HPGE WELL-TYPE DETECTORS FOR NEUTRON-ACTIVATION MEASUREMENTS ON THE FRASCATI TOKAMAK UPGRADE TOKAMAK

We describe an improvement of the neutron activation system in operati on on the Frascati Tokamak Upgrade (FTU) tokamak for the measurement o f the total neutron yield. A HPGe well-type detector (200 cm(3) active volume) is used to detect the photoemission from neutron activated sa mples (In-115m 336.2 k.eV gamma rays from DD neutrons on indium for FT U). Due to their high geometrical efficiency, HPGe well-type detectors are particularly suited to the FTU low-level activity measurements. A particular effort has been devoted to the calibration of the measurin g system. In particular, a multi-gamma calibration source (59-1332 keV energy range) with a density of 7.31 g/cm(3) consisting of a stack of indium foils has been prepared. This assures that the shape and volum e of the calibration source are the same as those of the samples used in the actual measurements. The full-energy-peak efficiency at the In- 115m 336.2 keV line is 0.197 with an overall uncertainty of 2% (1 sigm a). For a better characterization of the detector response as a functi on of the sample density, a further calibration source with the same g eometry has been prepared in a gel aqueous solution (density similar t o 1 g/cm(3)). The calibration curves for the well-type detector at the two different density values are compared. (C) 1997 American Institut e of Physics.