Proton Capture

Indirect techniques have been used to obtain information about reaction rates for several proton capture reactions that occur on short-lived nuclei. The techniques used to carry out the measurements are reviewed and the results obtained are presented. Also future prospects for further measurements with a new facility, T-REX, are discussed.

The gamma-rays from the N14(p, 𝛾)O15 reaction at the Ep=278 keV resonance, corresponding to the 7.56 MeV level in O15, were studied with a large crystal scintillation spectrometer. The direct ground state transition gamma-ray of about 3% in intensity of the total decay was found to exist in addition to the three known cascade lines. Angular distributions of these gamma-rays are all isotropic, supporting the J𝜋=1/2+ assignment to the resonance state. The transition strength (radiation width in units of Weisskopf single particle width) of the direct ground state transition E1 gamma-ray is calculated to be |M|2=4.15×10−6, which is by a factor of 10−4 smaller than the normal E1 transition strength found in light nuclei. Other transitions in O15 are estimated to be of normal strength compared with Wilkinson’s estimation.

The gamma-rays from the Si28(p, 𝛾)P29 reaction at the Ep=369 keV resonance
were studied with a large NaI (Tl) crystal scintillation spectrometer. The resonance
state, corresponding to the 3.116±0.012 MeV fourth excited state in P29, was found
to decay by (74±4)% to the first excited state at 1.384±0.008 MeV and by (26±4)%
to the second excited state at 1.961±0.013 MeV which decays predominantly to
the ground state. From the angular distribution measurements of each gamma-ray
line, spins and parities of these lower levels in P29 were unambiguously assigned
as follows: 1.38 MeV (1st), J=3/2+; 1.96 MeV (2nd), J=5/2+; 3.12 MeV (4th),
J=5/2+. These assignments are all consistent with the known character of the
corresponding levels in the mirror nucleus Si29, and are also in agreement with the
theoretical predictions. From the precise gamma-ray energy measurements, the Q-value of this reaction was estimated as 2.760±0.013 MeV, which is by about 36 keV
larger than the published value based on the -ray end-point energy measurement.
The resonance strength (2J+1)𝛤p𝛤 𝛾/(𝛤p+𝛤 𝛾) was found to be (4.7±0.8)×10−3 eV.