J. J. Carroll

Absolute calibration of low energy, thick target bremsstrahlung☆

Abstract The applicability of the Monte Carlo program GEANT3 for the calculation of low energy, thick target bremssstrahlung is investigated. The results of GEANT3, version 15, show very good correspondence to those of the well established EGS4 code. In contrast it is shown that GEANT3, version 11, leads to erroneous angular distributions, total intensities and even spectral shapes for angles far from the critical angle. The use of the 115In(γ, γ′)115Inm reaction for an absolute calibration of bremsstrahlung spectra is also demonstrated. This method relies on the reconstruction of the isomer yield excitation function from a set of 115In levels with well characterized properties. It can be utilized for continuous photon spectra up to about 3 MeV.

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.

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.

Nuclear activation techniques for measuring direct and backscattered components of intense bremsstrahlung pulses

Abstract High-voltage electron accelerators used for bremsstrahlung generation can produce intense pulses of radiation with different endpoint energies. The energy spectrum can be changed by varying the charging voltage or by softening the photons with Compton scattering in a low atomic number material. The high dose rate and the flexible spectrum capabilities of the Aurora accelerator have been used to investigate the potential for measuring the bremsstrahlung spectrum by photoactivation of nuclear isomeric states. Recent success in calibrating lower intensity sources has shown that gram-sized targets of isotopes, such as 115In, can be used to sample the incident X-rays at several discrete gateway energies. When irradiated at these energies the targets are excited to metastable states with lifetimes suitable for conventional counting after the flash.

Mössbauer effect measurement of the recoil‐free fraction for 57Fe implanted in a nanophase diamond film

The Mossbauer effect was used to investigate films of nanophase diamond (NPD) implanted with isotopically pure 57Fe at a dose of 5×1016 atoms/cm2 and an energy of 20 keV. When defects and voids created by the implantation were repaired with an overcoating layer of NPD, the recoil‐free fraction at room temperature for these samples was found to be fdia=0.94±0.06 with a corresponding Debye temperature of 1140 K. This relatively high value for f makes NPD films a promising host matrix for microgram quantities of Mossbauer isotopes.

Nuclear photoactivation cross sections for short-lived isomeric states excited with a 6 MeV linac

Photoactivation cross sections for short-lived (T12 < 20 s) isomeric states in 167Er, 79Br, 191Ir, 183W, 197Au, 77Se, 137Ba and 89Y were determined using bremsstrahlung from a 6 MeV medical therapy linac. A portable NaI(Tl) spectrometer and pneumatic transfer system was designed to avoid impacting normal use of the linac. Comparison with data obtained at the PITHON flash X-ray source (1.5 MeV endpoint energy) indicates that cross sections for populating these isomers through gateway states in the range 1.5–6 MeV are more than three orders of magnitude greater than corresponding cross sections below 1.5 MeV. The results are significant for the development of photoactivation techniques to measure short (<100 ns), nonreproducible bremsstrahlung pulses.

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.

Low-energy conversion electron Mössbauer spectroscopy using a chevron microchannel plate detector

A detection system for conversion electron Mossbauer spectroscopy is described. A chevron microchannel plate assembly attached to a two‐stage electrostatic lens is used to preferentially detect electrons with energies ≤15 eV. Mossbauer spectra collected with these electrons can provide information about a variety of solid‐state surface phenomena.