Vaibhav Bora

Numerical comparison of grid pattern diffraction effects through measurement and modeling with OptiScan software

Coatings of various metalized patterns are used for heating and electromagnetic interference (EMI) shielding applications. Previous work has focused on macro differences between different types of grids, and has shown good correlation between measurements and analyses of grid diffraction. To advance this work, we have utilized the University of Arizonas OptiScan software, which has been optimized for this application by using the Babinet Principle. When operating on an appropriate computer system, this algorithm produces results hundreds of times faster than standard Fourier-based methods, and allows realistic cases to be modeled for the first time. By using previously published derivations by Exotic Electro-Optics, we compare diffraction performance of repeating and randomized grid patterns with equivalent sheet resistance using numerical performance metrics. Grid patterns of each type are printed on optical substrates and measured energy is compared against modeled energy.

Continuous Phoswich™ detector for molecular imaging

We report on the development of a novel scintillator in which the decay time of its light emission varies continuously with the depth of an interaction in the crystal. The depth-of-interaction (DOI) information is thus encoded in the signal timing, which can be used to localize the position of the gamma interaction within the scintillator with high accuracy. This concept relies on the fact that decay times in certain scintillators vary considerably with the amount of dopant concentration. We are exploiting this property to create scintillators in which dopant concentration varies continuously and monotonically with depth in the crystals. Synthesis of such structures is accomplished using a specialized vapor deposition technique, which provides us with the control to vary the dopant concentration in the crystal during growth. Our technique also provides a reliable and cost-effective means to synthesize this seemingly complex structure in the large physical volumes required to provide the high absorption efficiency and large sensor areas required for PET and SPECT imaging, respectively. To date we have produced Continuous Phoswich™ scintillator (CPS™) structures measuring up to 7 cm in diameter and approaching 1 cm in thickness using cerium-doped lanthanum chloride (LaCl3:Ce). Controlled vapor deposition is used to create a Ce3+ concentration gradient of 1% to 30% over the specimen thickness. This paper discusses the fabrication and characterization of CPS LaCl3:Ce scintillators and Continuous Phoswich detectors (CPD™), and illustrates the continuous DOI capability of the CPS LaCl3:Ce/PMT detector.

Impact of the Fano Factor on Position and Energy Estimation in Scintillation Detectors

The Fano factor for an integer-valued random variable is defined as the ratio of its variance to its mean. Light from various scintillation crystals have been reported to have Fano factors from sub-Poisson (Fano factor ) to super-Poisson (Fano factor ). For a given mean, a smaller Fano factor implies a smaller variance and thus less noise. We investigated if lower noise in the scintillation light will result in better spatial and energy resolutions. The impact of Fano factor on the estimation of position of interaction and energy deposited in simple gamma-camera geometries is estimated by two methods - calculating the Cramér-Rao bound and estimating the variance of a maximum likelihood estimator. The methods are consistent with each other and indicate that when estimating the position of interaction and energy deposited by a gamma-ray photon, the Fano factor of a scintillator does not affect the spatial resolution. A smaller Fano factor results in a better energy resolution.

Estimation of Fano factors in inorganic scintillators

The correlation between outputs of two photodetectors on opposite faces of a scintillator crystal was used to deduce the Fano factor of the photoelectrons and the photons of scintillation light for different scintillators. The Fano factor of an integer-valued random process is defined as the ratio of its variance to its mean. A Poisson process has a Fano factor of one and a sub-Poisson process has a Fano factor of less than one. Correlation measurements were taken for a number of scintillators including YAP and SrI2, which have excellent energy resolution but very different light outputs. Measurements were also take for CsI:Na. From these measurements, the Fano factors for the photoelectrons as well as the optical photons were estimated.

Preliminary investigation fo the non-linear response of image intensifiers used for gamma-ray imaging

Image intensifiers combined with columnar scintillators have found application in x-ray and gamma-ray, biomedical imaging and other fields. In scintillator imaging, hundreds or thousands of optical photons can illuminate the faceplate of the image intensifier in a small area, essentially simultaneously. This is a situation not found in the typical design application for an image intensifier, night vision or low-light-level imaging. Microchannel plates (MCPs) are known to exhibit gain saturation that could result in non-linear signal response in scintillator imaging, limiting quantitative measurement capabilities. A calibrated LED photon source was developed that can provide a known average number of photons per unit area in a small spot size, similar to that seen due to a gamma-ray interaction in a BazookaSPECT imager. A BazookaSPECT imager is composed of a columnar scintillator and an image intensifier, with output light optically imaged onto a CCD camera. The calibrated source was used to investigate gain-saturation effects for two Proxivision, GmbH image intensifiers, a single-stage BV 2583 EZ and a two stage BV 2583 QZ-V 100N in a BazookaSPECT imaging configuration. No gain saturation was found for the single-stage image intensifier up to more than 100 optical photons per microchannel, but significant gain-saturation non-linearities were measured in the two-stage image intensifier at high gains for >12 optical photons per microchannel. Implications for scintillator imaging using such systems are discussed.

Fano factor of scintillation detectors from photoelectron correlations

In this work we propose a new experimental approach to studying the statistics of the scintillation light from scintillation detectors. Two photodetectors are used for detecting the light output from the two faces of a thin scintillator crystal. From the correlation of the signals from the two photodetectors, the Fano factor Fopt of the scintillation light is determined. Data on several inorganic crystalline scintillators (CsI:Na, LaBr3:Ce, and CdWO4) are presented, and in LaBr3:Ce it is observed that the correlations are negative and Fopt << 1 (sub-Poisson statistics).

Estimation of Fano factor in inorganic scintillators from time correlations

For a given energy deposited, the Fano factor of a scintillator is defined as the ratio of the variance of the number of scintillation photons to the mean number of scintillation photons. Correlations in time between the signals from two photomultiplier tubes collecting light from the same scintillation event were used to estimate the Fano factor of scintillators. At 662 KeV, LaBr3:Ce was found to be sub-Poisson, while YAP:Ce was found to be close to Poisson.

Negative temporal cross covariance in SrI2:Eu

Scintillation light is widely believed to be Poisson or super-Poisson. We tested this hypothesis by measuring the temporal correlation between two detectors detecting scintillation light resulting from the same gamma-ray event in SrI2:Eu . Poisson light is expected to yield zero temporal correlations, while super-Poisson light is expected to yield positive, and sub-Poisson light is expected to yield negative temporal correlation. Scintillation light in SrI2:Eu was found to be negatively correlated. Therefore, we conclude that the scintillation light in SrI2:Eu is sub-Poisson.

Impact of fano factor on position and energy estimation in scintillation detectors

The Fano factor for an integer-valued random variable is defined as the ratio of its variance to its mean. Light from various scintillation crystals has been reported to have Fano factors from sub-Poisson (Fano factor < 1), Poisson (Fano factor = 1) to super-Poisson (Fano factor > 1). For a given mean, a smaller Fano factor implies a smaller variance and thus less noise. We investigate if lower noise in the scintillation light results in better spatial and energy resolution in a scintillation imaging detector. The impact of Fano factor on estimation of position of interaction and energy deposited in simple gamma-camera geometries is estimated by calculating the Cramer-Rao bound. The calculated Cramer-Rao bound is quantitatively validated by estimating the variance of the maximum likelihood estimator.

The uniformity and imaging properties of some new ceramic scintillators

Results are presented of investigations into the composition, uniformity and gamma-ray imaging performance of new ceramic scintillators with synthetic garnet structure. The ceramic scintillators were produced by a process that uses flame pyrolysis to make nanoparticles which are sintered into a ceramic and then compacted by hot isostatic compression into a transparent material. There is concern that the resulting ceramic scintillator might not have the uniformity of composition necessary for use in gamma-ray spectroscopy and gamma-ray imaging. The compositional uniformity of four samples of three ceramic scintillator types (GYGAG:Ce, GLuGAG:Ce and LuAG:Pr) was tested using an electron microprobe. It was found that all samples were uniform in elemental composition to the limit of sensitivity of the microprobe (few tenths of a percent atomic) over distance scales from ~ 1 cm to ~ 1 um. The light yield and energy resolution of all ceramic scintillator samples were mapped with a highly collimated 57Co source (122 keV) and performance was uniform at mapping scale of 0.25 mm. Good imaging performance with single gamma-ray photon detection was demonstrated for all samples using a BazookaSPECT system, and the imaging spatial resolution, measured as the FWHM of a LSF was 150 um.