Answer to Question #210493 in Atomic and Nuclear Physics for asad

Question

Write down at least three precautions that must be adopted in order to perform the

experiment to study the characteristics of G.M tube

Answer 1

All nuclear radiations, whether they are charged particles or gamma rays, it will ionize

atoms/molecules while passing through a gaseous medium. This ionizing property of a

nuclear radiation is utilized for its detection. Geiger-Muller counter, commonly called as G-

M counter or simply as Geiger tube is one of the oldest and widely used radiation detectors. It

consists of a metallic tube with a thin wire mounted along its axis. The wire is insulated from the

tube using a ceramic feed-through (Fig. 1). The central wire (anode) is kept at a positive

potential of a few hundreds of volt or more with respect to the metallic tube, which is

grounded. The tube is filled with argon gas mixed with 5-10% of ethyl alcohol or halogens

(chlorine or bromine).

When an ionizing radiation enters the Geiger tube some of the energy of the radiation

may get transferred to a gas molecule within the tube. This absorption of energy results

in ionization, producing an electron-ion pair (primary ions). The liberated electrons move

towards the central wire and positive ions towards the negatively charged cylinder. The

electrons now cause further ionization by virtue of the acceleration due to the intense electric

field. These secondary ions may produce other ions and these in turn still other ions before

reaching the electrodes. This cascading effect produces an avalanche of ions. In an avalanche

created by a single original electron many excited gas molecules are formed by electron

collisions in addition to secondary ions. In a very short time of few nanoseconds these excited

molecules return to ground state through emission of photons in the visible or ultraviolet region.

These photons are the key element in the propagation of the chain reaction that makes up the

Giger discharge. If one of these photons interacts by photoelectric absorption in some other

region of the tube a new electron is liberated creating an avalanche at a different location in the

tube. The arrival of these avalanches at the anode causes a drop in the potential between

the central wire and the cylinder. This process gives rise to a very large pulse with an

amplitude independent of the type and energy of the incident radiation. The pulse is

communicated to the amplifier through an appropriate RC circuit, and then to a counter

which is called as scaler. Suitable arrangements are made to measure the counts for a preset

time interval. The schematic diagram of the G-M tube and the associated electronic components.

Precautions:-

1. Do not let anything touch the window. It is very fragile.

2.Do not ever apply a high voltage beyond the plateau region, as the tube will be damaged.

3.Count rates of more than 100000/min will likely be subject to some coincidence error .