The Gold Leaf Experiment
For this experiment a positively charged electroscope with a gold lead in a diverged position is used. A radium source is brought near to the cap of the electroscope. The radiation emitted ionises the surrounding air; as the cap is positively charged the positive ions repel and the negative ions are attracted to the cap. On reaching the cap the positively charged electroscope is neutralised by the negative ions and therefore, the gold leaf collapses.
The diffusion cloud chamber
Air containing alcohol vapour in a chamber is cooled by dry ice placed below a thin black metal plate. When a radioactive source is placed inside the chamber, so that the radioactive radiation passes through the vapour, tracks are produced in the alcohol vapour due to condensation of the alcohol vapour on the ions formed. The tracks can be seen by looking through the transparent plastic lid. They appear white against the black base plate of the chamber.
The Geiger-Muller (G-M) tube
When the ionising radiation enters the tube by penetrating the mica window the argon atoms become ionised into electron and argon-ion pairs. The free electrons will then accelerate towards the anode wire along the axis of the cylindrical cathode. The electrons will then cause further ionisation of the argon atoms by colliding with them, producing a cluster of electrons which will collect almost immediately at the anode. The positively charged ions will collect at the cathode. The electrons and argon ions at the electrodes produce a current pulse. This current pulse is amplified and then either fed into a rate meter or a scalar. A rate meter has a meter marked in counts by seconds (or per minute) from which the average pulse rate is read. A scalar counts the pulses and displays the total received in a certain time.
When the radioactive source is removed, the G-M tube usually registers a count of between 20 to 50 per minute. This is phenomenon is called background count which is due to background radiation. The background radiation may be caused by the radioactive
contamination of the defector or its’ surroundings, or because of cosmic radiation entering earth from space.