S.A. Payne

Scintillators With Potential to Supersede Lanthanum Bromide

New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI2, CaI2, SrBr2, BaI2 and BaBr2-SrI2 is the most promising. SrI2(Eu) emits into the Eu2+ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.

Crystal Growth and Scintillation Properties of Strontium Iodide Scintillators

Single crystals of SrI₂:Eu and SrI₂:Ce/Na were grown from anhydrous iodides by the vertical Bridgman technique in evacuated silica ampoules. Growth rates were of the order of 5-30 mm/day. Radioluminescence spectra of SrI₂:Eu and SrI₂:Ce/Na exhibit a broad band due to Eu²⁺ and Ce³⁺ emission, respectively. The maximum in the luminescence spectrum of SrI₂:Eu is found at 435 nm. The spectrum of SrI₂:Ce/Na exhibits a doublet peaking at 404 and 435 nm attributed to Ce³⁺ emission, while additional impurity-or defect-related emission is present at approximately 525 nm. The strontium iodide scintillators show very high light yields of up to 120 000 photons/MeV, have energy resolutions down to 3% at 662 keV (Full Width Half Maximum) and exhibit excellent light yield proportionality with a standard deviation of less than 5% between 6 and 460 keV.

New laser crystals based on KPb2Cl5 for IR region

Abstract The KPb2Cl5 single crystals, doped by different Rare Earth ions were grown. The RE segregation coefficient was found to decrease from 1.0 to 0.15 in the set from Nd to Yb. Spectroscopic properties and luminescence decay were studied, stimulated emission was demonstrated at 1.06 μm (Nd3+) and at 2.43 μm (Dy3+).

Transparent ceramic scintillators for gamma spectroscopy and radiography

Transparent ceramics combine the scintillation performance of single crystals with the ruggedness and processability of glass. We have developed a versatile, scaleable fabrication method, wherein nanoparticle feedstock is consolidated at temperatures well below melting to form inch-scale phase-pure transparent ceramics with optical scatter of α <0.1 cm-1. We have fabricated Cerium-doped Gadolinium Garnets with light yields of ~50,000 Ph/MeV and energy resolution of <5% at 662 keV. We have also developed methods to form sheets of the high-Z ceramic scintillator, Europium-doped Lutetium Oxide Bixbyite, producing ~75,000 Ph/MeV for radiographic imaging applications.

Design of a Facility for Measuring Scintillator Non-Proportionality

While the original Compton coincidence technique provided accurate measurements of electron response in scintillators, the data rate was low and measurements took weeks. We present the conceptual design for a high throughput version that is predicted to collect data at 65 cps, reducing measurement times from weeks to hours. In this design, a collimated 1 mCi 137Cs source will illuminate the scintillator sample from a distance of 18 cm and 5 high-purity germanium (HPGe) detectors placed 10 cm from the scintillator will measure the energy of the scattered gamma ray. The source can be placed in either of two positions spaced 15deg apart, allowing relatively uniform scattering angle coverage from 0deg to 146deg, corresponding to electron energies in the scintillator from 0 to 466 keV. The scintillator will be coupled to a hybrid photodetector (HPD), which has extremely linear response, and the HPDs ability to resolve single photoelectrons provides a built-in calibration mechanism. The output of each HPGe detector and the HPD will be digitized with a free-running 12-bit, 200 MHz ADC, providing accurate measurement of the signal amplitudes and the ability to measure the electron response for different temporal components of the scintillator signals. The facility will be located at Lawrence Livermore National Laboratory (LLNL) and is intended to be made available to the community at large. The goals are to facilitate scintillator development and to understand the nature of the light-yield non-proportionality and its effect on the energy resolution.

Room-temperature laser action at 4.3–4.4 µm in CaGa 2 S 4 :Dy 3+

We report room-temperature mid-IR laser operation in a new low-phonon-frequency nonhygroscopic host crystal, calcium thiogallate (CaGa(2)S(4)) . Laser action at 4.314.38 mum on the Dy(3+)H(11/2)(6)?H(13/2)(6) transition occurred with a maximum slope efficiency of 1.6%.

Bismuth-loaded plastic scintillators for gamma-ray spectroscopy

Exemplary embodiments of several new metal-loaded plastic scintillators are reported herein, comprising sterically and electronically isolated organotin additive complexes. Distance-dependent quenching relationships have been used as design criteria for the selection and synthesis of these organometallic additives, resulting in increased light yields and improved gamma-ray energy resolution values relative to previously reported PS/PVT examples. Fast scintillation decay properties have also been characterized in the prepared scintillators, rivaling the kinetics of stilbene-doped bibenzyl and BC-422Q-1% while providing higher light yields than these reference materials.

Nonlinear optical properties of LaCa(4)O(BO(3))(3).

We have grown LaCa(4)O (BO(3))(3) (LaCOB), an isostructural member of GdCa(4)O(BO(3))(3) (GdCOB) family and characterized its nonlinear optical properties. At 1064nm, d(eff) of 0.52+/-0.05 pm /V and an angular sensitivity of 1224+/-184(cm rad)(-1) for type I frequency doubling in LaCOB were determined relative to those of KTiOPO(4) , beta-BaB(2)O(4) , KD(2)PO(4) , LiB(3)O(5) , YCa(4)O(BO(3))(3) (YCOB), and GdCOB. The d(alphabetabeta) and d(gammabetabeta) coefficients of the nonlinear optical tensor for LaCOB, GdCOB, and YCOB were determined to be equivalent within the experimental uncertainty and have values of ?0.26+/-0.04?pm/V and ?1.69+/-0.17?pm /V , respectively. From phase-matching angle measurements at 1064 and 1047nm, we predict that LaCOB is noncritically phase matched at 1042+/-1.5 nm .

The Origins of Scintillator Non-Proportionality

Recent years have seen significant advances in both theoretically understanding and mathematically modeling the underlying causes of scintillator non-proportionality. The core cause is that the interaction of radiation with matter invariably leads to a non-uniform ionization density in the scintillator, coupled with the fact that the light yield depends on the ionization density. The mechanisms that lead to the luminescence dependence on ionization density are incompletely understood, but several important features have been identified, notably Auger-like processes (where two carriers of excitation interact with each other, causing one to de-excite non-radiatively), the inability of excitation carriers to recombine (caused either by trapping or physical separation), and the carrier mobility. This paper reviews the present understanding of the fundamental origins of scintillator non-proportionality, specifically the various theories that have been used to explain non-proportionality.

Diode arrays, crystals, and thermal management for solid-state lasers

We summarize our efforts in the development of solid-state lasers, including the laser diode arrays, pump light delivery, approaches to thermal management, and novel gain media. Our interests are in developing unique solid-state lasers, including those operating at higher powers, offering less common wavelengths, and having other specialized features. In this paper, we discuss high-power Tm:YAG and Yb:YAG lasers. The gas cooled slab laser concept using Yb:S-FAP, and side-pumped Er:YAG and Cr:ZnSe lasers. We address the optical and thermal physics of these systems and also mention several additional gain media that have the potential of offering unique performance characteristics: Ce:LiSAF, APG-2 laser glass, Dy:LaCl/sub 3/, and Yb:BCBF.