Progress on decays of extremely proton-rich nuclei based on the HIRFL-RIBLL1 facility

The decay of the nucleus far from the beta-stability line is a currently hot topic in the field of nuclear physics. Research progress on decays of extremely proton-rich nuclei in sd shell based on the HIRFL-RIBLL1 facility is reviewed in this paper. The detector array mainly consists of 3 double-sided silicon strip detectors for implanted heavy-ions as well as decayed protons, 4 quadrant silicon detectors for emitted beta-particles, and 5 Clover-type gamma-ray detectors. With this setup, a continuous beam implantation-decay method was developed for the beta-p-gamma coincident measurement. Thanks to the excellent performance of the detection system, high precision beta-decay spectroscopies of 15 proton-rich nuclei in sd shell have been obtained. For two typical cases, Si-22 and S-27, their decays as well as related topics are presented in detail. In the study of Si-22 decay, the half-life was determined as (28.6 +/- 1.4) ms, where the influence of beta p decay of daughter nucleus 21Mg was taken into account. An exotic beta 2p decay mode was discovered for the first time by tracking two decayed protons. Consequently, the mass of Si-22 was deduced by the Coulomb displacement energy of isobaric analogue state. The 2p separation energy is -108 (125) keV, indicating that Si-22 is a very marginal candidate for 2p ground-state emission. This result is in good agreement with the mass predication by the chiral effective field theory with 3-nucleon forces. Together with 3 branches of beta p decay, the decay scheme of Si-22 was established. Comparing with the beta decay of O-22, a hitherto largest mirror asymmetry up to 209% was found in transition to the first 1(+) excited state of the daughter nucleus. Shell-model calculations with isospin-nonconserving forces reproduced the observed data, attributing the large mirror asymmetry to the s(1/2) proton-halo state of Al-22. It provides another means to identify halo nuclei and a new clue to explore the isospin breaking. In the study of 27S decay, the half-life was determined as (16.3 +/- 0.2) ms, which is the most precise one so far. A total of 27 beta p decay branches were observed, and among them 24 with branch ratio less than 5% were new found branches. Besides, 1 beta 2p decay via the isobaric analogue state (T=5/2) of daughter nucleus P-27 to the ground state of Al-25 was confirmed. The level energies of P-27, branch ratios and log(ft)s of beta-feeding to these levels were determined, and therefore the decay scheme of 27S was established perfectly. Thanks to the high energy resolution of the detector system, the masses of P-27 and S-27 were determined with very high precision as Delta(P-27)=-659(9) keV and Delta(S-27)=17678(77) keV, respectively. Based on the new mass data, the thermonuclear reaction rate of Si-26(p,gamma)P-27 was deduced and its influence on the nucleosynthesis of galactic Al-26 in scenes involving nova and X-ray burst (XRB) was discussed. The abundance of A=26 in the burst ashes is increased by a factor of 2.4, while no substantial change was found in the XRB energy generation rate or the light curve. Results also suggest that S-27 is not a significant waiting point in the rapid proton capture process, and the change of the Si-26(p,gamma)P-27 reaction rate is not sufficiently large to affect the nova contribution to the synthesis of galactic Al-26.