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The Li ⁷ (α, γ) B¹¹ reaction and some other topics Singh, Prithe Paul

Abstract

The University of British Columbia Van De Graaff generator was used to study the resonant capture of α-particles by Li⁷ to form B¹¹ in the three excited states at 8.92 Mev, 9.19 Mev and 9.28 Mev. B¹¹, being in the middle of the p shell, has been rather extensively studied in the past decade with considerable disagreement between results obtained from the Li⁷ (α,γ) B¹¹ and B¹⁰ (d, p) B¹¹ reactions. With seven particles outside the closed shell theoretical calculations are difficult and the calculations of Kurath have been limited to the negative parity states only. The energies, intensities and angular distributions of the γ-rays which de-excite the three states of B¹¹ have been studied. It was found that the 9.28 and 9.19 Mev states cascade through the 6.76 Mev and 4.46 Mev states and negligibly, if at all, through the 6.8l Mev state. The γ-ray widths for many of the γ-ray transitions have been obtained and compared with the average radiative widths reported by Wilkinson. On the basis of the angular distribution results spins and parities have been assigned to some of the B¹¹ levels up to an excitation of 9.28 Mev. The results are in good agreement with recent work on the B¹⁰ (d, p) B¹¹ reaction. Tentative speculations concerning the nature of some of the transitions have been made, although no detailed comparison with theory seems possible at the moment. The assignments based upon the present work are [assignments omitted]. The assignment of 5/2⁻ to the 4.46 Mev state was well known previously. For the 8.92 Mev state the present work favours 5/2⁺ and in this is supported by recent results from the stripping reaction B¹⁰0 (d, p) B¹¹ which suggests positive parity for the level; however, the present results do not rule out the possibility of 5/2⁻ and formation of the state by d-wave α-particles. A three crystal pair spectrometer was used to determine accurately the energies of the γ-rays from Zn⁶⁵ and Na²². The energies of these γ-rays are above 1.022 Mev and since the accurately known rest mass of the pair electrons is subtracted from each incident photon by pair production, a measurement of the small remaining kinetic energy of the pair electrons with moderate accuracy gives the energy of the γ-rays with considerably greater percentage accuracy. The present results are, Zn⁶⁵ γ-ray Energy 1.1124 ± 0.0019 Mev Na²² γ-ray Energy 1.2736 ± 0.0018 Mev The pulse height spectrum and absolute efficiency of a ZnS-lucite fast neutron counter, consisting of a number of thin sheets of lucite coated with zinc sulphide and sandwiched together to form a rectangular block, was investigated using neutrons with energies from 280 Kev. to 16 Mev and γ-rays with energies of 1 Mev and 6 Mev. At a bias setting where the absolute neutron detection efficiency varied from 0.15% for 2 Mev neutrons to 0.3% for 4 Mev neutrons, the 6 Mev γ-ray sensitivity was less by a factor of 10⁹. Using this counter the yield and angular distribution of neutrons was measured from thick and thin heavy ice targets bombarded with protons below the D (p, n) 2p threshold. The yield and the angular distribution data fit very well with theoretical results calculated by Y.I. Ssu on the hypothesis that neutrons are produced by deuterons, scattered in the target by incident protons, which, then, collided with other target deuterons producing D (d, n)He³ neutrons. A semi-empirical method has been developed to calculate the γ-ray detection efficiency of NaI (TlI) crystals for γ-rays from 0.5 Mev to 12 Mev. The results were compared with the experimental efficiencies at .5 Mev, 1.25 Mev, 4 Mev, 6 Mev and 12 Mev, independently determined by absolute methods at .5 Mev, 1.25 Mev and 6 Mev and by relative comparison at 4 Mev and 12 Mev. The agreement is within 5% up to 6 Mev. The effects of scattered γ-rays by lead shielding was also investigated.

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