Misha Klugerman

LaBr/sub 3/:Ce scintillators for gamma ray spectroscopy

In this paper, we report on a relatively new scintillator - LaBr/sub 3/ for gamma ray spectroscopy. Crystals of this scintillator have been grown using Bridgman process. This material when doped with cerium has high light output (/spl sim/60,000 photons/MeV) and fast principal decay constant (/spl les/25 ns). Furthermore, it shows excellent energy resolution for /spl gamma/-ray detection. Energy resolution of 3.2% (FWHM) has been achieved for 662 keV photons (/sup 137/Cs source) at room temperature. High timing resolution (260 ps FWHM) has been recorded with LaBr/sub 3/-PMT and BaF/sub 2/-PMT detectors operating in coincidence mode using 511 keV positron annihilation /spl gamma/-ray pairs. Details of its scintillation properties, and variation of these properties with changing cerium concentration are reported. Potential applications of this material are also addressed.

Segmented crystalline scintillators: An initial investigation of high quantum efficiency detectors for megavoltage x‐ray imaging

Electronic portal imaging devices (EPIDs) based on indirect detection, active matrix flat panel imagers (AMFPIs) have become the technology of choice for geometric verification of patient localization and dose delivery in external beam radiotherapy. However, current AMFPI EPIDs, which are based on powdered-phosphor screens, make use of only approximately 2% of the incident radiation, thus severely limiting their imaging performance as quantified by the detective quantum efficiency (DQE) (approximately 1%, compared to approximately 75% for kilovoltage AMFPIs). With the rapidly increasing adoption of image-guided techniques in virtually every aspect of radiotherapy, there exist strong incentives to develop high-DQE megavoltage x-ray imagers, capable of providing soft-tissue contrast at very low doses in megavoltage tomographic and, potentially, projection imaging. In this work we present a systematic theoretical and preliminary empirical evaluation of a promising, high-quantum-efficiency, megavoltage x-ray detector design based on a two-dimensional matrix of thick, optically isolated, crystalline scintillator elements. The detector is coupled with an indirect detection-based active matrix array, with the center-to-center spacing of the crystalline elements chosen to match the pitch of the underlying array pixels. Such a design enables the utilization of a significantly larger fraction of the incident radiation (up to 80% for a 6 MV beam), through increases in the thickness of the crystalline elements, without loss of spatial resolution due to the spread of optical photons. Radiation damage studies were performed on test samples of two candidate scintillator materials, CsI(Tl) and BGO, under conditions relevant to radiotherapy imaging. A detailed Monte Carlo-based study was performed in order to examine the signal, spatial spreading, and noise properties of the absorbed energy for several segmented detector configurations. Parameters studied included scintillator material, septal wall material, detector thickness, and the thickness of the septal walls. The results of the Monte Carlo simulations were used to estimate the upper limits of the modulation transfer function, noise power spectrum and the DQE for a select number of configurations. An exploratory, small-area prototype segmented detector was fabricated by infusing crystalline CsI(Tl) in a 2 mm thick tungsten matrix, and the signal response was measured under radiotherapy imaging conditions. Results from the radiation damage studies showed that both CsI(Tl) and BGO exhibited less than approximately 15% reduction in light output after 2500 cGy equivalent dose. The prototype CsI(Tl) segmented detector exhibited high uniformity, but a lower-than-expected magnitude of signal response. Finally, results from Monte Carlo studies strongly indicate that high scintillator-fill-factor configurations, incorporating high-density scintillator and septal wall materials, could achieve up to 50 times higher DQE compared to current AMFPI EPIDs.

Characterization of polycrystalline TlBr films for radiographic detectors

Vapor deposited films of thallium bromide are evaluated as potential photoconductive layers in new large-area radiographic detectors. The attractiveness of the material lies in its inherent high effective atomic number and high density. Polycrystalline films up to 200 /spl mu/m have been grown and show a columnar structure with grains reaching 100 /spl mu/m in diameter. Current-voltage (IV) tests indicate a bulk resistivity of 10/sup 9/-10/sup 10/ /spl Omega//spl middot/cm, limited by ionic conduction. The instability of current with time is also observed, but it can be minimized with cooling. The films demonstrate high gain at relatively low field strengths as compared to other photoconductive layers. Benefits and drawbacks of TIBr are compared to other materials, and possible solutions are discussed.

PbI2 for high-resolution digital x-ray imaging

In this paper, we discuss recent progress that has been made in the development of high resolution X-ray imaging detectors using photoconducting films of lead iodide (PbI2). PbI2 is a wide bandgap semiconductor with high X- ray stopping efficiency. We have been investigating thick films of lead iodide which can be prepared in large areas in a cost effective manner. These films can be coupled to readout technologies such as amorphous silicon flat panel arrays and vidicon tubes to produce X-ray imaging detectors for applications such as mammography, fluoroscopy, X-ray diffraction and non-destructive evaluation. Recent results obtained when these PbI2 films are coupled to 512 X 512 flat panel a-Si:H array are reported. This includes dark current, signal and resolution measurements. Properties of lead iodide films which are relevant to imager performance are also discussed.

Large area x-ray image sensing using a Pbl2 photoconductor

We report the fabrication and evaluation of a Pbl2 imager using large area amorphous silicon technology. This approach uses a thick Pbl2 x-ray photoconductor to absorb x-rays and collect ionization charge under the action of an applied field, while amorphous silicon thin film transistors (TFT) provide a matrix-addressed read out of the signal to external electronics. The x-ray sensitivity of Pbl2 is high, and mobility-lifetime product is large enough to yield a high charge collection at low applied fields. The test arrays used to evaluate Pbl2 have 256 X 256 pixels of size 200 microns. Each pixel contains an amorphous silicon switching transistor, gate and data addressing lines, a charge storage capacitor and a metal pad to contact the Pbl2 layer. Early evaluation of the image sensor indicates the promise of Pbl2 but indicates that reduction of the leakage current is important.

RbGd/sub 2/Br/sub 7/:Ce scintillators for gamma-ray and thermal neutron detection

In this paper, we report on gamma ray and thermal neutron detection with RbGd2Br7:Ce scintillators. RbGd2Br7:Ce (RGB) is a new scintillator material, which shows high light output (56,000 photons/MeV) and has a fast principal decay constant (45 ns) when doped with 10 percent Ce. These properties make RGB an attractive scintillator for g-ray detection. Also, due to the presence of Gd as a constituent, RGB has a high cross section for thermal neutron absorption and can achieve close to 100 percent stopping efficiency with 0.5 mm thick RGB crystals. Crystals of RGB with three different Ce concentrations (0.1, 5, and 10 percent) have been grown and their basic scintillation properties such as light output, decay time, and emission spectrum have been measured. In addition, high efficiency thermal neutron detection has been confirmed in our studies.

Evaluation of CZT detectors with capacitive Frisch grid structure

Recent results have shown that capacitive Frisch grid structures significantly improve spectroscopic performance of planar CZT detectors especially at higher energies. This paper presents results obtained with larger detectors than those previously reported on. Devices with various aspect ratios and grid length-to-device thickness ratios were fabricated and evaluated. A FWHM energy resolution of approximately 2% at 662 keV was obtained for a device with dimensions of 5 mm x 5 mm x 9.2 mm.

Scintillator energy and flux linearity for RbGd/sub 2/Br/sub 7/:Ce, LaCl/sub 3/:Ce, and LaBr/sub 3/:Ce

The development of scintillator detectors made of dense ionic crystals with high light output and fast response times offers to improve radiation detection systems commonly employed in medical imaging. Three such cerium doped materials, rubidium gadolinium bromide (RbGd/sub 2/Br/sub 7/:Ce), lanthanum chloride (LaCl/sub 3/:Ce), and lanthanum bromide (LaBr/sub 3/:Ce) show promise as bright, fast scintillators with good energy resolution for gamma rays. To be useful as gamma-ray detectors for positron emission tomography (PET) and single photon emission computed tomography (SPECT) applications, the signal from these scintillators should have a linear energy response. To be useful as x-ray detectors in computed tomography (CT) applications, the output of these scintillators must be linear over a wide range of flux rates. We have therefor measured the response of these scintillators to different energy gamma-rays with a detector operated in pulse counting mode. We have also measured the range of linear response to diagnostic x-rays, of these materials, with a detector operated in current mode with a sigma-delta analog to digital converter (ADC). We find a linear energy response for all three materials from 60 keV to 662 keV. We find a linear x-ray flux response to five decades of flux for RbGd/sub 2/Br/sub 7/:Ce and four decades of flux for LaCl/sub 3/:Ce and LaBr/sub 3/:Ce. The current in the photodiode is proportional to the x-ray flux in the scintillators provided that direct interactions between x-rays and the photodiode are subtracted. This is accomplished by measuring the photodiode current with and without optical opaque tape between the scintillator and photodiode. Coupling dense scintillators to radiation hard solid state detectors may allow for configurations useful for both low flux gamma-ray and high flux x-ray detection which could improve dual-modality imaging techniques such as combined CT/SPECT and CT/PET.