Measurement of key resonances for the Al24(p,γ)Si25 reaction rate using in-beam

B. Longfellow

¹

,

A. Gade

²

,

B. A. Brown

³

et al.

Abstract

Energy levels and branching ratios for the rp-process nucleus 25 Si were determined from the reactions 9 Be( 26 Si, 25 Si)X and 9 Be( 25 Al, 25 Si)X using in-beam γ-ray spectroscopy with both highefficiency and high-resolution detector arrays. Proton-unbound states at 3695( 14) and 3802(11) keV were identified and assigned tentative spin-parities based on comparison to theory and the mirror nucleus. The 24 Al(p,γ) 25 Si reaction rate was calculated using the experimental states and states from charge-dependent USDA and USDB shell-model calculations with downward shifts of the 1s 1/2 proton orbital to account for the observed Thomas-Ehrman shift, leading to a factor of 10-100 increase in rate for the temperature region of 0.22 GK as compared to a previous calculation. These shifts may be applicable to neighboring nuclei, impacting the proton capture rates in this region of the chart.

Two-neutron knockout as a probe of the composition of states in Mg22,Al23 , and Si<

“…In the present work, this configuration sensitivity of the two-neutron knockout-residue p distributions is explored with three sd-shell cases where the incoming projectiles each have 12 neutrons: 9 Be( 24 Mg, 22 Mg+γ)X, 9 Be( 25 Al, 23 Al+γ)X, and 9 Be( 26 Si, 24 Si+γ)X. From analysis of the exclusive p distributions in two-neutron knockout, J π values are assigned and the dependence of the width on the L composition of the shell-model TNAs is explored, demonstrating the significant utility of this reaction as a spectroscopic tool.…”

“…The low-lying level scheme of 22 Mg is well known [11], allowing comparisons of the widths of p distributions for several states of the same spin. In 23 Al, only one excited state decays by γ-ray emission, a core-coupled 7/2 + state at 1616(8) keV [12], facilitating clean extraction of the exclusive p distributions for the two bound states.…”

“…Excitation energies and J π values in 24 Si are critical for the 23 Al(p,γ) rate, which has significant impact on energy generation in Type-I X-ray bursts [13,14]. Energy values differing by several 10 keV were reported originally [5,15] and only recently, a d( 23 Al, 24 Si+γ)n measurement resolved the discrepancy and, in addition to states at 1874(3) and 3449(5) keV, suggested a new level at 3471(6) keV to be either the 0 + 2 state with an extremely large Thomas-Ehrman shift [16,17] or the 4 + 1 state [18].…”

“…1 of Refs. [23,24]. For each event, the p of the reaction residue at the target was determined using the magnetic rigidity of the S800 spectrograph and the particle trajectory reconstructed from the position and angle measured in the S800 focal plane.…”

“…1(b) shows the Doppler-corrected γ rays detected in coincidence with 23 Al from two-neutron knockout. 23 Al has a low proton-decay threshold of 141.0(5) keV [11] and only one γ-ray transition is visible at 1622(6) keV, in agreement with Ref. [12].…”