Beta-delayed gamma decay of 26P: Possible evidence of a proton halo

Abstract

Background: Measurements of ${\hat{I}}^2$ decay provide important nuclear structure information that can be used to probe isospin asymmetries and inform nuclear astrophysics studies. Purpose: To measure the ${\hat{I}}^2$-delayed ${\hat{I}}^3$ decay of ${}^{26}$P and compare the results with previous experimental results and shell-model calculations. Method: A ${}^{26}$P fast beam produced using nuclear fragmentation was implanted into a planar germanium detector. Its ${\hat{I}}^2$-delayed ${\hat{I}}^3$-ray emission was measured with an array of 16 high-purity germanium detectors. Positrons emitted in the decay were detected in coincidence to reduce the background. Results: The absolute intensities of ${}^{26}$P ${\hat{I}}^2$-delayed ${\hat{I}}^3$-rays were determined. A total of six new ${\hat{I}}^2$-decay branches and 15 new ${\hat{I}}^3$-ray lines have been observed for the first time in ${}^{26}$P ${\hat{I}}^2$-decay. A complete ${\hat{I}}^2$-decay scheme was built for the allowed transitions to bound excited states of ${}^{26}$Si. $ft$ values and Gamow-Teller strengths were also determined for these transitions and compared with shell model calculations and the mirror ${\hat{I}}^2$-decay of ${}^{26}$Na, revealing significant mirror asymmetries. Conclusions: A very good agreement with theoretical predictions based on the USDB shell model is observed. The significant mirror asymmetry observed for the transition to the first excited state ($\hat{I}´=51(10)\%$) may be evidence for a proton halo in ${}^{26}$P.

15 pages, 10 figures, 7 tables