D.M. Drake

Radiative capture of energetic neutrons by 208Pb

Abstract The radiative capture cross section of neutrons with energies covering the range 6.2–14.7 MeV has been measured for the 208Pb(n, γ)209Pb reaction. Gamma-ray spectra were recorded with a NaI(Tl) anti-Compton spectrometer. A time-of-flight technique was used to reject undesirable background. Absolute cross sections were determined for γ-ray transitions to the g 9 2 ground state, the i 11 2 state at 0.78 MeV, the doublet of j 15 2 and d 5 2 states near 1.5 MeV, and the close pair of g 7 2 and d 3 2 states at about 2.5 MeV in 209Pb. All cross sections show a resonance behavior in accordance with the prediction of semi-direct capture theory as the neutron energy spans the giant-dipole resonance. The measured cross sections are a factor of 2–3 higher than the predicted values.

Double-Differential Beryllium Neutron Cross Sections at Incident Neutron Energies of 5.9, 10.1, and 14.2 MeV

We measured beryllium neutron-production cross sections using the time-of-flight technique at incident neutron energies of 5.9, 10.1, and 14.2 MeV, and at laboratory angles of 25, 27.5, 30, 35, 45,...

Optical-model analysis of antiproton-nucleus elastic scattering at 50 and 180 MeV

Abstract Accurate antiproton-nucleus elastic scattering angular distributions measured at two incident energies,∼50 and ∼180 MeV, on 12 C, 40 Ca, 208 Pb and at 178.4 MeV on 16,18 O have been analysed in the framework of the optical model. Extensive searches on the relative sensitivities to the different potential parameters have been done. Optical potentials with charge-distribution geometries failed in reproducing the experimental data; the finite range of the interaction has to be taken into account. The corresponding optical potentials have shallow real parts 5 MeV ⩽ V 0 ⩽ 105 MeV, their imaginary depths are at least twice larger than their real depths. These features rule out some models but are consistent with an analysis of recent p -atom data. The sensitivity of the optical-model analysis of elastic scattering data to the details of the radial potential has been investigated. Target mass and energy evolution of the strong-absorption radius and reaction cross section confirm that the antiproton penetrates more inside the target nucleus when the incident energy increases. Some features of the phenomenological potentials are compared with the predictions of microscopic potentials.

Radiative capture of fast neutrons by 89Y and 140Ce

The γ-ray spectra from the reactions 89Y(n, γ)90Y and 140Ce(n, γ)141Ce have been measured in the neutron energy range of 6.2–15.6 MeV. The pulse-height spectra were recorded with NaI(Tl) spectrometers and time-of-flight techniques were used to improve signal-to-background ratio. Capture cross sections were determined for γ-ray transitions to the two 2d52 levels at 0 and 203 keV of 90Y and to the 2f72 ground state of 141Ce as well as integrated cross sections to bound states in these nuclei. The observed γ-ray spectra and partial radiative capture cross sections were compared with predictions of the direct-semidirect capture theory. The resonance behaviour with neutron energy of both the ground-state and integrated partial capture cross sections shows the validity of the semidirect model for 89Y and 140Ce in the region of neutron energy encompassing the giant-dipole resonance. The observed symmetry of the cross sections about the peak of the resonance argues strongly for the complex form of the particle-vibration coupling interaction. A detailed comparison of the predictions of the DSD model using the complex coupling interaction shows that the capture cross sections are relatively insensitive to the real part of the interaction.

BEAM HEATING IN GAS TARGETS.

Abstract The correction for the change in density of molecules in gas targets as a function of beam current is investigated using both a pulsed and dc beam. The dominant physical mechanism is confirmed to be an overall heating of the gas for the type of target commonly used which has the incident beam stopping in the end foil of the gas cell. It is shown how this heating can be monitored and corrected for in a convenient and reliable manner. The effects of pulsing and possible temperature gradients are discussed.

Elastic and inelastic scattering of fast neutrons from 6Li and 7Li

Abstract The differential elastic and inelastic neutron scattering cross sections of 6Li and 7Li have been measured at incident neutron energies of 4.83, 5.74 and 7.5 MeV for 6Li and at energies of 3.35, 4.83, 5.74 and 7.5 MeV for 7Li. Scattered neutrons and gamma rays were observed independently. The cross sections were measured with a neutron time-of-flight spectrometer relative to the well-known cross section for neutron scattering from hydrogen. The gamma-ray spectra were measured with a NaI(Tl) spectrometer using time-of-flight techniques to eliminate the neutron background. The 2.184 MeV state in 6Li was excited by neutron inelastic scattering. The 3.56 MeV state in 6Li was not observed in either the neutron or gamma-ray studies. Scattered neutrons from the 0.478 MeV state in 7Li were resolved at both 3.35 and 4.83 MeV. Scattered neutrons from the 4.63 MeV state in 7Li were observed at incident neutron energies of 5.74 and 7.5 MeV. A continuum of neutrons due to three-particle break-up was observed for both 6Li and 7Li at 4.83, 5.74 and 7.5 MeV.

Neutron Capture Cross Sections of 182 W, 183 W, 184 W, and 186 W from 2.6 to 2000 keV

Neutron capture cross sections of four stable tungsten isotopes were measured as a function of energy by time of flight at the Oak Ridge Electron Linear Accelerator. The resolution achieved, ΔE/E o...

Radiative capture of fast neutrons by 40Ca

Abstract The radiative capture cross section for the reaction 40 Ca(n, γ) 41 Ca has been measured for the neutron energy range of 6.8–18.2 MeV. A NaI(Tl) anti-Compton spectrometer, a pulsed neutron beam, and time-of-flight techniques were utilized. The γ-ray spectra show strong transitions to the 1 f 7 2 ground state and a relatively high intensity of unresolved γ-rays to a group of levels at about 2 MeV, where two 3 2 − states are known to carry the 2 p 3 2 strength, and to levels around 4 MeV, which is the centroid energy of the 2 p 1 2 strength. Absolute cross sections were determined for these γ-ray transitions as well as the integrated cross section for transitions to bound states of 41 Ca. Only the cross section for the transition to the f 7 2 ground state of 41 Ca shows the resonance behavior predicted by the semidirect (collective) capture theory, but the agreement between the theoretical and measured f 7 2 cross sections is not satisfactory. The measured cross sections for the other γ-ray peaks are almost constant with neutron energy below about 15 MeV, in sharp contrast to the cross sections predicted by the semidirect theory for p 3 2 and p 1 2 transitions.

Inelastic scattering of antiprotons from 12C and 18O at 50 and 180 MeV

Abstract The angular distributions of the inelastic scattering of antiprotons for a few collective states of 12C and 18O have been measured. They exhibit a diffractive pattern, typical of strong absorption. Coupled channel analysis of these data has been carried out. The derived multipole moments agree fairly well with those obtained with other probes. Microscopic calculations performed in the framework of the DWIA formalism, using either the Paris or the Dover-Richard NN interaction are in good agreement with the data for the low-lying collective states. The present status of the experimental study of the unnatural parity levels is discussed.

The radiative capture of fast protons by medium-mass nuclei

Abstract High-energy photon spectra were obtained with moderate resolution for a number of neighboring nuclei: 56 Fe, 59 Co, 58 Ni, 60 Ni, 61 Ni, 63 Cu and 64 Zn in bombardments with protons (8 to 22 MeV). The shapes, magnitudes and energy dependence of the observed spectra are found to be consistent in most of their essential features with the implications of the semidirect model for fast nucleon capture.