Rene Reifarth

Inelastic scattering reactions including (n,2n) are particularly difficult to measure. Hardly any measurements on radioactive nuclei exist. The reason is that traditional methods require the detection of gammas or neutrons in a background of elastically scattered neutrons, which can interact with the detector or other materials. The 73As(n,2n) and the 73As(n,2n) reaction cross sections are the only neutron removal cross sections that can be measured with current methods. The corresponding cross sections for the short-lived isotopes 71,72,74As can only be directly measured in inverse kinematics. Since 75As is the only stable isotope of this element, the sample production is no problem at all. This report is therefore focused on the sample production for a possible 73As(n,2n) campaign.

The measurement of the bound-state decay of ²⁰⁵Tl at the Experimental Storage Ring (ESR) at GSI, Darmstadt, has recently been reported, with substantial impact on the use of ²⁰⁵Pb as an early Solar System chronometer and on the low-energy measurement of the solar neutrino spectrum via the LOREX project. Owing to the technical challenges in producing a high-purity ²⁰⁵Tl⁸¹⁺ secondary beam, a robust statistical method was developed to estimate the variation in the contaminant ²⁰⁵Pb⁸¹⁺ produced in the fragmentation reaction, which was subsequently transmitted and stored in the ESR. Here, we show that Bayesian and Monte Carlo methods produce comparable estimates for the contaminant variation, each with unique advantages and challenges given the complex statistical problems for this experiment. We recommend the adoption of such methods in future experiments that exhibit unknown statistical fluctuations.

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 M 1 photon strength function, especially in odd-odd nuclei. This work is focused on two main topics—deriving experimental Tm 169 ( n , γ ) cross section and studying statistical γ decay of Tm 170 , 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 Tm 170 cannot be reproduced without a scissors mode resonance centered at ≈ 3.3 MeV . The measured cross section in the unresolved-resonance region is generally lower than the latest evaluations. The derived Tm 169 s -process abundance is expected to increase by a factor of 1.26, while the changes of the abundances of elements heavier than Tm 169 are in the order of 0.2%. The scissors mode properties in Tm 170 are similar to those deduced in previous analyses of neighboring nuclei Er 168 and Ho 166 .