Electromagnetic Decay of the Second Excited State of C¹²

Citation

Seeger, Philip Anthony (1963) Electromagnetic Decay of the Second Excited State of C¹². Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/D1ET-DG62. https://resolver.caltech.edu/CaltechTHESIS:04272012-155927120

Abstract

It was desired to determine what fraction of C^(12) nuclei in the second excited state decay electromagnetically to the ground state instead of by alpha emission. The state was formed with the reaction N^(14)(d,α)C^(12)* by bombarding a thin vanadium nitride target on carbon foil with 1.8-MeV deuterons. Recoil C^(12)nuclei were observed in coincidence with the alpha particles from the reaction. The alphas were detected with a surface-barrier counter set at the appropriate unique angle, and the recoiling nuclei at 90° were analyzed in momentum with a magnetic spectrometer, in energy by a surface-barrier counter at the spectrometer focus, and in velocity by the time delay between the alpha and the C^(12) counts. Pulses of 40-nsec width were recorded photographically from a Tektronix type 555 dual-beam oscilloscope, providing a time resolution of 5 nsec, and also allowing pulse height analysis on the fast pulses. Of the C^(12) nuclei formed in the second excited state, 99.9+ per cent decay by alpha emission to Be^8 and are not detected; only those which decay to the C^(12) ground state by gamma or pair emission will be seen. Thus the experiment provides a direct measurement of Г_(rad)/Г. The value of Г_(rad) = (Г_γ + Г_e±) has astrophysical significance determining the rate of C^(12) formation in the He-burning process in red giant stars. Analysis of 170 observed events gives Г_(rad)/Г = (2.8±0.3) x 10^(-4) . Averaged with a value of (3.3 ± 0.9) X 10^(-4) obtained by Alburger, and combined with a value of Г =(8 ± 5) eV, the result is Г_(rad) = (0.0024 ± 0.0015) eV, which is two-and-one-half times the value used in previous astrophysical calculations.

Thesis Files