J.D. Anderson

Energy levels in Si27, Co56 and Zn63

Abstract The neutron spectra from the (p,n) reaction on Al, Fe 56 and Cu 63 have been measured at a laboratory angle of 23° to determine the energy levels of the residual nuclei. The protons were obtained from the Livermore variable energy cyclotron. The neutron energies were measured using time-of-flight techniques and a long (9 to 15 meter) flight path. In Si 27 eighteen levels were resolved. The agreement with the nine levels up to an excitation of 4.13 MeV previously reported from charged particle work is excellent. The additional levels are 4.29, 4.45, 4.70, 5.00, 5.08, 5.21, 5.30, 5.38 and 5.51 MeV with an error of ±0.04 MeV. Neutron groups were observed corresponding to the following excitations in Co 56 : 0.165, 0.560, 0.825, 0.980 and 1.105 MeV with an error of ±0.015 MeV. In Zn 63 the following levels were excited: 0.214, 0.640, 1.047, 1.246, 1.386, 1.640, and 1.697 MeV with an error of ±0.025 MeV.

Neutron spectrum from the T+T reaction

Abstract The neutron spectrum from the T+T reaction has been measured by time-of-flight techniques using the swept and bunched 0.5 MeV tritium beam from the Cockcroft-Walton accelerator. The 90° neutron spectrum shows evidence for the following reaction modes: (a) direct breakup into 2 neutrons plus alpha particle (T+T → n+n+He 4 , Q = 11.33 MeV) the three-body breakup shape being modified by the presence of the neutron-neutron interaction; (b) sequential decay proceeding via the He 5 ground state; and (c) sequential decay proceeding via a broad He 5 excited state. The branching ratios for the T+T reaction at 90° are approximately 70% for (a), 20% for (b) and 10% for (c). The neutron group leading to the He 5 ground state, estimated width 0.74±0.18 MeV, is isotropic to within an accuracy of ±10%.

The (α, n) cross sections on 17O and 18O between 5 and 12.5 MeV

Abstract The excitation functions for the (α, n) reaction on 17 O and 18 O have been measured from 5 to 12 MeV using long counters. Also, the neutron differential cross sections were measured at 9.8, 11.6 and 12.2 MeV, using the Livermore neutron time-of-flight facility. Neutron spectra were obtained every 15° between 3° and 135° with gas targets of enriched 17 O and 18 O. The angular distributions are roughly symmetric about 90°. Small forward peaking is observed at the higher bombarding energies for some of the states, mainly for the ground and first excited states.

Elastic and inelastic scattering of 14 MeV neutrons from the lithium isotopes

The angular distributions of elastically scattered 14-Mev neutrons have been measured for Li/sup 6/, Li/sup 7/, and natural Li from 20 deg to 130 deg . Inelastic scattering to the 2.18 Mev level in Li/sup 6/ and the 4.6 Mev level in Li/sup 7/ has been measured over a more restricted angular range. In Li/sup 6/ the 3.57 Mev level is not excited appreciably. The inelastic neutrons from the 2.18 Mev level in Li/sup 6/ and the 4.6 Mev level in Li/sup 7/ are peaked in the forward direction, which is indicative of a direct reaction. (auth)

The livermore 90-inch-cyclotron neutron-time-of-flight facility

Abstract The 90-inch cyclotron is a variable-frequency cyclotron operating at from 4 to 10 Mck. The standard neutron time-of-flight electronics are composed of a biased plastic scintillator for a detector, a time-reference signal derived from the cyclotron rf, a time-to-pulse-height converter, a slow amplifier and discriminator, and an Argonne-type 256-channel pulse-height analyzer. Although this basic system is extremely useful, several additions to the system have been found necessary. The replacement of the plastic scintillator with a stilbene crystal and the inclusion of a proton-electron differentiating circuit have reduced unwanted gamma ray background by a factor of 25. To increase the time between bursts of particles, electro-static sweeping is used. The analyzer data can be either printed out or punched on paper tape which is readily converted to punched cards or magnetic tape for computer-processing of data.

Inelastic scattering of 14 MeV neutrons

The inelastic scattering of 14 MeV neutrons from natural samples of Al, Fe, Ni, Cu, Cd, Sn Pb and Bi has been studied using the Livermore Cockcroft-Walton neutron time-of-flight facility. For each sample the absolute differential cross sections, σ(θ, E), were measured at laboratory angles of 60°, 90° and 120°. For the low-energy neutron spectra (1 < En < 5 MeV), many of the features observed were qualitatively in accordance with predictions of the statistical model. Quantitive comparison, however, required extending the statistical model to describe multiple neutron emission and the agreement was not satisfactory, independent of the choice of level density parameters.

A search for an excited state of B9 near 1.7 MeV

Abstract In a search for low-lying excited states of B9, the alpha-particle spectra from the C12(p, α) reaction were investigated for laboratory angles from 22 to 120° at four bombarding energies from 12.7 to 18.3 MeV. Also the neutron spectra from the Be9(p, n) reaction, at proton energies of 6.3 and 7.4 MeV, and from the Li6(α, n) reaction, at an alpha energy of 14.4 MeV, were measured over similar angular ranges. The three reactions produced no systematic evidence for excited states in B9 between 0.5 and 2.0 MeV excitation. Our results do not confirm a previously reported 1.7 MeV excited state in B9; an upper limit for the excitation of this level by the three reactions can be set at about 100 μb/sr. The accuracy of this result is limited because of the multibody breakup processes (mainly four-body breakup) competing with the reactions under study. Cross-sections for these multibody reactions will be presented. Evidence is also presented for the two-body decay of the 9.6 MeV level in C12 and the 3.1 MeV level in Be9.

Energy levels of 31S and 19Ne

Recently Colli et al., using the 32S(n, d)31P reaction, reported the existence of a 450 keV level in 31P. However, the investigation of other reactions has shown no evidence for such a level. Using the Livermore neutron time-of-flight facility and the (p, n) reaction on 31P, we have searched for the analogue of this proposed level in the mirror nucleus 31S. Neutron spectra were obtained for angles between 30° and 120° and for proton energies from 7.5 to 13.0 MeV. Although the correspondence between the observed levels in 31S and those reported for 31P was excellent up to the maximum excitation energy studied, viz., 3.7 MeV, no evidence for a level at 450 keV excitation was obtained. The reaction 19F(p, n)19Ne also was studied up to an excitation energy of 3.0 MeV in 19Ne. Besides confirming the results obtained by Freeman and West, a previously unreported level at 2.8 MeV was observed.

Elastic scattering of 14 MeV neutrons from nickel

Abstract The differential cross section for elastic scattering of 14 MeV neutrons from nickel has been measured in the angular range from 16 to 163°. Particular emphasis was placed on the investigation of large-angle scattering where definite deviations from optical model predictions have been reported previously in the proton elastic scattering process. Our results are in good agreement with the optical model calculations by Bjorklund and Fernbach, suggesting that the deviations in the charged-particle data are not due to shell effects, as anticipated, but probably to compound-nucleus elastic scattering.

ELASTIC SCATTERING OF 7- TO 14-MeV NEUTRONS FROM NITROGEN.

Abstract The differential cross sections for the scattering of neutrons with incident energies of 6.78, 7.41, 7.93, 8.35, 8.57, 9.38, 10.10, 10.93, 11.55, 12.25, 13.50 and 13.96 MeV from nitrogen have been measured for lab angles between 15° and 135° in 15° steps. The measurements were made with a liquid target in cylindrical geometry with a thickness of approximately one half of the total mean free path. The neutrons were produced by the D(d, n) 3 He reaction in the Livermore variable-energy cyclotron; the average neutron energy spread was ±200 keV. A multiple detector time-of-flight system was used for simultaneous recording of the data. The experimental differential cross sections are analysed in terms of the optical model, with only the real and imaginary potentials varied.