Evans Hayward

The nuclear photoeffect in holmium and erbium

Abstract The photoneutron yield and photon scattering cross sections have been measured in the giant resonance region for holmium and erbium. Both cross sections were found to be the same within the experimental uncertainties for the two target materials. The absorption cross section integrated to 23 MeV is 3.20±0.48 MeV·b. The intrinsic nuclear quadrupole moment determined from the giant resonance splitting is 7.6±1.1 b. The observed scattering cross sections were large compared to that calculated from the observed absorption cross section by means of the optical theorem and the dispersion relation. The additional contribution to the scattering cross sections results from the optical anisotropy of these nuclei and can be predicted quantitatively from the expression for the scattering by a three-dimensional classical oscillator. A justification for the classical formulation is given. The data also suggest that in the energy region between 10 and 23 MeV at least 80% of the integrated absorption cross section is associated with charge oscillations along the three spatially correlated nuclear axes.

Photoneutron production by Li6, Li7, B10, B11 and O16

Abstract The neutron emission cross sections have been measured from targets of Li 6 , Li 7 , B 10 , B 11 and O 16 when they were irradiated with X-rays in the energy range from 7 to almost 30 MeV. The integrated cross sections are respectively 0.40±0.03, 0.49±04, 0.44±0.32, 0.42±0.03 and 0.26±0.02 in units of the classical dipole sum. The average cross sections for the lithium and boron targets appear to be essentially constant in the energy range 25–30 MeV. The data also suggest that the boron cross sections contain a great deal of structure up to the highest energies studied.

The photodisintegration of bismuth and the lead isotopes

The photodisintegration of bismuth and the lead isotopes is studied by measuring the neutron emission cross sections. The lead targets are enriched in Pb/sup 206/, Pb/sup 207/, and Pb/sup 208/. The cross sections obtarned for all four isotopes are very similar. They peak near 13.8 Mev, have cross sections integrated to 18.5 Mev of 3.93 Mev x b and display some structure. The structure is shown to agree with the results of calculation. Below the ( gamma ,n) threshold the elastic scatterrng cross sections are measured for lead, radio- lead, and brsmuth. Self-absorption experiments are also performed. The analysis of these data shows that the participating levels have widths of a few volts. An experiment at liquid nitrogen temperature indicated that the levels in Pb/sup 206/ are narrower than those in the other two target materials. (auth)

The Backscattering of the Co60 Gamma Rays from Infinite Media

The radiation backscattered by semi‐infinite slabs of wood and steel wool, when irradiated by the Co60 gamma rays, has been investigated by means of a sodium iodide scintillation spectrometer. The main contributions to the scattered radiation are produced in one or two Compton collisions. However, the radiation that has suffered so many collisions as to have an isotropic angular distribution has also been observed.

Some experiences using a positron annihilation beam

Abstract The NBS positron annihilation-in-flight facility is described, along with the procedure for the calibration of the positron beam energy. Details are also given of a large NaI(Tl) spectrometer used with the annihilation photon beam. Response functions of this spectrometer for photons of 15 and 31 MeV have been measured using a photon tagging scheme based on the detection of the annihilation photons in coincidence. The variation in response function caused by the presence of absorbers in front of the spectrometer has been investigated. The response function measured at 15 MeV is compared with that obtained by resonance fluorescence of the 15.1 MeV level in 12C and the branching ratios for the decay of the 15.1 MeV level to the ground state and the excited states at 4.44 and 7.66 MeV have been extracted.

The energy dependence of polarization observables in the 2H(d,γ)4He reaction

Abstract Measurements of the tensor and vector analyzing powers, Ayy(130°) and Ay(130°), have been obtained for the 2H(d,γ )4He reaction for energies ranging from Ed(lab) = 0.3 MeV to Ed(lab) = 50 MeV. The Ayy(130°) data are sensitive to the D-state present in the ground state of 4He and are observed to have their maximum value near Ed = 30 MeV. The vector analyzing power data show a maximum near Ed = 3 MeV. The data are compared to the results of a microscopic multi-channel resonating group model calculation.

Electrodisintegration ofNi58,Ni60, andNi62

The (e,p) and (e,..cap alpha..) cross sections for targets of /sup 58/Ni, /sup 60/Ni, and /sup 62/Ni have been measured in the electron energy range 16 --100 MeV. They have been analyzed using the distorted-wave Born approximation E1 and E2 virtual photon spectra. Protons are emitted primarily following E1 absorption but ..cap alpha..-emission results from a combination of E1 and E2 absorption.

The absorption and scattering of photons by the Δ resonance

Recently, the experiment on the total photonuclear absorption cross sections have been extended to encompass the Δ resonance in complex nuclei. These important experiments involve at least four different techniques and have been performed in European Laboratories. These results are compared with the total cross sections measured in the giant resonance region and extending up to the meson threshold. The Gell‐Mann‐Goldberger‐Thirring sum rule provides a connection between the absorption cross sections in these two energy regions and the photopion cross sections of the nucleon.The total photonuclear absorption cross sections are related to the forward coherent scattering cross sections through the optical thoerem and dispersion relation. At backward angles, where measurements are possible, the scattering cross sections are strongly depressed by a form factor. The experimental cross sections do, however, exceed the prediction of a simple model.

Isovector Giant Quadrupole Resonance in (p↘,γ)

The 30Si(p↘,γ)31P reaction has been studied in the energy range Ep=20–36 MeV. A transition matrix element analysis of σ(θ) and σ(θ)Ay(θ ) at Ep=25.5 MeV reveals substantial E2 strength (σE2/σtot∼26%) in the (p,γ1) channel, in excess of a direct E2 capture estimate (∼7%). The energy dependence of Ay(90°) for γ1 shows a resonance structure which can be reproduced by a direct‐semidirect calculation including an E2 resonance (EIVGQR=38.6 MeV, ΓIVGQR=5.0 MeV, SIVGQR=50%) at the expected peak of the isovector giant quadrupole resonance built on the first excited state of 31P. Angular distributions of σ(θ) and σ(θ)Ay(θ) have also been obtained from the 89Y(p↘,γ)90Zr reaction at Ep=22.5 MeV. A preliminary analysis reveals large E2 contributions (∼28%) for the γ0 transition, as well as possible E3 strength.

On the polarizability of the proton

Abstract The relationship between the static polarizability of the proton and the electric and magnetic polarizabilities obtained from low-energy photon scattering is discussed. It is shown that a consistent picture emerges if one associates the static polarizability with the imaginary part of the forward scattering amplitude and the photon scattering cross section with the real part.