TOTAL ABSORPTION GAMMA-RAY SPECTROMETER FOR MEASUREMENT OF BETA-DECAY INTENSITY DISTRIBUTIONS FOR FISSION-PRODUCT RADIONUCLIDES

A total absorption gamma-ray spectrometer (TAGS), based on a 25.4 cm diameter x 30.5 cm long NaI(Tl) well detector, has been developed at the INEL on-line isotope separator facility. A Si detector in the well of the NaI(Tl) detector allows one to collect beta-particle-gated coincidence spectra as well as singles spectra. With this system, measurements of the total absorption spectra have been made for a number of fission product nuclei. The analysis of the measured spectra is based on response functions for single gamma-rays that are computed with a Monte Carlo code. Routines have been written to use these response functions to simulate the response to a cascade of several gamma-rays and by extension to a whole decay scheme. The bremsstrahlung from a ground-state beta-branch can also be simulated. The quality of the simulation is demonstrated for test spectra of CS-137, Na-24, Co-60, and Ag-110m. As examples of the operation of this system, the measurements and analyses of the beta-decay feeding distributions are described for the decay of Ba-141, Cs-139 and CS-140.