T.A. Tombrello

Excited states of Li6 observed through He4(d, d)He4

Angular distributions (18° ≦ θ_(c.m.) ≦156 °) for the elastic scattering of deuterons from He4 have been obtained at 500 keV intervals for bombarding energies between 2.9 and 11.5 MeV. A phase shift analysis of these data has been made yielding confirming evidence for the existence of D wave, J^π = 2^+ and 1^+ levels at excitation energies of 4.9 and 6.2 MeV, respectively. Single-level analyses of the P wave phase shifts indicate the possible presence of 2^−, 1^− and 0^− levels at 6.8, 7.8 and 9 MeV excitation energies. The moments of the tensor polarization of the scattered deuterons have been calculated and are compared to recent experimental results. The total reaction cross section is also calculated and is found to be in fair agreement with experiment.

The capture of protons by Be7

The capture of low-energy protons by Be^7 is calculated using a single-particle model for the ground state of B^8. All the model parameters are determined from the experimental data for the reactions Li^7(n,n)Li^7 and Li^7(n,γ)Li^8. The calculated results for Be^7(p,γ)B^8 are in agreement with the experimental data that are available and permit an extrapolation to energies corresponding to stellar temperatures.

Low-energy cross sections for 7Li(p, α)4He and 6Li(p, α)3He

Reaction cross sections for ^6Li(p, α)^3He and ^7Li(p, α)^4He were obtained using lithium beams and a CH^4 gas target at equivalent proton energies between 130keV and 560 keV. The Rutherford scattering of lithium on carbon was used as a cross-section standard. For ^6Li + p, an extrapolation of the cross section to the energy region of astrophysical interest is given.

A “hybrid” R-matrix-optical model parametrization of the 12C(α, γ)16O cross section☆

The ^(12)C(α,γ)^(16)O cross section at astrophysical energies has been determined from existing ^(12)C(α, α)^(12)C and ^(12)C(α,γ)^(16)O data by means of a “hybrid” R-matrix - optical model method; we obtain an S-factor of 0.08 _(−0.04)^(+0.05) MeV·b at E_(c.m) = 300 keV. The El γ-ray width of the 9.59 MeV state has been determined to be 23 ± 3 meV.

The ^9Be(p,α) and (p,d) cross sections at low energies

Cross sections for the ^9Be(p, d) and (p, α) reactions have been measured for proton lab energies of 30 to 700 keV. Angular distributions were measured for energies of 100 to 600 keV. Absolute normalization of the cross sections was done by measuring cross sections for Rutherford scattered α-particles. The maximum cross sections observed at the 330 keV (lab) J^π = 1^− resonance were 360 ± 20 mb for (p, α) and 470 ± 30 mb for (p, d). The value of S_(c.m)(E = 0) for the combined cross sections was estimated to be 35^(+45)_(−15) MeV · b. The reaction data were interpreted in terms of an R-matrix compound nucleus model. There were no obvious effects due to direct reactions. The low-energy behavior of the cross sections was fitted by using three ^(10)B resonance levels; a J^π = 2^+ level at −20 keV (c.m.), a 1^− level at 310 keV (c.m.) and a 1^+ level at 410 keV (c.m.).

Low-energy cross sections for 7 Li(p, α) 4 He and 6 Li(p, α) 3 He

Reaction cross sections for ^6Li(p, α)^3He and ^7Li(p, α)^4He were obtained using lithium beams and a CH^4 gas target at equivalent proton energies between 130keV and 560 keV. The Rutherford scattering of lithium on carbon was used as a cross-section standard. For ^6Li + p, an extrapolation of the cross section to the energy region of astrophysical interest is given.

The elastic scattering of 3He from tritium and 3He

The cross sections for the elastic scattering of ^3He from tritium and ^3He have been measured for bombarding energies from 4.3 to 21.4 MeV and from 11.9 to 18.9 MeV, respectively. The results agree well with a recent resonating-group calculation in which the marked change in the shape of the angular distributions above 15 MeV is attributed to a broad resonance in the l = 3 partial wave.

Neutron-neutron effective range parameters from the ^3H(^2H, ^3He)2n and ^3He(^2H, ^3H)2p reactions

The energy spectra of ^3H and ^3He nuclei from the ^3He(^2H, ^3H)2p, ^3H(^2H, ^3He)2n, ^3He(2H, 3He)pn and ^3H(^2H, ^3H)pn reactions were measured between 6° and 20° at a bombarding energy of 11 MeV. An upper limit of 5 µb/sr was determined for the cross section for producing a bound di-neutron at 6° and 7.5°. The ^3H(^2H, ^3He)2n and ^3He(^2H, ^3H)2p data together with previous measurements at higher energies have been used to investigate the reliability of information obtained about the two-nucleon system from such three-body final state reactions. As an aid to these investigations, we have made Born approximation calculations employing realistic nucleon-nucleon potentials and an antisymmetrized final state wave function for the five-particle system. These calculations reproduce many of the features observed in the experimental data and indicate that the role of complex exchange processes cannot be ignored. The results show that previous attempts to obtain information on the neutron-neutron effective range parameters from the ^3H(^2H, ^3He)2n reaction may have seriously overestimated the accuracy that could be obtained.

14C production by the reaction 11B(α, p)14C☆

To estimate the production rate of long-lived radio-nuclides during the controlled thermonuclear burning of exotic fuels, cross sections for the ^(11)B(α, p)^(14)C reaction have been measured for α-particle energies in the range 1.43 to 2.94 MeV. Angular distributions have been measured at 20 keV intervals within this energy range. Five resonances have been identified corresponding to states in ^(15)N at excitation energies from 12 to 13 MeV. A discussion of possible spin, parity assignments is presented.

Decay modes of 9Li and states of 9Be

The β-decay of the ^9Li nucleus and the subsequent particle decays of the unbound states of the ^9Be nucleus have been investigated with two complementary techniques, a recoil-particle method and a beta-neutron coincidence method. ^9Li nuclei were produced by the ^(18)O(^7Li, ^9Li) ^(16)O reaction at a bombarding energy of 20 MeV and by the ^3H(^7Li, ^9Li)p reaction with a 14 MeV bombarding energy. The energy spectrum of the delayed alphas and the time-of-flight spectrum of the delayed neutrons were obtained. The half-life was measured to be 177±3 ms. Besides the known decay branches to the ground state and to the 2.43 MeV state in ^9Be, a new decay branch to an unbound state in 9Be at an excitation energy of 2.78±0.12 MeV was observed. The branching ratios and the log ft values were found to be (65.0_(−2.4)^(+2.7)) %, (32.0_(−3.7)^(+2.7))%, (3.0_(−0.3)^(+2.7))% and 5.12_(−0.02)^(+0.01), 5.00_(−0.05)^(+0.04), 5.97_(−0.28)^(+0.05) to the ground state, the 2.43 MeV state and the 2.78 MeV state, respectively. The observed properties of the 2.78 MeV state favor an assignment of 1/2. This state decays mainly by neutron emission to the ground state of ^8Be and has a c.m. width of 1.10±0.12 MeV. The fraction of the decays of the 2.43 MeV state in ^9Be involving emission of a neutron to the ground state of ^8Be was found to be (6.4±1.2)%.