Constraining Neutron Capture Cross Sections for Unstable Nuclei with Surrogate Reaction Data and Theory

Abstract

Obtaining reliable data for nuclear reactions on unstable isotopes remains an extremely important task and a formidable challenge. Neutron capture cross sections-crucial ingredients for models of astrophysical processes, national security applications, and simulations of nuclear energy generation-are particularly elusive, as both projectile and target in the reaction are unstable. We demonstrate a new method for determining cross sections for neutron capture on unstable isotopes, using ^{87}Y(n,γ) as a prototype. To validate the method, a benchmark experiment is carried out to obtain the known ^{90}Zr(n,γ) cross section analogously. Our approach, which employs an indirect ("surrogate") measurement combined with theory, can be generalized to a larger class of nuclear reactions. It can be used both with traditional stable-beam experiments and in inverse kinematics at rare-isotope facilities.

Towards Neutron Capture on Exotic Nuclei: Demonstrating (d,pγ) as a Surrogate Reaction for (n,γ)Ratkiewicz1, Cizewski2, Escher3 et al. 2019Phys. Rev. Lett.Self Cite420360View full textAdd to dashboardCite

“…This approach moves beyond the WE approximation which has been previously shown, and here confirmed, to be inadequate for neutron capture [22,26,27]. We show that a robust model of the formation of the CN [13,14] and proper treatment of its decay [30,31] within the framework of the SRM [11,30,31] is necessary to extract from (d, pγ) data a capture cross section that agrees with the directly-measured (n, γ) cross section. We note that the kinematics of the (d, p) reaction are ideal for measurements with short-lived beams in inverse kinematics.…”

“…Specifically, nuclear level densities (NLD) and γ-ray strength functions (γSF) determine whether the CN decays primarily by neutron or γ-ray emission. A newly-developed method by Escher and colleagues [30,31] constrains the parameters in the decay models via Bayesian fits to the experimentally-measured surrogate coincidence probabilities. The constrained parameters are subsequently used to calculate the cross section for the (n, γ) reaction.…”

“…The experimentally-constrained parameters are used as inputs for an HF model which is subsequently used to calculate the (n, γ) cross section (Eq. 1) (for details see [30,31] sections in that it provides an experimentally-constrained cross section without relying on auxiliary data such as the average radiative width ( Γ γ ) or average s-wave neutron spacing (D 0 ), which are unavailable for exotic nuclei [30,31].…”

Novel techniques for constraining neutron-capture rates relevant for r-process heavy-element nucleosynthesis

“…Very recently, Escher et al [268] accomplished the surrogate method without exploiting the Weisskopf-Ewing approximation. By exploring a large parameter space, they demonstrated that a set of model parameters for the level density and γ-ray strength function could describe the experimental γ-decay probabilities P γ (E n , J, π).…”

“…For such calculations, there are several models and parameter sets that can be chosen from literature, and typically the spread in calculated cross sections may vary within factors of ≈ 5 − 10. To quantitatively determine this spread, Escher et al [268] have tested the sensitivity of the calculations using several input options [113,110,271,272,273]. The resulting systematic-error band is shown in panel (f) of Fig.…”