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Scintillation counters for gamma-ray measurements and a study of the gamma-rays from the D (p.Y) He[3] reaction Griffiths, George Motley

Abstract

The development of scintillation counters for measurements of Ƴ-ray energies and fluxes is described, with particular emphasis on Y-rays in the energy range 2 to 20 Mev. A comparison has been made between the experimentally observed and theoretically predicted properties of these counters. A three-crystal pair spectrometer has been developed which measures only those Ƴ-rays which interact with the detector crystal by producing electron-positron pairs. This device has proved useful for investigating the pair production process for Ƴ-rays with energies only 150 Kev. above the pair production threshold, as well as for the investigation of the complex Ƴ-ray spectrum produced by the F¹⁹(p,αƳ)0¹⁶ reaction. A detailed study of the D(p,Ƴ)He³ reaction has been made. The Ƴ-ray energy indicates that the transition goes to the ground state of He³. The shift in Ƴ-energy with changing proton, energy was, observed, and fits the following relation: E[subscript] Ƴ = (5.50 ± 0.03) + ⅔ E[subscript]p Mev. The Doppler shift of the Ƴ-ray energy (frequency) as a function of the angle between the directions of the incident protons and emitted Ƴ-rays has been measured. The observed shift has the full value expected, assuming that the Ƴ-rays are emitted in a time short compared to the slowing down time of the intermediate He³ nucleus in the target. From these results an upper limit of 2 x 10¹³sec. can be placed on the half-life for the Ƴ-ray transition. The cross section for the reaction for incident protons of 1 Mev. is estimated to be 6 x 10⁻³⁰ cm² (± 50%). The excitation function has been measured from 250 Kev. to 1.8 Mev. and the angular distribution of the Ƴ-rays has the form (Sin² θ + b) where b = 0.046 ± 0.005 for 1 Mev. protons and b = 0.025 ±0.006 for 1.75 Mev. protons. These results, when compared with recent theory, indicate that the reaction is due to direct radiative capture of the protons. Preliminary measurements have been made on the T(p,Ƴ)He⁴ reaction. The Ƴ-ray energy measured relative to the Ƴ rays from the Li⁷(p,Ƴ)Be⁸ reaction is 20.3 ± 0.4 Mev. for 800 Kev. incident protons, indicating that the transition goes directly to the ground state of the helium nucleus. The angular distribution of, the Ƴ-rays measured in a plane containing the incident proton beam direction is predominantly Sin² θ with the addition of a small isotropic component. The present approximate measurements show a greater isotropic component than that obtained at higher proton energy by other workers.

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