D.G. Richmond

Structure of intermediate states in the photoexcitation of the 89Y isomer

Abstract Resonant photon scattering off 89Y was investigated in a measurement of the 89Ym excitation function for bremsstrahlung endpoint energies E0 = 2−5 MeV and in a nuclear-resonance-fluorescence experiment with E0 = 5 MeV. The results are compared to a quasiparticle-phonon model calculation. Besides a well-known single-particle M1 transition at low energies, the photoexcitation spectrum is governed by transitions to states built by coupling of the dominant p 1 2 hole ground-state configuration to collective quadrupole phonons in the neighbouring 90Zr. The detailed decay cascade to the isomer reproduces the experimental finding of only two intermediate states with about equal strength and explains the suppression of other possible transitions due to the nature of the particular E1 matrix element. The theoretical isomer branching ratios are small compared to the experiment, but depend critically on details of the model.

Resonant photoexcitation of isomers. 115Inm as a test case

Abstract Large integrated cross sections for the photoexcitation of 115 In m have been detected around E x =3MeV. Complementary nuclear resonance fluorescence studies prove that the g.s. transition strength is largely confined to the same energy region and the intermediate excited states are identified. Unified-model calculations indicate magnetic dipole lg 9 2 → lg 7 2 spin-flip transitions as the major excitation mechanism.

Absolute measurement of spatial and spectral characteristics of bremsstrahlung using the photoexcitation of nuclear isomers

A method of obtaining absolute, direct measurements of the spatial and spectral characteristics of bremsstrahlung is discussed. This technique, called x‐ray activation of nuclei (XAN) is based on the use of well‐known photonuclear reactions which populate long‐lived nuclear isomers. These populations sample incident photon continua at discrete excitation energies and effectively store this information for convenient retrieval following the irradiation of gram‐sized targets. Recently a series of experiments has been conducted which has significantly expanded the available data for the photoexcitation of a wide range of isomers at higher energies. Thus it has become feasible to extend the use of XAN to energies approaching 4 MeV. The utility of this technique is demonstrated by the characterization of bremsstrahlung from the newly installed research linac at the University of Texas at Dallas.

Status and issues in the development of a γ-ray laser. II: Giant resonances for the pumping of nuclei

A γ-ray laser would stimulate the emission of radiation of wavelengths below 1 A from excited states of nuclei. However, the anticipation of a need for high pump powers tended to discourage early research and the difficulties in demonstrating a device were first assumed to be insurmountable. Over the past decade, advances in pulsed-power technology have changed these perceptions and studies have built a strong momentum. A nuclear analog of the ruby laser has been proposed and many of the component steps for pumping the nuclei have been demonstrated experimentally. A quantitative model based upon the new data and concepts has shown the γ-ray laser to be feasible if some real isotope has its properties sufficiently close to the ideals modeled. The greatest positive impact upon feasibility has come from the discovery of giant resonances for pumping nuclei that greatly reduce the levels of pump power needed.

Status and Issues in the Development of a Gamma-Ray Laser

A gamma-ray laser would stimulate the emission of radiation at wavelengths below 1 A from excited states of nuclei. However, the difficulties in realizing such a device were considered insurmountable when the first cycle of study ended in 1981. Nevertheless, research on the feasibility of a gamma-ray laser has taken a completely new character since then. A nuclear analog of the ruby laser has been proposed and many of the component steps for pumping the nuclei have been demonstrated experimentally. A quantitative model based upon the new data and concepts of this decade shows the gamma-ray laser to be feasible if some real isotope has its properties sufficiently close to the ideals modeled.