(M) In a field investigation to measure the population of mosquitoes within a given area, 10 4 radioactively labelled individuals were released. In a recovered sample of 2100 mosquitoes, 100 were found to be labelled. What is the population?
RADIOISOTOPE UNIT: Sc87101 P. 2 2. (A) Consider a 80 Co point source of 1.85 x 10 11 Bq set up for demonstrating the inactivation of an enzyme by radiation. The dose required for the reaction is 25 mGy and the irradiation is carried out at a distance of 0.5 m from the source. (i) Calculate the thickness required for a lead shield in~ wall to be built at 2 m from the source such that outside surface dose rate is 25 JJGY hr" . (ii) Determine the exposure time required to give the necessary reaction dose. (Half-value thickness for 80 Co = 138.6 kg m" 2 ; density of Pb = 1.13 X 10 4 kg m" 3 ; 1 c kg" 1 = 33.7 Gy in air; 1 C kg· 1 = 38.76 Gy in water; half-life of 80 Co = 5.3 yr and specific gamma ray constant for 80 Co = 9.2 x 1 o· 15 C kg· 1 hr" 1 Bq· 1 at 1 m) (1 o marks) (B) In liquid scintillation counting, what is the so-called balance point counting? Assume that a counter has been set to count 35 S under balance-point conditions to give approximately 80% efficiency. What changes in counting efficiency would be expected in the following circumstances assuming all instrument settings remained unchanged? Give your reasons.