The Stellar (n, gamma ) Cross Section of the Unstable Isotope 163Ho and the Origin of 164Er

Abstract

The stellar (n, y) cross section of the unstable isotope <SUP>163</SUP>Ho (t<SUB>½</SUB> = 4570 yr) has been experimentally determined relative to that of gold by means of the activation method. In the course of this experiment, a number of decay parameters for <SUP>164</SUP>Ho have been remeasured and improved. A sample containing 6.62 x 10<SUP>17</SUP> <SUP>163</SUP>Ho atoms was irradiated in a quasi-stellar neutron spectrum corresponding to a thermal energy of kT = 25 keV. This sample was produced from <SUP>162</SUP>Er, and the remaining impurity was sufficient to determine the <SUP>162</SUP>Er cross section simultaneously. For <SUP>163</SUP>Ho, a stellar cross section of σv/v<SUB>T</SUB> = 2125±95 mbarn at kT = 30 keV was found as the sum of the partial cross sections feeding the ground and isomeric states in <SUP>164</SUP>Ho. This value is of relevance for analyzing the abundance pattern in the s-process branchings at <SUP>163</SUP>Dy and <SUP>163</SUP>Ho that reflects the electron density during helium shell burning in low-mass stars.