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