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.
484 exam-style questions on CIE IGCSE Physics 5.2 Radioactivity, covering 5.2.1 Background radiation and measuring count rate, 5.2.2a Alpha, beta and gamma emission, 5.2.2b Deflection and ionising effects of nuclear radiation, 5.2.3a Radioactive decay, 5.2.3b Effects of decay on the nucleus and decay equations, 5.2.4a Half-life, 5.2.4b Choosing isotopes for applications, and 5.2.5 Effects of ionising radiation and safety precautions. Each one has a worked solution and a mark scheme showing where the marks go.