Radioactivity: Detection and Measurement

– In this subject, we will discuss the Detection and measurement of Radioactivity.

Detection and measurement of Radioactivity

– Radioactivity can be detected and measured by several methods.

– The important ones used in modern practice are listed below.

(1) Cloud Chamber 

(2) Ionization Chamber

(3) Geiger-Muller Counter

(4) Scintillation Counter

(5) Film Badges

(1) Cloud Chamber 

– This technique is used for detecting radioactivity.

– The chamber contains air saturated with water vapor.

– 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 a 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 the 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 sulfide 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 sulfide 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 produced scintillations.

– These fall on a photoelectric cell which produces a pulse of electric current for each flash of light. This is recorded on 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 darkens 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 / multicolor edition.