measurement of half-life period of thorium emanation

Radio activity is generally due to the disintegration of the nuclei of a number of atoms in the material concerned . the number of radioactive atoms diminish as time goes on , and the half-life period of a radioactive material is defined as the time for the number of radioactive atoms to diminish by half , or as the time taken for the intensity of the radioactivity to diminish by half .
The half-life period of thorium gas can be measured with the aid of a chamber attached to a plastic bottle containing thorium hydroxide . The chamber is clipped on to the support rod , and slid down the rod so that the top electrode of the electroscope enters the centre of the chamber as far as possible . The side electrode is kept at a potential of +3kV , and the gap between the side electrode and the flag is made 1-2 mm.
The bottle is shaken to release any thorium gas trapped , and then squeezed once or twice , keeping it upright . The thorium gas is transferred to the chamber , where it decays with the emission of α-particles and produces an ionization current . The flag should begin to beat at the rate of 1 beat or more per 10 seconds . The time of successive beats is noted , and this time lengthens as the radioactive gas decays .
The actual number of the beat is plotted against the time . A smooth curve is drawn through the points and extended to level off horizontally . If the number of the beats corresponding to the horizontal part of the curve is ten , as shown , the half-life period is the time to reach the fifth beat . This time is read from the graph .