half life

• the half life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve
• the half life is also the time it takes for the count rate (or activity) to halve to its initial level
• a long half life means the count rate is aproximately contant this why the radiation inside a smoke alarm should have a long life
• beta and gamma radiation will penetrate smoke so no change in the count rate will be detected by a smoke alarm
• alpha radiation is not very penetrating this means it is safe to use in the house
• ideal properties of a radioactive source to use for medical diagnosis? is short half life this means less damage to cells and low ionising power which means less damage to tissues aswell as highly penatrating meaning it can be detected outside the body