The quasiparticle rpa as a model for gamow-teller ß-decay

The validity of the quasiparticle random-phase approximation is studied by comparing it with full shell-model calculations in the sd-shell. Phenomenological interactions relevant for the sd-shell have been applied, and Gamow-Teller ß⁺ strengths are calculated. It is found that in the QRPA, the ß⁺ strength is a factor of two higher than the shell-model result. The shell-model results are less sensitive to the particle-particle interaction strength than the QRPA. With u, v factors from the shell-model wavefunctions, the ß⁺ strength in the QRPA reproduces the shell-model result. This suggests that the QRPA does not contain enough ground-state correlations.     

The structure of 26Mg investigated with the (d,p) reaction

Angular distributions have been measured for the ²⁵Mg(d, p)²⁶Mg reaction at 13 MeV leading to excited states between E_x = 0 and 8 MeV. Experimental cross sections are compared with DWBA calculations and extended shell-model calculations in the full sd shell. Spin and parity restrictions are obtained for several levels in the region E_x = 6?;8 MeV. Spectroseopic factors for transitions to the lowest four positive-parity states of each spin are well reproduced by the shell-model calculations; however, in mixed configurations the largest component is systematically underestimated by the shell model. Only 60% of the strength for $\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ transfer is observed.     

The 51V(p,n)51Cr reaction at Ep = 160 MeV

The ⁵¹V(p, n)⁵¹Cr reaction has been studied at E_p = 160 MeV using the Indiana University beam-swinger facility. Data have been obtained at several angles up to θ_L = 20°. The 0° spectrum is used to obtain a ΔL = 0 response function from which Gamow-Teller strength is derived. A shell-model calculation of the GT strength distribution is presented and compared with the experimental results. The M1 strength is also calculated and compared with available results from (e, e') and (p, p') experiments. A comparison is made with other N = 28 nuclei. Effects of a truncated shell-model space are presented.     

Renaissance of the Nilsson-model approach to light nuclei: The case of the A = 26 system

The farthest reaching interpretation of a light nuclear system in terms of the Nilsson-model is obtained for A = 26. The T = 1 and T = 0 levels of either parity in ²⁶Mg and/or ²⁶Al are shown to be grouped in more than 35 rotational bands together containing some 55 levels in ²⁶Mg and 170 in ²⁶Al. The Nilsson-model configurations and deformations of bands are deduced from measured spectroscopic factors and electromagnetic transition rates. The experimental data are amended by shell-model calculations in the complete s-d basis space based on the universal s-d Hamiltonian. The rotational structure of positive-parity states is completely contained in the shell-model results. The Nilsson diagram gives a natural explanation of all bands. An important facet is the observation of particle-hole configurations with large K which arise from the complete alignment of particle and hole angular momenta along the major axis of deformation.     

Effects of proton pair vibrations in the low-lying spectrum of closed-shell nuclei: Structure of 48Ca

The importance of proton pair vibrations in the low-lying spectra of closed-shell nuclei with Z ≠ N is emphasized. Indications for the occurrence of these states in ⁴⁸Ca are obtained from a restricted shell-model calculation with both active neutrons and protons.     

A new quantum number in the description of 8Be

We introduce a schematic Hamiltonian to describe ⁸Be. A new quantum number emerges from a shell-model and configuration interaction calculation for this nucleus. The theoretical introduction of this quantum number is accounted for by a new classification of states in terms of the unitary irreducible representations of the non-compact group Sp(2, R). The classification of states is shown to be physically relevant.     

The (3He,n) reaction on 16O and 18O

The (³He, n) reaction on ¹⁶O and ¹⁸O has been used to study low-spin states in ¹⁸Ne and ²⁰Ne up to E_x ≈ 8 and 20 MeV, respectively. The measured neutron angular distributions have been analysed using DWBA. By a comparison with shell-model calculations in the (s, d) shell it is found that most of the two-proton transfer strength can be explained within that shell. Important contributions, however, from the (f, p) shell in low-lying negative parity states are also present.     

High-spin states in109Sn and their decay to the ground state

An extended level scheme of¹⁰⁹Sn is presented showing high-spin states up to E_x≈ 8 MeV and spins up toJπ=(41/2+). Their decay to the 5/2+ ground state has been observed identifying a 12.8 keV 7/2⁺ → 5/2⁺ transition. A half-life of T_1/2=7(1) ns has been measured for the 17/2⁺ state atE _x =2114 keV. The experimental data are compared with the predictions of shell-model calculations.     

Neutrino-12C reactions and the LSND and KARMEN experiments on neutrino oscillations

We present new theoretical results of the flux-averaged ¹²C(ν _e , e ^?)¹²N and ¹²C(ν _μ , μ ^?)¹²N cross sections with ν _μ (ν _e ) coming from the decay-in-flight (decay-at-rest) of π ⁺(μ ⁺). These cross sections are relevant for the interpretation of the recent experiments on neutrino oscillation performed by the LSND and KARMEN collaborations. The microscopic approaches used are charge-exchange random phase approximation (RPA), charge-exchange RPA among quasiparticles (QRPA), and the Shell Model. We show that the exclusive cross sections are in nice agreement with the experimental values for both reactions when a large-scale shell-model calculation is performed. Concerning the inclusive cross section for ν _μ coming from the decay-in-flight of π ⁺, the calculated value keeps overestimating the experimental one by 20–30%, while the inclusive cross section due to ν _e coming from the decay-at-rest of μ ⁺ is in agreement within experimental error bars with the measured values. The shell-model prediction for the decay-in-flight neutrino cross section is reduced compared to the RPA one because of the different kind of correlations in the calculation of the spin modes (in particular, the quenching of the 1⁺) and partially due to the shell-model configuration basis, which is not large enough, as we show using arguments based on sum rules.     

The ΔTz = + 1 Gamow-Teller excitation of N = 28 isotones

Results of shell-model calculations of Gamow-Teller strength distributions are presented for ⁵⁰Ti → ⁵⁰Sc, ⁵²Cr → ⁵²V and ⁵⁴Fe → ⁵⁴Mn. The results support the previous shell-model estimates that a large fraction of Gamow-Teller strength is concentrated at low excitation energy in the daughter nuclei. The strength distributions are in fairly good agreement with those of forward-angle cross sections of T₀ + 1 isospin states of the intermediate-energy (p, p') reaction on N = 28 isotones. An estimate of quenching in the T₀ → T₀ + 1 τσ mode transition is attempted. Calculations are performed also for the first 2⁺ and 4⁺ parent states in the context of electron capture at the late stages of stellar evolution.     

Weak isoscalar response of 11B

Weak-interaction responses of ¹¹B were studied by measuring the ¹¹B(³He, t) and ¹¹B(p, p′) reactions. Obtained nuclear transition matrix elements B(GT), B(στ), and B(σ) were compared with the shell-model calculations. The shell-model calculations, which explained the isovector parts B(GT) and B(στ) reasonably well if the quenching factors of 0.5–0.7 were taken into account, did not describe the isoscalar part B(σ).     

Many-body terms in the folded diagram expansion

The folded diagram expansion for the effective hamiltonian to be used in shell-model calculations for a system with A valence nucleons yields folded diagrams connecting up to A nucleons. The importance of such effective many-nucleon forces is investigated for the example of 4 valence nucleons in the 1s0d shell (²⁰Ne). The folded diagram terms turn out to reduce the binding energy for the low-lying states. The effects of the folded diagrams involving three nucleons are of the same importance as those of the two-body terms. The four-nucleon forces originating from folded diagrams are considerably weaker. This suggests that three-body operators have to be considered in a microscopic calculation of an effective energy-independent shell-model hamiltonian, whereas terms involving four nucleons may be ignored.     

Intrinsic Structure of Light Nuclei in Monte Carlo Shell Model Calculation

Wavefunctions obtained from the Monte Carlo shell model (MCSM) calculation are investigated. It has been difficult to discuss the intrinsic structure of nuclei in conventional shell-model calculations. We propose a way to describe an intrinsic state in the MCSM and demonstrate the appearance of two-α-cluster structure in the ⁸Be ground state. The changes of cluster shape with respect to the number of major shell and basis is discussed. The shape of the ¹⁰Be ground state is also investigated. The behavior of valence neutron is consistent with the picture of molecular orbit state. The method is also applied to the ⁶He ground state, which is expected to have the asymmetric distribution of valence neutrons.     

Properties of the quasiparticle excitations in 207Pb and 209Pb from an extrapolation of the optical-model potential

A previously developed dispersion relation approach is used to calculate the shell-model potential in the case of neutrons in ²⁰⁸Pb, in the energy domain (-50 MeV, 0). This potential contains a dispersive contribution besides a Hartree-Fock type component, and thereby includes correlation and polarization effects. The shell-model and the Hartree-Fock type potentials are assumed to have Woods-Saxon shapes with diffuseness a_v = 0.70 fm; the energy dependence of their depths and radii is calculated. The energy dependence of the shell-model potential is characterized by the effective mass, whose dependence upon radial distance and neutron energy is determined. The effective mass is a sensitive function of energy, in contrast to its Hartree-Fock type component which is nearly independent of energy. Attention is drawn to the fact that the effective mass in nuclear matter cannot be straightforwardly identified with the effective mass at the nuclear centre. The effective mass presents a sharp peak at the nuclear surface near the Fermi energy and a dip at the surface for energies 10 to 20 MeV away from the Fermi energy. The spectroscopic factors of single-particle excitations in ²⁰⁷Pb and ²⁰⁹Pb are calculated from the difference between the effective mass and its Hartree-Fock type component. The predicted values of the valence single-particle wave functions at large radial distances are in fair agreement with experimental values deduced from analyses of sub-Coulomb pickup reactions. It is shown that the dispersive contribution increases the level density parameter by about 25%, in agreement with previous microscopic or semi-phenomenological models; the calculated level density parameter is in good agreement with the empirical value.     

Beta-delayed proton spectroscopy of 24Si

In an experiment at the LISE3 facility of GANIL, we used the projectile fragmentation of a ³⁶Ar primary beam at 95 MeV/nucleon to produce the isotope ²⁴Si. The beta decay half-life of ²⁴Si has been determined to be T_1/2 = (140 ± 8) ms, in agreement with earlier measurements. In addition to the decay of the isobaric analog state, several proton peaks are observed for the first time allowing to identify most of the Gamow-Teller transitions to unbound states and to present a partial decay scheme for ²⁴Si. These results are compared with shell-model predictions.     

High-spin states in 208Rn

The yrast decay scheme of ²⁰⁸Rn has been investigated up to spin $\text{≈ 20}\text{h}\text{?}$ and an excitation energy of ≈ 6 MeV. Several different γ-ray spectroscopic techniques were used to determine the properties of excited states and transitions in the nucleus. Significant changes to the previously established level scheme are proposed, based on the existence of an unobserved 3.1 keV transition. Simple empirical shell-model calculations of level energies aided in the assignment of shell-model configurations to excited states and the decay scheme is discussed in terms of these configurations. The energy level systematics for the even radon isotopes, from A = 206 to 212 are discussed, as are core polarization effects in the even radon isotopes (A = 204 to 210) and polonium isotopes (A = 202–208).     

High-spin states in 92Tc

High-spin states in ⁹²Tc have been studied using the 33 MeV ⁹²Mo(³He, p2nγ)⁹²Tc reaction. Levels up to J = (13) are identified. The results are compared with various shell-model calculations.     

Structures and decay of deep-hole states in light nuclei populated by the (p, 2p 2p) reactions

Decay particles from the s-hole states in ¹¹B and ¹⁵N have been measured in coincidence with the quasifree ¹²C(p, 2p) and ¹⁶O(p, 2p) reactions at E _p =392 MeV. Triton decay is found to be dominant for the ¹¹B(s-hole) state and also found to be larger than α decay for the ¹⁵N(s-hole) state despite its smaller Q value compared to α decay. Measured decay branching ratios are discussed in comparison with the results of statistical-model, SU(3)-model, and shell-model calculations. The energy spectra around the s-hole states in both ¹¹B and ¹⁵N exhibit some bumplike structures, which can be qualitatively explained by recent shell-model calculations for both nuclei.     

Exact shell-model and projected Hartree-Fock calculations for Mg

The results of a projected Hartree-Fock calculation are compared with those of an exact shell-model calculation for states of J ? 6 in the nucleus ²⁴Mg. The realistic Kuo interaction is used, and gives reasonable agreement with experiment.     

Truncation of the nuclear shell-model space using spectral strength distribution

A procedure, based on spectral distribution methods, is introduced to truncate a large shell-model matrix and to renormalize the matrix for the effects of the truncation. The basis states are selected depending on their contribution in the ground-state region and the renormalization is achieved by requiring that the strength distributions of each configuration in the truncated space to have the same widths as those in the full space. The calculation starts with a given effective Hamiltonian in the original space without having to introduce any free parameters due to truncation. Tests against full shell-model calculations in the ds shell are performed for energy levels of ²⁸Si and for electromagnetic transitions in the (ds)⁶T = 0 space.