Explain the principle of particle detection. Draw log n-V graph showing different regions and discuss the significant physical processes taking place in these regions.
Ans.: Principle of Particle Detection: There are three gas filled detectors based on the production of ionization in a gas and the separation and collection of the ions by means of electrostatic fields. Ionization chamber, proportional counter and Geiger-Muller counter are such detectors. Here a cylindrical chamber carries electrode located along the axis of the cylinder and insulated from it The chamber is filled with a gas a pressure of one atmosphere of less. A voltage V is applied so that central wire is at positive potential and the chamber is at negative potential. R is resistance and C as a capacitance. Let us assume that an ionizing particle passes through the chamber producing ions and electrons in the gas. The electrons reach the central electrode and the positive ions towards the walls of the chamber. The current of the voltage signal thus developed across R shall depend on the applied voltage. At a given potential V there will be a certain number of electrons reaching the central electrode produced by an initial ionization. As this V varies the number of electrons reaching central wire would vary. This variation is shown graphically in the fig. (b) Where the logarithm of the number of ions is plotted against the applied voltage.
(1) Region A (Recombination region): Initially when the applied voltage is less the electric field is not so effective in removing the ions for collect in at the electrodes means the recombination of ion pairs take place and all the ions formed by nuclear radiation are not detected. Hence this region is known as ion recombination region.
(2) Region B (Saturation region): As the voltage is increased further, we reach in region B where the voltage is sufficiently high so that the loss of ions through recombination is almost negligible and ion pairs move to the electrode. So rapidly that almost energy ion pair reaches the electrodes. So the pulse height remains independent of the voltage applied. This region is known as saturation region of ionization region.
(3) Region C (proportional): When applied voltage is increased to limit of saturation region the pulse height increases due to secondary ionization. When the voltage is increased the electrons which are liberated by the primary ionizing agent get sufficient kinetic energy to produce secondary ion pairs due to collision with gas molecules. The primary and the secondary electrons thus produced are again accelerated by the electric field and produce more secondary ion pairs by collision. In this region the detector will give rise to pulses of different height depending on whether the initial ionization is caused by particles of particle. This proportionality between the pulse height and the initial ionization allows us to use the detector to distinguish between particles of different energies and different ionizing powers. As the applied voltage is increased this proportionality breaks down. The region C is called proportional region. The apparatus operating in this region is known as proportional counter. Here pulse height remains proportional to the amount generally lost in the chamber by the primary ionizing particle of nuclear radiation.
(4) Region D (limited proportional region): When voltage increases in the region the pulse height continuously increase but it is not proportional to initial ionizing intensity. This region is termed as region of limited proportionality and is not much used for measuring. The limitation may be because of: (a) Ultraviolet photons may be formed (b) New electrons may be formed when positive ions reach the cathode. (c) The space charge may distort the electric field.
(5) Region E (Geiger-Muller region)- In this region pulse height is very large and also completely independent of the initial ionization and all particles produce pulses of same height irrespective to their energy and primary ionization. This region is called Geiger Muller region and the apparatus operating in this region is known as Geiger Muller counter.
(6) Region F (Continuous discharge region) If the voltage is increased beyond the region F there will be an onset of continuous by consists of a large number multiple pulses the region has no interest here because discharge is indifferent to the presence of incident charge particles. This region is known as continuous discharge region.(A)Recombination region (B)ionization chamber region (C)Proportional counter region (D)Limited proportionality region (E)Geiger Muller counter region (F)Continuous discharge. In the fig. (b), there are two curves. Curve is due to the passage of small energy particle i.e. low number of primary electrons; and (b) is due to a high energy particle i.e. large number of primary electrons. We find that the curves are almost parallel even beyond V2 up to a certain value of potential V3. Between V and V each electron acts independently and produces its own avalanche, not being affected be the presence of other electrons. The number of secondary electrons will thus be more if the initial number is more. The number of ions collected between V2 and V3 is proportional to the initial ionization. This region Cm is thus called proportional counter region. For still higher values of potential, beyond V3 the gas multiplication effect continuous to increase vary rapidly. The avalanche produced by electrons start interacting and the proportionality feature is lost. The avalanche with large primary ionization grows rapidly capered to the one with low primary ionization. The region D, between V3 and V4 is known as region of limited proportionality. Beyond V3 the charge collected becomes independent of the ionization producing it. In this region even a very low energy ionizing particle will produce a large effect. Thus beyond V4 the curves (a) and (b) become identical. The region E is called Geiger Muller counter region.