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A simple Monte Carlo algorithm for the simulation of the passage of [gamma] rays of about 1 MeV in a medium is presented. In this energy range the only relevant processes are Compton scattering and photoelectric absorption. The algorithm allows the visualization of the photon tracks as well as the calculation of many quantities of interest. Several problems for a layer and a cylinder are proposed. In particular, the energy transferred to electrons in a water cylinder as a function of depth and the line shape of a NaI scintillator is calculated. [DOI: 10.1119/1.1509416]

In the scope of construction of a PET apparatus based on detection of Compton scattering in silicon (Compton camera), timing properties of 1-mm-thick silicon pad and double-sided microstrip detectors are studied. Timing in pad detectors is also investigated for 140.5 keV [sup.99m]Tc and 364.5 keV [sup.131]I gamma rays in view of a SPECT application. Compton scattering and energy loss of the Compton electron in silicon detector are simulated using the GEANT package. Electric field in the detector is calculated numerically for a fully depleted detector in the abrupt junction approximation, taking into account the geometry and varying the reverse voltage. Signal formation is studied using Ramo's theorem and pulse shaping properties of the trigger circuit. A time-walk cut is seen to be directly corresponding to a deposited-energy cut. At 10 keV threshold in 1-mm-thick detectors, 10 ns timing windows are shown to reject a significant portion of events, degrading efficiency or limiting the angular range in a prohibitive way. More involved techniques are therefore suggested, either in the electronics circuit or in later stages of the trigger. Index Terms--Charge collection, Compton camera, PET, silicon detectors, simulation, SPECT, timing.

A technique for validating electron energy resolution [[eta].sub.e] results measured using the modified Compton coincidence technique (MCCT) has been developed. This technique relies on comparing measured gamma-ray energy resolution [[eta].sub.[gamma]] with calculated values that were determined using the measured [[eta].sub.e] results. These [[eta].sub.[gamma]] calculations were based on Monte Carlo photon transport simulations, the measured NaI(Tl) electron response, a simplified cascade sequence, and the measured [[eta].sub.e] results. To demonstrate this technique, MCCT-measured NaI(Tl) [[eta].sub.e] results were used along with measured [[eta].sub.[gamma]] results from the same NaI(Tl) crystal. Agreement to within 5% was observed for all energies considered between the calculated and measured [[eta].sub.[gamma]] results for the NaI(Tl) crystal characterized. The calculated [[eta].sub.[gamma]] results were also compared with previously published [[eta].sub.[gamma]] measurements with good agreement (< 10%). In addition to describing the validation technique that was developed in this study and the results, a brief review of the [[eta].sub.e] measurements made using the MCCT is provided. Based on the results of this study, it is believed that the MCCT-measured [[eta].sub.e] results are reliable. Thus, the MCCT and this validation technique can be used in the future to characterize the [[eta].sub.e] of other scintillators and to determine NaI(Tl) intrinsic energy resolution. Index Terms--Electron energy resolution, electron response, light yield nonproportionality.

In mammography, the first cause of image contrast reduction arises from the photons scattered inside the examined organ. The amount of Compton scattering strongly depends on the irradiation area and on the distance between the organ and the X-ray detector. We have experimentally evaluated how these geometrical conditions affect the scattering fraction. Our experimental setup includes a single photon counting device based on a silicon pixel detector as X-ray sensor, a lucite cylinder to simulate the breast tissue, and a lead collimator to define the irradiation area. We have evaluated the contrast and the signal-to-noise ratio for images acquired in different conditions. Index Terms--Contrast, mammography, pixel detector, scattered photons, signal-to-noise ratio (SNR), single photon counting chip.

In this paper, we investigate the effects of spatial resolution and relative orientation of detectors on the accuracy of the directional localization in two-detector Compton cameras. We derive a general expression that relates the angular uncertainty to spatial resolution and geometry. Appropriate expressions for specific configurations are obtained as special cases of the general expression. Finally, we present a simple formula for estimating the position uncertainty of Compton cameras in terms of relative spatial resolution and distances.

Compton telescopes have been an important tool in gamma-ray astronomy, most recently with the COMPTEL instrument on NASA's COMPTON Gamma Ray Observatory (CGRO). Scientific objectives in low/medium energy gamma-ray astronomy place highest priority on significant improvement in sensitivity relative to CGRO and ESA's planned INTEGRAL mission. The observational requirements include both discrete and extended sources, and narrow, broad, and continuum spectral features. A high spectral and spatial resolution Compton telescope is the preferred instrument for a future mission. The broad range of scientific objectives that such a mission will address include: mapping radioactivity across the Galaxy (26Al, 44i, 60Fe), detection of several Type Ia supernovae per year (56Ni, 56Co), study nuclear burning in novae (22Na, 7Be, 511 keV), study the accretion of matter onto galactic and massive extra-galactic black holes and the associated formation of relativistic jets, and provide unique insights into neutron stars, cosmic gamma-ray bursts, and solar flares. A mission to achieve these goals should have discrete source, narrow line sensitivities approaching [10.sup.-7] [Gamma]/[cm.sup.2]-s for the important astrophysical gamma-ray lines from 511 keV to 2 MeV. Detectors which combine state-of-the-art energy resolution (few keV) and spatial resolution (1 mm) are required.

We describe recent work on the development of a Compton scatter polarimeter for measuring the polarization of hard X-rays (100-300 keV) from astrophysical sources. Results from measurements with a laboratory prototype are summarized, along with comparisons to Monte Carlo simulations. We also present our latest design concept, which envisions a complete polarimeter module on the front end of a 5-inch position-sensitive PMT Although the emphasis of our development effort is towards measuring hard X-rays from solar flares, our latest design has the advantage that it is sensitive over a rather large field-of-view (> 1 steradian), a feature that makes the design especially attractive for [Gamma]-ray burst studies.

An energy-subtraction Compton scatter camera (ESCSC) was previously proposed for in-vivo imaging of radiopharmaceuticals used as bio-tracers in Nuclear Medicine. To further evaluate the usefulness of this ESCSC design, studies pertaining to image reconstruction are explored and presented. Generally speaking, a Compton scatter camera works on the principle that an emitted gamma ray undergoes a Compton scatter interaction in a primary detector system and then is subsequently absorbed by a secondary detector system. Using the measured interaction energies and positions, a cone surface can be backprojected which intercepts the emission space near the point of the gamma-ray emission (proximity depends on resolution). When backprojecting and linearly superposing multiple cones into a source space, calculations should include normalizing the total weight contributed by each cone as well as how the differentially intercepted area increases as you move farther away from the vertex of the cone (i.e., intercepted voxels farther away from the vertex are given less weight). Backprojected 'point kernel profiles', based upon simulated data, are presented corresponding to point sources located at several positions (revealing the degree of spatial variance) within the ESCSC camera geometry. From these results the spatially variant point kernel function may be deduced for future use in image reconstruction. Additionally, two different algorithms for backprojection are compared.

Many practical applications of Compton Camera Imager (CCI) systems require imaging of extended sources in diffuse backgrounds. This paper presents image reconstructions of simulated CCI system data from extended source distributions with and without a diffuse background. The simulations were performed using a biased photon transport Monte Carlo code which was validated with MCNP results. The reconstruction code used an iterative back-projection technique based upon the Expectation Maximization Maximum Likelihood method. Examples are shown of two clearly resolved cylindrical sources, an extended source both with and without background, and extended sources at multiple depths within the volume. The results demonstrate the ability to image extended sources in high backgrounds with a limited number of events.

To study the light yield nonproportionality and intrinsic energy resolution of inorganic scintillators, a Compton coincidence technique was previously designed, implemented and benchmarked. This technique provides the ability to accurately measure the electron response of scintillation materials and thus provides an accurate means of studying these characteristics. In this study, the electron responses of two 'classic' scintillators (Ca[F.sub.2](Eu) and [Bi.sub.4][Ge.sub.3][O.sub.12] (BGO)) and a newer scintillator ([Lu.sub.2]Si[O.sub.5]:[Ce.sup.3+] (LSO)) have been measured over the energy range from 5 keV to approximately 450 keV using this technique. Electron responses for all three scintillation materials are observed to increase monotonically with electron energy. BGO is observed to have the least nonproportionality, with its electron response varying about 20% over this energy range. Conversely the LSO electron response is observed to vary about 50% over this energy range and thus exhibits the largest nonproportionality.

In this study we investigated the potential of Gd-153 and Co-57 as transmission sources. Since both of these sources have higher energy photopeaks higher than that of the predominant T1-201 photons, they will result in contamination of the emission data. The amount of contamination and the ease of compensating for it are important factors in selecting an isotope for the transmission source. These factors were investigated and, based on these studies, we conclude that both isotopes result in a level of contamination that is significant in comparison to typical T1-201 emission count levels. While Gd-153 results in approximately 50% more contamination of the emission data than Co-153, the object dependence of the contamination for both isotopes is such that contamination compensation will be difficult. In fact, compensation may be somewhat easier for Gd-153. As a result, selection of one of these isotopes may be based on other considerations such as cost and usability with other emission isotopes.

In simultaneous Tc-99m TCT and T1-201 SPECT data acquisition, the emission images are degraded by not only scatter from the emission source, but also cross-contamination from the transmission photons. Also, the transmission images are degraded by scatter from the transmission source and cross-contamination from the emission photons. In this work, a criterion based on maximizing a Figure-of-Merit (FOM) was used to reduce image degradations by optimizing the position and width of the energy windows used in the data acquisition in a triple-camera fan-beam SPECT system. The FOM determines a compromise between increased detection efficiency of primary photons and reduction of scatter and cross-contamination photons. Scatter in the projection data was modeled using Monte Carlo (MC) simulation methods. Experimental studies were performed to verify the simulations and to estimate the amount of Pb x-rays and scatter in the collimator which were not modeled in the MC program. Results suggest that cross-contamination degrades the emission and transmission data. However, the contamination does not significantly alter the optimal energy window settings, which should be centered at [approximately]77 keV with a window width of [approximately]34% for detecting the Tl emission photons, and centered at 140 keV with a window width of [approximately]20% for detecting the Tc transmission photons.

We propose to use a tellurium-123m line source to determine attenuation maps for reconstruction of technetium-99m images. To experimentally evaluate the performance of the transmission line source for the fast sequential approach for emission and transmission data acquisition, a prototype tellurium-123m line source was designed and manufactured. The line source has a length of 8 cm with total activity of 36-39 mCi at the time of the studies. Phantom and patient studies were conducted to evaluate the performance of the line source. The results of this study demonstrate that the tellurium-123m line source can be used to accurately determine attenuation maps for transmission studies with Tc-99m activity in the patient's myocardium.

Energy information is used to generate emission (99mTc, [E.sub.[Gamma]] = 140 keV) and transmission (57Co, [E.sub.[Gamma]] = 122 keV) images from a combined projection data set. These components are separated from each other and from Compton scatter by fitting the spectra at each pixel with a simple model. The method is tested using a cylindrical phantom containing inserts with differing attenuation. Emission and transmission position and energy (xyE) data were acquired in list mode and later summed to simulate simultaneous acquisition. Therefore, the true emission and transmission images are known. The quality and quantitative accuracy of the technique are evaluated by comparing images extracted from the combined data set to the 'true' scatter-corrected transmission and emission images, respectively. Compared to using two energy windows, the spectrum fitting technique substantially reduced crosstalk. Although the resulting bias in the emission images was typically less than a few percent, the bias in the transmission images varied from -11% to +16% as the emission activity was increased with constant transmission source activity. Also, the variance added to the emission images obtained by the fitting procedure was found to be less than the variance caused by the stochastic nature of the emission and detection processes.

A Monte Carlo simulation has been developed to simulate and correct for the effect of Compton scatter in 3-D acquired PET brain scans. The method utilizes the 3-D reconstructed image volume as the source intensity distribution for a photon-tracking Monte Carlo simulation. It is assumed that the number of events in each pixel of the image represents the isotope concentration at that location in the brain. The history of each annihilation photon's interactions in the scattering medium is followed, and the sinograms for the scattered and unscattered photon pairs are generated in a simulated 3-D PET acquisition. The calculated scatter contribution is used to correct the original data set. The method is general and can be applied to any scanner configuration or geometry. In its current form the simulation requires 25 hours on a single Sparc10 CPU when every pixel in a 15-plane, 128x128 pixel image volume is sampled, and less than 2 hours when 16 pixels (4x4) are grouped as a single pixel. Results of the correction applied to 3-D human and phantom studies are presented.

Variations in detector efficiency induce significant artifacts in reconstructed images unless accurate normalization methods are used. Scattered events are detected with significantly different efficiencies than unscattered events because the photons in scattered coincidences arrive at the detector at lower energies and over a wide range of angles of incidence. Current methods work well for septa-extended imaging because the scatter fractions are low. The large scatter fractions encountered in fully 3D imaging, however, require that these differences be accounted for. We propose a normalization technique that accounts for them and show that it yields more uniform images than two other potential methods.

Tomographic reconstruction of electron density images requires the recording of scattered photons as a function of detector location and energy during Compton scattering. Analytic solutions for reconstruction are similar to filtered-backprojection algorithms involved in computed tomography (CT). The new image reconstruction method is more efficient than CT methods for industrial nondestructive imaging.

A new method has been developed for estimating the Compton-scatter component within the photopeak for local energy spectra measured by an Anger camera in SPECT. Assuming that the measured energy spectrum is the source scatter energy distribution convolved with a known camera energy-resolution function plus an unscattered spectral component, a least square inverse operation is performed to recover the source scatter distribution. Since this inverse operation is ill-posed, the regularization technique, with a parameter to be specified, is applied for stabilization. With the new method, scatter fractions similar to those from polynomial spectral fitting (PSF) have been observed for experimentally measured, high-count data with a hot (Tc to the 99m) or I to the 131) sphere in a cold cylinder, and the inverse (Tc to the 99m only). Monte-Carlo simulations of the hot-sphere phantom (Tc to the 99m) show that the average error of corrected counts decreases by a factor of 1.6 or more, compared to cubic PSF, when the measured count within a 20% window ranges from 100 to 5000. The method is also less sensitive to the width of the fitting window. A regularization parameter from 1 to 10 is recommended for practical cases. Finally, the shape of a recovered source scatter distribution matches that determined by a high-resolution semiconductor-detector measurement as well as by Monte-Carlo simulation.

The occurrence of ferromagnetism is studied in the Ca[Ru.sub.1-x][Fe.sub.x][O.sub.3] system by means of magnetic Compton scattering. The magnetic Compton scattering experiment revealed the occurrence of a large magnetic moment on Ru and an antiferromagnetic coupling between Fe and Ru spin moments.

The magnetism of Ca[Ru.sub.1-x][Mn.sub.x][O.sub.3] is studied by using magnetic Compton scattering measurements for polycrystalline specimens at 10 K under 2.5 T by using a synchrotron-radiation x-ray. The results have shown that a sort of ferrimagnetism is established in the Ca[Ru.sub.1-x][Mn.sub.x][O.sub.3] system.

The simple physics of Compton scattering, combined with the slowing down characteristics of highly relativistic electrons and the general properties of most cargos is presented. The studies provide a path for the development of an effective and sensitive screening mechanism for actinides in elemental form.

The X-ray emission processes of extragalactic jets on scales resolvable by the subarcsec resolution of the Chandra X-ray Observatory are reported. The spatially resolved X-ray emission from kiloparsec-scale jets is considered and the radio observations of parsec-scale jets and broadband observations of spatially unresolved but highly variable core emission from subparsec jets of blazer-type active galactic nuclei (AGN) are discussed.

Polyamide 6,12 crystalline fibers, commercially known as Nylon 6,12, were irradiated using gamma radiation and characterised by Infrared Spectroscopy (FT-IR), Raman, Differential Scanning Calorimeter (DSC) and Thermogravimetric Analysis (TGA). The analysis of the data, by using Flory's Theory and the principles of the Compton effect, allowed establishment of a reaction mechanism to explain how the polyamide chains are modified.

The densities of tissues for medical applications and those materials for industrial applications are determined by using the Compton scattering. The features of a successive approximation process (SAP) for the reconstruction of the density image from the Compton scattered energy spectral data are discussed.

In our previous work, an efficient slice-by-slice blurring model was developed based on the Klein-Nishina formula to model first order Compton scatter in both parallel begun and converging begun SPECT. The first order Compton scatter projection was generated by first forming an effective scatter source imaging (ESSI) corresponding to first order Compton scatter and then forward-projecting the ESSI to the detector surface. using a projector that modeled both the non-uniform attenuation effect and the depth-dependent detector response effect. In tiffs work. the technique was extended to model multiple order Compton scatter. At each scattering voxel, first order Compton scattered photons confined in a scattering cone which was determined by the energy window used for emission projection acquisition were used as the source for second order Compton scatter. The effective scatter source image (ESSI) corresponding to the second order Compton scatter was then obtained using the slice-by-slice blurring model. Forward-projecting this ESSI to the detector gave the second order Compton scatter projection. For a point source in a non-uniform scattering object for fan beam geometry, the second order Compton scatter point responses generated using the proposed method matched well with those using Monte Carlo simulations at the peak region, but not as well at the mils. This work proposed an efficient method to model multiple order Compton scatter in both parallel beam and converging begun SPECT.

Observations of Nuclear Reactors on Satellites with a Balloon-Borne Gamma-Ray Telescope RECEN T ARTICLES HAVE DISCUSSED background signals observed by Solar Maximum Mission and Ginga satellite instruments that appear to [...]