Nuclear Radiation DetectorUnit-V (B.Tech-II sem)

Dr. Vishal JainAssistant ProfessorDepartment of Physics

Radiation

Radiation is the kind of energy, that comes from a source & travels through space & may be able to penetrate various materials OrIs 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

The three basic types of gaseous ionization detectors are1) Ionization chambers, 2) Proportional counters, and 3) Geiger-Müller tubes

• 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.

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

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.

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

IONIZATION CHAMBERThe 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. 

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.

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

tne

tqI

RVI 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

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.

Puls

e H

eigh

t

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.

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.

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.

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

abr

VrElog

)(

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

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.

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.

Puls

e H

eigh

t

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

nM1

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.

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

cmcmcm

volt

abr

VrEe

/22.32566

01.01log303.201.0

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

Geiger-Muller CounterIn 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.

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

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 1100VApplied Voltage (V)

Threshold Voltage

n1n2

Cou

ntin

g R

ate

(Cou

nts/

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.

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.

Self Quenching by alcoholSelf 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 DecayThe 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

VVT

Time

Working Voltage

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

Nn

Counting Efficiency :- The counting efficiency is defined as the ability of a counter of its counting at least one ion pair produced

)1( slpeficiencyCountingEf 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 NnτNumber of counts missed = Error in counting, Nnτ = N-n

Real Count Rate n

nN

1

Efficiency of Counter )1( n

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

%4.94100500472

Nn

Q.4: The efficiency of a GM counter is 90 %. If it counts maximum 6000 counts /minute, then calculate the paralysis time of counterSol:- Given efficiency of counter is 90% =0.9 n=6000 counts/min = 6000/60= 100 count/sec

so )1( n

sec10

)1001(9.03

Scintillation CounterThe 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.

ConstructionLike 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.

WorkingWhen 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 .

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   (6) 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. 

(7) 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

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