2.6 - Half life

What is half life?

The time it takes for the activity of a radioactive source to halve, or the time it takes for half the radioactive nuclei to decay.

How can half-life be used?

To calculate the age of a sample or to predict the amount of a sample that will be left after a certain time.

How does a tracer work?

Gamma radiation is injected into the body and cameras are used to take an image of where the radiation is. Gamma radiation is used because it is weakly ionising so it will have less effect on cells in body yet it is very penetrating so it can pass through the body to be detected by the camera. It has a short half life which means activity decays to a safe level quickly.

How are cancer cells treated from inside the body?

Beta radiation is placed inside a tumour to kill cancer cells, Beta radiation is used because it is strongly ionising and will kill cells it is absorbed by yet is moderately penetrating so can pass through the whole tumour and kill all cells without effecting healthy cells. It has a short half life which means activity decays to a safe level quickly.

How are cancer cells treated from outside the body?

Gamma radiation is aimed at the tumour from outside the body. Gamma radiation is used because it is very penetrating so can pass through the body to reach cancer cells yet is weakly ionising so it has less effect on healthy cells. It has a long half life as the source is outside the body and can be stored safely and used time and time again.

How can the thickness of paper or foil be monitored?

A beta radiation source is placed on one side of a sheet of paper or foil and a detector on the other side. If the count is too low, not enough radiation could penetrate and paper is too thick. Beta radiation is used because it is moderately penetrating and can pass through paper but count would be affected by the thickness. It has a long half life as the source can be stored safely and used time and time again.

How does a smoke detector work?

Alpha radiation in the detector ionises the air which creates a current. If there was smoke in the detector it would block the radiation and the current would change, setting off the alarm. Alpha radiation is used as it is very ionising which creates a current yet is the least penetrating so will be blocked by smoke particles. It has a long half life so the activity does not change enough to change the current produced.