Speaker
Description
The decay of $^8$B into $^8$Be is of great interest for both nuclear structure and astrophysics. For astrophysics, the decay of $^8$B is the main source of solar neutrinos with energy higher than 2 MeV mainly coming from the intense (88%) beta branch of the $^8$B decay to the 3 MeV state of $^8$Be.
From the nuclear structure point of view, the 2$^+$ ground state of $^8$B is the only well-established proton halo state known. The $\beta^+$/EC decay of this stage could give access to the 2$^+$ doublet at 16.6 and 16.9 MeV in $^8$Be observed in reaction studies more than fifty years ago [1]. These states have dominant configurations as $^7$Li+p and $^7$Be+n, respectively and constitute the highest mixed isospin doublet known [2] (almost 50%), this interpretation aligns well with experimental reaction data, though a direct confirmation remains elusive. The beta decay of $^8$B into $^7$Be offers a valuable tool to probe the isospin composition of this doublet by analyzing the selective contributions from Fermi and Gamow-Teller components. Nonetheless, resolving the feeding to the 2+ doublet poses challenges due to low beta feeding.
Experiment IS633 was performed at the ISOLDE-CERN facility's decay station (IDS), focused on investigating the 2$^+$ doublet of $^8$Be through the beta decay of $^8$B. In this process, $^8$B feeds the excited states of $^8$Be which subsequently break up into two particles or a proton plus $^7$Li, depending on the level fed. Detection of charged particles was achieved through four particle telescopes, each comprising a Double-Sided Silicon Strip Detector (DSSD) stacked with a thick Si-PAD detector. The present data constitute a two-order-of-magnitude improvement in statistics over the preceding benchmark experiment JYFL08 [3] achieving a good separation of the two states for the first time.
In this contribution, a comprehensive description of the IS633 experiment will be presented. The obtained $^8$Be excitation spectrum has been analyzed using a convolution function for the detector response [4] and a four-level R-matrix approach. From it, the feeding to the levels and the deduced fermi and GT transitions to the doublet are extracted. A complementary method based on beta recoil has also been applied. From these two analyses, the isospin mixing of the doublet is determined.
[1] C. P. Browne et al., Phys. Lett. 23 (1966) 371.
[2] P. von Brentano, Phys. Rep. 264 (1996) 57.
[3] O. Kirsebom et al., Phys. Rev. C 83 (2011) 065802.
[4] S. Viñals et al., Eur. Phys. J. A 57 (2021) 49.
