Absolute activity of gold foil measurement using coincidence method to determine absolute neutron flux of zero power reactor

To perform an effective experimental implementation in a zero- power reactor or more generally, critical assemblies, it is necessary to measure the total neutron flux to calibrate the reactor power. The 4πβ-γ coincidence method via the activation of gold foil is a well known standard method for absolute neutron flux measurements. The absolute neutron flux is obtained by measuring the absolute activity of the irradiated gold foil, ranged in 102</sup>-104</sup> Bq. In this method, the gold foil is irradiated at the desired position. The beta, gamma, and the coincidence rates are by using the 4πβ-γ coincidence system, and the absolute activity of gold foil is then obtained. The β-γ rays emitted from the irradiated gold foil are recorded in a concise period of tim (of the order 10-8</sup> seconds). The advantage of this method is that the efficiency of either detector is directly dependent on the counting rates. Changing the beta detector efficiency due to the high voltage change and then measuring  the beta, gamma, and coincidence counting rates, the plot of beta counting rates against the beta detector efficiencies can be introduced. By extrapolating the efficiencies to a unit value, the absolute activity of the gold foil can be obtained.