Detection of Radiation by Geiger-Muller & Scintillation Counter, Diffusion Cloud Chamber and Photographic Plate

Radioactive radiations can be detected by so many devices. But the most common devices used are the Geiger-Muller counter, scintillation counter, diffusion cloud chamber and photographic plate coated with silver halide. Here, we will be discussing each of them in the simplest way possible.

Geiger-muller counter function on the principle of ionization effect of radiation on gasses. It is a simple device consisting of a central chamber filled with gas (usually Argon gas and a small amount of Bromine vapour) and fitted with two electrodes: Cathode and Anode. The chamber is maintained at low pressure and a potential difference of about 450 volts between the cathode and anode.

Ionizing radiations or particles enters the chamber to initiate ionization by first ionizing some of the atoms of the argon gas to produce electrons. The electrons produced then move towards the anode while protons move in opposite direction to the cathode. This creates a short flow of small current that is amplified and detected by suitable devices that may include any of the following: loud speaker where it is observed as audible clicks, ratemeter where the indicator is the extent of movement of the needle, scaler where it is expressed as reading recorded in the register.

An important observation is that Geiger-muller tube and the connect detectors can be used to detect the anount or intensity of the radiation. This is because the count rate is proportional to the intensity of the radiation. Hence, the estent of movement of the needle of a Ratemeter can actually tell how much radiation is in the surrounding.

Scintillation counter is used especially for the detection of Gamma-emissions. This is the reason why it is mostly important in security applications. It is used for nuclear waste monitoring and detection where it is mounted on vehicles and helicopters, in border security, freight terminals, scrap metals yard,e.t.c.

Scintillation counter works on the principle that certain gasses fluorescence when exposed to ionization radiations. An example is zinc sulphide that glow or fluorescent when exposed to radiation. This can be seen with a microscope or counted with other devices. Sodium Iodide (NaI) with a small amount of Thallium is most especially used for the detection of Gamma-ray, while crystalline Cesium Iodide (Csl) is used to detect protons and α-particles.

The diffusion cloud chamber is the simplest form or Cloud chamber or Wilsom chamber. There are specific patterns formed by α-and β-particles, and γ-rays in a diffusion cloud chamber. These patterns can be used to tell the presence of radioactivity and which radiation is present. The chamber is filled with gas that is supersaturated with water or alcohol vapour and the ionization particles are allowed to pass through the gas where ions are formed in their track, serving as centres or nuclei for condensation of the water vapour or alcohol. It is this part that is photographed as a permanent record.

The use of photographic plate is an important way of detecting radioactivity even though it is not a technical device like the three above. Radioactive samples placed on a photographic plate coated with silver halide will cause the plate to become darker. The higher the radiation, the more radiation absorbed by the plate, and the darker the plate.