Cross Section Measurement of 9Be(\gamma,n)8Be and Implications for \alpha+\alpha+n -> 9Be in the r-Process

TL;DR

This study measures the cross section of the 9Be(gamma,n)8Be reaction with high precision, revealing larger values than previous data, which could impact models of the r-process in explosive astrophysical environments.

Contribution

It provides the first high-accuracy, absolute cross section measurements for 9Be(gamma,n)8Be over a broad energy range, refining the reaction rate used in r-process nucleosynthesis models.

Findings

Cross sections are 40% larger than previous measurements.

Measurement accuracy is within +/- 10%.

Results suggest revised reaction rates could influence r-process modeling.

Abstract

Models of the r-process are sensitive to the production rate of 9Be because, in explosive environments rich in neutrons, alpha(alpha n,gamma)9Be is the primary mechanism for bridging the stability gaps at A=5 and A=8. The alpha(alpha n,gamma)9Be reaction represents a two-step process, consisting of alpha+alpha -> 8Be followed by 8Be(n,gamma)9Be. We report here on a new absolute cross section measurement for the 9Be(gamma,n)8Be reaction conducted using a highly-efficient, 3He-based neutron detector and nearly-monoenergetic photon beams, covering energies from E_gamma = 1.5 MeV to 5.2 MeV, produced by the High Intensity gamma-ray Source of Triangle Universities Nuclear Laboratory. In the astrophysically important threshold energy region, the present cross sections are 40% larger than those found in most previous measurements and are accurate to +/- 10% (95% confidence). The revised thermonuclear alpha(alpha n,gamma)9Be reaction rate could have implications for the r-process in explosive environments such as Type II supernovae.