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Nuclear spectroscopy in light nuclei Neilson, George Croydon

Abstract

Measurements of the angular distributions of the gamma-rays from the reaction N¹⁵ (p,αɤ)C¹² indicate that the 4.43 Mev excited state of C¹² has spin 2 and even (+) parity and that the 12.51 Mev and 12.95 Mev levels of O¹⁶ have spin 2 and odd (-) parity. The level in O¹⁶ at 13.24 Mev was not determined uniquely since the theoretical distributions for assignments 3- and 4+ differ only by small amounts. The angular distribution of the ground-state alpha-particles from the reaction N¹⁵(pα) C¹²has been studied for proton energies from 500 kev to 975 kev. The most striking feature of the angular distribution patterns is the strong forward-backward asymmetry which indicates interference between levels of the compound nucleus of opposite parity. The distributions can be explained most simply by assuming two levels in O¹⁶; one at 12.43 Mev excitation with spin O and even parity; the other at 13.09 Mev excitation with spin 1 and odd parity. The strong interference term can also be explained by assuming that there exist broad unresolved O+ and 2+ energy levels in this region, as suggested by recent work on the elastic scattering of alpha-particles by C¹². A fast neutron spectrometer, using the time-of-flight method, which may also be used to study reactions involving the simultaneous emission of neutrons and gamma-rays, is described. One stilbene scintillation counter is placed close to the source, the other a suitable distance away. The two counters are connected to a new type of coincidence time-sorter which converts delay-time between associated events into a pulse-amplitude distribution which may be analysed by a "kicksorter," enabling the whole range of delay times or energies to be displayed and recorded at one time. The delay time may result from the neutron flight-time following scattering in the first counter or from the time-of-flight of a neutron from the target when a coincident gamma-ray has been detected in the first counter. Typical results are presented for the reactions D(dn)He³ and Be⁹(dn i )B¹⁰. The reaction Li¹⁶ (dn ɤ)Be¹⁷ has been studied with the new type of neutron spectrometer. This spectrometer has made possible the- study of the angular correlations between the monoenergetic neutron group to the 0.43 Mev state of Be⁷ and the gamma-rays which follow neutron emission. The results of these angular correlations indicate that the first excited state of Be⁷ has spin ½ and odd parity in agreement with values found for the mirror nucleus Li⁷. The excitation function for the neutron group to the first excited state of Be⁷ has also been studied for deuteron-bombarding energies from 300 kev to 1500 kev and was found to be very similar in shape to that found by other workers for the total neutron yield from the reaction Li⁶+ d.

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