The Beta-Delayed Proton and Gamma Decay of 27P
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The first and one of the most well observed gamma-ray lines in the interstellar medium was that of 1.809 MeV that comes from the β–decay of the ground state of ^(26)Al to excited states in its daughter (^(26)Mg), which then undergoes gamma decay. Since the half-life of ^(26)Al (7.2 x 105 yr) is much less than the time elapsed from the formation of the solar system (4.57 × 10^(9) years ago), its presence is a clear indication of ongoing nucleosynthesis in the Milky Way. The creation and destruction sites of ^(26)Al is still under debate, but is thought to be produced mainly in Wolf-Rayet (WR) stars. The destruction of ^(26)Al can be accomplished by proton capture on either the ground state or the metastable-state, the later is the one studied in this thesis. Energetically speaking, stars are too cold to study their reaction rates directly, that is, the actual cross sections are too low to allow us to measure them easily in the lab. So, an indirect method is required to gain the necessary information. In the case of interest, the indirect method used was the study of β–delayed γ and proton decay. The direct method involves a proton tunneling through the Coulomb barrier of ^(26m)Al to form excited states in ^(27)Si, which then decay through γ–emission. The indirect method starts with ^(27)P, which then β–decays to the same excited states in ^(27)Si (due to selection rules). The states that are populated above the proton threshold (E^(∗) > S_(p) + E(0^(+)) = 7.463 + 0.228 = 7.691 MeV) can then decay by proton emission to ^(26m)Al. These states represent the resonances of interest in the direct proton capture process. While no new proton lines were observed, because the region of interest was dominated by the β–background, a slightly higher total proton branching ratio was estimated (0.155(3)% – 0.163(3)%). There was, however, a noticeable ’bulge’ in the energy region around 300–400 keV, suggesting that there is potentially valuable information yet to be properly identified. This will require a detector system less sensitive to the β particles. Several new γ lines were seen, mostly γ’s emitted from the IAS, which itself had a new and more accurate energy value assigned (6638(1)). Also, the 7463(3) level has been positively identified as a spin 1 level. The resonance strength for the one level where both protons and γ’s were visible (assuming that the 1614 keV γ was correctly placed) was estimated to be in a range of 0.433 meV to 15 meV (large range is due to the unknown level lifetime). This level might possibly contribute to the reaction rate, but only at temperatures in the region of core collapse supernovae.
Simmons, Ellen Nicole (2013). The Beta-Delayed Proton and Gamma Decay of 27P. Doctoral dissertation, Texas A & M University. Available electronically from