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The four crystal pair spectrometer and its use in the study of nuclear reactions Dosso, Harry William


A four crystal pair spectrometer has been constructed and used to study several nuclear reactions. The spectrometer selects those events for which pulses corresponding to annihilation radiation in two side counters are coincident in time with the pulses from the recording centre counter. A fourth counter has been incorporated to decrease low energy counts in the pulse height distribution resulting from energy loss in the form of Bremsstrahlung and electron escape. The spectrometer has been used to measure low energy pair production cross sections. The relative cross sections for 1.114, 1.173, 1.332 and 2.62 Mev. gamma rays were measured. Absolute cross sections were obtained for 1.114, 1.173 and 2.62 Mev. gamma ray energies with the aid of results obtained by West (1956). The absolute cross sections obtained were: .317 ± .03 x 10⁻²⁶ cm.², 1.34 ± .12 x 10⁻²⁶ cm.², and 116 ± 10 x 10⁻²⁶ cm.² for 1.114, 1.173 and 2.62 Mev. respectively. The relative intensity of the 6.94 Mev. and 7.1 Mev. gamma ray with respect to the 6.13 Mev. gamma ray for the F¹⁹ (p , α, γ)0¹⁶ reaction at the 340 Kev. resonance was measured and found to be .023 ± .002. The absolute efficiency of the spectrometer for 6 Mev. gamma rays for a distance of 9.72 cm. was measured to be 2.05 ± .09 x 10⁻³%. The spectrometer was used to observe the gamma radiations from the B¹⁰ (d, p, γ)C¹¹ and B¹⁰ (d, n, γ)C¹¹ reactions at 1.5 Mev. bombarding energy. The high energy resolution and efficient background rejection made it possible to resolve the complex gamma ray spectrum in the presence of an intense neutron flux.

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