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Register a new userLow-energy structure of 61Mn populated following $beta$ decay of 61Cr
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$beta$ decay of the $^{61}$Cr$_{37}$ ground state has been studied. A new half-life of 233 +/- 11 ms has been deduced, and seven delayed $gamma$ rays have been assigned to the daughter, $^{61}$Mn$_{36}$. The low-energy level structure of $^{61}$Mn$_{36}$ is similar to that of the less neutron-rich $^{57,59}$Mn nuclei. The odd-A $_{25}$Mn isotopes follow the systematic trend in the yrast states of the even-even, Z + 1 $_{26}$Fe isotopes, and not that of the Z - 1 $_{24}$Cr isotopes, where a possible onset of collectivity has been suggested to occur already at N = 36.
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The nature of $J^{pi}=1^-$ levels of $^{96}$Zr below the $beta$-decay $Q_{beta}$ value of $^{96}$Y has been investigated in high-resolution $gamma$-ray spectroscopy following the $beta$ decay as well as in a campaign of inelastic photon scattering experiments. Branching ratios extracted from $beta$ decay allow the absolute $E1$ excitation strength to be determined for levels populated in both reactions. The combined data represents a comprehensive approach to the wavefunction of $1^-$ levels below the $Q_{beta}$ value, which are investigated in the theoretical approach of the Quasiparticle Phonon Model. This study clarifies the nuclear structure properties associated with the enhanced population of high-lying levels in the $^{96}$Y$_{gs}$ $beta$ decay, one of the three most important contributors to the high-energy reactor antineutrino spectrum.
Total absorption spectroscopy was used to investigate the beta-decay intensity to states above the neutron separation energy followed by gamma-ray emission in 87,88Br and 94Rb. Accurate results were obtained thanks to a careful control of systematic errors. An unexpectedly large gamma intensity was observed in all three cases extending well beyond the excitation energy region where neutron penetration is hindered by low neutron energy. The gamma branching as a function of excitation energy was compared to Hauser-Feshbach model calculations. For 87Br and 88Br the gamma branching reaches 57% and 20% respectively, and could be explained as a nuclear structure effect. Some of the states populated in the daughter can only decay through the emission of a large orbital angular momentum neutron with a strongly reduced barrier penetrability. In the case of neutron-rich 94Rb the observed 4.5% branching is much larger than the calculations performed with standard nuclear statistical model parameters, even after proper correction for fluctuation effects on individual transition widths. The difference can be reconciled introducing an enhancement of one order-of-magnitude in the photon strength to neutron strength ratio. An increase in the photon strength function of such magnitude for very neutron-rich nuclei, if it proved to be correct, leads to a similar increase in the (n,gamma) cross section that would have an impact on r-process abundance calculations.
Sub-barrier Coulomb-excitation was performed on a mixed beam of $^{62}$Mn and $^{62}$Fe, following in-trap $beta^{-}$ decay of $^{62}$Mn at REX-ISOLDE, CERN. The trapping and charge breeding times were varied in order to alter the composition of the beam, which was measured by means of an ionisation chamber at the zero-angle position of the Miniball array. A new transition was observed at 418~keV, which has been tentatively associated to a $(2^{+},3^{+})rightarrow1^{+}_{g.s.}$ transition. This fixes the relative positions of the $beta$-decaying $4^{+}$ and $1^{+}$ states in $^{62}$Mn for the first time. Population of the $2^{+}_{1}$ state was observed in $^{62}$Fe and the cross-section determined by normalisation to the $^{109}$Ag target excitation, confirming the $B(E2)$ value measured in recoil-distance lifetime experiments.
The structure of $^{34}$Si was studied through $gamma$ spectroscopy separately in the $beta^-$ decays of $^{34}$Mg and $^{34}$Al at the ISOLDE facility of CERN. Different configurations in $^{34}$Si were populated independently from the two recently identified $beta$-decaying states in $^{34}$Al having spin-parity assignments $J^pi = 4^-$ dominated by the normal configuration $pi (d_{5/2})^{-1} otimes u (f_{7/2})$ and $J^pi = 1^+$ by the intruder configuration $pi (d_{5/2})^{-1} otimes u (d_{3/2})^{-1}(f_{7/2})^{2}$. The paper reports on spectroscopic properties of $^{34}$Si such as an extended level scheme, spin and parity assignments based on log($ft$) values and $gamma$-ray branching ratios, absolute $beta$ feeding intensities and neutron emission probabilities. A total of 11 newly identified levels and 26 transitions were added to the previously known level scheme of $^{34}$Si. Large scale shell-model calculations using the {sc sdpf-u-mix} interaction, able to treat higher order intruder configurations, are compared with the new results and conclusions are drawn concerning the predictive power of {sc sdpf-u-mix}, the $N=20$ shell gap, the level of mixing between normal and intruder configurations for the 0$_1^+$, 0$_2^+$ and 2$_1^+$ states and the absence of triaxial deformation in $^{34}$Si.
The radionuclide $^{22}$Na is a target of $gamma$-ray astronomy searches, predicted to be produced during thermonuclear runaways driving classical novae. The $^{22}$Na(p,$gamma$)$^{23}$Mg reaction is the main destruction channel of $^{22}$Na during a nova, hence, its rate is needed to accurately predict the $^{22}$Na yield. However, experimental determinations of the resonance strengths have led to inconsistent results. In this work, we report a measurement of the branching ratios of the $^{23}$Al $beta$-delayed protons, as a probe of the key 204--keV (center-of-mass) $^{22}$Na(p,$gamma$)$^{23}$Mg resonance strength. We report a factor of 5 lower branching ratio compared to the most recent literature value. The variation in $^{22}$Na yield due to nuclear data inconsistencies was assessed using a series of hydrodynamic nova outburst simulations and has increased to a factor of 3.8, corresponding to a factor of $sim$2 uncertainty in the maximum detectability distance. This is the first reported scientific measurement using the Gaseous Detector with Germanium Tagging (GADGET) system.
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