A radiation detector is used to monitor the radiation from a radioactive source. A ratemeter records the total number of counts per minute.
(a) The source is brought close to the detector. The table below shows how the ratemeter reading varies with time:
Time / s0306090120150180210240Ratemeter reading / (counts/min)4303332602041621301068774Count rate due to source / (counts/min)400(i)230(ii)132(iii)765744 \begin{array}{|l|c|c|c|c|c|c|c|c|c|}\hline\text{Time / s} & 0 & 30 & 60 & 90 & 120 & 150 & 180 & 210 & 240 \\ \hline\text{Ratemeter reading / (counts/min)} & 430 & 333 & 260 & 204 & 162 & 130 & 106 & 87 & 74 \\ \hline\text{Count rate due to source / (counts/min)} & 400 & \text{(i)} & 230 & \text{(ii)} & 132 & \text{(iii)} & 76 & 57 & 44 \\ \hline\end{array} Time / sRatemeter reading / (counts/min)Count rate due to source / (counts/min)043040030333(i)6026023090204(ii)120162132150130(iii)1801067621087572407444The bottom row shows the count rate due only to the source. Fill in the missing values for (i)\text{(i)}(i), (ii)\text{(ii)}(ii), and (iii)\text{(iii)}(iii).
From the table, estimate the half-life of the radioactive source.
Even when the source is a long way from the detector, the ratemeter registers a reading of 30 counts/min30\text{ counts/min}30 counts/min. Suggest a cause of this 30 counts/min30\text{ counts/min}30 counts/min reading.
144 exam-style questions on CIE IGCSE Physics 5.2.4a Half-life. Each one has a worked solution and a mark scheme showing where the marks go.