Ga measurement to inform i-process nucleosynthesis using GODDESS at FRIB*

In a recent study of the metal-poor star HD94028, several anomalous high-Z isotopic abundance ratios (As/Ge, Se/As among others) that diverge from summed s- and r-process abundance distributions were reported [1]. A new neutron-capture driven process named the i-process has been proposed as an explanation, taking place at neutron densities intermediate to the s- and r-processes. A subsequent sensitivity study [2] using a one zone simulation to match the observed HD94028 abundances identified the ⁷⁵Ga(n,γ)⁷⁶Ga reaction rate to be the most significant when varied, showing a strong anti-correlation to the predicted As abundance. The GODDESS detector system was used in May 2024 at the Facility for Rare Isotope Beams (FRIB) to inform the ⁷⁵Ga(n,γ)⁷⁶Ga reaction rate using the ⁷⁵Ga(d,pγ)⁷⁶Ga reaction as a surrogate. Protons, heavy recoils, and gamma rays were detected in coincidence, permitting the use of both traditional surrogate analysis using discrete gamma spectroscopy, and a novel ‘no-gamma’ surrogate (NGS) reaction analysis that studies protons and heavy recoils in coincidence to arrive at the (n,γ) reaction rate as a function of excitation energy. Preliminary results from the experiment will be discussed.

[1]  I. U. Roederer, et al. , Ap.J. 821, 37 (2016)

[2]  J.E.McKay, et al. , MNRAS 491, 5179 (2019)