Search for Nova Presolar Grains: γ -Ray Spectroscopy of Ar 34 and its Relevance for the Astrophysical Cl 33 (p,γ) Reaction

The discovery of presolar grains in primitive meteorites has initiated a new era of research in the study of stellar nucleosynthesis. However, the accurate classification of presolar grains as being of specific stellar origins is particularly challenging. Recently, it has been suggested that sulfur isotopic abundances may hold the key to definitively identifying presolar grains with being of nova origins and, in this regard, the astrophysical Cl33(p,γ)Ar34 reaction is expected to play a decisive role. As such, we have performed a detailed γ-ray spectroscopy study of Ar34. Excitation energies have been measured with high precision and spin-parity assignments for resonant states, located above the proton threshold in Ar34, have been made for the first time. Uncertainties in the Cl33(p,γ) reaction have been dramatically reduced and the results indicate that a newly identified ℓ =0 resonance at Er=396.9(13) keV dominates the entire rate for T=0.25-0.40 GK. Furthermore, nova hydrodynamic simulations based on the present work indicate an ejected S32/S33 abundance ratio distinctive from type-II supernovae and potentially compatible with recent measurements of a presolar grain

Direct Mass Measurements to Inform the Behavior of <math display="inline"><mrow><mmultiscripts><mrow><mi>Sb</mi></mrow><mprescripts/><none/><mrow><mn>128</mn><mi mathvariant="normal">m</mi></mrow></mmultiscripts></mrow></math> in Nucleosynthetic Environments

International audienceNuclear isomer effects are pivotal in understanding nuclear astrophysics, particularly in the rapid neutron-capture process where the population of metastable isomers can alter the radioactive decay paths of nuclei produced during astrophysical events. The β-decaying isomer Sb128m was identified as potentially impactful since the β-decay pathway along the A=128 isobar funnels into this state bypassing the ground state. We report the first direct mass measurements of the Sb128 isomer and ground state using the Canadian Penning Trap mass spectrometer at Argonne National Laboratory. We find mass excesses of -84564.8(25)  keV and -84608.8(21)  keV, respectively, resulting in an excitation energy for the isomer of 43.9(33) keV. These results provide the first key nuclear data input for understanding the role of Sb128m in nucleosynthesis, and we show that it will influence the flow of the rapid neutron-capture process

Using spin alignment of inelastically-excited fast beams to make spin assignments: the spectroscopy of 13^{13}O as a test case

International audienceExcited states in O13 were investigated using inelastic scattering of an E/A=69.5 MeV O13 beam off of a Be9 target. The excited states were identified in the invariant-mass spectra of the decay products. Both single-proton and sequential two-proton decays of the excited states were examined. For a number of the excited states, the protons were emitted with strong anisotropy where emissions transverse to the beam axis are favored. The measured proton-decay angular distributions were compared to predictions from distorted-wave Born-approximation calculations of the spin alignment which was shown to be largely independent of the excitation mechanism. The deduced O13 level scheme is compared to ab initio no-core shell model with continuum predictions. The lowest-energy excited states decay isotropically consistent with predictions of strong proton 1s1/2 structure. Above these states in the level scheme, we observed a number of higher-spin states not predicted within the model. Possibly these are associated with rotational bands built on deformed cluster configurations predicted by antisymmetrized molecular dynamics calculations. The spin alignment mechanism is shown to be useful for making spin assignments and may have widespread use

Discovery of the new isotope 251Lr^{251}\mathrm{Lr}: Impact of the hexacontetrapole deformation on single-proton orbital energies near the Z=100Z=100 deformed shell gap

International audienceThe products of the Tl203,205(Ti50,2n) fusion-evaporation reactions were studied using the recently commissioned Argonne Gas-Filled Analyzer at Argonne National Laboratory. Two α-decay activities with energies of 9210(19) and 9246(19) keV and half-lives of 42−14+42 and 24.4−4.5+7.0 ms were observed which were followed by the known α decays of Md247 and Es243. They are interpreted as originating from the 1/2−[521] and 7/2−[514] single-proton Nilsson states in the hitherto unknown isotope Lr251. From the measured Qα values the 1/2− level was placed 117(27) keV above the 7/2− level in Lr251 in contrast to Lr255 where the 1/2− level is the lowest. Also, the α decay of Lr253 was studied in more detail and a new α line at 8660(20) keV was found and a new half-life value of 2.46(32) s for an isomeric state in Lr253 was measured. The Lr251,253,255Qα values were compared with predictions of various mass models. The relative energies of the 1/2−[521] and 7/2−[514] single-proton Nilsson states in Lr251,253,255 isotopes were compared with results of the cranking shell model with pairing treated using the particle-number-conserving method. The level separation and, in particular, the level order change between Lr251 and Lr255 was reproduced only when the hexacontetrapole deformation ɛ6 was included in the calculations

57^{57}Zn β\beta-delayed proton emission establishes the 56^{56}Ni rprp-process waiting point bypass

We measured the $^{57}$Zn $\beta$-delayed proton ($\beta$p) and $\gamma$ emission at the National Superconducting Cyclotron Laboratory. We find a $^{57}$Zn half-life of 43.6 $\pm$ 0.2 ms, $\beta$p branching ratio of (84.7 $\pm$ 1.4)%, and identify four transitions corresponding to the exotic $\beta$-$\gamma$-$p$ decay mode, the second such identification in the $f p$-shell. The $p/\gamma$ ratio was used to correct for isospin mixing while determining the $^{57}$Zn mass via the isobaric multiplet mass equation. Previously, it was uncertain as to whether the rp-process flow could bypass the textbook waiting point $^{56}$Ni for astrophysical conditions relevant to Type-I X-ray bursts. Our results definitively establish the existence of the $^{56}$Ni bypass, with 14-17% of the $rp$-process flow taking this route

57^{57}Zn β\beta-delayed proton emission establishes the 56^{56}Ni rprp-process waiting point bypass

International audience•In our letter, we address the dominant uncertainty regarding the extent of the rp-process flow bypass around Ni56 waiting point for astrophysical conditions relevant to Type-1 X-ray bursts.•We removed the final significant uncertainty, the beta-delayed proton branching ratio for Zn57, definitively determining that there is a Ni56 bypass.•In addition, we also identified the second case of the exotic β−γ−p decay mode in the fp-shell. We measured the Zn57β-delayed proton (βp) and γ emission at the National Superconducting Cyclotron Laboratory. We find a Zn57 half-life of 43.6±0.2 ms, βp branching ratio of (84.7±1.4)%, and identify four transitions corresponding to the exotic β-γ-p decay mode, the second such identification in the fp-shell. The p/γ ratio was used to correct for isospin mixing while determining the Zn57 mass via the isobaric multiplet mass equation. Previously, it was uncertain as to whether the rp-process flow could bypass the textbook waiting point Ni56 for astrophysical conditions relevant to Type-I X-ray bursts. Our results definitively establish the existence of the Ni56 bypass, with 14-17% of the rp-process flow taking this route