Effect of anisotropic D–D fusion neutron emission on counter calibration using activation techniques

Detailed plasma and neutron transport simulation is needed for accurate measurements of the total neutron emission by means of foil activation during deuterium neutral beam injection into toroidal fusion plasmas. Results from a dedicated package of computer codes for simulation of neutron production and neutron transport are presented. The simulation procedure comprises spatially resolved Fokker–Planck calculations of the deuteron velocity distribution, the calculation of absolute neutron spectra as functions of the direction of emission and, finally, a detailed Monte Carlo neutron transport simulation. The simulation procedure is used to calculate the influence of anisotropic neutron emission on activation foil measurements for neutral-beam-heated toroidal fusion plasmas due to the variation of the differential D–D neutron production with angle of emission. Theoretical results predict effects in the order of a few percent. This is consistent with experimental results which show that the total neutron yield is underestimated by about 5% if effects of anisotropic emission are being neglected.