Michael J. Longo

Demonstration of megavoltage and diagnostic x-ray imaging with hydrogenated amorphous silicon arrays.

Flat-panel imagers consisting of the first large area, self-scanning, pixelated, solid-state arrays made with hydrogenated amorphous silicon (a-Si:H) are under development by the authors for applications in diagnostic x-ray and megavoltage radiotherapy imaging. The arrays, designated by the acronym MASDA for multi-element amorphous silicon detector array, consist of a two-dimensional array of a-Si:H photodiodes and thin-film transistors and are used in conjunction with scintillating materials. Imagers utilizing MASDA arrays offer a variety of advantages over existing technologies. This article presents initial megavoltage and diagnostic-quality x-ray images taken with several such arrays including the first examples of anatomical-phantom images. The external readout electronics and imaging techniques required to obtain such images are outlined, the construction, operation, and advantages of the arrays briefly reviewed, and the future potential of this new technology discussed.

Detection of a dipole in the handedness of spiral galaxies with redshifts z∼0.04

Abstract A preference for spiral galaxies in one sector of the sky to be left-handed or right-handed spirals would indicate a parity violating asymmetry in the overall universe and a preferred axis. This study uses 15,158 spiral galaxies with redshifts − 0.0408 ± 0.011 with a probability of occurring by chance of 7.9 × 10 − 4 . A similar asymmetry is seen in the southern Galaxy spin catalog of Iye and Sugai. The axis of the dipole asymmetry lies at approx. ( l , b ) = ( 52 ° , 68.5 ° ) , roughly along that of our Galaxy and close to alignments observed in the WMAP cosmic microwave background distributions. The observed spin correlation extends out to separations ∼ 210 Mpc / h , while spirals with separations 20 Mpc / h have smaller spin correlations.

Development of hydrogenated amorphous silicon sensors for high energy photon radiotherapy imaging

Measurements with a-Si:H n-i-p photodiodes with sensitive areas of approximately 0.6 mm/sup 2/ exposed to a 6 MV radiation therapy treatment beam have been performed. Such photodiodes can be configured into large two-dimensional arrays of addressable sensors suitable for real-time imaging of megavoltage treatment beams as well as other applications such as diagnostic X-ray imaging. Signal sizes per radiation pulse up to 3.9*10/sup 6/ and 69*10/sup 6/ electrons are observed when the sensors are used with cronex (CaWO/sub 4/) and lanex (gadolinium-oxysulfide) intensifying screens, respectively. For the cronex and lanex screens the size of the measured signals can be enhanced by reducing the thickness of the top p-layer, thereby reducing the attenuation of the incident light. However, this is a smaller effect for the lanex screen, which provides substantially more light signal. The variation in cronex signal size with p-layer thickness is consistent with calculations based on known attenuation coefficients. >

Large area amorphous silicon photodiode arrays for radiotherapy and diagnostic imaging

Abstract Amorphous silicon imaging devices consisting of two-dimensional pixel arrays of photodiodes and field effect transistors can now be fabricated over areas as large as 30 cm by 30 cm. Such imagers can offer considerable advantages for real-time radiotherapy megavoltage and diagnostic X-ray imaging applications. The design, operation, and advantages of such imagers are discussed, and sensor signal data are presented.

Observation of the decay K- ---> pi- mu+ mu- and measurements of the branching ratios for K+- ---> pi+- mu+ mu-

Using data collected with the HyperCP (E871) spectrometer during the 1997 fixed-target run at Fermilab, we report the first observation of the decay K--->pi(-)mu(+)mu(-) and new measurements of the branching ratios for K+/--->pi(+/-)mu(+)mu(-). By combining the branching ratios for the decays K+-->pi(+)mu(+)mu(-) and K--->pi(-)mu(+)mu(-), we measure Gamma(K+/--->pi(+/-)mu(+)mu(-))/Gamma(K+/--->all) = (9.8+/-1.0+/-0.5)x10(-8). The CP asymmetry between the rates of the two decay modes is [Gamma(K+-->pi(+)mu(+)mu(-))-Gamma(K--->pi(-)mu(+)mu(-))]/[Gamma(K+-->pi(+)mu(+)mu(-))+Gamma(K--->pi(-)mu(+)mu(-))] = -0.02+/-0.11+/-0.04.

Radiation response characteristics of amorphous silicon arrays for megavoltage radiotherapy imaging

Signal measurements for a variety of pixelated a-Si:H imaging arrays irradiated by megavoltage beams are reported. Data are presented for 6-, 10-, and 15-MV beam energies for screens containing gadolinium-oxysulfide and calcium tungstate. These results are the first quantitative radiation signal sizes reported for such arrays. In addition, the initialization times of these arrays are reported. The performances of arrays of various sizes and pitches with respect to these measured values are compared. These results are part of an ongoing effort to develop flat-panel, real-time gamma and X-ray imagers.<<ETX>>

Factors Affecting Image Quality for Megavoltage and Diagnostic X-Ray a-Si:H Imaging Arrays.

Two factors which could affect the image quality of the a-Si:H arrays under development by our collaboration are the temporal drift of the leakage current of the sensors and the capture and release of charge in deep trapping states in the sensors. Data for both of these factors are reported, the implications for imaging are discussed, and strategies for reducing or eliminating their effects are suggested.

Light-Response Characteristics of Amorphous Silicon Arrays for Megavoltage and Diagnostic Imaging

Light-sensitive hydrogenated amorphous silicon pixel arrays are now under development for real-time megavoltage and diagnostic fluoroscopic imaging. Such applications place stringent demands upon a variety of array properties including the uniformity of the light-response function of the pixels. It is desirable that the design and fabrication of these imaging arrays maximize such uniformity. The implications of uniformity for imaging are reviewed, and data obtained from small arrays are presented and discussed.

Radiation damage studies of amorphous-silicon photodiode sensors for applications in radiotherapy X-ray imaging

Abstract The high radiation tolerance of hydrogenated amorphous silicon (a-Si:H) is one reason it has become a candidate for high-energy physics applications and for radiotherapy and diagnostic imaging. The performance of 1 μm and 5 μm a-Si:H n-i-p photodiode sensors used in conjunction with Lanex (Gd 2 O 2 S:Tb) intensifying screens has been measured as a function of high-energy photon dose. Over the course of irradiation with a 60 Co source to a total dose of ∼10 4 Gy the output signal due to the sensor-screen combinations experienced maximum variations of −1.3% and +2.7% for the 1 μm and 5 μm sensors, respectively. Transient effects associated with the sensors and screens are also reported.

Search for disoriented chiral condensate at the Fermilab Tevatron

We present results from MiniMax (Fermilab T-864), a small test/experiment at the Fermilab Tevatron designed to search for the production of a disoriented chiral condensate (DCC) in p-p(bar sign) collisions at {radical}(s)=1.8 TeV in the forward direction, {approx}3.4<{eta}<{approx}4.2. Data, consisting of 1.3x10{sup 6} events, are analyzed using the robust observables developed in an earlier paper. The results are consistent with generic, binomial-distribution partition of pions into charged and neutral species. Limits on DCC production in various models are presented. (c) 2000 The American Physical Society.