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Register a new userBeta-decay study of $T_z=-2$ proton-rich nucleus $^{20}$Mg
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The $beta$ decay of the drip-line nucleus $^{20}$Mg gives important information on resonances in $^{20}$Na, which are relevant for the astrophysical $rp$-process. A detailed $beta$ decay spectroscopic study of $^{20}$Mg was performed by a continuous-implantation method. A detection system was specially developed for charged-particle decay studies, giving improved spectroscopic information including the half-life of $^{20}$Mg, the excitation energies, the branching ratios, and the log $ft$ values for the states in $^{20}$Na populated in the $beta$ decay of $^{20}$Mg. A new proton branch was observed and the corresponding excited state in $^{20}$Na was proposed. The large isospin asymmetry for the mirror decays of $^{20}$Mg and $^{20}$O was reproduced, as well. However, no conclusive conclusion can be draw about the astrophysically interesting 2645~keV resonance in $^{20}$Na due to the limited statistics.
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A very exotic decay mode at the proton drip-line, $beta$-delayed $gamma$-proton decay, has been observed in the $beta$ decay of the $T_z$ = -2 nucleus $^{56}$Zn. Three $gamma$-proton sequences have been observed following the $beta$ decay. The fragmentation of the IAS in $^{56}$Cu has also been observed for the first time. The results were reported in a recent publication. At the time of publication the authors were puzzled by the competition between proton and $gamma$ decays from the main component of the IAS. Here we outline a possible explanation based on the nuclear structure properties of the three nuclei involved, namely $^{56}$Zn, $^{56}$Cu and $^{55}$Ni, close to the doubly magic nucleus $^{56}$Ni. From the fragmentation of the Fermi strength and the excitation energy of the two populated 0$^{+}$ states we could deduce the off-diagonal matrix element of the charge-dependent part of the Hamiltonian responsible for the mixing. These results are compared with the decay of $^{55}$Cu with one proton less than $^{56}$Zn. For completeness we summarise the results already published.
Background: The $^{15}$O($alpha ,gamma$)$^{19}$Ne bottleneck reaction in Type I x-ray bursts is the most important thermonuclear reaction rate to constrain experimentally, in order to improve the accuracy of burst light-curve simulations. A proposed technique to determine the thermonuclear rate of this reaction employs the $^{20}$Mg($beta palpha$)$^{15}$O decay sequence. The key $^{15}$O($alpha ,gamma$)$^{19}$Ne resonance at an excitation of 4.03 MeV is now known to be fed in $^{20}$Mg($beta pgamma$)$^{19}$Ne; however, the energies of the protons feeding the 4.03 MeV state are unknown. Knowledge of the proton energies will facilitate future $^{20}$Mg($beta p alpha$)$^{15}$O measurements. Purpose: To determine the energy of the proton transition feeding the 4.03 MeV state in $^{19}$Ne. Method: A fast beam of $^{20}$Mg was implanted into a plastic scintillator, which was used to detect $beta$ particles. 16 high purity germanium detectors were used to detect $gamma$ rays emitted following $beta p$ decay. A Monte Carlo method was used to simulate the Doppler broadening of $^{19}$Ne $gamma$ rays and compare to the experimental data. Results: The center of mass energy between the proton and $^{19}$Ne, feeding the 4.03 MeV state, is measured to be 1.21${^{+0.25}_{-0.22}}$ MeV, corresponding to a $^{20}$Na excitation energy of 7.44${^{+0.25}_{-0.22}}$ MeV. Absolute feeding intensities and $gamma$-decay branching ratios of $^{19}$Ne states were determined including the 1615 keV state. A new $gamma$ decay branch from the 1536 keV state in $^{19}$Ne to the ground state is reported. The lifetime of the 1507 keV state in $^{19}$Ne is measured to be 4.3${^{+1.3}_{-1.1}}$ ps resolving discrepancies in the literature. Conflicting $^{20}$Mg($beta p$) decay schemes in published literature are clarified.
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B. E. Glassman (Department of Physics
, Astronomy
, Nationaln Superconducting Cyclotron Laboratory
2019
textbf{Background}: Superallowed $0^+ rightarrow 0^+$ $beta$ decays of isospin $T=2$ nuclides can be used to test theoretical isospin symmetry breaking corrections applied to extract the CKM matrix element $V_{ud}$ from $T = 0,1$ decays by measuring precise $ft$ values and also to search for scalar currents using the $beta- u$ angular correlation. Key ingredients include the $Q_{textrm{EC}}$ value and branching of the superallowed transition and the half life of the parent. textbf{Purpose}: To determine a precise experimental $Q_{textrm{EC}}$ value for the superallowed $0^+ rightarrow 0^+$ $beta$ decay of $T=2$ $^{20}$Mg and the intensity of $^{20}$Mg $beta$-delayed $gamma$ rays through the isobaric analog state in $^{20}$Na. textbf{Method}: A beam of $^{20}$Mg was produced using the in-flight method and implanted into a plastic scintillator surrounded by an array of high-purity germanium detectors used to detect $beta$-delayed $gamma$ rays. The high-resolution $gamma$-ray spectrum was analyzed to measure the $gamma$-ray energies and intensities. textbf{Results}: The intensity of $^{20}$Mg $beta$-delayed $gamma$ rays through the isobaric analog state in $^{20}$Na was measured to be $(1.60 pm 0.04_{textrm{stat}} pm 0.15_{textrm{syst}} pm 0.15_{textrm{theo}}) times 10^{-4}$, where the uncertainties are statistical, systematic, and theoretical, respectively. The $Q_{textrm{EC}}$ value for the superallowed transition was determined to be $4128.7 pm 2.2$ keV based on the measured excitation energy of $6498.4 pm 0.2_{textrm{stat}} pm 0.4_{textrm{syst}}$ keV and literature values for the ground-state masses of $^{20}$Na and $^{20}$Mg. textbf{Conclusions}: The $beta$-delayed $gamma$-decay branch and $Q_{textrm{EC}}$ value are now sufficiently precise to match or exceed the sensitivity required for current low-energy tests of the standard model.
The results of a study of the beta decays of three proton-rich nuclei with $T_z=text{-}2$, namely $^{48}$Fe, $^{52}$Ni and $^{56}$Zn, produced in an experiment carried out at GANIL, are reported. In all three cases we have extracted the half-lives and the total $beta$-delayed proton emission branching ratios. We have measured the individual $beta$-delayed protons and $beta$-delayed $gamma$ rays and the branching ratios of the corresponding levels. Decay schemes have been determined for the three nuclei, and new energy levels are identified in the daughter nuclei. Competition between $beta$-delayed protons and $gamma$ rays is observed in the de-excitation of the $T=2$ Isobaric Analogue States in all three cases. Absolute Fermi and Gamow-Teller transition strengths have been determined. The mass excesses of the nuclei under study have been deduced. In addition, we discuss in detail the data analysis taking as a test case $^{56}$Zn, where the exotic $beta$-delayed $gamma$-proton decay has been observed.
Beta-decay properties of neutron-rich Ca isotopes have been obtained. Half-life values were determined for the first time for 54Ca [86(7) ms], 55Ca [22(2) ms], and 56Ca [11(2) ms]. The half-life of 230(6) ms deduced for 53Ca is significantly longer than reported previously, where the decay chain 53K -> 53Ca -> 53Sc was considered. A delayed gamma ray with energy 247 keV as identified following beta decay of 54Ca, and is proposed to depopulate the first 1+ level in 54Sc. The beta-decay properties compare favorably with the results of shell model calculations completed in the full pf-space with the GXPF1 interaction. The half-lives of the neutron-rich Ca isotopes are also compared with gross beta-decay theory. The systematic trend of the neutron-rich Ca half-lives is consistent with the presence of a subshell gap at N=32.
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