Chopped-beam PGAA facility at Budapest

The use of the chopped-beam technique is an option for in-beam cross-section measurements at the Budapest PGAA-NIPS facility. If the half-life of the interesting decay is between seconds and days, the time evolution of the activation can be followed on-line and the gamma decay peaks can be separated from the prompt neutron capture peaks by means of the beam chopper. The background conditions and the selectivity of the gamma ray spectrometry have also been substantially improved recently. For short half-lives, the on-line detection is advantageous over the conventional decay measurement; hence the longer acquisition time improves the statistical precision. The technique has an important advantage over activation in a reactor core: as the beam is free of epithermal neutrons, the effect of low-energy resonances can be completely avoided. Successful standardization measurements are reported on many stable targets, as well as on 99Tc, 129I and natural uranium. The latter have been used as input data to level scheme and cross-section studies for transmutation. The selection criteria for the ideal targets were discussed. The cross-section measurements at thermal and sub-thermal energies are feasible on a couple of actinide isotopes of Am, Pu, Np, provided that sufficient target is available. We are also investigating the possibility of measuring the cross-section deviation from the 1/v-law in the region of 10-25 meV, by using a time-of-flight setup with a small duty cycle chopper disk and a comparator nuclide with proven 1/v-behaviour