The beta-Oslo method: experimentally constrained (n,γn,\gamma) reaction rates relevant to the rr-process

Unknown neutron-capture reaction rates remain a significant source of uncertainty in state-of-the-art $r$-process nucleosynthesis reaction network calculations. As the $r$-process involves highly neutron-rich nuclei for which direct ($n,\gamma$) cross-section measurements are virtually impossible, indirect methods are called for to constrain ($n,\gamma$) cross sections used as input for the $r$-process nuclear network. Here we discuss the newly developed beta-Oslo method, which is capable of providing experimental input for calculating ($n,\gamma$) rates of neutron-rich nuclei. The beta-Oslo method represents a first step towards constraining neutron-capture rates of importance to the $r$-process.Comment: 4 pages, 1 figure, conference proceedings Nuclei in the Cosmos XV 2018, Italy

Na-29: Defining the edge of the island of inversion for Z=11

The low-energy level structure of the exotic Na isotopes 28 ; 29 Na has been investigated through -delayed spectroscopy. The N 20 isotones for Z 10 – 12 are considered to belong to the ‘‘island of inversion’’ where intruder configurations dominate the ground state wave function. However, it is an open question as to where and how the transition from normal to intruder dominated configurations happens in an isotopic chain. The present work, which presents the first detailed spectroscopy of 28 ; 29 Na , clearly demonstrates that such a transition in the Na isotopes occurs between 28 Na ( N 17 ) and 29 Na ( N 18 ), supporting the smaller N 20 shell gap in neutron-rich sd shell nuclei. The evidence for inverted shell structure is found in -decay branching ratios, intruder dominated spectroscopy of low- lying states, and shell model analysis.status: publishe

beta decay studies of the neutron-rich V56-58 isotopes

b decay of 56,57,58V has been used to populate low-energy levels of 56,57,58Cr, respectively. The low-energy levels of the even-even Cr isotopes and b-decay half-lives and branching ratios of the parent V isotopes are compared to results of shell-model calculations. The Cr energy level calculations were completed in the full p f -model space, using several different interactions. The systematic variation of E(21 1), which is indicative of a subshell closure at N532, was best reproduced in calculations using a pf-shell-model interaction based on effective two-body matrix elements with some replacement by the G matrixstatus: publishe

Shape coexistence in Ni-68

The internal-conversion and internal-pair-production decays of the first excited 0+ state in Ni68 are studied following the β decay of Co68. A novel experimental technique, in which the ions of Co68 were implanted into a planar germanium double-sided strip detector and which required digital pulse processing, is developed. The values for the energy of the first excited 0+ state and the electric monopole transition strength from the first excited 0+ state to the ground state in Ni68 are determined to be 1605(3) keV and 7.6(4)×10-3, respectively. Comparisons of the experimental results to Monte Carlo shell-model calculations suggest the coexistence between a spherical ground state and an oblate first excited 0+ state in Ni68. © 2014 American Physical Society.status: publishe

Lowest excitations in Ti-56 and the predicted N=34 shell closure

Recent experimental characterization of the subshell closure at N 32 in the Ca, Ti, and Cr isotones has stimulated shell-model calculations that indicated the possibility that the N 34 isotones of these same elements could exhibit characteristics of a shell closure, namely, a high energy for the first excited 2 level. To that end, we have studied the decay of 56 Sc produced in fragmentation reactions and identified new rays in the daughter N 34 isotone 56 Ti . The first 2 level is found at an energy of 1127 keV , well below the expected position that would indicate the presence of an N 34 shell closure in 56 Ti .status: publishe

β-decay studies of the transitional nucleus 75Cu and the structure of 75Zn

The β decay of 75Cu [t1/2 = 1.222(8) s] to levels in 75Zn was studied at the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory. The γγ and βγ data were collected at the Low-energy Radioactive Ion Beam Spectroscopy Station using the high-resolution isobar separator to obtain a purified 75Cu beam with a rate of over 2000 ions per second. The excited states in 75Zn have been identified for the first time. A total of 120 γ-ray transitions were placed in a level scheme containing 59 levels including two states above the neutron separation energy and a previously unknown 1/2− isomeric state at 127 keV. Spins and parities of several states were deduced and interpreted based on the observed β feeding and γ-decay pattern.status: publishe

Beta decay of the proton-f5/2 ground state of 77Cu studied with 255 MeV and 0.2 MeV purified radioactive beams

Isobarically purified beams of 77Cu with energies of 225 and 0.2 MeV were used at the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory to study beta decay into states in 77Zn. Data taken at 225 MeV allowed the determination of absolute branching ratios relative to the decay of 77Cu for this beta decay as well as its daughters. From these we obtained a refined beta-delayed neutron emission probability of 30.3(22)% and a probability that the decay proceeds through 77Zn(g) of 49.1(26)%. A total of 64 gamma rays were placed in a level scheme for 77Zn containing 35 excited states including one state above the neutron separation energy, whereas two gamma rays were observed for the βn branch to states in 76Zn. The growth and decay curves of some prominent gamma rays indicate a single beta-decaying state with a half-life of 480(9) ms. The decay pattern for 77Cu, with observed feeding of 8(3)% to 7/2+ 77Zn(g) and 6(3)% to 1/2− 77Zn(m), in contrast to the large feeding observed for decay of proton-p3/2 73Cu(g) to 1/2− 73Zn(g), strongly suggests a proton-f5/2 ground state for the studied 77Cu activity.status: publishe

Low-energy level schemes of 66,68Fe and inferred proton and neutron excitations across Z = 28 and N = 40

Background: The nuclei in the region around 68Ni display an apparent rapid development of collectivity as protons are removed from the f7/2 single-particle state along the N = 40 isotonic chain. Proton and neutron excitations across the Z = 28 and N = 40 gaps are observed in odd-A 27Co and 26Fe isotopes. Little spectroscopic information beyond the excited 2+ and 4+ is available in the even-even (66,68)26Fe nuclei to compare with shell model calculations. Purpose: Our goal is to determine the low-energy level schemes of 66,68Fe and compare the observed excitations with shell model calculations to identify states wherein a contribution from excitations across Z = 28 and N = 40 are present. Method: The low-energy states of 66,68Fe were populated through the beta decay of 66,68Mn produced at the National Superconducting Cyclotron Laboratory. Beta-delayed gamma-ray transitions were detected and correlated to the respective parent isotope to construct a low-energy level scheme. Results: The low-energy level schemes of 66,68Fe were constructed from observed gamma-ray coincidences and absolute gamma-ray intensities. Tentative spin and parity assignments were assigned based on comparisons with shell model calculations and systematics. The two lowest 0+ and 2+ states were characterized in terms of the number of protons and neutrons excited across the respective shell gaps. Conclusion: The removal of two protons from 68Ni to 66Fe results in an inversion of the normal configuration and the one characterized by significant excitation across the Z = 28 and N = 40 gaps. Approximately, one proton and two neutrons are excited across their respective single-particle gaps in the ground state of 66Festatus: publishe

beta decay of Si-38,Si-40 (T-z =+5,+6) to low-lying core excited states in odd-odd P-38,P-40 isotopes

Low-lying excited states in 38,40P have been identified in the β decay of Tz=+5,+6, 38,40Si. Based on the allowed nature of the Gamow-Teller (GT) decay observed, these states are assigned spin and parity of 1+ and are core-excited 1p1h intruder states with a parity opposite to the ground state. The occurrence of intruder states at low energies highlights the importance of pairing and quadrupole correlation energies in lowering the intruder states despite the N=20 shell gap. Configuration interaction shell model calculations with the state-of-art SDPF-MU effective interaction were performed to understand the structure of these 1p1h states in the even-A phosphorus isotopes. States in 40P with N=25 were found to have very complex configurations involving all the fp orbitals leading to deformed states as seen in neutron-rich nuclei with N≈28. The calculated GT matrix elements for the β decay highlight the dominance of the decay of the core neutrons rather than the valence neutrons.status: publishe

β decay of Si 38,40 (Tz=+5, +6) to low-lying core excited states in odd-odd P 38,40 isotopes

Low-lying excited states in P38,40 have been identified in the β decay of Tz=+5,+6, Si38,40. Based on the allowed nature of the Gamow-Teller (GT) decay observed, these states are assigned spin and parity of 1+ and are core-excited 1p1h intruder states with a parity opposite to the ground state. The occurrence of intruder states at low energies highlights the importance of pairing and quadrupole correlation energies in lowering the intruder states despite the N=20 shell gap. Configuration interaction shell model calculations with the state-of-art SDPF-MU effective interaction were performed to understand the structure of these 1p1h states in the even-A phosphorus isotopes. States in P40 with N=25 were found to have very complex configurations involving all the fp orbitals leading to deformed states as seen in neutron-rich nuclei with N≈28. The calculated GT matrix elements for the β decay highlight the dominance of the decay of the core neutrons rather than the valence neutrons