Search Results (24 total)

We determined the absolute branch of the T=2 superallowed decay of ³²Ar by detecting the β⁺-delayed protons and γ decays of the daughter state. We obtain b_SA^β=(22.71±0.16)%, which represents the first determination of a proton branch to better than 1%. Using this branch along with the previously determined ³²Ar half-life and energy release, we determined ft=(1552±12) s for the superallowed decay. This ft value, together with the corrected Ft value extracted from previously known T=1 superallowed decays, yields a measurement of the isospin symmetry breaking correction in ³²Ar decay δ_C^exp=(2.1±0.8)%. This can be compared to a theoretical calculation δ_C=(2.0±0.4)%. As by-products of this work, we determined the γ and proton branches for the decay of the lowest T=2 state of ³²Cl, made a precise determination of the total proton branch and relative intensities of proton groups that leave ³¹S in its first excited state and deduced an improved value for the ³²Cl mass.

Angular distributions for the elastic scattering of ⁸Li on ⁹Be and the neutron transfer reactions ⁹Be(⁸Li,⁷Li)¹⁰Be and ⁹Be(⁸Li,⁹Li)⁸Be were measured with a 27 MeV ⁸Li radioactive nuclear beam. Spectr- oscopic factors for ⁸Li ⊗n= ⁹Li and ⁷Li ⊗n= ⁸Li bound systems were obtained from the comparison between the experimental differential cross section and finite-range distorted-wave Born approximation calculations with the code FRESCO. The spectroscopic factors obtained were compared to shell model calculations and to other experimental values from (d,p) reactions. Using the present values for the spectroscopic factor, cross sections for the direct neutron-capture reactions ⁷Li(n,γ)⁸Li and ⁸Li(n,γ)⁹Li were calculated in the framework of a potential model.

Angular distributions for the ¹¹B+²⁵Mg elastic scattering, ²⁵Mg(¹¹B,¹²C)²⁴Na proton pickup, and ²⁵Mg(¹¹B,¹⁰Be)²⁶Al stripping reactions have been measured at E_¹¹B=35 MeV. The angular distributions have been analyzed by the distorted-waves Born approximation calculations using the code fresco. The spectroscopic factors for the overlaps 〈²⁵Mg|²⁶Al〉,〈²⁵Mg|²⁴Na〉 for the ground state and excited states of ²⁶Al and ²⁴Na have been obtained and compared to previous measurements and shell-model calculations.

Full coupled channels calculations were performed for the ¹⁶O(d,n)¹⁷F and ¹⁶O(d,p)¹⁷O transfer reactions at several deuteron incident energies from E_lab=2.29  MeV up to 3.27  MeV. A strong polarization effect between the entrance channel and the transfer channels ¹⁶O(d,n)¹⁷F(1∕2⁺,0.495) and ¹⁶O(d,p)¹⁷O(1∕2⁺,0.87) was observed. This polarization effect had to be taken into account in order to obtain realistic spectroscopic factors from these reactions.

The ground state and low-lying levels of the unbound ¹¹N nucleus were investigated with the three-neutron pickup reaction ¹⁴N(³He,⁶He)¹¹N. The energies and widths of these, experimentally observed, levels are compared with other measurements and calculations. Angular distributions were measured for the first time for this reaction. The distorted-wave Born approximation analysis confirms the spin assignments for the lowest levels.

The structure of the particle unbound nucleus ¹⁰Li was investigated in a kinematically complete experiment using the ⁹Li(d,p)¹⁰Li reaction in inverse kinematics at an incident ⁹Li energy of 20 MeV/nucleon. The experiment utilized the S800 Spectrograph at the National Superconducting Cyclotron Laboratory to measure the outgoing ⁹Li from the breakup of ¹⁰Li in coincidence with the recoiling protons from the (d,p) reaction which were measured using an array of silicon detectors. Based on the kinematics of the recoiling protons from the ⁹Li(d,p) reaction, we measured a lower limit of Δ=33.10±0.08 MeV (i.e., Δ>33.02 MeV) for the mass excess of ¹⁰Li which is consistent with previous measurements. Through the complete reconstruction of the breakup kinematics, the structure of ¹⁰Li associated with a ground state ⁹Li core was distinguishable from structure associated with a ⁹Li core in its first excited state. The observed ratio of ⁹Li^* core events to the total number of ¹⁰Li events that were detected in the experiment was 0.10±0.04 at forward center of mass angles (2.7° to 9.5°), and 0.24±0.03 at more backward center of mass angles (11° to 26°). The resulting Q-value spectra was best fit with either a single resonance located at Q=-2.58(11) MeV [corresponding to a neutron separation energy S_n=-0.35(11) MeV] or two resonances located at Q=-3.00(24) MeV [S_n=-0.77(24) MeV] and Q>-2.43 MeV (S_n>-0.2 MeV).

Angular distributions for the elastic scattering of ^4,6He on ⁵⁸Ni have been measured at near-barrier energies. The present data, combined with others for the ⁴He+⁵⁸Ni system at intermediate energies, allowed the determination of the ^4,6He ground-state nuclear densities through an unfolding method. The experimentally extracted nuclear densities are compared with the results of theoretical calculations.

An attempt was made to measure the excitation function of the cross section for the ⁸Li(n,γ)⁹Li reaction by performing the inverse reaction ⁹Li(γ,n)⁸Li, with the equivalent photons in the electric field of nuclei in a Pb target providing the γ rays for the reaction. The energy spectrum of lithium nuclei in coincidence with neutrons had no discernible peak where any beam-velocity ⁸Li’s would be located. Statistically, a Gaussian-shaped ⁸Li peak could have been present with 30±29 counts, which we interpreted as consistent with zero, with a two-standard-deviation upper limit of 87 counts. Using the fact that neutron capture on ⁸Li must be dominantly s-wave capture, and applying detailed balance, we obtained, with E in eV, σ_n,γ<930E^-1/2 μb. The corresponding limit on the astrophysical reaction rate is <790 cm³ mol^-1 s^-1. Theoretical predictions of the reaction rate have exceeded our upper limit by factors of 3–50.

For the first time evidence of the ground state of the proton-rich, unbound nucleus ¹⁰N has been found in the multinucleon transfer reaction ¹⁰B(¹⁴N,¹⁴B)¹⁰N. The observed resonance of ¹⁰N has a mass excess of 38.8(4) MeV and a width of Γ=2.3(16) MeV, close to the Audi-Wapstra estimation of 38.5(4) MeV. ¹⁰N is the last missing A=10 multiplet partner.

We report measurements of the cross section of the ⁸Li(d,α)⁶He reaction in the energy range E_c.m.=2.3–3.5 MeV using a ⁸Li-radioactive beam on a CD₂ foil. The astrophysical S factor and reaction rate were calculated from the measured cross section. The ⁶He nuclei produced in the reaction were detected in solid-state detector telescopes. This reaction might have affected the primordial abundance of ⁶Li in big bang nucleosynthesis, since ⁶He beta decays to ⁶Li. However, several big bang nucleosynthesis network calculations were found to be insensitive to this reaction, suggesting that the ⁸Li(d,α)⁶He reaction does not affect ⁶Li primordial production.

The ratio of L- to K-shell electron captures in light nuclei is particularly sensitive to electron overlap and exchange effects. Calculations of these effects in ⁷Be disagree by more than 20%. We report a measurement of the L/K ratio in ⁷Be, using a cryogenic microcalorimeter which clearly separates L- and K-shell captures. The obtained L/K ratio of 0.040(6) is less than half that of existing predictions for free ⁷Be. The discrepancy is likely due to in-medium effects distorting the L-shell electron orbitals.

Energy distributions for outgoing ⁷Be fragments from breakup of ⁸B on a ⁵⁸Ni target have been measured at an incident energy of 25.75 MeV, which is below the Coulomb barrier. The data are compared with coupled-channels calculations that include higher-order couplings in the continuum.

The angular momentum distribution of the compound nucleus is a fundamental characteristic of the reaction dynamics and can provide insight into reactions involving neutron- or proton-rich projectiles. Specifically, following the fusion of ⁶He with ²⁰⁹Bi (at center-of-mass energies of 18 to 27 MeV), ²¹²At is formed by the evaporation of three neutrons from the compound nucleus. The decay process leaves the residual ²¹²At in either the ground state (J^π=1^-, T_1/2=314 ms) or a metastable state (J^π=9^-, T_1/2=119 ms). The ratio of the number of residual ^212mAt to the total number of ²¹²At residual nuclei is sensitive to the original momentum distribution of the compound nucleus. The measured isomer ratio is consistent with that predicted by standard models. This agreement is observed even at the lower energies where the measured three neutron evaporation cross section is greatly enhanced compared to model calculations. While the inclusion of coupling to the neutron-transfer channels improves the agreement with the observed cross-section data somewhat, the predicted isomer ratio then diverges from the measured ratio.

Reaction products from the interaction of ⁶He with ²⁰⁹Bi have been measured at energies near the Coulomb barrier. A ⁴He group of remarkable intensity, which dominates the total reaction cross section, has been observed. The angular distribution of the group suggests that it results primarily from a direct nuclear process. It is likely that this transfer and/or breakup channel is the doorway state that accounts for the previously observed large sub-barrier fusion enhancement in this system.

The single-nucleon knockout reactions ⁹Be(¹⁴B,¹³B+γ)X and ¹⁹⁷Au(¹⁴B,¹³B+γ)X, at an incident energy of 60 MeV per nucleon, have been used to probe the structure of ¹⁴B and of the core fragment ¹³B. A dominant 2s configuration is deduced for the neutron in the ground state of ¹⁴B. The longitudinal momentum distribution for this state is consistent with “neutron halo” structure. Spin assignments for ¹³B excited states at 3.48 and 3.68 MeV are proposed based on the observed spectroscopic factors for one-neutron removal.

The angular distribution for the breakup of ⁸B→⁷Be+p on a ⁵⁸Ni target has been measured at an incident energy of 25.75 MeV. The data are inconsistent with first-order theories but are remarkably well described by calculations including higher-order effects. The comparison with theory illustrates the importance of the inclusion of the exotic proton halo structure of ⁸B in accounting for the data.

The structure of the halo nucleus ¹¹Be has been studied using the reaction ⁹Be(¹¹Be,¹⁰Be+γ)X at 60 MeV/nucleon. The ground state structure of ¹¹Be is determined by comparing the experimental cross sections to a calculation combining spectroscopic factors from the shell model with l-dependent single-particle cross sections obtained in an eikonal model. This experiment shows the dominant 1s single-particle character of the ¹¹Be ground state and indicates a small contribution of 0d admixture in the wave function. After correction for the approximately 22% intensity to excited levels, a clean and precise distribution of parallel momentum for knockout from the 1s halo wave function is obtained for the first time.

This report details a greatly improved measurement of the four-neutron evaporation cross section following the fusion of ⁶He+²⁰⁹Bi for center-of-mass energies between 23.5 and 30.7 MeV. The results, for energies above the Coulomb barrier, are interpreted within the context of the standard statistical model.

The fusion of ⁶He with a ²⁰⁹Bi target has been studied at energies near to and below the Coulomb barrier. Despite the weak binding of the valence neutrons in ⁶He, little evidence is found for suppression of fusion due to projectile breakup. Instead, a large enhancement of sub-barrier fusion is observed. It is suggested that this enhancement may arise from coupling to positive Q value neutron transfer channels, resulting in “neutron flow” between the projectile and the target.

The nuclear structure of ¹⁷Ne has been studied by the ²⁰Ne(³He,⁶He)¹⁷Ne reaction at 70 MeV. Thirteen levels were identified in ¹⁷Ne, and angular distributions have been measured for the first time for this reaction. Based on the observed transferred angular momentum dependence of the angular distributions, a spin-parity assignment has been made for several states in ¹⁷Ne. With the inclusion of the data on ¹⁷Ne, T=3 / 2 quartet analog states have been completed for six levels in the A=17 isobar system. The level shifts in these A=17 nuclei are analyzed in terms of the isobaric multiplet mass equation (IMME). The results of such an analysis show a slight linear dependence of the b and c coefficients of the equation on the excitation energy. The coefficients for the positive parity states seem to follow a different systematics than the negative parity states, suggesting that these parity-dependent level shifts are reflecting the structure change. These coefficients of the IMME are discussed.

A spectroscopic study of the proton-rich, particle unstable nucleus ¹¹N has been performed using the multinucleon transfer reaction ¹²C(¹⁴N,¹⁵C)¹¹N at 30A MeV incident energy at GANIL. Levels of ¹¹N are observed as well defined resonances in the spectrum of the ¹⁵C ejectiles. They are localized at 2.18(5), 3.63(5), 4.39(5), 5.12(8), and 5.87(15) MeV above the ¹⁰C+p threshold. The comparison of the measured widths with R-matrix calculations allows the estimation of spins and parities for these resonances.

Fission following the fusion of ⁶He with a ²⁰⁹Bi target has been studied near the Coulomb barrier, at excitation energies of 32 and 34 MeV in the compound system. These new experimental data are in disagreement with previous work which reported an anomalously large fusion-fission yield, when compared with ⁴He-induced fission of ²⁰⁹Bi at similar excitation energies. In fact, the ⁶He-induced fusion-fission yield appears to be smaller than that for ⁴He, in qualitative agreement with conventional statistical model calculations.

Lifetimes of low-lying levels in the one- and three-quasiparticle bands in ¹³³Ce have been measured using the recoil-distance Doppler-shift technique. The E2 transition strengths extracted for the negative parity yrast states are well described by the triaxial-rotor-plus-quasiparticle and the geometrical models, but the interacting-boson-plus-fermion predictions are too small by about a factor of 3. The B(M1) values extracted for the levels in the positive parity three-quasiparticle band are consistent with the previous νh_11/2⊗πh_11/2⊗πg_7/2 configuration assignment to this band.

The angular distributions of the cross sections of reactions to levels up to E_X=11 MeV in ⁴⁴Ti were measured for the reaction ⁴⁰Ca(⁶Li,d) at E=37 MeV. These angular distributions for the unbound states, particularly the well populated 7.67(10⁺), 8.04(12⁺), 8.54, 8.96, 9.58, and 10.86 MeV states, were analyzed together with the data at E=50 MeV using α-particle wave functions calculated at the resonance energies using the squared Woods-Saxon potentials. The strongly excited state at E_X=10.86 MeV, whose spin parity was not identified from the data at 50 MeV, was found to be a candidate for the bandhead state of the higher nodal K^π=0⁺ band with principal quantum number N=14. © 1996 The American Physical Society.