Ionization chamber,G.M. counter

Ionization chamber

The ionization chamber is the simplest of all gas-filled radiation radiation detectors,and is widely used for the detection and measurement of certain type of ionizing radiation:X-rays,gamma rays and beta particles. Conventionally,the"ionizing chamber" is used exclusively to describe those detectors which collect all the charges created by direct ionizationwithin the gas through the application of an electric field.It only uses the discrete charges created by each the interation between the incident radiation and the gas, multiplication mechanism used by other radiation instruments,such as the Geiger-Muller counter or the proprotional counter.

Princple of operation :

An ionization chamber measures the charge from the number of ion pairs within the gas caused by incident radiation.It cosists of a gas filled chamber with two electrodes;knwon as anode and cathode.The elctrodes may be in the form of parallel plates (Parallel Plate Ionization chambers:PPIC),or cylinder arrragement with a coaxially located internal anode wire.a volatge potetial is applied between the electrodes is ionized by incident ionizing radiation, ions pairs are created and the resultant positive ions and dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field.This generates an ionization current which is measured by an electrometer circuit.The electrometer must be capable of measuring the very small output current which is in the region ofamperesto picoampers,depending on the chamber desig,radiation dose and applied voltage.

The electrometer must be capable of measuring the very small output current.This continual gneration of chage produces an ionization current, which is a measure of the total ionizing dose entering the chamber cannot discriminate between radiation types (beta and gamma) and cannot produces an energy spectrum of radation.

The electric field also enables the device to work continuously by mopping up electrons,which prevents the fill gas from becoming saturated,where no more ions could be collected,and by preventing the recombination of ion pairs which would diminish the ion current. This mode of operation is reffered to as"current" mode,meaning that the output signal is a continous current,and not a pulse output as in the Geiger-Muller tube or the propertional counter.

Reffering to the accompanying ion pair collection graph,it can be seen that in the "ion chamber" operating region the collection of ion pairs is effectively constant over a range of applied voltagae ,as due to it's relaively conatant low electric field strength the ion chamber dose not have any"multiplication effect ", This is in distinction to the Geiger-Muller tube or the proportional counter whereby secondary electrons,and ultimately multiple avalanches,greatly amplify the original ion-current charge.

(a) Pulse ionization chamber or non-integrating type:In this type,the time constant RC is small and each pulse produced by ionizing event may be detected as explained above.

(b)Current ionization chamber or integrating type:In this type,the time constant RC is comparitively large and current pulse are allowed to commbine into a steady flow of current i.e.direct current may be measured.

Uses:

(1) Ionization chambers have been used to study the \(\alpha\)-particle,\(\beta\)-particle,protons,electrons and nuclie of lighter elements.

(2)They were extensively used in the early studies of cosmic rays phenomena.

(3) They can be used for measurement on X-rays and \(\gamma\) rays.

G.M. Counter

In 1928 Geiger Muller developed the conter which was first described in 1908 by Rutherford and Geiger when they evacuated the charge carried by an \(\alpha\) - particle . This counter is thus named as Geiger-Muller counter.

Construction and Working;

it consist of a tungsten fine wire serving as anode supported inside a metal cylinder along its axis by an insulating plug EE as shown in figure. The cylinder contains inert gas like argon mixed with the vapour of some volatile compound like ethyl alcohol ( quenchingg vapour ) at a pressure of about 10 cm of Hg. The cylinder is connected to the negative terminal of H.T. battery. The P.D. between the cathode and the anode through a high resistance R is usually large enough ( 1000V to 4000V ), so that even a single ion pair produced by a single incident particle can produce an electric discharge.

Figure Here:

When an ionizing particle enters the counter , ionization takes place and a few ions are produced. If the applied P.D. is strong enough, these ions are multiplied by further collisions. An flows through the resisrtance R. The P.D. ,thus, developed acroos R is amplified by vaccum tube circuits and is made to operate a mechanical counnter. In this way, single single particle can be resister. Then sudden pulse of dischahrge sweeps away hte ions from the chamber and the counter is ready to resister the arrival of the next particle.

Plateau Characteristics of Counter

If a G.M. counter tube is exposed to a source of constant radiation intensity and the rate of counting is recorded as a function of the voltage on the counter tube, a graph is obtained as shown in the figure above. It is known as the characteristic curve for the counter. It is seen that te unless the voltage across the counter tube exceeds a certain a minimum value \(V_s\), the counting rate icreases gradually until the point B is reached. From htis point onwards, the counting rate becomes consytant until the point C is reached. This flat region of the curve is called the plateau. This is the region of the counter operation where the counting rate is, more or less, independent of small change in P.D. across the tube. The G.M. counter must be operated in this region. In actual working, a voltage corresponding to the centre of the plateau is applied is called working voltage. Beyond the plateau region, the applied voltage is so large that a continuous discharge takes place in the tube and the count rate increases very rapidly. It does not require any incident ionizing particle passing through it for this to happen.

figure: Plateau characteristics of a G.M. couner

Quenching, Dead Time And Recovery TIme

The anode wire is surrounded by the slow moving pisitive ion sheath, which reduces the electric field, making the discharge stop within a few microseconds ( \(10^{-6}\) ). As the positive io sheath moves away towards the bube's wall (cathode), the value of the electric field rises tov permit another electric discharge due to production of electrons when the positive ion hit the wallof the tube. This result in a continuous discharge and the tube is not ready to receive another incident particle. Therefore ,some mechanism must be deviced to terminate the discharge after each event (quenching of the counter).

The simplest method for achieving this is to makre the resister R very large. For a current (at the time of discharge), this resistance has a large voltage drop (IR) which makes the P.D. between anode and cathode fall off to sucjh an extent that the discharge in the counter can not be maintained. However, this has the effect of prolongation of time before the counter becomes ready to accept another pulse. This time interval iscalled dead time and it is 200\(\mu\)s. After the dead time interval, the counter may operate with a reduced pulse size till the positive ion reach the cylinder and the counter potential regions its original value for operation in the Geiger regions. The recovery time is the time after which the original pulse levels are restored. During this recovery time, the pulses are recorded but are of similar size.

Reference:

Reviews of Modern Physics. Lancaster, P.A.: Published for the American Physical Society by the

American Institute of Physics, 1952. Print.

Wehr, M. Russell, and James A. Richards. Physics of the Atom. Reading, MA: Addison-Wesley

Pub., 1984. Print.

Young, Hugh D., and Roger A. Freedman. University Physics. Boston, MA: Pearson Custom,

2008. Print.

Adhikari, P.B. A Textbook of Physics. 2070 ed. Vol. II. Kathmandu: Sukunda Publication, 2070.

Print.