G.A. Bartholomew

Gamma-Ray Strength Functions

The study of the energy dependence of multipole transition probabilities is a broad subject in which certain subfields have progressed much more rapidly than others. Thus for gamma rays ≲2 MeV, E1, M1, and E2 transition probabilities have been well studied (Per 66, SHR 66, SG 65), and at higher energies, ≳10 MeV, the absorption cross section and other properties of the E1 giant resonance are well known (Fir 70), but between these energies there are large gaps in our knowledge. In the present review we are concerned with the distribution of reduced radiation width, particularly for E1 and M1 radiation, in the less well-known “tail region” well below the peak of the E1 giant resonance. We will concentrate on elements with mass number A≳ 90, where a statistical description of radiative processes is appropriate, and on γ-ray energies above ~2 MeV.

Spins of levels in N15, Fe57, Cu64, Zr92 and Hg200 by neutron capture γ-ray directional correlations

Abstract Neutron capture γ-ray directional correlations, with the assumption of negligible M2 admixture in high energy E1 transitions, are used to deduce spins of levels in N15, Fe57, Cu64, Zr92 and Hg200. For several of the correlations studied, multipolarities of primary γ-rays, and hence parities of certain levels, are inferred from the systematics of neutron capture γ-ray radiation strengths and other information. On this basis, the following assignments are made: N15: 6.32 MeV level, 3 2 or 5 2 ; Fe57: 0.365 MeV level, 3 2 ; Cu64: 0.277 and 0.607 MeV levels, 1 or 2+; Zr92: 2.33 MeV level, 3-, negative energy resonance dominating thermal capture, 2; Hg200: 1.59 MeV and 2.10 MeV levels, 1+, resonance at −2 eV, 0. In Hg200, as a consequence of the spin 0 capturing state assignment and E1 multipolarity assignments for primary γ-rays, levels at 2.39, 2.64, 2.97, 3.09, 3.21 and 3.38 MeV are tentatively concluded to be 1+. Information concerning γ-ray multipole mixtures is also obtained in N15 and Fe57.

The Y89 (n, γ) Y90 reaction

Abstract The thermal neutron capture gamma rays from the Y89(n, γ)Y90 reaction were investigated in the energy region between 0.18 and 7.0 MeV with a pair spectrometer, a flat crystal spectrometer, and an angular correlation arrangement using two sodium iodide crystals. The prominent gamma rays in the spectrum, which is unusually simple for a nucleus of this mass number, can be fitted to a level diagram for Y90 with levels at 0.2024±0.0003, 0.247±0.002, 0.7767±0.0002, 1.215±0.002 and 2.741±0.012 MeV. Angular correlation studies between cascading gamma rays, with certain assumptions based on the gamma ray intensities, lead to tentative assignments of 2+ and 3− for the levels at 0.7767 and 0.2024 MeV respectively. The neutron binding energy of Y90 is found to be 6.849±0.009 MeV. The observed level scheme is compared with shell model predictions.

Properties of low lying levels in Mg25

Abstract The reaction Mg25 (pp′γ) Mg25 and the negaton decay of Na25 have been used to study the first four excited states of Mg25 at 0.58, 0.98, 1.61, and 1.96 MeV. The 0.98 MeV level decays to the level at 0.58 MeV or to ground by gamma emission in the intensity ratio 51:49. The 1.61 MeV level decays to ground > 96% of the time. The 1.96 MeV level decays to the level at 0.98 MeV or 0.58 MeV or to ground in the intensity proportions 25:50:25. A study of the gamma rays following negaton decay of Na25 shows that the levels in Mg25 at 0.98 and 1.61 MeV are fed by negaton transitions. Gamma rays due to negaton transitions to the 0.58 state are less than 3% of the total. Comparisons between the gamma-ray branching ratios of levels in Mg25 and those in Al25 indicate that the 0.58, 0.98 and 1.96 MeV levels in Mg25 have spins and parities 1 2 +, 3 2 + and 5 2 + respectively, while that at 1.61 MeV is probably 7 2 + . The most likely spin and parity assignments for the ground state of Na25 are 5 2 + .

Resonance neutron capture in 198,199,201Hg

Abstract Partial radiation widths of γ-rays of energies E γ > 4.1 MeV following capture of neutrons with E n 199 Hg, 200 Hg and 202 Hg. Gamma-ray tables and decay schemes are given. Gamma-ray strength functions and level density distributions are derived. Departures from statistical behaviour are observed in 199 Hg. It is found that the energy dependence of the level density below 3 MeV differs in functional form in the three isotopes.

The 115In(n,γ)116In reaction

Abstract High-energy γ-rays following neutron capture at thermal energies and at the 1.46, 3.86, 9.12, 12.1, 23.0, 39.9 and (46.3+48.6) eV resonances in natural In are examined. Details of the level structure of 116 In are deduced and the partial radiation widths of primary transitions determined. The neutron separation energy is found to be 6781±3 keV. Strength functions are derived E1 and M1 transitions. The E1 strength is found to remain constant and the M1 strength to decrease with γ-ray energy from 5.2 to 6.7 MeV.

Gamma rays following μ− capture in heavy elements

Abstract The spectra of γ-rays > 0.6 MeV following the capture of μ − mesons in Bi, 208 Pb(NO 3 ) 2 , radiogenic Pb, Au, Ag and Zr targets are measured with a large NaI detector and the intensity and energy distributions are deduced. The spectra are distinguished by a few resolved lines at low energies and, for Bi, radiogenic Pb and Au, by a distinct group of γ-rays centered at 4.2 MeV. In gross shape, the spectra resemble those following inelastic neutron scattering. There is no prominent γ-ray anomaly near 5.5 MeV similar to that in the spectra following neutron transfer reactions in the same mass region. The average number of γ-rays per σ-capture and mean excitation energy prior to γ-emission are estimated.

The 205Tl γ-ray strength function below 7.5 MeV

Abstract The 205 Tl γ-ray strength function below 7.5 MeV is derived from a measurement with 3% energy resolution of the 205 Tl(γ, γ) 205 Tl cross section between 3.5 and 7.5 MeV. This strength function is found to be similar to those extracted from (n, γ) data in the neighbouring Tl isotopes. The structure observed in the Tl strength function below 7.5 MeV is consistent with that predicted from recent particle-hole calculations for 208 Pb.