Year 13 Applied Science: Thickness of aluminium to stop beta

In this experiment, we first measured the background count because there is always this natural radioactivity in the air. We will never get a reading of zero. We got a reading of 16 counts in 20 seconds, a bit higher than normal because the lead-lined box containing the source was a bit too close. I then used 30cm tongs to get the source from the box and was careful to make sure that the source was pointing away from people. You were all over 2 metres behind the experiment and we have previously shown that our beta source cannot be distinguished from background at about 1.5 metres. Another safety precaution is to have the source out of the box for as little time as possible. We measured the count rate for the beta source through air. Then we measured the thickness of an aluminium sheet with a micrometer screw gauge. A reading of 40 means 0.40mm thick. We measured the count rate with 1 sheet, then added another and then a third. With the third sheet the count rate was down the background level, so we found that 3 x 0.40mm = 1.2mm of aluminium will stop all radiation from our beta source.