Indirect studies of neutron capture: the surrogate method in the storage ring featuring Guy Leckenby (LP2i Bordeaux, France)

Hosted by: Borbála Cseh (Konkoly Observatory)

Abstract: Neutron capture cross sections on radioactive nuclei are needed to understand the astrophysical neutron-capture processes (s, i, n, and r) that synthesise the elements heavier than iron (though we must choose judiciously which nuclei to study). Unfortunately, direct measurements on nuclei that cannot be made into a target are not feasible because, when you go to inverse kinematics for radioactive beams, there is (presently) no free neutron target. Theoretical predictions are also not precise enough; state of the art reaction models exhibit ~factor 2 uncertainty around stability that increases to ~factor 10-100 as we depart from stability. The bulk of this uncertainty originates in understanding the compound nuclear decay, where different de-excitation modes compete to determine the reaction outcome. This is where indirect methods, that populate the same compound nucleus through alternative reaction or decay pathways, can provide valuable experimental constraints on compound-nucleus models.

In this talk, I will focus on the surrogate reaction method that we have recently implemented in the Experimental Storage Ring at GSI, Darmstadt. Doing reactions in the storage ring offers many advantages in terms of clean, thin targets and near-perfect efficiency. In our most recent experiment on 238U(d,p), this has allowed us to measure five de-excitation channels simultaneously so we can constrain 238U(n,y)/(n,f)/(n,n')/(n,2n)/(n,3n) all in one experiment! Correctly handling both parameter and model uncertainties will be essential to extracting reliable neutron-induced cross sections for our method, but I will also give some perspectives on how the nuclear astrophysics community can achieve better uncertainty quantification with reaction models in general. Finally, I will introduce our plans for moving towards experiments on radioactive beams, which will unlock a vast swathe of nuclei using the pristine conditions of the storage ring.