U. Giesen

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.

The reaction branching 31P(p, γ)/31P(p, α) in the RP-process

The reactions ^(31)P(p, γ)^(32)S and ^(31)P(p, α)^(28)Si have been studied in the energy range 280–620 keV to investigate the influence of low-energy resonances on the stellar reaction rates. Several new resonances have been observed and the resonance strengths for both reaction channels have been determined. The reaction rates have been calculated from the present results and are compared with the results of Hauser-Feshbach calculations.

Direct proton capture on 32S

The ^(32)S(p,γ)^(33)Cl reaction has been measured in the proton-energy range E_p = 0.4–2.0 MeV. Non-resonant γ-transitions were observed to the final states in ^(33)Cl at E_x = 0,811 and 2846 keV. The corresponding spectroscopic factors have been extracted from fits to the excitation functions and are compared to values from stripping data as well as theoretical model calculations. The astrophysical aspects of the ^(32)S(p,γ)^(33)Cl reaction are also discussed.

Beta-delayed particle decay of 17Ne

Abstract Protons and α-particles emitted from 36 Ar levels following the β-decay of 36 K ( T 1 2 = 0.34 s ) have been investigated. Delayed particles were detected using a surface barrier detector and a Microchannel plate in a back-to-back geometry. Five new proton groups and eight new α-particle groups at E p = 623, 1169, 1199, 1272, 1409 keV and E α = 1522, 1562, 1963, 2982, 3613, 3748, 3849, 4086 keV were identified. Particle branching ratios and corresponding upper limits for the log ƒt values of the precursor β + transitions were calculated. The observed particle groups were assigned to excited levels in 36 Ar. Ratios of partial widths for proton and α-particle emission could be determined for two levels in 36 Ar. New evidence for spin-parity assignments is presented. One of the new delayed α-particle groups corresponds to a previously unknown natural-parity state near the proton threshold in 36 Ar. The implications for the explosive hydrogen burning of 35 Cl are discussed.

Investigation of the 12C(α, γ)16O reaction via the β-delayed proton decay of 17Ne

Abstract We have determined the branching ratios for the β-delayed proton decay of excited states in 17 F to excited states in 16 O by measuring proton-γ-ray coincidences. We have observed transitions to the 2 + state at 6.917 MeV in 16 O from 17 F states at 11.193, 10.0, 9.45, 8.83 and 8.44 MeV. In particular, the transition from the 9.45 MeV state is an order of magnitude stronger than the accompanying transitions to the 1 − state at 7.117 MeV and the 3 − state at 6.130 MeV, and seems a favourable case for the observation of 16 O break-up into α + 12 C .

{beta}-delayed {alpha} emission of {sup 18}N: Broad J{sup {pi}}=1{sup -} states in the {sup 14}C +{alpha} system

The {beta}-delayed {alpha}-spectrum of {sup 18}N has been measured at the TISOL facility at TRIUMF resolving the structure of a broad {alpha}-particle distribution at {alpha}-energies of 1.5-5.5 MeV into several states that show a distinct interference pattern. R-matrix fits assuming a solely J{sup {pi}}=1{sup -} partial wave confirm this finding. The extrapolated low energy part of this structure constitutes the natural parity background in a possible experiment to observe the parity forbidden {alpha}-decay of the E{sub x}=6.880 MeV, J{sup {pi}}=0{sup -} state in {sup 18}O that is estimated from the fit to be 2x10{sup -8} of the intensity of the E{sub {alpha}}=1081 keV peak. In addition, the tails of the broad 1{sup -} states observed likely contribute to the astrophysical {sup 14}C ({alpha},{gamma}){sup 18}O reaction rate though the experimental information available is too incomplete to allow precise predictions. The {beta}-delayed {alpha}-spectrum of {sup 18}N also excludes complex {beta}-feeding amplitudes at a significant level. The states seen in the {beta}-delayed {alpha}-decay produce a pattern of approximately equally spaced 1{sup -} states in {sup 18}O consistent with J{sup {pi}}=1{sup -} states seen at the same and higher excitation energies of {sup 18}O.