Detection and measurement of Radioactivity

The radioactive radiation can be detected and measured by a number of methods. The important ones used in modern practice are listed below.

(1) Cloud Chamber

** This technique is used for detecting radioactivity.

** The chamber contains air saturated with water vapour.

** When the piston is lowered suddenly, the gas expands and is supercooled.

** As an α- or β-particle passes through the gas, ions are created along its path. These ions provide nuclei upon which droplets of water condense.

** The trail or cloud thus produced marks the track of the particle. The track can be seen through the window above and immediately photographed.

** Similarly, α- or β-particles form a trail of bubbles as they pass through liquid hydrogen.

** The bubble chamber method gives better photographs of the particle tracks.

(2) Ionization Chamber

** This is the simplest device used to measure the strength of radiation. 

** An ionization chamber is fitted with two metal plates separated by air. 

** When radiation passes through this chamber, it knocks electrons from gas molecules and positive ions are formed. 

** The electrons migrate to the anode and positive ions to the cathode. Thus a small current passes between the plates. This current can be measured with an ammeter, and gives the strength of radiation that passes through the ionization chamber. 

** In an ionization chamber called Dosimeter, the total amount of electric charge passing between the plates in a given time is measured. This is proportional to the total amount of radiation that has gone through the chamber.

(3) Geiger-Muller Counter

** This device is used for detecting and measuring the rate of emission of α- or β particles.

** It consists of a cylindrical metal tube (cathode) and a central wire (anode).

** The tube is filled with argon gas at reduced pressure (0.1 atm).

** A potential difference of about 1000 volts is applied across the electrodes.

** When an α- or β-particle enters the tube through the mica window, it ionizes the argon atoms along its path.

** The argon ions (Ar⁺) are drawn to the cathode and electrons to anode. Thus for a fraction of a second, a pulse of electrical current flows between the electrodes and completes the circuit around.

** Each electrical pulse marks the entry of one α- or β-particle into the tube and is recorded in an automatic counter.

** The number of such pulses registered by a radioactive material per minute, gives the intensity of its radioactivity.

(4) Scintillation Counter

** Rutherford used a spinthariscope for the detection and counting of α-particles.

** The radioactive substance mounted on the tip of the wire emitted α-particles.

** Each particle on striking the zinc sulphide screen produced a flash of light.

** These flashes of light (scintillations) could be seen through the eye-piece.

** With this device it was possible to count α-particles from 50 to 200 per second.

Modern scintillation counter

** A modern scintillation counter also works on the above principle and is widely used for the measurement of α- or β-particles.

** Instead of the zinc sulphide screen, a crystal of sodium iodide with a little thallium iodide is employed.

** The sample of the radioactive substance contained in a small vial, is placed in a ‘well’ cut into the crystal.

** The radiation from the sample hit the crystal wall and produce scintillations.

** These fall on a photoelectric cell which produces a pulse of electric current for each flash of light. This is recorded in a mechanical counter.

** Such a scintillation counter can measure radiation upto a million per second.

(5) Film Badges

** A film badge consists of a photographic film encased in a plastic holder. When exposed to radiation, they darken the grains of silver in photographic film.

** The film is developed and viewed under a powerful microscope.

** As α- or β-particles pass through the film, they leave a track of black particles. These particles can be counted.

** In this way the type of radiation and its intensity can be known. However, γ-radiation darken the photographic film uniformly. The amount of darkening tells the quantity of radiation.

** A film badge is an important device to monitor the extent of exposure of persons working in the vicinity of radiation.

** The badge-film is developed periodically to see if any significant dose of radiation has been absorbed by the wearer.

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.

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