1. Absorption and ‘Spreading Out’ of ionising radiation. There are two mechanisms which cause the intensity of ionising radiation t decrease as distance from the source increases: 1. Absorption. All ionising radiation is absorbed to a certain extent by the material it is passing through. Gamma rays from a particular source all have the same KE (reasons later), but absorption is not the important factor in the reduction of intensity with range for Gamma radiation (reasons soon!) Alpha particles from a particular source all have the same amount of KE (Remember Mass Defect from L6). They are absorbed by about 10cm of air or about 0.1 mm of paper. Beta particles have KE which varies from 0 up to a maximum dictated by the mass defect in the decay process (Remember neutrinos in L6). The KE of Beta particles is randomly distributed, so their absorption works as shown below

2. Range which absorbs half the particles Emitter emitting narrow beam of particles at n per second The maths for absorption n/2 particles per second detected n/4 particles per second detected n/8 particles per second detected As range increases, absorption causes an exponential decrease of intensity.

3. 2. Spreading Out. It is difficult to produce a narrow beam of ionising radiation, so almost all ionising radiation spreads out in all directions. This means that the intensity of ionising radiation decreases with range in the same way as an ‘E’ or ‘G’ field. It obeys a 1/r 2 law. The maths for spreading out. Source emitting in all directions Intensity of detected radiation proportional to 1/r 2.

4. Experiment. In most situations, the relationship between intensity of ionising radiation and range is caused by a combination of absorption and spreading out, so it is mathematically complicated. But, for Beta particles passing through aluminium, the particles do not have time to spread out much before they are absorbed, so we can test the exponential relationship. And, for Gamma particles in air, they are hardly absorbed at all, so we can test the 1/r 2 law. Design, carry out, and do the necessary analysis on these two experiments