Studies On Exotic Decay Of The Extremely Proton-Rich Nuclei ^26,27P And ²⁷S

The study on the synthesis and properties of the exotic nuclei far from the stable line has been the frontier of nuclear physics for a long time,providing important information for the crucial issues such as the effective interaction and basic symmetry in nuclei,the nuclear structure,the astrophysical nucleosynthesis,and the chances to access existed limit of nuclear stability.Due to the high?-decay energies and low separation energies of nucleons adjacent to the proton dripline,many decay channels including?-delayed particle emission will be open which offer new opportunities to study the exotic nuclear structures and properties under the extreme conditions to improve the existing shell-model theories and understanding of the microstructure of matter,the evolution in the cosmos and the origin of the elements.The?decay experiment of ²⁶P,²⁷P and ²⁷S was performed at the National Laboratory of Heavy Ion Research Facility in Lanzhou(HIRFL).The nuclei of interest were produced via the projectile fragmentation of³²S¹⁶⁺primary beam at 80.6Me V/nucleon impinging upon a 1581?m-thick ⁹Be target.The fragmentation was separated and purified through the Radioactive Ion Beam Line in Lanzhou(RIBLL1)and identified by the energy loss(?E)and the time-of-flight(To F)information obtained with two quadrant silicon detectors(QSDs)and two plastic scintillators,respectively.Under a continuous-beam mode,the isotopes of interest were implanted into three W1-type double-sided silicon strip detectors(DSSDs)with thicknesses of142?m(DSSD1),40?m(DSSD2),and 304?m(DSSD3)in a certain proportion,where the subsequent decay signals were measured and correlated by the energy,position,and time information.Three QSDs were installed downstream to serve as an anticoincidence of the penetrating heavy ions and light particles coming along with the beam.Besides,?rays were detected by five Clover-type high-purity Germanium(HPGe)detectors surrounding the silicon array.Several techniques such as printed circuit boards,cryogenic cooling system,leading edge discrimination and front-back coincidence of DSSDs were used to improve the signal-to-noise ratio and enhance the identification ability to the rare decay events.The direct and accurate measurement of decay events with high detection efficiency and low detection energy threshold were also achieved.In this²⁶P experiment,the experimental results including the half-lives,the charged-particle spectrum,the decay branching ratios,the log ft values,Gamow-Teller transition strengths,?ray spectrum and the decay scheme are consistent with the literature values.The half-life of²⁶P 43.6±0.3 ms was obtained which is in good agreement with previous results.The various decay components are identified based on proton-?ray coincidence,and the corresponding decay branching ratios were given by the number of decay events and the total number of²⁶P particles injected into the DSSDs.The decay paths of some protons in previous work are reassigned,the new initial and final states corresponding to two proton peaks with energies of 1998(2)ke V and 4837(7)from the?-delayed decay of ²⁶P were identified by proton-?ray coincidence.Meanwhile,two new charged-particle peaks at 7842(6)ke V and 4205(11)ke V were observed for the first time.The half-lives of two new charged-particle peaks were obtained by gating on these two peaks,verifying that the two newly discovered charged-particle peaks originate from the?-delayed proton decay of ²⁶P.Since the7842(6)ke V proton is higher than the energy of the proton peaks from the Isobaric Analogue State(IAS)in²⁶Si to the ground state of²⁵Al,it should be proton emitted from an excited state above the IAS of ²⁶Si.A new proton peak at 4205(11)ke V was observed clearly in coincidence with the 1367 ke V?transition which could be considered as a two-proton emission from?-delayed two-proton decay of²⁶P to the first excited state[E_x=1369(1)ke V,J_?=2⁺]of ²⁴Mg.The results showed that 4205(11)ke V and 7842(6)ke V came from the same excited state by calculation the excitation energy of²⁶Si which was deduced to be E_x=13357(12)ke V above the IAS[E_x=13042(9)ke V]of ²⁶Si.The observed branching ratio and the log ft value of this state from ²⁶P decay were determined to be 0.78(5)%and 3.78(6),respectively.The log ft value of 3.78(6)is rather small among those of the allowed Gamow-Teller transitions.Unlike the previous observation on Fermi delayed two-proton emission from IAS,the present work clearly identified a novel decay mode,Gamow-Teller delayed two-proton(GT2p)emission.The?-decay strength of the newly observed excited state above IAS is unexpectedly strong comparing with the results of shell-model calculations using various Hamiltonians.The branching ratio of the two-proton emission is unexpectedly larger than that of the one-proton emission,though latter has larger decay energy.Both unexpectedly strong Gamow-Teller transition rates and the unexpectedly large branch ratios of two-proton emission suggest that the present theory may globally be underestimated the probabilities of the GT2p emission which will bring new opportunity for future experimental and theoretical study.The shell-model calculations were also performed,and more sophisticated quantitative analyses on the spectroscopic information of ²⁶P are still in progress.In this experiment,²⁶P and ²⁷P were produced simultaneously,due to extremely low?-delayed proton decay branching ratio of ²⁷P and the high?background,low energy protons were not observed.As a result,no new proton could be identified from the charged-particle spectrum of ²⁷P in this experiment.However,a large time-correlation window between implantation and decay events has been applied to accurately estimate the contribution of the background caused by uncorrelated events.A fit with a function composed of an exponential part and a constant background yielded the half-life of ²⁷P to be 263.1±10.9 ms.The uncertainty contained statistic and systematic uncertainties.The?values of the analog state transitions between ²⁷P and ²⁷Mg were also given,showing that there is no isospin-symmetry breaking in the mirror decay of ²⁷P and ²⁷Mg within the error range(uncertainties).The shell-model calculations were also performed.Meanwhile,Two-proton angular correlations were measured by the silicon array to study the mechanism of two-proton emission of ²⁷S.An energy peak at 6372(15)ke V with a branching ratio of 2.4(5)%in the?-delayed charged-particle spectrum above 5 Me V was identified as a two-proton transition via the isobaric analog state in ²⁷P to the ground state of ²⁵Al in the?decay of ²⁷S.Two-proton angular correlations of ²⁷S?2p were measured by the silicon array for the first time to study the mechanism of two-proton emission.Based on experimental results and Monte Carlo simulations,it was found that the main mechanism for the?2p emission of ²⁷S is of sequential nature.