Woon-Seng Choong

Strontium and barium iodide high light yield scintillators

Europium-doped strontium and barium iodide are found to be readily growable by the Bridgman method and to produce high scintillation light yields.

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.

Nonproportionality of Scintillator Detectors: Theory and Experiment. II

We report measurements of electron response of scintillators, including data on 29 halides, oxides, organics, and fluorides. We model the data based on combining the theories of: Onsager, to account for formation of excitons and excited activators; Birks, to allow for exciton-exciton annihilation; Bethe-Bloch, to relate electron stopping to its energy; and Landau, to describe how fluctuations in the linear energy deposited (dE/dx) lead to nonproportionalitys contribution to resolution. In general there is satisfactory agreement with experiment, in terms of fitting the electron response data and reproducing the literature values of resolution. We find that the electron response curve shapes are more affected by the host lattice than by the activator or its concentration.

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.

Scintillator Non-Proportionality: Present Understanding and Future Challenges

Scintillator non-proportionality (the fact that the conversion factor between the energy deposited in a scintillator and the number of visible photons produced is not constant) has been studied both experimentally and theoretically for ~50 years. Early research centered on the dependence of the conversion factor on the species of the ionizing radiation (gamma, alpha, beta, proton, etc.), and researchers during the 1960s discovered a strong correlation between the scintillation efficiency and the ionization density. In more recent years, non-proportionality has been proposed as the reason why the energy resolution of most scintillators is worse than that predicted by counting statistics. While much progress has been made, there are still major gaps in our understanding of both the fundamental causes of non-proportionality and their quantitative link to scintillator energy resolution. This paper summarizes the present state of knowledge on the nature of the light-yield non-proportionality and its effect on energy resolution.

Evidence for the decay sigma+ --> pmu+ mu-.

We report the first evidence for the decay Sigma(+)-->pmu(+)mu(-) from data taken by the HyperCP (E871) experiment at Fermilab. Based on three observed events, the branching ratio is B(Sigma(+)-->pmu(+)mu(-))=[8.6(+6.6)(-5.4)(stat)+/-5.5(syst)]x10(-8). The narrow range of dimuon masses may indicate that the decay proceeds via a neutral intermediate state, Sigma(+)-->pP(0),P0-->mu(+)mu(-) with a P0 mass of 214.3+/-0.5 MeV/c(2) and branching ratio B(Sigma(+)-->pP(0),P0-->mu(+)mu(-))=[3.1(+2.4)(-1.9)(stat)+/-1.5(syst)]x10(-8).

The timing resolution of scintillation-detector systems: Monte Carlo analysis

Recent advancements in fast scintillating materials and fast photomultiplier tubes (PMTs) have stimulated renewed interest in time-of-flight (TOF) positron emission tomography (PET). It is well known that the improvement in the timing resolution in PET can significantly reduce the noise variance in the reconstructed image resulting in improved image quality. In order to evaluate the timing performance of scintillation detectors used in TOF PET, we use Monte Carlo analysis to model the physical processes (crystal geometry, crystal surface finish, scintillator rise time, scintillator decay time, photoelectron yield, PMT transit time spread, PMT single-electron response, amplifier response and time pick-off method) that can contribute to the timing resolution of scintillation-detector systems. In the Monte Carlo analysis, the photoelectron emissions are modeled by a rate function, which is used to generate the photoelectron time points. The rate function, which is simulated using Geant4, represents the combined intrinsic light emissions of the scintillator and the subsequent light transport through the crystal. The PMT output signal is determined by the superposition of the PMT single-electron response resulting from the photoelectron emissions. The transit time spread and the single-electron gain variation of the PMT are modeled in the analysis. Three practical time pick-off methods are considered in the analysis. Statistically, the best timing resolution is achieved with the first photoelectron timing. The calculated timing resolution suggests that a leading edge discriminator gives better timing performance than a constant fraction discriminator and produces comparable results when a two-threshold or three-threshold discriminator is used. For a typical PMT, the effect of detector noise on the timing resolution is negligible. The calculated timing resolution is found to improve with increasing mean photoelectron yield, decreasing scintillator decay time and decreasing transit time spread. However, only substantial improvement in the timing resolution is obtained with improved transit time spread if the first photoelectron timing is less than the transit time spread. While the calculated timing performance does not seem to be affected by the pixel size of the crystal, it improves for an etched crystal compared to a polished crystal. In addition, the calculated timing resolution degrades with increasing crystal length. These observations can be explained by studying the initial photoelectron rate. Experimental measurements provide reasonably good agreement with the calculated timing resolution. The Monte Carlo analysis developed in this work will allow us to optimize the scintillation detectors for timing and to understand the physical factors limiting their performance.

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.

Performance of a Facility for Measuring Scintillator Non-Proportionality

We have constructed a second-generation Compton coincidence instrument, known as the Scintillator Light Yield Non-proportionality Characterization Instrument (SLYNCI), to characterize the electron response of scintillating materials. While the SLYNCI design includes more and higher efficiency HPGe detectors than the original apparatus (five 25%-30% detectors versus one 10% detector), the most novel feature is that no collimator is placed in front of the HPGe detectors. Because of these improvements, the SLYNCI data collection rate is over 30 times higher than the original instrument. In this paper, we present a validation study of this instrument, reporting on the hardware implementation, calibration, and performance. We discuss the analysis method and present measurements of the electron response of two different NaI:Tl samples. We also discuss the systematic errors of the measurement, especially those that are unique to SLYNCI. We find that the apparatus is very stable, but that careful attention must be paid to the energy calibration of the HPGe detectors.

Characterization of the LBNL PEM camera

We present the tomographic images and performance measurements of the LBNL positron emission mammography (PEM) camera, a specially designed positron emission tomography (PET) camera that utilizes PET detector modules with depth of interaction measurement capability to achieve both high sensitivity and high resolution for breast cancer detection. The camera currently consists of 24 detector modules positioned as four detector banks to cover a rectangular patient port that is 8.2/spl times/6 cm/sup 2/ with a 5 cm axial extent. Each LBNL PEM detector module consists of 64 3/spl times/3/spl times/30 mm/sup 3/ LSO crystals coupled to a single photomultiplier tube (PMT) and an 8/spl times/8 silicon photodiode array (PD). The PMT provides accurate timing, the PD identifies the crystal of interaction, the sum of the PD and PMT signals (PD+PMT) provides the total energy, and the PD/(PD+PMT) ratio determines the depth of interaction. The performance of the camera has been evaluated by imaging various phantoms. The full-width-at-half-maximum (FWHM) spatial resolution changes slightly from 1.9 mm to 2.1 mm when measured at the center and corner of the field of the view, respectively, using a 6 ns coincidence timing window and a 300-750 keV energy window. With the same setup, the peak sensitivity of the camera is 1.83 kcps//spl mu/Ci.