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Direct radiative capture of Alpha-particles by Tritium Ottewell, David Frederick

Abstract

Improved absolute cross section and angular distribution measurements have been made for the direct radiative capture of alpha-particles by tritium. Gamma-ray yields have been obtained for laboratory energies ranging from .360 to 1.883 MeV for capture to the P[sub 3/2-] ground state and to the P[sub 1/2-] first excited state. Angular distributions have been measured at three angles and two energies (.853 MeV and 1.883 MeV). These measurements are more accurate than previous ones due to the use of better gamma-ray detectors, improved targets, and a more accurate determination of the tritium content of these targets. An approximate direct capture calculation has been done which incorporates a simple two-body model to estimate the cross section of the T([Mathematical Values])⁷Li reaction. An averaged local potential was used to describe the interaction between the alpha-particle and the triton, where the interior structure of both particles was assumed to play no role in the capture interaction. The model involves the calculation of electromagnetic matrix elements between initial and final state wave functions evaluated for Saxon-Woods potentials with parameters adjusted to fit both elastic scattering data and the binding energies for the ground and first excited states of ⁷Li. The scattering phase shifts were extrapolated from data measured at energies higher than those used for the present experiment. Some characteristics implied by a more detailed treatment of the wave functions, that incorporate exchange effects, have affected the choice of parameters for the two-body model. The model gives a reasonable fit to the measured cross sections considering that no free parameters were incorporated in the calculation once the wave functions had been defined by the phase shifts and the binding energies. There are some unresolved discrepancies in the theoretical and experimental angular distributions.

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