Stewart D. Bloom

Nuclear shell model calculations of neutralino-nucleus cross sections for 29Si and 73Ge.

We present the results of detailed nuclear shell model calculations of the spin-dependent elastic cross section for neutralinos scattering from \si29 and \ge73. The calculations were performed in large model spaces which adequately describe the configuration mixing in these two nuclei. As tests of the computed nuclear wave functions, we have calculated several nuclear observables and compared them with the measured values and found good agreement. In the limit of zero momentum transfer, we find scattering matrix elements in agreement with previous estimates for \si29 but significantly different than previous work for \ge73. A modest quenching, in accord with shell model studies of other heavy nuclei, has been included to bring agreement between the measured and calculated values of the magnetic moment for \ge73. Even with this quenching, the calculated scattering rate is roughly a factor of 2 higher than the best previous estimates; without quenching, the rate is a factor of 4 higher. This implies a higher sensitivity for germanium dark matter detectors. We also investigate the role of finite momentum transfer upon the scattering response for both nuclei and find that this can significantly change the expected rates. We close with a brief discussion of the effects of some of the non-nuclear uncertainties upon the matrix elements.

The (p,n) reaction and the nucleon-nucleon force

Session I: Derivation of the Effective from the Free Nucleon-Nucleon Interaction.- What We Think We Know About the Free N-N Interaction.- Properties and Applications of Effective Interactions Derived from Free Nucleon-Nucleon Forces.- Determination of the Effective Interaction for Bound State Calculations in Nuclei.- Linear Least-Squares Analysis and Effective Interactions.- Effective Interactions: Derivation of Moment Criteria.- Field-Theoretic Origin of the Isovector Central (Lane) and Spin-Orbit Potentials for Quasielastic (p,n) Reactions.- Session II: Nuclear Structure Effects.- Microscopic Structure of ?J? = 1+ Excitations in sd-Shell Nuclei.- Elements of Comparison Between Electron-Nucleus and Nucleon-Nucleus Scattering.- The (p,n) Reaction at Intermediate Energies.- Experimental Test of One-Pion Exchange and PCAC in Proton-Nucleus Charge Exchange Reactions at 144 MeV.- Evidence for Gamow-Teller Strength in Broad Bumps in (p,n) and (3He,t) Spectra.- Session III: More on Effective Interactions, Sensitivity Studies, Meson Probes.- An Empirical Effective Interaction.- Determination of the Macroscopic Isovector Potential from Nucleon-Nucleus Scattering.- Reaction Mechanism/Structure Sensitivity Studies of Cross Sections, Asymmetries, and Polarization in (p,p?) and (p,n).- The Role of the n-p Interaction in Microscopic Calculations of Collective Motion.- Pion Single Charge Exchange.- K+-Nucleus Inelastic and Charge Exchange Reactions.- Effective Interactions and Reaction Dynamics.- Session IV: Experimental Techniques and Facilities. Reaction Comparisons.- Experimental Capabilities - Present and Future.- Calculations of Neutron Detector Efficiencies.- Experimental Determinations of Neutron Detector Efficiencies.- A Spectrometer for Neutron-In, Charged-Particle-Out Reactions.- The (n,p) Reaction on Light N=Z Nuclei at 60 MeV.- The (n,p) Reaction at 60 MeV on N&Z Targets.- Excitation of Spin-Flip, Isospin-Flip States in (p,n), (e,e?) and (?,??): A Comparative Study on 24,25,26Mg.- Comparisons of (p,p?) and (p,n) Reaction Measurements.- Session V: The (p,n) Reaction Below 100 MeV A.- Folding Model Analysis of the (p,n) Quasielastic Reaction.- Properties of Multistep Amplitudes in Charge-Exchange Reactions.- The Sequential Transfer Mechanisms in (p,n) Reactions.- Energy Dependence of V? in the (p,n) Reaction 10-30 MeV.- Polarization Observables in (p,n) Reactions.- Self-Consistent Application of the Lane Model.- Theoretical Studies of the Polarization Analyzing Power Difference in 3H(p,n)3He and 15N(p,n)15O.- Session VI: The Broad Picture.- Effective Interactions at Low and Medium Energies.- Hadronic and Electromagnetic Nuclear Probes.- Participants.

Gamow-Teller electron capture strength distributions in stars: Unblocked iron and nickel isotopes

Abstract Results of shell-model calculations of the Gamow-Teller strength distributions for the (n, p) or electron capture direction are presented for 54 Fe → 54 Mn, 56 Fe → 56 Mn, 56 Ni → 56 Co, 60 Fe → 60 Mn, and 64 Fe → 64 Mn . In the cases of 56Fe, 60Fe, and 64Fe calculations were performed on both the ground and first excited states. These and similar transitions are characterized by large amounts of Gamow-Teller strength at low excitation energy in the daughter nuclei. The question of quenching in the (n, p) direction remains open. It is expected that the late stages of presupernova stellar evolution (before neutron shell blocking) will depend on the character and distribution of Gamow-Teller strengths for electron capture transitions such as these.

Missing Gamow-Teller strength in mass 42

Abstract The reaction 42 Ca(p,n) 42 Sc at E p = 160 MeV is used to measure the Gamow-Teller (GT) strength function. Normalization of the dominant peak in the (p,n) spectrum to B (GT) determined from the analogous transition in the beta decay of 42 Ti renders the strength function absolute, and the total measured stregth is about half of the minimum value required for a T = 1 nucleus. Shell model calculations are presented which reproduce the shape of the strength function, but overpredict the absolute measured strength by about a factor of two. Evidently the missing strength has been moved out of the region of nucleon particle-hole excitations, and quenching, due possibly to Δ 33 coupling, is indicated. Symmetry implications of an observed strong suppression of the T > component of the GT strength are discussed.

Isospin conservation in the β-decay of Sc44, Zr95 and Nb95

The angular correlation between β rays and circularly polarized γ rays has been measured by the method of rapid alternation in the cases of Sc44, Zr95 and Nb95. The present result for Sc44 indicates that the Fermi matrix element is very small or zero, within experimental error. This agrees with results obtained previously at this laboratory for Sc46 and A41 (also by the method of rapid alternation), but disagrees with the measurement of Boehm and Wapstra. In the case of Zr95 excellent agreement is obtained with the results of Appel and Schopper, but a different interpretation of the results is offered here which eliminates the need for their conclusion that interference exists between the Gamow-Teller and Fermi transitions. Our interpretation requires (as theirs does) that at least one of the pertinent excited states in Nb95 has spin 72 (preferably the 726-keV state). The agreement between theory and the measurement is then very close and requires almost no mixingin any of the radiations. Nb95, measured for the first time here, decays a pure Gamow-Teller (ΔJ = 1) transition followed by an almost pure M1 radiative transition. In this case no other alternative has any reasonable probability. The spin of the level at 762 keV in Mo95 is 72, and the spins of Sc44 and the level at 1.16 MeV in Ca44 are definitely 2.

Observation of radiation from Δn=3 transitions for planar-channeled electrons

Abstract Planar-channeled electrons make spontaneous electric-dipole transitions between states of opposite parity. Measured values for the relative intensities of radiation from Δn = 3 and Δn = 1 transitions are shown to be in reasonable agreement with calculated values for 54 MeV electrons channeled along {110} planes in silicon.

High-resolution Ge(Li) study of the decay of 129gTe and 128mTe

Abstract The decay of 129m Te and 129g Te to levels in 129 I has been studied using Ge(Li) detectors for high-resolution γ-ray spectroscopy with some supporting data from NaI(Tl) detectors in coincidence. A total of 46 gamma rays is accounted for by 16 excited states (one tentative) in 129 I up to 1401.6 keV. These include ten new gamma rays not previously reported, a new level in 129 I at 1282.1 keV ( 5 2 + ) and a probable new level at 1204.2 keV ( 3 2 + , 5 2 + ) (values of J π in parentheses). The decay scheme was arrived at primarily on the basis of precise γ-ray energy determinations (to ±50 eV in many cases and never worse than ±200 eV). Beta-decay log ƒt values were determined from our γ-ray intensity balances coupled with published β-branchings. Most-probable spins and parities were deduced based on the γ-ray branching, the log ƒt value and published angular correlations of other workers. The J π = 1 2 + state predicted by Kisslinger and Sorensen in the region of 200 eV and observed in 125 I and 127 I is not seen; we find log ƒt ≧10.6 for the allowed β-decay from 129g Te to such a level. We likewise find no evidence for the 11 2 − state at ≈ 930 keV as predicted by Kisslinger and Sorensen; the allowed β-decay to this state would have log ƒt ≧ 11.0 .

64Ga spin from β-γ(CP) correlation data and decay to 64Zn levels☆

Abstract The decay of 2.6 min 64Ga produced by the reaction 64Zn(p, n)64Ga has been studied by Ge(Li) γ-ray spectroscopy, NaI(TI) crystals both singly and in coincidence, and β-γ(CP) correlation measurements. A transmission type of polarimeter was used in the correlation measurements to give good discrimination against lower energy γ-rays. The decay was found to populate excited states in 64Zn at 991.3, 1799.2, 1909.8, 2608.5, 3186.2, 3261.7, 3365.9 3425.0, 3795.0, 4453.5 keV, and possibly 3623 keV. The decay properties of these states were generally found to agree well with the (p, p′γ) work of Van Patter and co-workers. Beta-gamma (CP) correlation measurements were made on β rays in coincidence with strong ground state γ-ray transitions from the states at 3365.9 and 3425.0 keV. The combined asymmetry parameter of −1.00±0.05 strongly favours a spin of 0 for the 64Ga ground state; a spin of 1 would be possible only if the mixing into the 64Ga ground state involves a very large Coulomb matrix element of ⪸ 344 keV . Possible explanations of the unusualy large Coulomb matrix element (35 keV) between the analogue of the 64Zn ground state and the 64Ga ground state are discussed.

Difference in the electronic and fission decay modes for muonic atoms

Abstract A simple model is presented which predicts a difference in the electronic and fission decays of ground-state muonic heavy atoms. It is interpretable in terms of the relative population of two nuclear states: the isomeric fission state and the ground state. The application of the model to 238U indicates there could be a significant population of the isomeric state at the end of the muonic atomic cascade.

Analog and configuration states in 49Sc (Jπ = 32− and 12−) and the low-lying level structure in 48Sc

Abstract The J π = 3 2 − and 1 2 − states in 49Sc generated by the configurations {(1 f 7 2 ) 8 (2 p 3 2 , 1 2 )} are discussed. Excitation energies. M1 transitions, and E2 transitions among the seven 3 2 − and five 1 2 − levels, including the 3 2 − analog state at Ex = 11.56 MeV and a putative excited analog 1 2 − state at ≈ 13.6 MeV, were calculated and compared with experiment where possible. The β-decay of 49Ca to the 3 2 − level at 3.08 MeV and the (presumably) 1 2 − level at 4.49 MeV (the lowest-lying 3 2 − and 1 2 − levels) were also calculated. Two interactions were used, the well-known Kuo-Brown (KB) force and a new interaction which we call the PMM force, the latter being derived mainly from direct-reaction cross sections of nucleons on various nuclei. Both KB and PMM interactions lead to cancellations which cut down both the β-decay of 49Ca and the M1 decay of the A-state to the lowest-lying 3 2 − and 1 2 − states, as observed experimentally. In addition, strong M1 decays to several 3 2 − and 1 2 − levels with excitations of 7–11 MeV are predicted, as is observed experimentally. The E2 decays of all the predicted 3 2 − states to the 7 2 − ground state were also calculated. In the case of the E2 decay of the A( 3 2 − ) state, comparison with experiment is hampered by the role of fine structure; experiment is weaker than theory by a factor of ≈ 50. The (1 f 7 2 ) 8 group of levels in 48Sc were also calculated (Jπ = 0+ → 7+). The PMM force gave excellent agreement with recent experimental results while the KB force gave relatively poor agreement, as might be expected from the severe truncation of the shell-model basis. The suggestion is that the phenomenological basis of the PMM force corresponds physically to the (1 f 7 2 ) 8 space of 48Sc as well as the (1 f 7 2 − ) 8 (2 p 3 2 or 2 p 1 2 ) space of 49S