Studying the (α, p)-process in X-ray bursts using radioactive ion beams

In type I X-Ray Bursts (XRBs) the nuclear flow is driven towards the proton-drip line by the triple-α reaction, the (α, p)-process, and the rp-process. Along the nucleosynthetic path, the reaction flow can be stopped at so-called waiting-point nuclei. The low Qpγ value of a waiting-point nucleus leads to (p; γ)-(γ, p) equilibrium causing the flow to stall and await a β decay. However, if the temperature is high enough the competing (α, p) reaction can bypass the waiting point. This can have significant effects on the final elemental abundances, energy output, and observables such as double-peaked luminosity profiles. In the intermediate mass region 22Mg, 26Si, 30S, and 34Ar have been identified as possible candidates for waiting-point nuclei in XRBs. A method to study the (α, p)-process on intermediate mass waiting-point nuclei has been developed whereby the time-inverse reaction is studied in inverse kinematics using radioactive ion beams produced by the in-flight method at the ATLAS facility at Argonne National Laboratory. The three reactions p(29P,26Si)a, p(33Cl, 30S)a, and p(37K,34Ar)a have been studied for the first time to determine cross sections for 26Si(α, p) 29P, 30S(α, p)33Cl, and 34Ar(α, p)37K, respectively. The results and future plans will be discussed. © Copyright owned by the author(s)

In-beam γ -ray spectroscopy of 63^{63}Mn

The measurement was performed at the National Superconducting Cyclotron Laboratory (NSCL).International audienceBackground: Neutron-rich, even-mass chromium and iron isotopes approaching neutron number N = 40 havebeen important benchmarks in the development of shell-model effective interactions incorporating the effectsof shell evolution in the exotic regime. Odd-mass manganese nuclei have received less attention, but provideimportant and complementary sensitivity to these interactions.Purpose:We report the observation of two newγ -ray transitions in 63Mn,which establish the (9/2−) and (11/2−)levels on top of the previously known (7/2−) first-excited state. The lifetime for the (7/2−) and (9/2−) excitedstates were determined for the first time, while an upper limit could be established for the (11/2−) level.Method: Excited states in 63Mn have been populated in inelastic scattering from a 9Be target and in thefragmentation of 65Fe. γγ coincidence relationships were used to establish the decay level scheme. A Dopplerline-shape analysis for the Doppler-broadened (7/2−) → 5/2−, (9/2−)→ (7/2−), and (11/2−) → (9/2−)transitions was used to determine (limits for) the corresponding excited-state lifetimes.Results: The low-lying level scheme and the excited-state lifetimes were compared with large-scale shell-modelcalculations using different model spaces and effective interactions in order to isolate important aspects of shellevolution in this region of structural change.Conclusions: While the theoretical (7/2−) and (9/2−) excitation energies show little dependence on the modelspace, the calculated lifetime of the (7/2−) level and calculated energy of the (11/2−) level reveal the importanceof including the neutron g9/2 and d5/2 orbitals in the model space. The LNPS effective shell-model interactionprovides the best overall agreement with the new data

In-beam γ -ray spectroscopy of 38,40,42Si

Excited states in the nuclei 38,40,42 Si have been studied using in-beam Γ-ray spectroscopy following multi-nucleon removal reactions to investigate the systematics of excitation energies along the Z=14 isotopic chain. The most probable candidates for the transition from the yrast 4 + state were tentatively assigned among several γ lines newly observed in the present study. The energy ratios between the 21 + and 41 + states were obtained to be 2.09(5), 2.56(5) and 2.93(5) for 38,40,42Si, respectively, indicating a rapid development of deformation in Si isotopes from N=24 to, at least, N=28. © Owned by the authors, published by EDP Sciences, 2014.link_to_subscribed_fulltex

Well developed deformation in Si42

Excited states in Si38,40,42 nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of S40 and S44 provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742(8) keV in Si42 confirms the 2 + state reported in an earlier study. Among the γ lines observed in coincidence with the 2 +→0 + transition, the most probable candidate for the transition from the yrast 4 + state was identified, leading to a 41+ energy of 2173(14) keV. The energy ratio of 2.93(5) between the 21+ and 41+ states indicates well-developed deformation in Si42 at N=28 and Z=14. Also for Si38,40 energy ratios with values of 2.09(5) and 2.56(5) were obtained. Together with the ratio for Si42, the results show a rapid deformation development of Si isotopes from N=24 to N=28. © 2012 American Physical Society.link_to_subscribed_fulltex