Absolute measurement of sources activities in the cross-section determination—Improvement by simulation

A calibration system composed of a 4 π β - γ gas-flow proportional counter coupled to a pair of NaI(Tl) crystals operating in coincidence was used for absolute measurement of Am 242 g sources activities. The samples were produced by irradiating dried aliquots of 241Am solution with epithermal neutrons in a research reactor. Efficiency tracing methodology using 60Co as tracer was applied. Thick external absorbers were used to change the Am 242 g beta detection efficiency and to avoid counting alpha particles from the dominating activity of 241Am. The resulting complex behavior of the beta detection efficiency curve was simulated by the Monte Carlo method, and the Am 242 g activity was obtained by least-squares method fitting. The activity was determined with an uncertainty of about 1%.