A Geiger counter placed close to a sealed radioactive source records 1 020 counts per minute at the initial instant. When the counter is moved far away from any source, it registers a steady background of 20 counts per minute.
After the counter has been operating beside the source for 5 hours, the total reading falls to 260 counts per minute.
(i) Determine the decay constant of the radioactive material, correct to four significant figures.
(ii) Using the value of the decay constant obtained in part (i), predict the total reading of the counter (including background) 10 hours after the initial instant.
(iii) After how many hours from the start will the net activity of the source (i.e. reading after subtracting background) fall to 5 % of its initial net activity?