Radioactive Detectors

Radioactive Detectors

Geiger-Muller Tube

1. The Geiger-Muller tube is an effective radioactive detector. It can trace alpha particles, beta particles and gamma rays.


2. The outer part of the G-M tube is made of aluminium which acts as the cathode.


3. The middle part of the G-M tube is a metal wire which acts as the anode.


4. The G-M tube is filled with argon gas at low pressure.


5. Initially, the G-M tube must be connected to a high voltage before being used.


6. This high voltage causes some ionization of argon gas.

Cloud Chamber

1. The cloud chamber is made by using a transparent plastic box. The space in it is divided into two parts by a metal.


2. The lower part is filled with solid carbon dioxide. Sponge is used to push the solid carbon dioxide towards the metal plate.


3. The upper part is filled with molecules of alcohol vapour released from the felt which is initially soaked in alcohol.


4. When the alcohol vapour diffuses downwards, it will become colder. Thus, a supersaturated condition will be produced in the space in the lower part of the chamber.


5. When the radioactive rays enter the upper part, the ionization of air will occur. Saturated alcohol vapour will move above the ions. Droplets of liquid alcohol on the ions will cause the formation of misty tracks.


6. Steps to ensure clear tracks:




• The transparent Perspex cover is rubbed with a soft cloth to produce charges which will remove all ions in the chamber before any radioactive rays enter.


• The cloud chamber must be placed horizontally to ensure smooth flow of particles in it.


• If light is used, it must shine on the area superated with vapour and not on the black base of the chamber in order to avoid heating it.


7. Normally, the tracks produced are not uniform. This shows that the radioactive rays are produced randomly.


8. There are three types of tracks as shown in Table below.

Types of radioactive rays	Explanation
Tracks of alpha particles	The alpha tracks are thick and straight. This shows that alpha particles have the strongest ionizing power and the biggest mass.
Tracks of beta particles	The beta tracks are thin and curvy. This shows that beta particles have low ionizing power and small mass.
Tracks of gamma ray	Their tracks are short, curvy and spiky from the middle. It shows that it has the lowest ionizing power.

9. The number of radioactive tracks produced will decrease after a while. This is because after some time, the condensation of alcohol vapour on the radioactive source will block the emission of radioactive rays.

Spark counter

The wire gauze and thin wire are connected to a voltage of more than 2000 V.

The voltage is increased slowly until sparks are produced in between.

The sparks are formed due to ionisation of the air.

The voltage is then decreased until no sparks are formed.

The radioactive source is brought close to the wire gauze.

The radioactive rays will ionize the air molecules between the wire gauze and thin wire. Positively charged ions will be attracted to the negatively charged gauze and the negatively charged ions will be attracted to the positively charged thin ions.

Secondary ionization will occur due to the collision between the ions and the air molecules.

Therefore, sparks are formed.

The number of sparks measured the intensity of radioactive rays from its source randomly.

The spark counter can only trace alpha particles which have high ionizing power.

Electroscope

When charged plate of the electroscope is exposed to the source of alpha particles, the gold leaf will collapse slowly.

This is due to the ions and electron are produced by the alpha particles which will neutralize the charge in the electroscope.

The rate of collapse of the gold leaf indicates the strength of the radioactive source.

Photographic Plate

All types of radioactive rays will darken the photo film. The effect is like sunlight acting on it.

The ionization effect by the radioactive rays will decompose silver bromide crystals on the film.

Films which are exposed to sunlight will show white lines representing radioactive tracks.

Films are kept in the badges worn by workers as a tracer device of radioactive rays.

The main disadvantage of using a film as a radioactive tracer is that it needs to be processed in order to prove the presence of radioactive rays.