Detectors of radioactivity

In addition to the spark counter, the Geiger counter and the cloud chamber that are dealt with fully in separate files. (See spark counter, Geiger Mueller tube and cloud chamber) the following instruments are used for detecting radioactivity:

(a) Photographic plates. Though these are not good for gamma-radiation, as there is insufficient ionisation, alpha and beta particles produce visible tracks in the plate where they pass.

(b) Electroscope. The leaf falls, due to ionisation of the surrounding air. This is rather a crude method and is not good for gamma radiation.

(c) Ionisation chamber. This is a can with a coaxial wire electrode. A 5 μC radioactive source will give a current of about 10^-9 μA in a small chamber.

(d) Scintillation counter. Particles or gamma rays produce a flash of light in a crystal (zinc sulphide and silver for alpha and beta radiation, sodium iodide and tellurium for gamma rays). This is detected by a photomultiplier tube and the electrical pulse recorded.

(e) Bubble chamber. Radiation creates tracks of bubbles in a superheated liquid such as hydrogen or propane. The bubble chamber is a much more effective detector of radiation than the cloud chamber, because of the much greater number of atoms per unit volume of the liquid in it. This means that there is a much greater chance of a collision occurring between an incoming particle and a nucleus.

(f) Solid-state detector. This is a reverse biased p-n junction of semiconductor material, and when ionising radiation falls on it ion pairs are formed at the junction, thus producing a current through it.

Student investigation 1
The ionising ability that forms the basis of many detectors of radioactivity may be investigated by the following experiment.
Use a d.c. amplifier as shown in Figure 4, with a wire gauze connected around a central brass electrode. Set the meter to the 10^-11 A range and connect the output to a 1 mA range meter.
(a) Hold a lighted splint just outside the gauze and blow the flame gently towards the electrode.
(b) Hold a mC radium source, just above the rod using a pair of tweezers.
In each case observe the meter current.

Student investigation 2
The absorption of gamma-radiation is of crucial importance in the nuclear power industry and this investigation is designed to measure this.
(a) Absorption in air Set up the Geiger tube as shown in the diagram, and compare the count rates at different distances from the source. Plot a linear graph of the results. (b) Absorption in building materials and soil Prepare a suitable series of absorbers of concrete, brick and soil.
Using the Geiger tube and scaler as before, measure the intensity of the gamma-beam with different thicknesses in place.
Plot linear graphs relating count rate to absorber thickness, and deduce the thickness of each absorber required to reduce the gamma-ray flux to one-tenth of the original value.

Observe the normal radioactivity safety procedures when carrying out these investigations.