DETERMINATION OF TFTR FAR-FIELD NEUTRON DETECTOR EFFICIENCIES BY LOCAL NEUTRON-FLUX SPECTRUM MEASUREMENT

Neutron detectors have often been located on the tokamak fusion test r eactor (TFTR) test cell floor 3 m or more from the vacuum vessel for e ase of detector access, to reduce radiation damage, minimize count sat uration problems, and to avoid high magnetic fields. These detectors i nclude Si surface-barrier diodes, fission chambers, natural diamond de tectors, and T-2 production in a moderated He-3 cell. To evaluate the performance of these detectors during deuterium-tritium (D-T) operatio n, we determined the neutron flux spectrum incident on the principal d etector enclosure using nuclide sample sets containing Al, Ti, Fe, Co, Cu, Zn, Ni, Zr, Nb, In, and Au activation foils. Foils were installed and then removed after ample exposure to TFTR D-T neutrons. High effi ciency, high purity Ge detectors were used for gamma spectroscopy of t he irradiated foils. The incident neutron fluence and spectral distrib ution were unfolded from the measured results, and used to derive abso lute detector efficiencies. (C) 1997 American Institute of Physics.