing the voltage drop across the series resistance.
Provided the ion current does not change for
several values of the time constant RC, its steady
state value is given by
t
ne
t
qI
R
V
I R
 ,
Iion=n.e Thus the measurement of Iion can gives as the integrated effect of the total
ionization event or the intensity of ionizing radiation

16. The pulse coming out of the ionization chamber is taken through an amplifier for
further counting. The voltage time dependent of the pulse in shown in figure. The
upper curve for RC = Infinity shows the nature of large value of time constant
Here the initial fast rise of the pulse upto the point A, is because of arrival of the
fast moving electrons. There is then a slow rise upto the point B, Which is due to
the arrival of slow moving positive ions. For finite time constant RC=1 micro
second the pulse shape is shown in figure.
PulseHeight
Time (Sec)
RC = 0.1µs
RC = 1µs
RC = ∞
Voltage-time characteristics of the Pulse
Properties
(a) The ionization chamber have been used to
study alpha particles, proton and nuclei of
lighter element
(b) The ionization chamber is not useful to
detect electrons because of their low
ionisation.
(c) There is a draw back, its take time to
detect next particle after detection first
one. And B-ray, Gamma Ray are not
detected.

17. Proportional Counter
The proportional counter is a type
of gaseous ionization detector device used
to measure particles of ionizing radiation.
The key feature is its ability to measure
the energy of incident radiation, by
producing a detector output that
is proportional to the radiation energy;
hence the detector's name.
Proportional counter produces a large
voltage pulse because the number of ion
pairs produced are greatly amplified by the
phenomena gas multiplication. Gas
multiplication is induced by a strong applied
electric field. The operation of gas filled
chamber in the voltage region where gas
multiplication is present but a strong
dependence on the initial ionization is still
maintained has result a very useful detector
called proportional counter.

18. Construction
The basic cylindrical geometry with a central anode wire and the outer walls acting as the
cathode is a very common design for proportional counter. Proportional counter usually
consist of a metallic cylindrical shape. This cylinder C works as outer electrode (Cathode)
having dimensions 20 cm long and 2cm in diameter. Afine tungsten wire also used which
work as central anode. An entrance window has to be provided for weakly penetrating
radiation but the thickness can mean that low energy α rays, X-rays and charged charged
particles cannot be enter the counter. The Cylinder C is filled with a mixture of noble Ar and
CH4 methane gas in ratio of 9:1 at 1 atmospheric pressure. The schemetic diagram shown in
figure.

19. Working
The polarity is important, the electrons must be attracted to the central axis wire. There
are two main reasons why the cylindrical geometry is used with this polarity. The first
main region is that this design allows for practical gas multiplication. Gas multiplication
only occurs when the applied electric field is high because the electrons must be
accelerated to high kinetic energy. In parallel plate geometry the electric field is uniform
between the plates. If the gap between the plates is 1.0cm, then to create applied field of
5.16 x 106 V/m, it is necessary to apply 51800V . Such a high volt is practically
impossible
In a cylindrical geometry with the anode at the center, the electric field at radius r from
the anode is given by







a
b
r
V
rE
log
)(
Here V= the applied voltage, a = anode wire radius, b= cathode tube inner radius
For large electric field r is required to be small, therefore a needs to be small i.e. the
anode wire have to be thin. However E(r) will vary with changes in a, so E(r) will
note be constant

20. When the charge particle or radiations α, β, γ-rays photon enters an ionization chamber,
ionization of gas take place resulting in an ion pair formation. The positive ion move
towards the chamber wall while the electron move towards the central wire. In the
proportional region the applied voltage is so high that the primary ion gains sufficient
kinetic energy, to produce secondary ion by collision with the atom of gas, resulting into
gas multiplication. In this ion multiplication or gas multiplication, the number of ions
increase exponentially this process is cumulative and is called avalanche. This avalanche
occurs at a certain point (as shown in figure) of the anode and depends on the radius of
anode wire, radius of tube, nature of the gas and applied voltage.

21. As avalanche takes place near the anode, all the secondary electrons are accumulated at the
anode within about 1ns time. This produces a very small voltage pulse. Main voltage pulse
is generated by the flow of secondary positive ions towards the cathode, which takes about
500 µ sec. The voltage pulse generated by the ionizing particle in the proportional counter
in shown in fig.
PulseHeight
Time (µSec)
RC = 0.03µs
RC = 0.15µs
RC = ∞
Shape of the Pulse
Initially voltage pulse increases quickly and
then decrease according to the time constant
τ=RC. The value of gas multiplication factor M
is given by








np
n
M
1
Here n=number of secondary ion produced by the
primary electrons, P is the probability of
production of photoelectrons
USES :- (A) Proportional counter permits both the counting and energy determination of
particles even of very low energy. (B). It can be used as a spectrometer particularly for
Beta rays.
DISADVANTAGE: - The major disadvantage of this counter is that the amplification factor
depends on applied voltage. So the applied voltage must be maintain constant within the
narrow limit because a slight change in voltage change the gas multiplication.

22. Problems
Q.1: Calculate the electric field at the surface of the wire of a proportional counter with a
wire radius 0.1mm and a cylinder radius 1cm, when 1500 volt is applied between the two.
Sol:- Given the voltage V=1500 Volt, Radius of the wire a= 0.1 mm=0.01cm
Radius of the cylinder b= 1cm
for large value of Electric Field r must be same as a so r = 0.01cm
As we know cmvolt
cm
cm
cm
volt
a
b
r
V
rE
e
/22.32566
01.0
1
log303.201.0
1500
log
)(
10















Q.2: An alpha particle stopped in an ionization chamber in which it produces 15x104 ion
pairs. Each time the alpha particle produce an ion pair, it lose 35eV of energy. What is the
kinetic energy of the alpha particle? Calculate the amount of charge collected by each plate.
Sol:- Given the number of ion pair produced in ionization chamber = 15x104
The energy lost by alpha particle to produce an ion pair =35eV
So the kinetic energy of the alpha particle is equal to = 15x104 x 35eV= 5.25 MeV
The amount of charge collected by each plate
q = ne = 15x104 x1.6 x 10-19
=2.4 x 10-14 coulomb

23. Geiger-Muller Counter
In 1928, Geiger and Muller in Germany
developed this counter, The main difference
between proportional counter and GM
Counter is that in GM counter gas
multiplication so large that an avalanche
does not form at one point but spreads over
the entire length of central wire. Therefore
the amplification does not depend on the
initial ionization produced by the ionizing
particle. It does not distinguish the type of
radiations that enter in the chamber i.e.
output pulse is independent of energy and
nature of the particle detected.

24. Construction: GM Counter consist of a glass tube with a thin central wire located along its
axis as shown in figure. Thin wire made of tungsten acts as anode and a copper cylinder
surrounding it acts as a cathode. A high voltage between 800-2000 volts corresponding to
the Geiger region or plateau region is maintained between the wire and surrounding
cylinder. The electric field in the vicinity of the central wire is always very high. The tube
is filled with argon gas at a pressure about 10-3 torr. A small quantity of ethyl alcohol
(10% ) is introduced in the tube as a quenching agent. The glass tube is provided with a
window of very thin mica foil or cellophane or glass so that particles of small penetration
power such as alpha particle or Beta particle can enter inside. A resistance R is connected
in the counter circuit so that current pulse produce a voltage across it. The voltage pulse is
amplified and counted by an electronic counter.
Geiger Muller Counter

25. Working
When the counter operates in the Geiger region, the ionizing particle passing through the
tube ionize the gas and the electron released by ionization is accelerated towards the central
wire. This electron acquire very high velocity and produces large number of ion pairs by
repeated collision with the atom of gas. The secondary electron so liberated are also
accelerated and more ion pair are produced. This multiplication action is very rapid and an
avalanche results throughout the central wire. Thus a large pulse of ionization current is
produced and it is independent of the number of primary ion pairs formed by the incident
particle.
Geiger Plateau
Break down
Voltage
Starting
Voltage Operating
VoltageVS V1 V2
900V 1100V
Applied Voltage (V)
Threshold
Voltage
n1
n2
CountingRate(Counts/min)
Variation in counting rate with voltage in G.M. Counter
When the G.M Counter connected with
electronic circuit which records pulse height
(0.25V) proper to this region and note the
small pulses, it is seen that until the voltage
reaches the starting voltage Vs and shown in
figure, the pulse are too small to be detected .
As the voltage increases above this limit. The
count rate increases until the threshold voltage
Vg of GM region is reached. Above Vg, for
about 200-300 volts, counting rate almost
remains constant. The range of potential over
which counting rate becomes constant is
Geiger Plateau region.

26. Beyond the plateau region a continues discharge takes place and counting is not
possible. In the GM region the ionization pulse depends on the physical
dimensions, voltage, type of gas employed and does not depend on initial
ionization. Only a single particle is sufficient to start the process which gives 50
volt pulse height. This pulse can be detected without preamplification.
The operating voltage in the GM region is high enough for the electrons. They
rise some atom to excited states followed by ultraviolet radiation. The absorption
of which gives rise to new avalanche. Thus in a sort time avalanche spreads over
the whole length of the tube but the positive ions move slowly and they form an
ion sheath (space charge) around the anode for a sort while. This ion sheath drop
the voltage below Vg and no further pulse can be detected. The instrument
becomes in operative for the time while sheath removed from the anode. These
positive ion reach to cathode and produce fresh avalanche of electrons so at the
anode a state of confusion produced, one due to continuous avalanche and another
due to fresh avalanche. Hence to remove states of confusion, the continuous
avalanching is suppressed. The method of separation of continuous avalanching is
known as Quenching. The suppression by adding a quenching agent in
counter gas (alcohol, polyatomic gas) is known as Self Quenching.

27. Self Quenching by alcohol
Self quenching by adding ethyl alcohol vapor to Ar gas ( 90 % argon gas and 10% ethyl
alcohol. The ionization potential of alcohol (11.3 eV) is lower than that of argon (15.7 eV) as
a result the argon ions moving towards cathode are neutralized by acquiring an electron from
the alcohol molecule and alcohol ion are formed. These alcohol ions however do not gives
rise to secondary avalanche, when they are neutralized at the cathode. Thus, there is no
multiple pulsing and the discharge is quenched. The halogen atoms used instead of alcohol to
increase life time of self quenching.
Pulse formation and Decay
The presence of positive ions sheath around the
anode makes the GM counter inoperative for a period
of time, which is known as dead time. During this
time the field around the wire reduce to a sufficiently
low value, so that more electrons cannot be produced
thus the counter remains insensitive, till the positive
ions have moved away or the counter fails to record
another ionizing particle entering during this period.
To avoid large dead time effect , the counting rate of
the detector must be kept sufficient low, such that the
probability of a second event occurrence during a
dead time period is small.
Dead
Time
Recovery
Time
Paralysis
Time
V
VT
Time
Working Voltage

28. After dead time, the detector takes few micro seconds, before it regains its original working
condition. This time is known as recovery time and it lasts about 10-4 sec. The sum of dead
time and recovery time will be the resolving time during which the counter is inactive. The
high counting rate is generally reduced by reducing the high potential supply to the counter
for a definite time interval called paralysis time.
Efficiency of the GM Counter :- Efficiency is defined by the ratio of the number
of observed counts per unit time (n) to the number of ionizing particles entering the counter
tube, during that time, thus
N
n

Counting Efficiency :- The counting efficiency is defined as the ability of a counter of
its counting at least one ion pair produced
)1( slp
eficiencyCountingEf 
Where s= specific ionization at one atmosphere, p pressure in atmosphere and l is the path
length of the ionization particle in the counter
Paralysis Time and Real Counts :- Let us assume that τ be the paralysis time of
counter and it responds at a rate n counts per minute and N particles enter per minute and
number of counter missed will be Nnτ
Number of counts missed = Error in counting, Nnτ = N-n
Real Count Rate
n
n
N


1
Efficiency of Counter )1(  n

29. Properties of GM Counter :-
(i) GM Counter are relatively inexpensive
(ii) GM Counter is durable and easily portable
(iii) It can detect all type of radiation
(iv) It cannot be differentiate which type of radiation in being detected
(v) It has very low efficiency
(vi) It cannot used to determine the exact energy of the detected radiation
Q.3: A GM Counter reads 472 counts per minutes when 500 charged particles are incident
per minute on it . Find the efficiency of GM Counter.
Sol:- Given the number of counts n= 472 per min
Number of charged particle incident on it N = 500 per min
So the efficiency %4.94100
500
472

N
n

Q.4: The efficiency of a GM counter is 90 %. If it counts maximum 6000 counts /minute,
then calculate the paralysis time of counter
Sol:- Given efficiency of counter is 90% =0.9
n=6000 counts/min = 6000/60= 100 count/sec
so )1(  n
sec10
)1001(9.0
3





30. Scintillation Counter
The modern electronic scintillation counter was invented in 1944 by Sir Samuel Curran. A
scintillation counter is an instrument for detecting and measuring ionizing radiation by
using the excitation effect of incident radiation on a scintillator material, and detecting the
resultant light pulses. It consists of a scintillator which generates photons in response to
incident radiation, a sensitive photomultiplier tube (PMT) which converts the light to an
electrical signal and electronics to process this signal. Scintillation counters are widely used
in radiation protection, assay of radioactive materials and physics research because they can
be made inexpensively yet with good quantum efficiency, and can measure both the
intensity and the energy of incident radiation.

31. Construction
Like the other radiation detectors it has three basic components the detector tube, power
source and a measuring device. The detector tube consists of the scintillation material (
such as sodium iodide thallium activated crystal) and a photomultiplier tube with positively
charged dynodes in it. At one end of the photomultiplier tube is a photocathode and at the
other end in an anode. The circuit is connected between the photocathode and the anode. If
there is a flow of electrons between these two electrodes, there will be a current flow that
can be measured on the measuring device
The brief operation of components is explained below
(a) Photocathode:- It convert the light photons into electrons
(b) Dynodes Assembly :- A series of electrodes used to amplify the signal. Dynodes are
connected at intermediate voltage, typically about 50 or 100 volt per step. Using
dynodes assembly, initial signal is amplified by 105 to 106 times
(c) Anode:- It collects the electrons and generates as output.
(d) Voltage divider network:- It split the high voltage supply into the various potentials
required by dynodes.
(e) Shell:- It prevents the component from electric and magnetic field.

32. Working
When the Gamma ray interacts with the scintillation material, visible light is emitted. This
visible light interacts with the photocathode, and electrons are emitted and attracted to the
first positively charged dynodes. When it strikes the dynode, more electrons are emitted.
The first dynode is shaped so that it directs the emitted electrons to the next dynode. The
electrons are multiplied again by the second dynode and sent to the third dynode. The
electron multiplication continues throughout all the dynodes in the photomultiplier tube.
The result is a large flow of electrons striking the anode. Typically each electron emitted
from the photo cathode will end up as about a million electrons striking the anode.
Afterward the anode collected the electrons . A measurable electric current is the result. The
measuring device measure the current. The output of scintillation detector is a pulse of
electrons that is proportional to the energy of the original radiation interacting with the
scintillation material .

33. Advantages
(1) With large size and highly transparent phosphor it displays very high frequency.
(2) The pulse height is proportional to the energy dissipated in the phosphor by the
incident radiation. Hence it is possible to determine the energies of individual
incoming particles.
(3) The time of pulse being very short so that resolving power is high. It can detect
particles whose time of arrival is separated considerably by less than 10-8-sec.
(4) Because of very small dead time. Scintillation counter is capable for fast counting
rate.
(5) It is more efficient for ray counting with a large scintilla or the scattered rays also
counted and get a increased photo peak efficiency.
Disadvantage
(1) Poor energy resolution. In spots of its high detection efficiency the recovering
energy in the process of f converting it into light flashes and into photoelectrons.
Such detectors are capable of handling high counting rates in spectroscopy work
also because of (1) Time resolution: The time resolution is dependent on the spread
in the transit time of the electrums in the photomultiplier tube. The spreading time is
2-5 ns. As the electrons are collected in the anode we get negative pulse from the
anode.
(2) The decay time of the anode pulse is around 250 ns. Hence such detectors are
capable of handling high counting rates in nuclear spectroscopy work

34. Previous Year Question 2014-15