Astrophysical analysis of the measurement of (alpha,gamma) and (alpha,n) cross sections of 169Tm

TL;DR

This study measures the cross sections of 169Tm reactions with alpha particles at various energies, compares them with theoretical models, and discusses implications for astrophysical gamma-process nucleosynthesis.

Contribution

It introduces a novel method combining activation with X-ray counting for measuring reaction cross sections and analyzes energy dependence of alpha widths in astrophysical contexts.

Findings

Measured (alpha,gamma) and (alpha,n) cross sections at lower energies than before.

Observed deviations from theoretical predictions below 14 MeV, indicating the need for modified alpha widths.

Revised astrophysical reaction rates are higher than previous estimates by factors of 2 to 37.

Abstract

Reaction cross sections of 169Tm(alpha,gamma)173Lu and 169Tm(alpha,n)172Lu have been measured in the energy range 12.6<=E_alpha<=17.5 MeV and 11.5<=E_alpha<=17.5 MeV, respectively, using the recently introduced method of combining activation with X-ray counting. Improved shielding allowed to measure the (alpha,gamma) to lower energy than previously possible. The combination of (alpha,gamma) and (alpha,n) data made it possible to study the energy dependence of the alpha width. While absolute value and energy dependence are perfectly reproduced by theory at energies above 14 MeV, the observed change in energy dependence at energies below 14 MeV requires a modification of the predicted alpha width. Using an effective, energy-dependent, local optical alpha+nucleus potential it is possible to reproduce the data but the astrophysical rate is still not well constrained at gamma-process temperatures. The additional uncertainty stemming from a possible modification of the compound formation cross section is discussed. Including the remaining uncertainties, the recommended range of astrophysical reaction rate values at 2 GK is higher than the previously used values by factors of 2-37.