Ionizing radiation enters the detector chamber and ionizes the
mixture of gas in it. The electrons drift towards the positive
electrode and ions move towards the negative electrode. Thus,
ampere meter detects a current. The number of ion pairs is
proportional to the number of ionizing particles entering the
detector chamber. Thus, the current is proportional to the
intensity of ionizing radiation. The light electrons drift
100,000 times faster than the heavy ions. The motion of
electrons is mostly responsible for the current. Rutherford
placed a thorium oxide (ThO2) sample directly in the detector
chamber. He noticed that the radioactivity increased with time.
After removing the sample carefully without changing the air in
the chamber, he found the air remained radioactive. The
radioactive air decayed with a specific half life. Other
radioactive samples did not give the same observation. His
experiment suggested the existence of a radioactive gaseous
element. The experiment was later interpreted as due to the