Neutronics simulations of ST40 tokamak operations for activation foils in DD plasma operation

Accurate neutron yield provides critical information on plasma performance in tokamaks. Additionally, these measurements are essential for radiation protection activities and shielding verification. This paper discusses the neutron transport simulations and presents a method for measuring the deuterium-deuterium neutron yield using neutron induced activity in indium foil within Tokamak Energy's experimental spherical tokamak, ST40. The neutron transport simulations, carried out with an application of the Geant4 toolkit, provide the differential neutron flux at the indium foil location. A cadmium zinc and tellurium (CZT) detector has been used to measure the neutron activation of indium foil at the end of each plasma operation day. The CZT detector measures the gamma rays from the activated indium foil. However, a typical operation day includes multiple plasma pulses, and activity measured includes contributions from all these plasma pulses throughout the day. Hence, an algorithm has been developed to reproduce the induced activity in indium foil resulting from individual plasma pulses. This algorithm utilizes the diamond detector's response, placed next to the activation foil and measures neutron emission from ST40 via elastic scattering of 12 C nuclei as a secondary source of neutron diagnostics.