R. E. Azuma

The3He(α,γ)7Be reaction and the solar neutrino problem

The capture reaction3He(α,γ)7Be has been investigated in the energy range ofEc.m. =107 to 1,266 keV. The4He or3He beams of up to 300 μA particle current were incident on3He or4He gas targets, respectively. The gas target systems were all of the windowless and recirculating type. Excitation functions have been obtained with the use of an extended-static gas target, while the measurements ofγ-ray angular distributions involved a quasi-point supersonic jet system. The determination of absolute cross sections has been carried out with both types of gas target systems. Theγ-ray yields in the3He(α,γ)7Be reaction were detected using 80 cm3 Ge(Li) detectors. The data lead to a zero-energy intercept of the astrophysicalS(E) factor ofS(0)=0.30±0.03 keV-b. This result reduces the calculated solar neutrino rate by a factor of 1.76.

21Na(p,gamma)22Mg reaction and oxygen-neon novae.

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in oxygen-neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E(c.m.)=205.7+/-0.5 keV with a resonance strength omegagamma=1.03+/-0.16(stat)+/-0.14(sys) meV.

The beta decay of 9Li to levels in 9Be: A new look

Beta-delayed neutrons and alpha particles from the decay of 9Li have been studied with a 3He neutron spectrometer and silicon surface barrier detectors. The data show that the (50 ± 3)% feeding to particle unbound states mainly proceeds to the four excited states in 9Be at 2.43 MeV (30 ± 3)%, 2.78 MeV (16 ± 3)%, and 11.28 and 11.81 MeV (3.8 ± 0.2)%. The strong feeding to the 11.81 MeV state observed corresponds to a log ft value of 2.84. The beta-decay rates are strongly affected by the broad levels populated in the decay. Corrections involving R-matrix calculations have been applied for the extraction of the reduced transition probabilities. We confirm the recently determined strong asymmetry in the beta decays of the case32− ground states of 9C and 9Li to the excited case52− states in the daughters 9B and 9Be.

Information about the 12C(α,γ)16O reaction from the β-delayed proton decay of 17Ne

We are studying the β-delayed proton decay of 17Ne with the goal of determining the E2 part of the 12C(α,γ)16O cross section at energies relevant to helium burning in stars. We have determined branching ratios for proton and α-decay for states in 17F from 8.08 to 11.193 MeV. In addition, we have observed the break-up of the isobaric analogue state (IAS) at 11.193 MeV into three particles via two channels: proton decay to the 9.59 MeV state in 16O which breaks up into an α-particle plus 12C; and α-decay to the 2.365 MeV state in 13N which breaks up into a proton plus 12C. This is the first reported observation of the decay of the IAS to the 1− state in 16O at 9.59 MeV.

Astrophysical S factor of 3H(α,γ) 7Li

Abstract The reaction 3 H(α,γ) 7 Li has been investigated for E CM = 79–464 KeV with the use of Ge(Li) detectors. The astrophysical S ( E ) factor is found to be energy-dependent, rising smoothly to S (0) = 0.14±0.02 keV b at zero energy. This is a factor of two higher than the value incorporated in the compilations. The energy dependence of the data is consistent with recent microscopic and direct capture model calculations. The higher 7 Li production rate is of interest to primordial nucleosynthesis.

Cross section and astrophysical S-factor for the 13C(p, γ)14N reaction☆

Abstract We have studied the 13C(p, γ)14N reaction in the energy range Ep = 120–950 keV and obtained cross section curves for capture into the first six states of 14N. The 1−, T = 1 state in 14 N was observed at laboratory energy ER = 557.6 ± 0.5 keV (Ex = 8068.1 ± 0.5 keV) with width ΓR = 40 ± 1 keV, and the 2−, T = 0 state at ER = 450.4 ± 0.5 keV (Ex = 7968.6 ± 0.5 keV). Spectroscopic factors have been obtained from observation of direct capture to all states; they are in fair agreement with results from stripping reaction studies. The S-factor curve for the transition to the ground state is in excellent agreement with previous results; S-factor curves for the other transitions are reported for the first time. The total S-factor extrapolated to 25 keV is 7.7 ± 1.0 keV·b, somewhat higher than previous values.

Spin response of magnetic dipole transitions in 156Gd and 164Dy

Abstract Intermediate energy proton scattering has been used to probe the spin part of the recently discovered low-lying isovector magnetic dipole transitions in the rotational rare earth nuclei 156 Gd and 164 Dy. A large spin response is found in 164 Dy, whereas in 156 Gd the results are consistent with the picture of a predominantly convective excitation. The results are discussed in the context of the IBA-2 model and recent RPA calculations.

Stellar reaction rate of 20Ne(α, γ)24Mg☆

Abstract The reaction 20Ne(α, γ)24Mg has been investigated at Eα(lab) = 0.55–3.20 MeV. Neon gas enriched to 99.95% in 20Ne was recirculated in differentially pumped gas target systems of the extended and quasipoint jet types. New resonances were found at Eα(lab) = 958, 1226, 1260, 1704 and 2277 keV, which correspond to known states in 24Mg. Excitation energies, γ-ray decay schemes, γ-ray angular distributions, resonance widths and strengths as well as Jπ and T-assignments are reported for all the resonances. Information on low-lying states in 24Mg is also obtained. The nuclear and astrophysical aspects of the results are discussed.

The influence of low-energy resonances on the reaction rate of 18O(α, γ)22Ne☆

Abstract The 18 O( 6 Li,d) 22 Ne α-transfer reaction has been used to study the level structure of 22 Ne around the α-threshold and to identify natural-parity states in that excitation range. The 18 O(α, γ) 22 Ne reaction has been studied in the energy range between 340 and 800 keV to determine the existence of possible resonances and to investigate their influence on the reaction rate of 18 O(α, γ) 22 Ne.

The astrophysical implications of low-energy resonances in 22Ne + α

Abstract The 22 Ne( 6 Li, d) α-transfer reaction has been used to search for α-unbound levels in 26 Mg of importance for resonant α-capture on 22 Ne in stellar helium burning. To determine the resonance strengths of the observed states the 22 Ne(α, n) 25 Mg reaction was investigated in the energy range between 600 and 900 keV. One resonance was identified and its strength determined. The astrophysical implications of the present results are discussed.