P. van der Merwe

A study of (3He, n) reactions on 24Mg, 28Si and 32S

Abstract Using a time-of-flight spectrometer, measurements have been made on the neutron groups from the 24 Mg( 3 He, n) 26 Si reaction at 4.9, 5.2 and 5.6 MeV, the 28 Si( 3 He, n) 30 S reaction at 5.14 and 5.6 MeV and the 32 S( 3 He, n) 34 Ar reaction at 5.6 MeV. The ground state Q -values for the reactions were measured to be 75±30, −579±25 and −760±30 keV, respectively, Groups were measured corresponding to levels at 2.78, 3.35, (3.77), 3.89 and 4.16, all ±0.04 MeV in 26 Si, 2.21, 3.43 and 3.71, all ±0.03 MeV and at 2.19±0.04 and 3.59±0.06 MeV in 34 Ar. There is a close correspondence between some of the new levels reported here and analogous states in the A = 26, 30 and 34 isobaric triplets. Angular distributions and absolute cross sections have been measured for several neutron groups from these reactions. The results show that the reaction is predominantly a direct one at these energies. Plane wave and distorted wave double stripping calculations give reasonable fits in the forward direction particularly for L = 0 transitions. Simple double stripping theory does not explain the relatively large cross sections at backward angles observed for some transitions.

Fast neutron scattering from 139La and 141Pr

Abstract Using time-of-flight techniques, the isotones 139 La and 141 Pr were studied by means of elastic and inelastic neutron scattering in the energy range 230–2580 keV. The energies of more than 20 levels in 139 La and of more than 15 levels in 141 Pr were determined. Contrary to results from Coulomb excitation experiments, no evidence was found for levels between 200 and 1100 keV in either nucleus. Excitation functions and angular distributions for scattering to a number of levels in both nuclei were measured. Optical-model parameters were determined from fits to elastic scattering angular distributions and to the excitation functions for scattering to the first excited states. A Hauser-Feshbach analysis of the inelastic data yielded probable spin values for some higher levels. For other “levels”, the calculations indicated unresolved structure.

Levels excited in73Ge by (d, p) and (p, p′) reactions

Angular distributions from the reactions72Ge(d, p)73Ge and73Ge(p, p′)73Ge have been studied with 8.0 MeV deuterons and 12.0 MeV protons, respectively, using the Aldermaston Tandem Van de Graaff generator and multi-angle magnetic spectrograph. The ground stateQ-value of the (d, p) reaction was measured to be 4.571±0.004 MeV. The energies, values of the transferred orbital angular momenta, parities and transition strengths in both reactions were determined by use of distorted-wave Bornapproximation calculations, for levels up to 3 MeV excitation energy. The sums of the transition strengths are compared with shell model calculations as well as with values obtained in neighbouring nuclei. Discrepancies between parity and possible spin assignments of states observed in the two reactions are discussed.

A study of the Ca(p, n)Sc reactions: (I). Energy levels in scandium isotopes

Abstract Time-of-flight measurements have been made on the neutrons emitted from enriched targets of 43, 44, 48 Ca bombarded by 4.0 to 5.5 MeV protons. Two additional levels have been found in 43 Sc at 1347±10 and 1424±10 keV in agreement with recent work on the 40 Ca(α, p) reaction; in 48 Sc, knowledge of the level structure has been extended up to 4169 keV with the observation of 34 additional levels; and the measurements for the low-lying states in 44 Sc agree with previous work. The measured (p, n 0 ) Q -values for 43 Ca, 44 Ca and 48 Ca are −2998±10, −4447±10 and −534±15 keV, respectively.

An investigation of 73As energy levels by means of (p, n) and (p, nγ) reactions

Abstract The energy levels of 73 As were studied by means of the 73 Ge(p, n) 73 As reaction at proton energies between 2.1 and 3.3 MeV. Neutron energies were measured by time of flight, and γ-ray energies with Ge(Li) detectors. The γ-decay of levels was studied by means of γ-n coincidences. Angular distributions were measured for the strongest de-excitation γ-rays, and relative excitation strengths of levels were derived from the neutron spectra. These are compared with predictions of the statistical theory to derive spins for a number of levels. A level and decay scheme for 73 As up to 1.98 MeV excitation is presented.

The energy levels of 139La and 141Pr from (n, n′ γ) measurements

Abstract Using time-of-flight to discriminate against neutron induced backgrounds ina Ge(Li) detector, the energy levels of 139 La and 141 Pr were studied by means of (n, n′γ) reactions. The observed γ-rays were assigned to transitions from level up to an excitation of 1835 keV in 139 La and 1844 keV in 141 Pr. Most of these levels had previously been observed in (n, n′) measurements. Some weakly excited new levels were also observed. Structure which could not be resolved in the neutron scattering experiments but for which Hauser-Feshbach calculations had indicated higher multiplicity was shown to consist of closely spaced levels.

Fast neutron scattering from rubidium

A study was made of the (n, n), (n, n′), and (n, n′ γ) reactions which occur when a natural rubidium scattering sample is exposed to mono-energetic neutron fluxes of energies ranging from 120 to 1910 keV. Total cross sections, elastic scattering angular distributions and excitation functions for inelastic scattering were measured with a neutron time-of-flight spectrometer. Results are compared with Hauser-Feshbach theory. A 40 cc Ge(Li) detector was used in the time-gated mode to measure γ-rays from the (n, n′ γ) reaction. The latter measurements were used to refine and extend the energy level schemes derived from neutron spectroscopy. Several new levels were discovered in the low-lying (below 1900 keV) energy level spectra of85Rb and87Rb. Gamma decay schemes and branching ratios were determined for the low-lying levels of85Rb and87Rb.

A study of the Ca(p, n)Sc reactions: (II). Fluctuation analysis of the reaction 48Ca(p, n)48Sc

Abstract The 48Ca(p, n)48Sc reactions to the first 11 excited states in 48Sc have been studied as a function of angle and proton energy from 4.03 to 5.63 MeV. Absolute cross sections (±25%) have been obtained using a calibrated, time-of-flight detection system. The cross sections are observed to have random fluctuations in accordance with the Ericson fluctuation theory. There is no indication of any direct reaction contributions to the cross sections. The average angular distributions are symmetric about 90°. The coherence energy Γ is observed to be ≦ 9 keV at 15 MeV excitation in 49Sc. Autocorrelation analysis of the cross sections for exciting the third to 11th excited states of 48Sc leads to probable spins of (3,4), (1,2), 2, (1,2), 1, (2,3), 2 and 0, respectively.

The energy levels of79Br and81Br

The low-lying energy level and decay schemes of the stable bromine isotopes were studied by observing the γ-rays from the (n, n′γ) reactions in a natural bromine sample with the aid of a Ge (Li) detector. A pulsed neutron source and time-of-flight methods were used to reduce background due to neutron events in the detector. Most of the γ-rays observed could be assigned to transitions in either79Br or81Br. In addition to those levels previously known from γ-decay or Coulomb excitation experiments, several new levels were excited in each isotope.

Energy levels of 71Ge from 71Ga(p, nγ)71Ge

Abstract Energy levels of 71Ge were populated by the 71Ga(p, n) reaction at proton energies between 1.72 and 2.96 MeV and their decay studied by both γ-γ and γ-n coincidences. Spins were determined within the framework of the statistical theory from angular distributions of de-excitation γ-rays and from excitation strengths of levels derived from γ-ray intensities and branching. A level and decay scheme for 71Ge up to an excitation of 1.3 MeV is presented.