M.A. Lone

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.

Transient magnetic fields for large-Z atoms recoiling through gadolinium

We have utilized Coulomb excitation of the 52− first excited state in 207Pb, whose g-factor is well known, to calibrate the transient field for Z = 82 recoils in ferromagnetic gadolinium at 77 K over a wide range of velocities, 2.4Aν0 ≦ ν ≦ 10.2ν0. The transient field is found at first to increase with increasing recoil velocity, and then to assume a nearly constant value of ≈ 6.2kT for ν ≳ 5.5ν0. The precession measurements in 207Pb, 232Th and 238U are combined with the results of other recent experiments to examine the atomic number dependence of the transient field. We find a roughly linear increase of the transient field with Z of the recoil. n nThe observed transient fields for Pb recoils were interpreted as arising from single ns electrons. Calculations that use simple estimates for the single vacancy lifetimes and for the cross sections for electron capture and loss indicate a dominant contribution to the transient field from the 4s shell, with a polarization ξ, ≈ 0.09.

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.

Single-particle states at very high spin in the light dysprosium isotopes

Abstract High-spin states in 150–152 Dy have been investigated. Gamma-gamma coincidences in all three isotopes have been studied with Compton-suppressed Ge(Li) detectors. For 151 Dy, lifetimes of the high-spin states have been measured by the recoil-distance technique. The spin and parity assignments, transition rates, and level systematics support the interpretation of the observed states as proton particle-hole excitations coupled to aligned valence neutron configurations. Shell-model calculations, based on effective interactions and a 146 Gd core, quantitatively reproduce the observed level patterns, and the predicted transition rates qualitatively agree with the observed values.

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.

Doubly radiative thermal neutron capture in 2H and 16O: Experiment and theory

Abstract Measurements and calculations are presented for the cross sections for two-photon emission following thermal neutron capture in 2H and 16O. Upper limits for σ2γ were measured in both cases. For 2H, σ2λ = 8 ± 15 μb, for γ-rays in the energy region 700

Resonance averaged channel radiative neutron capture cross sections

Abstract In order to apply Lane and Lynns channel capture model in calculations with a realistic optical model potential, we have derived an approximate wave function for the entrance channel in the neutron-nucleus reaction, based on the intermediate interaction model. It is valid in the exterior region as well as the region near the nuclear surface, and is expressed in terms of the wave function and reactance matrix of the optical model and of the near-resonance parameters. With this formalism the averaged channel radiative neutron capture cross section in the resonance region is written as the sum of three terms. The first two terms correspond to contributions of the optical model real and imaginary parts respectively, and together can be regarded as the radiative capture of the shape elastic wave. The third term is a fluctuation term, corresponding to the radiative capture of the compound elastic wave in the exterior region. On applying this theory in the resonance region, we obtain an expression for the average valence radiative width similar to that of Lane and Mughabghab. We have investigated the magnitude and energy dependence of the three terms as a function of the neutron incident energy. Calculated results for 98 Mo and 55 Mn show that the averaged channel radiative capture cross section in the giant resonance region of the neutron strength function may account for a considerable fraction of the total (n, γ) cross section; at lower neutron energies a large part of this channel capture arises from the fluctuation term. We have also calculated the partial capture cross section in 98 Mo and 55 Mn at 2.4 keV and 24 keV, respectively, and compared the 98 Mo results with the experimental data.

An interference effect in the channel radiative neutron capture process

Abstract This study shows that the small thermal neutron radiative capture cross sections in 12 C and neighbouring nuclides are the result of destructive interference between the potential scattering wave and the resonance scattering wave near the nuclear surface, resulting in a drastic cancellation in the radial integral. The behaviour of the scattering wave function is examined, and the general condition for the occurrence of such cancellation is discussed. The expression for the channel radiative capture cross section which has been derived has the same structure as the Lane-Lynn formula but is expressed in terms of different parameters. In addition, this investigation shows that if the optical model well depth is adjusted so that the binding energy of the p 1 2 orbit in 12 C is kept at the experimental value, then the calculated results for the potential and channel radiative neutron capture cross sections are insensitive to the value of the nuclear radius.

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.

Laser-induced neutron radiative capture reaction

Abstract Laser-stimulated enhancement of the p-wave radiative capture of a low-energy neutron is investigated. Conceptually, this process proceeds as follows: in the presence of a strong laser field, the capture of a low-energy neutron and a simultaneous low-energy photon transition may populate a p-wave resonance near the binding energy; this will result in an enhanced E1 radiative transition to a low-lying s-wave neutron bound state. The cross section for such a process is expressed in terms of the intensity of the laser radiation and nuclear matrix elements. Numerical estimates show that an appreciable enhancement of the radiative capture will not be observed until the laser electric field strength exceeds a magnitude of 10 5 to 10 8 V/cm. The lower value corresponds to the existence of a p-wave resonance in the intermediate state and an s-wave resonance in the entrance channel, whereas the higher value corresponds to the case when the s-wave resonance is absent. They are also much higher than an estimate reported recently in the literature; an explanation is given for possible sources of discrepancy.