Isotopic yield determination for thermal Neutron induced fission of 235U, 239Pu and 241Pu in the light, symmetry and heavy mass regions using calorimetric low temperature detectors

Abstract

Isotopic yield investigations for thermal neutron induced fission reactions are presented in this work, which were performed using calorimetric low temperature detectors (CLTDs) with the passive absorber method in a series of experiments at the LOHENGRIN spectrometer at the ILL Grenoble. The present work expands the isotopic yield determination with the passive absorber method from the light fragment mass region to the symmetry and heavy fragment mass region where the isotopic yield determinations were rather challenging or hardly accessible until now. A new CLTD array of 25 detector pixels was constructed and a rotatable disk with several positions for SiN absorber foil stacks was installed in front of the detectors at only 5 mm distance in order to optimize the absorber thickness for respective mass regions and to improve the detection efficiency. The operation of the new detector setup was tested at the Munich tandem accelerator at the MLL Garching with the heavy ion beams 130Te and 127I in the energy range of 45 MeV to 80 MeV where the CLTDs measured the ion beams with energy resolutions of less than 1%. Energy loss measurements with the heavy ions $^{130}Te$ and $^{127}I$ were performed at different SiN absorber thicknesses to estimate the expected quality of nuclear charge separation in the heavy mass region of fission fragments and to gain insights in the shape of energy loss distributions. This was in particular helpful for the analysis of overlapping peaks in the heavy fragment mass region. In the experiments performed at the ILL reactor, isotopic yields were determined in the light, symmetry and heavy fragment regions with 235U, 239Pu and 241Pu targets. A first set of measurements was performed for the precise yield determination of 92Rb for the three targets and and of 96Y for 235U and 241Pu targets due to its importance for the understanding of the reactor anti-neutrino anomaly studies. The 92Rb and 96Y yields thus determined allowed to resolve the discrepancy between recent independent measurements and the nuclear data libraries like JEFF for the 92Rb yield, as well as confirms the 96Y yields from an independent measurement. Yield measurements towards the symmetry region were performed for 239Pu(nth, f) and 241Pu(nth, f). Isotopic yields were determined for 24 masses in the range 89<A<112$ for 241Pu(nth, f) for the first time with LOHENGRIN. Towards the mass symmetry known Z-yield data were supplemented for masses A = 110 to 112 for 241Pu(nth, f), and for masses A = 110 to 113 for 239Pu(nth, f). The even-odd staggering in the isotopic yields thus determined towards the symmetry region for the first time for 241Pu(nth, f) and 239Pu(nth, f) provides insights in the even-odd effect conjectured in different theoretical fission models for symmetric fission and contrasting predictions based on the previously partially measured data at the LOHENGRIN. Finally, isotopic yields were determined in the heavy fragment region for masses A = 128 to 137 for the first time with the passive absorber method for 239Pu(nth, f). Besides the possibility of cross-checking available experimental isotopic yield data with an alternative technique, it allowed to complete the experimental data sets with masses A = 130 - 132 and 135, which are either not easily or not at all accessible with gamma-ray spectroscopy.