Tm169(n,γ) cross section and statistical decay properties from measurements at the DANCE facility

Abstract

Radiative neutron capture on thulium, which is a monoisotopic element, plays a role in different applications such as nuclear astrophysics or nuclear burning environments. Considerable discrepancies—reaching 20%—exist between evaluations in the unresolved-resonance region. Furthermore, experimental data on statistical γ decay in odd-odd rare-earth nuclei is scarce. There are still open questions about the systematics of the so-called scissors mode in the M1 photon strength function, especially in odd-odd nuclei. This work is focused on two main topics—deriving experimental Tm169(n,γ) cross section and studying statistical γ decay of Tm170, in particular properties of the scissors mode. The capture experiments to obtain experimental cross section were performed at the Los Alamos Neutron Science Center using the time-of-flight technique and employing the Detector for Advanced Neutron Capture Experiments. Measured coincident γ-ray spectra were also compared with statistical simulations using the code to test different models of level density and photon strength functions. The capture cross section was determined from 1.8 eV to 0.97 MeV, the broadest neutron-energy range ever measured for this isotope. Several new resonances have been observed. The statistical γ decay of Tm170 cannot be reproduced without a scissors mode resonance centered at ≈3.3MeV. The measured cross section in the unresolved-resonance region is generally lower than the latest evaluations. The derived Tm169 s-process abundance is expected to increase by a factor of 1.26, while the changes of the abundances of elements heavier than Tm169 are in the order of 0.2%. The scissors mode properties in Tm170 are similar to those deduced in previous analyses of neighboring nuclei Er168 and Ho166.