Detection and Measurement of Radioactivity

 

RADIOACTIVITY:

"The disintegration or decay of unstable atoms accompanied by emission of radiation is called Radioactivity."

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 (Fig. 4.4) 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) Ionisation Chamber

    This is the simplest device used to measure the strength of radiation. An ionisation 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 ionisation 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 (Fig. 4.6) 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 ionises 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 (Fig. 4.7) for the detection and counting of ---------- Rutherford used a spinthariscope (Fig. 4.7) 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. 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 produces 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 up to 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.

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