[en] Neutron-capture reactions on very neutron-rich nuclei are essential for heavy-element nucleosynthesis through the rapid neutron-capture process, now shown to take place in neutron-star merger events. For these exotic nuclei, radiative neutron capture is extremely sensitive to their γ-emission probability at very low γ energies. In this work, we present measurements of the γ-decay strength of ⁷⁰Ni over the wide range 1.3 ≤ Eγ ≤ 8 MeV. A significant enhancement is found in the γ -decay strength for transitions with Eγ < 3 MeV. At present, this is the most neutron-rich nucleus displaying this feature, proving that this phenomenon is not restricted to stable nuclei. We have performed E1-strength calculations within the quasiparticle time-blocking approximation, which describe our data above Eγ ≃ 5 MeV very well. Moreover, large-scale shell-model calculations indicate an M1 nature of the low-energy γ strength. This turns out to be remarkably robust with respect to the choice of interaction, truncation, and model space, and we predict its presence in the whole isotopic chain, in particular the neutron-rich 72,74,76Ni.

[en] Shape coexistence in neutron-rich nuclei in the N = 20 island of inversion, along the N = 28 isotone line, and in the region around neutron number N = 40 is reviewed. The present status, emerging experimental opportunities and challenges in the interpretation are discussed. (paper)

[en] Nuclear reactions on unstable fission products are of interest to nuclear non-proliferation efforts and basic science. While these reactions have historically been extremely difficult to measure, new experimental facilities are beginning to make beams of fission products available for the first time, enabling exciting experiments. The opening of this new area of the nuclear chart for measurements presents the opportunity to test, refine, and expand theories developed to explain behavior closer to stability, deepening our knowledge of the fundamental physics at play. We will present a summary of our white paper on the work needed, and the investments required to enable this work, to make measurements of nuclear reactions away from stability and the theoretical developments required to understand the underlying physics.

[en] Precision measurements of correlation observables in nuclear beta decay provide sensitive means to test the standard electroweak model (SM) and to search for new physics beyond. The presence of new physics would manifest itself in the correlation observables through phenomenological scalar or tensor couplings which are excluded by the SM. In the beta decay of nuclei and in neutron decay, the Fierz interference term is one of the most sensitive parameters to such exotic couplings since it is linear in those couplings and provides therefore competitive constraints as compared to those obtained from high energy physics [1]. Stringent constraints on scalar couplings have been obtained from the contribution of the Fierz term to the Ft-values in pure Fermi transitions [2] but the constraints on tensor couplings obtained from pure Gamow-Teller transitions are significantly weaker [1]. The Fierz term has rarely been measured directly in the past, mainly because of difficulties related with the back-scattering or out-scattering of electrons from detectors or with the control of possible dead-layers which induce distortions in the energy spectrum of beta particles. We have performed an experiment at the National Superconducting Cyclotron Laboratory, using a high purity beam of 6He produced by fragmentation of 18O, with the purpose to explore the measurement of the Fierz term in a geometry where the beta particles do not have to cross any interface and where the detection system has no moving parts. The extracted 6He ions, with an initial energy of 77 MeV/nucleon, have first been degraded down to 46 MeV/nucleon and then implanted into CsI(Na) and NaI(Tl) scintillation detectors, at about 12 mm from the detector surface. Since the range straggling of the beam is about 2 mm and the range of 3.5 MeV electrons (end point of 6He decay) in these materials is about 6-7 mm, no beta particle can escape from the scintillators, depositing thereby their full energy in the detectors and eliminating the problems of partial energy deposition. Particular attention has been devoted to identify possible beam contaminants as well background produced by beam induced reactions in the detectors. This contribution will describe the experiment and present the status of the data analysis

[en] Numerous scientific fields including astrophysics, nuclear power, and nuclear forensics require a knowledge of basic nuclear properties for large numbers of short-lived, radioactive isotopes far removed from stable nuclei. Neutron-capture cross sections are one such piece of nuclear data where direct measurements are not possible and theoretical predictions can vary by orders of magnitude. A recently developed indirect technique for inferring neutron capture rates, the β-Oslo method, has been introduced but not compared against a known, directly measured neutron capture cross section. To provide this benchmark, two indirect methods based on β decay and charged-particle reactions were used to extract the nuclear level density and γ-ray strength function of ⁵¹Ti. The nuclear level density and γ-ray strength function from the two data sets were found to be equivalent and were used to extract the neutron capture cross section of ⁵⁰Ti which agrees with previous direct measurements at high neutron energies. Here, the results demonstrate the validity of the β-Oslo method for extracting neutron capture cross sections of short-lived nuclei and provide a sufficiently small uncertainty to be used in various applications.

[en] The β-decay intensity of "7"0Co was measured for the first time using the technique of total absorption spectroscopy. The large β-decay Q value [12.3(3) MeV] offers a rare opportunity to study β-decay properties in a broad energy range. Two surprising features were observed in the experimental results, namely, the large fragmentation of the β intensity at high energies, as well as the strong competition between γ rays and neutrons, up to more than 2 MeV above the neutron-separation energy. The data are compared to two theoretical calculations: the shell model and the quasiparticle random phase approximation (QRPA). Both models seem to be missing a significant strength at high excitation energies. Possible interpretations of this discrepancy are discussed. The shell model is used for a detailed nuclear structure interpretation and helps to explain the observed γ-neutron competition. The comparison to the QRPA calculations is done as a means to test a model that provides global β-decay properties for astrophysical calculations. Our work demonstrates the importance of performing detailed comparisons to experimental results, beyond the simple half-life comparisons. Finally, a realistic and robust description of the β-decay intensity is crucial for our understanding of nuclear structure as well as of r-process nucleosynthesis.

No abstract available

[en] A novel type of gaseous ionization detector - Optical Time Projection Chamber (OTPC) - developed to study rare nuclear decays is presented. The OTPC records tracks of charged particles ionizing a counting gas by optical imaging of the light generated by electrons multiplied in the amplification structures. By combining an electron drift-time profile measured by a photomultiplier and a CCD camera image we reconstruct three-dimensional trajectories of particles, energies and charges. The capabilities of the OTPC detector to study various decay modes are demonstrated by observation of beta-delayed proton emission from ¹³O, two-alpha break-up of ⁸Be, triple-alpha decay of ¹²C excited states and two-proton radioactivity of ⁴⁵Fe

[en] Sensitive searches for exotic scalar and tensor couplings in nuclear and neutron decays involve precision measurements of the shape of the β-energy spectrum. We have performed a high statistics measurement of the β-energy spectrum in the allowed Gamow-Teller decay of "6He with the aim to first find evidence of the contribution due to the weak magnetism form factor. We review here the motivation, describe the principle of the measurement, summarize the theoretical corrections to the allowed phase space, and anticipate the expected statistical precision.

[en] Low-spin states were studied in ³⁰Al following the β decay of ³⁰Mg produced in the fragmentation of 140-MeV/AMU ⁴⁸Ca. Analysis of the β-γ and β-γ-γ coincidences revealed a new 1⁺ state at 2413 keV, confirmation of the level scheme, and a more accurate half-life measurement of 315(6) ms for the ³⁰Mg ground state. Higher-spin states were investigated in the reaction of ¹⁴C on ¹⁸O at 22 MeV. Protons and deuterons were detected in a segmented E-ΔE Si telescope in coincidence with one or two γ rays in the FSU Ge detector array. A comparison of the resulting level and decay scheme with predictions of the sd shell model shows good agreement with all but six of the states in both excitation energy and γ-decay branching ratios. The root-mean-square deviations in energy of these states using the older USD and newer USDA and USDB interactions were 265, 176, and 173 keV, respectively. The remaining six states are well described as 4^- to 7^- states, similar in relative energy to those in ²⁸Al but shifted down by about 1200 keV. These states also agree well with the predictions of shell-model calculations using the WBP interaction. A comparison of the lowest 4^- states in even A Na, Al, and P isotopes shows a systematic decrease in energy with increasing N and with decreasing Z. The energies of the 4^- states are almost identical in nuclei with the same N-Z values