Search

We report here the results of a study of the $\beta$ decay of the proton-rich Ge isotopes, $^{60}$Ge and $^{62}$Ge, produced in an experiment at the RIKEN Nishina Center. We have improved our knowledge of the half-lives of $^{62}$Ge (73.5(1) ms), $^{60}$Ge (25.0(3) ms) and its daughter nucleus, $^{60}$Ga (69.4(2) ms). We measured individual $\beta$-delayed proton and $\gamma$ emissions and their related branching ratios. Decay schemes and absolute Fermi and Gamow-Teller transition strengths have been determined. The mass excesses of the nuclei under study have been deduced. A total $\beta$-delayed proton-emission branching ratio of 67(3)% has been obtained for $^{60}$Ge. New information has been obtained on the energy levels populated in $^{60}$Ga and on the 1/2$^-$ excited state in the $\beta p$ daughter $^{59}$Zn. We extracted a ground state to ground state feeding of 85.3(3)% for the decay of $^{62}$Ge. Eight new $\gamma$ lines have been added to the de-excitation of levels populated in the $^{62}$Ga daughter.

We report on new γ-ray spectroscopy results from β decays of In99 and Sn101. 30 new γ rays were observed following the β decay of In99, and inconsistencies in the literature with respect to the γ rays following the β decay of Sn101 were addressed with two confirmed cases and two new transitions. The experimental γ-ray energies, intensities, and coincidence relationships are discussed with shell model calculations, where theoretical β-decay branching ratios from the parent nuclei and γ-ray cascades of excited states from the daughter nuclei were combined to generate hypothetical βγ spectra and βγγ coincidence matrices. The most intense β-delayed γ-ray branches in both Cd99 and In101 were well reproduced with this approach, and several γ rays were assigned to new excited states based on their good agreement with shell model predictions.

We report first time measurements of the beta-neutrino angular correlation based on the kinetic energy shift of protons emitted in parallel or anti-parallel directions with respect to the positron in the beta decay of $^{32}$Ar. This proof of principle experiment provided simultaneous measurements for the superallowed 0$^+$~$\rightarrow$~0$^+$ transition followed by a 3356~keV proton emission and for a Gamow-Teller transition followed by a 2123~keV proton emission. The results, respectively ${\tilde a_{\beta\nu}}=1.01(3)_{(stat)}(2)_{(syst)}$ and ${\tilde a_{\beta\nu}}=-0.22(9)_{(stat)}(2)_{(syst)}$, are found in agreement with the Standard Model. A careful analysis of the data shows that future measurements can reach a precision level of 10$^{-3}$ for both pure Fermi and pure Gamow-Teller decay channels, providing new constraints on both scalar and tensor weak interactions.

A decay spectroscopy experiment on proton-rich nuclei in the vicinity of the doubly magic Sn100 was carried out at RIKEN Nishina Center. More than 20 nuclei with 43≤Z≤50 and N≤51, produced by fragmentation reactions were investigated via analyses of β-decay, βp-decay, and subsequent γ-ray data. Owing to higher statistics, the precision on the half-lives of many of the ground states and isomers was improved. β-decay endpoint energies of 11 states in 8 nuclei were measured for the first time, and the corresponding QEC and excitation energies were generally consistent with various mass models. Many β-delayed proton emission branching ratios were measured either for the first time or with higher precision compared to literature values, and some of them differed by more than 2σ. Many of the large discrepancies were associated with nuclei with long-lived isomeric states, highlighting large systematic uncertainties involved in these measurements. Twenty-five new γ rays were observed, and ten new states are proposed with unambiguous excitation energies, spins, and parities. Most of the energies of the excited states were consistent within 300 keV or 20%, whichever was greater, compared to shell model predictions in the proton/neutron (p1/2,g9/2) model space assuming a Sr76 core. A signature of a new (1/2-) isomer in Cd97 with T1/2=0.73(7) s was found, in good agreement with shell model predictions.

J. Park et al. -- 1 pag. -- Original Article: Properties of γ-decaying isomers and isomeric ratios in the 100 Sn region. Phys. Rev. C 96, 044311 (2017)

Half-lives and energies of $\ensuremath{\gamma}$ rays emitted in the decay of isomeric states of nuclei in the vicinity of the doubly magic $^{100}\mathrm{Sn}$ were measured in a decay spectroscopy experiment at Rikagaku Kenkyusho (The Institute of Physical and Chemical Research) of Japan Nishina Center. The measured half-lives, some with improved precision, are consistent with literature values. Three new results include a 55-keV $E2\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$ ray from a new $({4}^{+})$ isomer with ${T}_{1/2}=0.23(6)\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ in $^{92}\mathrm{Rh}$, a 44-keV $E2\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$ ray from the $({15}^{+})$ isomer in $^{96}\mathrm{Ag}$, and ${T}_{1/2}({6}^{+})=13(2)$ ns in $^{98}\mathrm{Cd}$. Shell-model calculations of electromagnetic transition strengths in the $({p}_{1/2},{g}_{9/2})$ model space agree with the experimental results. In addition, experimental isomeric ratios were compared to the theoretical predictions derived with an abrasion-ablation model and the sharp cutoff model. The results agreed within a factor of 2 for most isomers. From the nonobservation of time-delayed $\ensuremath{\gamma}$ rays in $^{100}\mathrm{Sn}$, new constraints on the ${T}_{1/2}$, $\ensuremath{\gamma}$-ray energy, and internal conversion coefficients are proposed for the hypothetical isomer in $^{100}\mathrm{Sn}$.

The $\beta$ decay of the odd-odd nucleus $^{70}$Br has been investigated with the BigRIPS and EURICA setups at the Radioactive Ion Beam Factory (RIBF) of the RIKEN Nishina Center. The $T=0$ ($J^{\pi}=9^+$) and $T=1$ ($J^{\pi}=0^+$) isomers have both been produced in in-flight fragmentation of $^{78}$Kr with ratios of 41.6(8)\% and 58.4(8)\%, respectively. A half-life of $t_{1/2}=2157^{+53}_{-49}$ ms has been measured for the $J^{\pi}=9^+$ isomer from $\gamma$-ray time decay analysis. Based on this result, we provide a new value of the half-life for the $J^{\pi}=0^+$ ground state of $^{70}$Br, $t_{1/2}=78.42\pm0.51$ ms, which is slightly more precise, and in excellent agreement, with the best measurement reported hitherto in the literature. For this decay, we provide the first estimate of the total branching fraction decaying through the $2^+_1$ state in the daughter nucleus $^{70}$Se, $R(2^+_1)=1.3\pm1.1\%$. We also report four new low-intensity $\gamma$-ray transitions at 661, 1103, 1561, and 1749 keV following the $\beta$ decay of the $J^{\pi}=9^+$ isomer. Based on their coincidence relationships, we tentatively propose two new excited states at 3945 and 4752 keV in $^{70}$Se with most probable spins and parities of $J^{\pi}=(6^+)$ and $(8^+)$, respectively. The observed structure is interpreted with the help of shell-model calculations, which predict a complex interplay between oblate and prolate configurations at low excitation energies.

We report the first observation of the ${2}^{+}$ isomer in $^{52}\mathrm{Co}$, produced in the $\ensuremath{\beta}$ decay of the ${0}^{+}$, $^{52}\mathrm{Ni}$ ground state. We have observed three $\ensuremath{\gamma}$ rays at 849, 1910, and 5185 keV characterizing the $\ensuremath{\beta}$ de-excitation of the isomer. We have measured a half-life of 102(6) ms for the isomeric state. The Fermi and Gamow-Teller transition strengths for the $\ensuremath{\beta}$ decay of $^{52m}\mathrm{Co}$ to $^{52}\mathrm{Fe}$ have been determined. We also add new information on the $\ensuremath{\beta}$ decay of the ${6}^{+}$, $^{52}\mathrm{Co}$ ground state, for which we have measured a half-life of 112(3) ms.

The results of a study of the $\ensuremath{\beta}$ decays of three proton-rich nuclei with ${T}_{z}=\ensuremath{-}2$, namely $^{48}\mathrm{Fe},^{52}\mathrm{Ni}$, and $^{56}\mathrm{Zn}$, produced in an experiment carried out at GANIL, are reported. In all three cases we have extracted the half-lives and the total $\ensuremath{\beta}$-delayed proton emission branching ratios. We have measured the individual $\ensuremath{\beta}$-delayed protons and $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ 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 $\ensuremath{\beta}$-delayed protons and $\ensuremath{\gamma}$ rays is observed in the de-excitation of the $T=2$ isobaric analog 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 $^{56}\mathrm{Zn}$, where the exotic $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-proton decay has been observed.

A multihit capacity setup was used to study the decay of the dripline nucleus Ar-31, produced at the ISOLDE facility at CERN. A spectroscopic analysis of the beta-delayed three-proton decay of Ar-31 is presented for the first time together with a quantitative analysis of the beta-delayed 2p gamma decay. A new method for determination of the spin of low-lying levels in the beta p daughter 30S using proton-proton angular correlations is presented and used to determine that the spin of the 5.2-MeV level is most likely 3(+) with 4(+) also possible. The half-life of Ar-31 is found to be 15.1(3) ms. An improved analysis of the Fermi beta strength including the beta 3p-decay mode gives a total measured branching ratio of 3.60(44)%, which is lower than the theoretical value found to be 4.24(43)%. Finally, a previously unidentified. transition from the isobaric analog state in the decay of Ar-33 has been found.

The half-life of the superallowed Fermi ${\ensuremath{\beta}}^{+}$ emitter $^{18}\mathrm{Ne}$ has been determined via two high-precision $\ensuremath{\beta}$ counting experiments at TRIUMF's Isotope Separator and Accelerator facility. Using two different $4\ensuremath{\pi}$ continuous-flow gas-proportional counters, two independent measurements were made, yielding a weighted average of ${T}_{1/2}=1.{66400}_{\ensuremath{-}0.00048}^{+0.00057}$ s, which is consistent with but approximately 2 times more precise than a previous high-precision measurement based on $\ensuremath{\gamma}$-ray counting. The present work achieves $\ifmmode\pm\else\textpm\fi{}0.034%$ precision for the $^{18}\mathrm{Ne}$ half-life and will enable the inclusion of $^{18}\mathrm{Ne}$ among the set of precisely measured superallowed Fermi $\ensuremath{\beta}$-decay $ft$ values once improved measurements are made for its $Q$ value and superallowed branching ratio. As a test of systematic uncertainties, the half-life of $^{23}\mathrm{Ne}$ was measured and the world average has been improved, by a factor of 1.7, to $37.148\ifmmode\pm\else\textpm\fi{}0.032$ s.

Resonances just above the proton threshold in S-30 affect the P-29(p, gamma)S-30 reaction under astrophysical conditions. The (p,gamma)-reaction rate is currently determined indirectly and depends on the properties of the relevant resonances. We present here a method for finding the ratio between the proton and gamma partial widths of resonances in S-30. The widths are determined from the beta 2p- and beta p gamma-decay of Ar-31, which is produced at the ISOLDE radioactive ion beam facility at the European research organization CERN. Experimental limits on the ratio between the proton and gamma partial widths for astrophysical relevant levels in S-30 have been found for the first time. A level at 4689.2(24) keV is identified in the gamma spectrum, and an upper limit on the Gamma(p)/ Gamma gamma ratio of 0.26 (95% C.L.) is found. In the two-proton spectrum two levels at 5227(3) keV and 5847(4) keV are identified. These levels were previously seen to gamma decay and upper limits on the Gamma(gamma)/Gamma(p) ratio of 0.5 and 9, respectively, (95% C.L.) are found, where the latter differs from previous calculations.

A decay spectroscopic study of the neutron-rich isotopes has been performed using fragmentation of a 86Kr primary beam. Fragments from this reaction have been selected by the LISE2000 spectrometer at the Grand Acc el erateur National d Ions Lourds (GANIL). Half-lives of 29 isotopes, including the first ones identified for 61Ti (15 4 ms), 64V (19 8 ms), and 71Fe (28 5 ms), have been determined and compared with model predictions. 67,68Mn -delayed rays were observed for the first time. The branching for the -delayed neutron emission was measured to be greater than 10(5)% in the 67Mn decay. The 67Fe isomeric level is firmly determined at higher energy than assigned in previous works. The excitation energies of the first (2+) and (4+) states of 68Fe are suggested to lie at 522(1) and 1389(1) keV, respectively, thus bringing confirmation of assignments based on in-beam -ray spectroscopy. Beyond-mean-field calculations with the Gogny D1S force have been performed for even-mass nuclei through the Fe isotopic chain. Not only 68Fe but most of the neutron-rich Fe isotopes with neutron numbers below N = 50 are interpreted as soft rotors. The calculated mean occupancy of the neutron g9/2 and d5/2 orbitals in correlated ground statesmore » is steadily growing with increasing neutron number throughout the isotopic chain. Interpretation of 67Fe data is based upon the present calculations for the 66Fe and 68Fe even cores.« less

The half-life, the branching ratio, and the decay $Q$ value of the superallowed $\ensuremath{\beta}$ emitter $^{42}\mathrm{Ti}$ were measured in an experiment performed at the JYFLTRAP facility of the Accelerator Laboratory of the University of Jyv\"askyl\"a. $^{42}\mathrm{Ti}$ is the heaviest ${T}_{z}=\ensuremath{-}1$ nucleus for which high-precision measurements of these quantities have been tried. The half-life (${T}_{1/2}=208.14\ifmmode\pm\else\textpm\fi{}0.45$ ms) and the $Q$ value [${Q}_{\mathrm{EC}}=7016.83(25)$ keV] are close to or reach the required precision of about 0.1%. The branching ratio for the superallowed decay branch [$\mathrm{BR}=47.7(12)%$], a by-product of the half-life measurement, does not reach the necessary precision yet. Nonetheless, these results allow one to determine the experimental $\mathit{ft}$ value and the corrected $\mathcal{F}t$ value to be 3114(79) and 3122(79) s, respectively.

In an experiment at the SISSI/LISE3 facility of GANIL, we have studied the decay of the two proton-rich nuclei $^{45}\mathrm{Fe}$ and $^{48}\mathrm{Ni}$. We identified 30 implantations of $^{45}\mathrm{Fe}$ and observed for the second time four implantation events of $^{48}\mathrm{Ni}$. In 17 cases, $^{45}\mathrm{Fe}$ decays by two-proton emission with a decay energy of 1.154(16) MeV and a half-life of ${T}_{1/2}={1.6}_{\ensuremath{-}0.3}^{+0.5}$ ms. The observation of $^{48}\mathrm{Ni}$ and of its decay allows us to deduce a half-life of ${T}_{1/2}={2.1}_{\ensuremath{-}0.7}^{+2.1}$ ms. One out of four decay events is completely compatible with two-proton radioactivity and may therefore indicate that $^{48}\mathrm{Ni}$ has a two-proton radioactivity branch. We discuss all information now available on two-proton radioactivity for $^{45}\mathrm{Fe}$ and $^{48}\mathrm{Ni}$ and compare it to theoretical models.

The \ensuremath{\beta} decay of $^{31}\mathrm{Mg}$ has been investigated at GANIL. The ions were implanted in a silicon detector array surrounded by germanium \ensuremath{\gamma} detectors. \ensuremath{\beta} particles and subsequent \ensuremath{\gamma} rays have been observed. The $^{31}\mathrm{Mg}$ half-life has been measured (${T}_{1/2}=237\ifmmode\pm\else\textpm\fi{}25$ ms) in good agreement with previously reported values. The decay scheme of $^{31}\mathrm{Mg}$ has been enriched with two new low-spin levels and seven new \ensuremath{\gamma} transitions in $^{31}\mathrm{Al}$. Most importantly, very weak feeding to the $^{31}\mathrm{Al}$ ground and lowest excited states has been established. These results are compared to new shell-model calculations in the $\mathit{sd}\text{\ensuremath{-}}\mathit{fp}$ valence space, using an interaction that was modified in order to reproduce the recently established low-energy level scheme of $^{31}\mathrm{Mg}$. A good agreement for all observables is found, supporting the idea that $^{31}\mathrm{Al}$ is outside and $^{31}\mathrm{Mg}$ is inside the so-called ``island of inversion.''

The yrast J(pi)=8(+) states in neutron-rich Ni-70,Ni-72,Ni-74,Ni-76 nuclei are predicted to be isomeric. The present paper describes two GANIL experiments. In the first of them a search was made for the 8(+) isomeric states in Ni-72,Ni-74 nuclei via fragmentation of Ge-76 using the ion gamma-decay correlation technique. Although these states were not observed, limits for their lifetimes were determined. In the second experiment the decay spectroscopy of Co-70,Co-72 nuclei was performed using fragmentation of a Kr-86(36+) beam and the new LISE2000 spectrometer. The beta delayed gamma rays from the decay of Co-70,Co-72 to Ni-70,Ni-72 were observed using the EXOGAM germanium detectors. The half life of Co-72 was measured to be 62(3) ms and the level sequence of the lowest excited states in Ni-72 was suggested, with the 2(+) state at 1096 keV. An attempt to reproduce the level scheme in terms of shell-model calculations was undertaken. The reasons for the disappearance of the 8(+) isomer in Ni-72 are discussed.

The ${\ensuremath{\beta}}^{+}$ decay half-lives of 22 neutron-deficient nuclei in the cobalt-to-krypton region have been measured following the fragmentation of a primary ${}^{78}\mathrm{Kr}$ beam at an energy of 73 MeV per nucleon. The half-lives of the ${T}_{z}=\ensuremath{-}1$ nuclei ${}^{62}\mathrm{Ge},$ ${}^{64}\mathrm{As},$ and ${}^{66}\mathrm{Se}$ are determined for the first time with values of (129\ifmmode\pm\else\textpm\fi{}35) ms, ${18}_{\ensuremath{-}7}^{+43}\mathrm{ms},$ and (33\ifmmode\pm\else\textpm\fi{}12) ms, respectively. The impact of these results on the nucleosynthesis and time scale of the astrophysical rapid proton-capture process are discussed.

A systematic study of the population probabilities of microsecond isomers produced following the fragmentation of 208Pb projectiles at 1 GeV/nucleon has been undertaken at the SIS/FRS facility at GSI Darmstadt. Gamma decays from approximately 20 isomeric states, mainly in the rare-earth and transitional nuclei with A∼180, were identified and the corresponding isomeric ratios deduced. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning gamma cascades in the final nucleus (sharp cutoff). The model is found to represent an upper limit for the population of isomeric states in relativistic projectile fragmentation.

The ${\ensuremath{\beta}}^{+}$-decay half-lives of the neutron-deficient odd-odd $N=Z$ nuclei ${}^{74}\mathrm{Rb},$ ${}^{78}\mathrm{Y},$ ${}^{82}\mathrm{Nb},$ and ${}^{86}\mathrm{Tc}$ have been measured following the fragmentation of a primary ${}^{92}\mathrm{Mo}$ beam at an energy of 60 MeV per nucleon at the GANIL laboratory, France. This was achieved by correlating ${\ensuremath{\beta}}^{+}$ decays with the implantation of unambiguously identified fragments at the final focus of the LISE3 separator. The deduced ${\mathrm{log}}_{10}{\mathrm{ft}}_{1/2}$ values are consistent with ${0}^{+}\ensuremath{\rightarrow}{0}^{+},$ Fermi superallowed transitions, which together with the measured ${\ensuremath{\beta}}^{+}$-detection efficiencies, suggest $T=1,$ ${I}^{\ensuremath{\pi}}{=0}^{+}$ ground states for these odd-odd $N=Z$ nuclei. These data represent the heaviest $N=Z$ systems for which Fermi superallowed decays have been established.

\ensuremath{\gamma}-ray decays depopulating isomeric states have been observed in a number of very neutron deficient nuclei around $A\ensuremath{\sim}80$ following the projectile fragmentation of a ${}^{92}\mathrm{Mo}$ primary beam. Previously unobserved decays have been identified in the $N=Z+2$ nuclei ${}_{39}^{80}\mathrm{Y}$ and ${}_{41}^{84}\mathrm{Nb}$ and the $N=Z$ nucleus, ${}_{43}^{86}\mathrm{Tc},$ making the latter the heaviest $N=Z$ nucleus to date in which a discrete \ensuremath{\gamma}-ray transition has been assigned. The lifetime of the previously reported ${I}^{\ensuremath{\pi}}={\frac{9}{2}}^{+}$ isomeric state in ${}^{73}\mathrm{Kr}$ has also been measured and a clearer picture of its decay properties has been deduced. Isomeric ratios have been measured and have been interpreted in terms of the yrast or nonyrast nature of the isomeric state.