[en] The rearrangements of protons and neutrons amongst the valence single-particle orbitals during double-beta decay of Mo-100 have been determined by measuring cross sections in (d, p), (p, d), (He-3, a), and (He-3, d) reactions on Mo-98,Mo-100 and Ru-100,Ru-102 targets. The deduced nucleon occupancies reveal significant discrepancies when compared with theoretical calculations; the same calculations have previously been used to determine the nuclear matrix element associated with the decay probability of double-beta decay of the Mo-100 system.

[en] A study of low-lying excited states in Z = 51 isotopes has been performed using single-proton adding reactions, (α,t) and (³He,d), on the series of stable, even mass Z = 50 isotopes. Our goal was to build upon results from a previous (α,t) study [1] by examining the fragmentation of high-j (g_7/2 and h_11/2) single-proton strengths utilising greater statistics. Data from the (³He,d) measurements provide further information regarding the low-j orbitals within this nuclear shell. Preliminary findings from the analysis are presented in this report.

[en] The distribution of neutron-hole strength has been studied in the N = 81 isotones ¹³⁷Ba, ¹³⁹Ce, ¹⁴¹Nd and ¹⁴³Sm through the single-neutron removing reactions (p,d) and (³He,α), at energies of 23 and 34 MeV, respectively. Systematic cross section measurements were made at angles sensitive to the transferred angular momentum, and spectroscopic factors extracted through a distorted-wave Born approximation analysis. Application of the MacFarlane-French sum rules indicate an anomalously low summed g_7/2 spectroscopic factor, most likely due to extensive fragmentation of the single-particle strength. Single-particle energies, based upon the centroids of observed strength, are presented.

[en] The single-neutron adding (d,p) reaction has been performed on ¹³⁶Xe in inverse kinematics at 10 MeV/u. The position, time-of-flight, and energy of the outgoing protons were analyzed by the new helical orbit spectrometer, HELIOS, at Argonne National Laboratory. An excitation-energy resolution of ≤ 100 keV was obtained in the outgoing proton spectra. The experimental setup is described, along with a technique of extracting absolute cross sections. Data are shown which illustrate the performance of the device. This measurement clearly demonstrates the potential of HELIOS for future heavy radioactive-beam studies.

[en] Pair correlations in the ground state of ¹³⁰Te have been investigated using pair-transfer experiments to explore the validity of approximations in calculating the matrix element for neutrinoless double-β decay. This nucleus is a candidate for the observation of such decay, and a good understanding of its structure is crucial for eventual calculations of the neutrino mass, should such a decay indeed be observed. For proton-pair adding, strong transitions to excited 0⁺ states had been observed in the Te isotopes by Alford et al. [Nucl. Phys. A 323, 339 (1979)], indicating a breaking of the BCS approximation for protons in the ground state. We measured the neutron-pair removing (p,t) reaction on ¹³⁰Te and found no indication of a corresponding splitting of the BCS nature of the ground state for neutrons.

[en] If a reliable measurement of a neutrinoless double beta decay (0v2β) rate is made, the effective neutrino masses can be determined from the nuclear matrix element. Theoretical calculations of nuclear matrix elements, however, show some disagreement. To test the suitability of various theoretical models, they should be benchmarked against experimentally measured nuclear properties, such as the ground-state distribution of nucleons in the parent-daughter nuclei, and how they change as a result of the decay process. Single neutron-adding reactions have been performed on the 0v2β candidate nucleus, ¹³⁰Te. The Macfarlane-French sum rules have then been used to determine the single-particle vacancies. Some quasi-random phase approximations (QRPA) can greatly simplify theoretical calculations by describing the ground state of even-even nuclei using a BCS wavefunction. This assumption has been tested using two-neutron removal, (p,t) reactions. The BCS wavefunction appeared to be a valid approximation for valence neutrons.

[en] The energies of the g_7/2 and h_11/2 neutron orbitals in N = 51 isotones have been investigated. The single-neutron adding reactions (d,p) and (α, ³He) have been performed on ⁸⁸Sr, ⁹⁰Zr and ⁹²Mo targets, at beam energies of 15 MeV and 50 MeV, respectively. These measurements were supplemented by studying the d(⁸⁶Kr,p)⁸⁷ Kr reaction at an energy of 10 MeV/u, in inverse kinematics. Absolute cross sections were measured, l assignments made and spectroscopic factors extracted. The energy centroids of the single-particle strength have been deduced and the observed trends are discussed.

[en] The helical orbit spectrometer, HELIOS, at Argonne National Laboratory has been developed to measure transfer reactions in inverse kinematics with good Q-value resolution. The technique is discussed alongside examples of measurements with medium-mass beams, the first exploration of reactions in the the forward hemisphere, and a future outlook.

[en] The single-neutron properties of the N=83 nucleus ¹³⁷Xe have been studied using the ¹³⁶Xe(d,p) reaction in inverse kinematics at a beam energy of 10 MeV/u. The helical-orbit spectrometer, HELIOS, at Argonne National Laboratory was used to analyze the outgoing protons, achieving an excitation-energy resolution of ∼100 keV. Extraction of absolute cross sections, angular distributions, and spectroscopic factors has led to a more complete understanding of the single-neutron strength in ¹³⁷Xe. In particular, the centroids of the νh_9/2 and νi_13/2 strengths appear to evolve through the N=83 isotones in a manner consistent with the action of the tensor force.