R.H. Bartram

The F+ center in reactor-irradiated aluminum oxide*†

Abstract A point-ion calculation has been performed for the F + center in α-Al 2 O 3 (one electron in an O 2− vacancy). Optical transitions are predicted at 2·26, 3·39 and 5·15 eV. Contact hyperfine interactions with the two nearest pairs of Al 3+ ions are calculated to be 151 and 39 G. Single crystals of α-Al 2 O 3 were reactor-irradiated up to doses of 10 20 fast neutrons per cm 2 , and studied by electron spin resonance (ESR). A broad ESR spectrum with 13 resolved components at g =2·0029±0·0005 was interpreted as the interaction of an unpaired electron with two pairs of Al 3+ nuclei with hyperfine constants of 49·2 and 13·5 G. These values are in the same ratio as the values calculated for the F + center, to which this ESR spectrum is attributed. The discrepancy of a factor of three is typical of point-ion calculations. The optical absorption spectrum for heavily-irradiated samples is not available for comparison with calculated transition energies.

Efficiency of electron-hole pair production in scintillators

Abstract Phenomenological models for electron-hole pair production in scintillators are reviewed. All models provide similar results when compared on the basis of consistent definitions. The assumption of constant loss parameter K is untenable. The loss parameter K ( E ) was calculated from first principles by employing a dielectric response function derived from the Callaway model. The rapid variation of K ( E ) suggests replacement of K by an effective threshold displacement, ΔE , which is appreciable for semiconductors but negligible for insulators. The prediction of high conversion efficiency, β ≅ 1.0, implies low transfer efficiency, S

Many‐body perturbation theory electronic structure calculations for the methoxy radical. I. Determination of Jahn–Teller energy surfaces, spin–orbit splitting, and Zeeman effect

Many‐body perturbation theory calculations of the electronic structure are reported for C3v and Jahn–Teller distorted conformations of the methoxy radical CH3O. The Jahn–Teller distortion reduces the energy relative to the minimum energy for the C3v structure by −0.64 kcal/mol. Furthermore, the dynamic Jahn–Teller effect reduces the calculated spin–orbit splitting from 78 to 37 cm−1. An analysis of the Jahn–Teller energy surface yields the e mode vibrational frequencies (ν4 = 2314, ν5 = 1066, ν6 = 792 cm−1) and Coriolis coupling coefficients (ζ4 = 0.065, ζ5 = −0.152, and ζ6 = 0.186) for the ground state. The orbital g factor g0 = 0.647 was calculated and used to determine the components of the g tensor for free methoxy and matrix‐trapped methoxy. For free methoxy, g∥ = 2.645 and g⊥ = 0; for the matrix‐trapped radical, experimental data was used to calculate the splitting 1.7 kcal/mol of the methoxy energy level caused by its site environment. This splitting quenches g∥ to a value of 2.096.

ESR of Exchange Coupled Cr3+ Ions in Phosphate Glass

A study of the ESR spectrum of a phosphate glass heavily doped with Cr2O3 is reported. The spectrum is attributed to exchange coupled Cr3+ ion pairs and is interpreted with reference to a spin Hamiltonian containing isotropic exchange, Zeeman, and fine structure terms. From the temperature dependence of the ESR intensity, it is inferred that the coupling is antiferromagnetic with an average coupling constant J=24±2 cm−1. The effects of fine structure interactions on the lineshape and breadth of the glass spectrum are considered, and the magnitudes of the fine structure parameters are estimated from the observed linewidths and dependence of the lineshape on microwave frequency. It is found that the fine structure constants are distributed in the range from 0.03 to 0.3 cm−1. The majority of the pairs are characterized by values near the lower end of the range, but the higher values are required to account for the absorption in the wings of the line.

Electron traps and transfer efficiency of cerium-doped aluminate scintillators

Abstract Comparative measurements of thermoluminescence and scintillation light outputs of gamma-ray irradiated Ce;LuAlO 3 (LuAP) and Ce:YAlO 3 (YAP) reveal that electron trapping significantly depresses transfer efficiency in these scintillator materials, but fails to explain fully either their performance differential or their departures from ideal efficiency. In the limit of short radiation times, the ratio of integrated thermoluminescence light output to integrated scintillation light output is 0.14 in LuAP and 0.02 in YAP.

Pressure and temperature dependence of chromium photoluminescence spectra in fluoride elpasolites

Abstract Photoluminescence spectra of K 2 NaGaF 6 Cr 3+ and K 2 NaScF 6 :Cr 3+ , measured as functions of hydrostatic pressure, reveal a blue shift of the emission band culminating in a pressure-induced transition from a broad-band fluorescence spectrum to a highly structured phosphorescence spectrum. The blue shift is interpreted in terms of a pressure dependent local compressibility. The high-pressure, low-temperature emission spectrum of K 2 NaScF 6 :Cr 3+ reveals a splitting of the zero-phonon line indicative of a transition to a lower symmetry phase.

Inhomogeneous broadening of the optical spectra of Yb3+ in phosphate glass

Abstract The observed widths and asymmetries of the spectral lines of Yb 3+ in phosphate glass are attributed to systematic site-to-site variations in the crystal field at the rare earth. A model rare earth site consisting of a Yb 3+ ion coordinated by three PO 4 tetrahedra is considered. A compliant mode of distortion of the complex is identified, and it is shown that a distribution of distorted sites can lead to lineshapes which reproduce many of the features of the observed spectra. Using a point-ion model, the crystal field at the rare earth site is calculated as a function of a single configuration coordinate which describes the distortion. The electrostatic energy of the complex is also determined as a function of the configuration coordinate, and the lineshapes are computed from the crystal-field levels assuming a Boltzmann distribution. Because of uncertainty in the structural parameters of the glass, the effect on the lineshapes of variations in the dimension of the complex and in the charge states of the ligands is considered. The crystal-field parameters are scaled to compensate deficiencies in the point-ion approximation, and the effect on the lineshapes of changes in the scale factors is discussed.

Computer modeling of the optical properties of substitutional chromium impurities in halide elpasolites

Abstract Ab initio embedded-cluster molecular-orbital calculations were performed with the MELD program on both ground ( 4 A 2g ) and excited ( 4 T 2g ) electronic states of substitutional Cr 3+ in the halide elpasolites K 2 NaGaF 6 , K 2 NaScF 6 and Cs 2 NaYCl 6 . External interactions of the molecular cluster were represented by pair potentials, and lattice relaxation was accomplished by means of a modification of the HADES lattice-statics program. The calculations account successfully for the pressure dependence of photoluminescence spectra and of vibration frequencies inferred from their vibronic structure.

ESR of N2− in uv‐Irradiated Single Crystals of Anhydrous Barium Azide

Single crystals of anhydrous barium azide have been irradiated at room temperature with 2537 A ultraviolet light. Upon cooling to 77°K, an ESR spectrum is observed, the angular variation of which is expressed by the spin‐Hamiltonian H = βH·g·S + I·A·S with gx≈gy = 1.997 ± 0.001, gz = 1.979 ± 0.0001, Ax = 3.6 ± 0.2 G, Ay = 20.0 ± 0.2 G (see text), Az = 4.1 ± 0.2 G. The spectrum is interpreted in terms of the theoretical predictions of Gelerinter and Silsbee for an N2− molecular ion in a field of rhombic symmetry. The ESR data are used to speculate on the orientation of the azide ions in the Ba(N3)2 lattice, the structure of which is as yet unknown. The optical absorption spectrum of thin single crystals in the range 200‐300 mμ has been measured at room temperature and is correlated with the wavelength threshold for production of the paramagnetic center. It is postulated that the initial step in production of the center is the creation of an excited azide ion by the ultraviolet light.

Scintillation Properties of CsI:Tl Crystals Codoped With

Despite the acknowledged advantages of CsI:Tl for most scintillator applications, its use for CT and other high-speed imaging has been hindered by a high degree of afterglow in its scintillation decay. We have found that a particularly effective way to suppress this afterglow is to codope the material with certain dipositive rare earth ions capable of trapping the vagrant carriers that give rise to it. We have extensively studied the manner in which one such ion, Eu2+, alters the spectroscopic and kinetic properties of the scintillation, and have developed a coherent mathematical model consistent with the experimental results. But the beneficial effect of Eu2+ appears to be restricted only to relatively short times (say les 200 ms ) after the end of the excitation pulse. To be effective at longer times, the codopant should also provide some nonradiative means to annihilate the trapped carriers before their escape can enhance the low-level long-term emission. And, as predicted by the model, this is exactly what Sm2+ does. In this paper we describe the experimental effort to characterize the behavior of the CsI:Tl, Sm material system. Spectroscopically, we find that the familiar broad Tl emission becomes distorted and progressively shifted to shorter wavelengths. Kinetically, we find that the afterglow of the emission is substantially reduced relative to conventional CsI:Tl (no codopant), and that this effect is always found, regardless of the conditions of excitation. And finally, we find that the material shows virtually no memory of its previous excitation history (the so-called hysteresis phenomenon), in stark contrast with both conventional CsI:Tl and the corresponding material codoped with Eu. Various aspects of these effects and their dependence on the concentrations of the dopants are also discussed.