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This work investigates the cumulative dose 60Co gamma ( $\gamma$ ) – ray irradiation effects on enhancement mode HEMT devices inheriting 3D – Electron and Hole Gases for dosimeter applications. The devices are irradiated through a 60Co source and demonstrate the enhancement in the drain current metrics. To elucidate, the said devices were irradiated through different mechanisms and the Compton effect was investigated through contour plots via TCAD simulations. The degradation of Schottky Gate contact and insulator charging affects the 2D – Hole Gas at the GaN cap and thereby affects the bottleneck at the 3DEG sheet. This significantly affects the OFF – state leakage components of the said device and can therefore be exploited for potential use in sensing and dosimeter applications. The leakage components can be exploited further to improve the linearity of the dosimeter by considering different grading profiles of the AlGaN layer. In this regard, a workflow for optimizing the sensitivity and linearity of the dosimeter through different graded profiles is also presented. Amongst all, gaussian graded profiles have been identified as the best – case scenario considering the sensitivity and exhibiting a linear operation.

Obtaining directional information is required in many applications such as nuclear homeland security, contamination mapping after a nuclear incident and radiological events, or during the decontamination work. However, many directional radiation detectors are based on directional shielding, made of lead or tungsten collimators, introducing two main drawbacks. The first is the size and weight, making those detectors too heavy and irrelevant for utilization in handheld devices, drone mapping, or space applications. The second drawback is the limited field of view (FOV), which requires multiple detectors to cover the whole required FOV or machinery to rotate the detector’s narrow FOV detector. We propose a novel $4\pi $ directional detector based on a segmented hollow cubic detector, which uses the Compton effect interactions with no heavy collimators. The symmetrical cubical design provides both higher efficiency and $4\pi $ detection ability. Instead of the traditional two types of detectors (scatterer and absorber) structure, we use the same type of detector, based on Gd3Al2Ga3O12 (GAGG) (Ce) scintillator coupled to silicon photomultiplier. An additional advantage of the proposed detector is obtained by locating the photon sensors inside the detector, behind the scintillators, which improves the radiation hardness required for space applications. Furthermore, such an arrangement flattens the temperature variation across the detector, providing better gain stability. The main advantage of the proposed detector is an efficient $4\pi $ radiation detection for high-energy gamma rays without the use of heavy collimators. [ABSTRACT FROM AUTHOR]

The ability to localize and map the distribution of gamma-ray emitting radionuclides in 3D has applications ranging from medical imaging to nuclear security. In the case of radiological source search and nuclear contamination remediation, the deployment of freely moving detection systems such as handheld instruments or ground/aerial-based vehicles is critical in overcoming the inverse square law and complex shielding scenarios. Using contextual sensors, these systems can simultaneously generate 3D maps of the surrounding environment and track the position and orientation of the gamma-ray sensitive detectors in that environment. The fusion of contextual scene data and gamma-ray detector data to facilitate real-time 3D gamma-ray image reconstruction has been demonstrated with mobile high purity germanium (HPGe) and CdZnTe-based Compton cameras for gamma-ray energies ranging from a few hundred keV to several MeV. Here we apply this approach for lower energy (50–400 keV) gamma rays, using a handheld CdZnTe-based omnidirectional imaging system and an active coded mask imaging modality. We present our approach to real-time reconstruction using a scene-data-constrained graphics processing unit (GPU)-accelerated list-mode maximum likelihood expectation maximization (MLEM) algorithm and show results from several measurements in the lab and in the field. [ABSTRACT FROM AUTHOR]

In this paper, we propose a double-layered cadmium zinc telluride Compton camera using the weighted list-mode maximum likelihood expectation maximization (MLEM) method. We performed a Monte Carlo simulation of the performance. The detection system comprised two detector arrays arranged in parallel. Each array consisted of $6 \times 6$ crystals with a 7.5 mm pixel pitch. The dimension of a single crystal was $6 \times 6 \times 15$ mm. For Compton imaging, the 137Cs activity was set to 78 kBq and the acquisition time was set to 2 hours. By applying the weighted MLEM method, the events with small angular uncertainty were more emphasized than those with large angular uncertainty. Each angular uncertainty was calculated based on the energy and geometry uncertainties of each event. We compared the traditional list-mode MLEM and the weighted list-mode MLEM for single and double detector arrays. The results showed that the combination of a double-layered imager with the weighted MLEM method represented the best image performance compared to other combinations. The sources at the offset positions were also simulated to estimate the directional dependency of the camera system. [ABSTRACT FROM PUBLISHER]

This study evaluated the performance of a multiple-scattering Compton camera (MSCC) for in vivo monitoring of the dose distribution of therapeutic photons in various phantoms. When 6-MV photons interacted with materials in the phantom, most photons deposited part of their energies on the voxels and were scattered out of the phantom. The 3D distribution of the scattered photons detected by the MSCC was reconstructed by using image processing methods based on Compton kinematics. The optimized design parameters from previous studies were used in the current system. In order to verify the accuracy of dose estimation by using MSCC, the results of depth-dose distribution curves in the phantom were compared with theoretical values calculated by MCNPX for various phantoms. The dose distribution of scattered photons showed similar trends to the deposited dose of incident photons in various phantoms. For realistic visualization, the reconstructed Compton images were combined with Computed Tomography (CT) images of the phantom. The results of this study demonstrated the feasibility of using Compton imaging as a means of monitoring the 3D dose distribution of the therapeutic photons in radiation therapy. The combination of Compton and CT images of the human phantom highlighted the effectiveness of our MSCC for monitoring patient doses in 3D photon therapy. [ABSTRACT FROM AUTHOR]

An index is presented of the periodical which includes author index such as Aneesh Kumar, K V. Ravikumar, and J.M. Benlloch and subject index such as Computerized tomography, Computerized instrumentation, and Compton effect.

Proton and light ion beams are applied to the therapeutic irradiation of cancer patients due to the favorable dose deposition of these particles in tissue. By means of accelerated ions, a high dose can be accurately deposited in the tumor while normal tissue is spared. Since minor changes in the patient’s tissue along the beam path can compromise the success of the treatment, an in-vivo monitoring of the dose deposition is highly desired. Cameras detecting the prompt \gamma -rays emitted during therapy are under investigation for this purpose. Due to the energy spectrum of prompt \gamma -rays with a range between a few keV and several MeV, it is reasonable to consider the utilization of electron-positron pair production events to reconstruct the origin of these prompt photons. The combined use as a pair production and Compton camera is expected to increase its efficiency. We evaluated if a pair production camera could be suitable in this context by means of Monte-Carlo simulations. Modelling of the pair production events taking place in a prototype detector dedicated to Compton imaging were performed. We analyzed the efficiency of the detector system regarding pair production and Compton events. The most crucial property of this pair production camera is the angular resolution. The results of this work indicate that the spatial resolution of the considered detection system used as pair production camera is, for principal reasons, insufficient for an application to range assessment in particle therapy. Furthermore, the efficiency of the pair production camera under study is one order of magnitude lower than the efficiency of the setup applied to the detection of Compton events. [ABSTRACT FROM AUTHOR]

Compton imaging is a gamma-ray imaging technique useful for photons with energies in the range of a hundred keV to several MeV. Measuring gamma rays with a Compton camera results in cone data that needs to be mathematically inverted to determine the incident flux distribution. In the past, filtered back-projection solutions for Compton telescope data required sums of spherical harmonics or stereographically mapping the back-projection, which can result in imaging artifacts. We present a solution to this inversion problem that removes these complexities by embedding the 2-D directional image on the surface of a sphere S^2 into R^3 where it is easily solvable. In this manner we relate 2-D Compton 4\pi imaging to the 3-D Radon transform, which has known solutions. To accomplish this, the cone data is converted to planar data. Additionally we show how the planar geometry can be used to produce a computationally efficient implementation. This reconstruction is demonstrated with a two-plane, double-sided strip, HPGe Compton camera. [ABSTRACT FROM AUTHOR]

The ionization processes and temporal evolutions of atmosphere ionized by prompt \gamma rays from high-altitude nuclear explosions are studied by numerical simulations. With the increasing of the burst point, the peak energy density of the Compton electrons decreases, but the corresponding altitude of the peak rises, which is 30 km directly below the burst point, and 40 km–50 km below the burst height 1000 km from the burst point following Earth’s surface curvature. It is shown that the prompt \gamma rays have significant ionization effects on the atmosphere from the burst point to several kilometers from the burst point, and the affected area increases with the rising of the burst point. By the secondary ionization, the Compton electrons produce many more low-energy electrons whose lifetime increases with the rising of the altitude. [ABSTRACT FROM AUTHOR]

Current pulses flowing along a metallic target obliquely irradiated by Z pinch-generated X-rays are measured. The leading interaction mechanism shows a strong dependence on incident X-ray intensity in addition to photon energy. Photoelectric effect expected to explain the observed current is not vital in the case of intense irradiation used in our experiments. The measured results are attributed to Compton current and thermocurrent on the same level. Simple analytical modeling of radiation demonstrates that obliquely incident X-ray–metal interaction is an effective method for generating directional electromagnetic pulse with its energy and intensity being of interest for practical purposes. [ABSTRACT FROM PUBLISHER]

The Compton camera is a relevant imaging device for the detection of prompt photons produced by nuclear fragmentation in hadrontherapy. It may allow an improvement in detection efficiency compared to a standard gamma-camera but requires more sophisticated image reconstruction techniques. In this work, we simulate low statistics acquisitions from a point source having a broad energy spectrum compatible with hadrontherapy. We then reconstruct the image of the source with a recently developed filtered backprojection algorithm, a line-cone approach and an iterative List Mode Maximum Likelihood Expectation Maximization algorithm. Simulated data come from a Compton camera prototype designed for hadrontherapy online monitoring. Results indicate that the achievable resolution in directions parallel to the detector, that may include the beam direction, is compatible with the quality control requirements. With the prototype under study, the reconstructed image is elongated in the direction orthogonal to the detector. However this direction is of less interest in hadrontherapy where the first requirement is to determine the penetration depth of the beam in the patient. Additionally, the resolution may be recovered using a second camera. [ABSTRACT FROM AUTHOR]

By using a prototype Compton camera consisting of silicon (Si) and cadmium telluride (CdTe) semiconductor detectors, originally developed for the ASTRO-H satellite mission, an experiment involving imaging multiple radiopharmaceuticals injected into a living mouse was conducted to study its feasibility for medical imaging. The accumulation of both iodinated (^131I) methylnorcholestenol and ^85Sr into the mouse's organs was simultaneously imaged by the prototype. This result implies that the Compton camera is expected to become a multi-probe tracker available in nuclear medicine and small animal imaging. [ABSTRACT FROM PUBLISHER]

Likelihood-based test statistics for the task of detecting a radioactive source in background using a gamma-ray imaging system often have intractable distributions. This complicates the tasks of predicting detection performance and setting thresholds that ensure desired false-alarm rates. Asymptotic distributions of test statistics can aid in predicting performance and in setting detection thresholds. However, in applications with complex sensors, like gamma-ray imaging, often only approximate statistical models for the measurements are available. Standard asymptotic approximations can yield inaccurate performance predictions when based on misspecified models. This paper considers asymptotic properties of detection tests based on maximum likelihood (ML) estimates under model mismatch, i.e., when the statistical model used for detection differs from the true distribution. We provide general expressions for the asymptotic distribution of likelihood-based test statistics when the number of measurements is Poisson, and expressions specific to gamma-ray source detection that one can evaluate using a modest amount of data from a real system or Monte Carlo simulation. Considering a simulated Compton imaging system, we show that the proposed expressions yield more accurate detection performance predictions than previous expressions that ignore model mismatch. These expressions require less data and computation than conventional empirical methods. [ABSTRACT FROM AUTHOR]

Polarimetry has been recognized as a very important observational parameter for high energy astrophysics (>100 keV). For this reason the capability to perform accurate polarimetric measurement of high energy cosmic sources should be included in future space mission proposals. This idea has become very appealing in the perspective of new high energy focusing telescope implementing Laue lenses. In order to optimize the focal plane design of such mission as a Compton scattering gamma-ray polarimeter, a CdZnTe detector prototype has been tested at the ESRF (European Synchrotron Radiation Facility) under a \sim100% polarized gamma-ray beam. The prototype detector tested is a 5 mm thick CdZnTe array with the anode segmented into 16 \times 16 pixels of 2.5\times 2.5\ mm^2. One of the most important factors that can affect significantly the polarimetric measurements is the direction of the incoming photons from the polarized radiation source with respect to the focal plane of the polarimeter. We have studied and analyzed the polarimetric response of the CdZnTe prototype as a function of the inclination of a polarized beam with respect to the optical axis of the detector from 0^\circ to 10^\circ tilt angle. [ABSTRACT FROM AUTHOR]

Three-dimensional-position-sensitive semiconductor detectors provide both energy and locations of gamma-ray interactions in the detector. From these data, Compton-imaging methods can reconstruct the spatial source distribution around the detector as a function of energy. Here, the generalized likelihood ratio test (GLRT) and the source-intensity test (SIT) are applied to automate detection and identification of point sources of radiation in distributed background by using the combined energy and direction information from these detectors. Throughout, the model makes no assumptions of the background intensity or energy distribution. However, directional information enables detection and identification without prior knowledge of the background spectrum and the detection of isotopes also appearing in the background. Spatially, background is modeled as uniform, but only slight degradation in performance was observed when the background was twice as intense in one hemisphere. Experimental detection and identification performance as a function of false-alarm probability was measured for three weak isotopes using experimental data collected on a 20 mm\,\times\,20 mm\,\times\,15 mm CdZnTe detector. Both the GLRT and SIT performed better than the commercial software Genie 2000 (Canberra), which uses only spectral information, when the possible source direction was unknown. Prior knowledge of the possible source direction further improves both detection and identification. [ABSTRACT FROM AUTHOR]

Iterative image reconstruction of data measured by a Compton scattering camera has to overcome various difficulties, e.g., large amount of data, noise arising from both low counts recorded, and the imaging response. Image estimation by the Maximum Likelihood (ML) criterion induces noise amplification, so a denoising step is needed. The proposed solution is a denoising technique using wavelet-based thresholding of the ML Expectation-Maximization (EM) update factors, called WTDEM. The thresholds are scale dependent, and proportional to the standard deviation of the high-frequency sub-band coefficients at the respective scale. It results in lower reconstruction errors than the MLEM algorithm, and the Gaussian smoothing, and, in addition, it is stable. The WTDEM algorithm is illustrated by computer experiments. [ABSTRACT FROM AUTHOR]

The detection of radioactive contraband is a critical problem in maintaining national security for any country. Gamma-ray emissions from threat materials challenge both detection and measurement technologies significantly. The development of a sequential, model-based Bayesian processor that captures both the underlying transport physics of gamma-ray emissions including Compton scattering and the measurement of photon energies offers a physics-based approach to attack this challenging problem. The inclusion of a basic radionuclide representation of absorbed/scattered photons at a given energy along with interarrival times is used to extract the physics information available from noisy measurements. It is shown that this representation leads to an “extended” physics-based structure that can be used to develop an effective sequential detection technique. The resulting model-based processor is applied to data obtained from a controlled experiment in order to assess its feasibility. [ABSTRACT FROM AUTHOR]

In hadrontherapy in order to fully take advantage of the assets of the ion irradiation, the position of the Bragg peak has to be monitored accurately. Here, we investigate a monitoring method relying on the detection in real time of the prompt \gamma emitted quasi instantaneously during the nuclear fragmentation processes. Our detection system combines a beam hodoscope and a double scattering Compton camera. The prompt-\gamma emission points are reconstructed by intersecting the ion trajectories given by the hodoscope and the Compton cones reconstructed with the camera. We propose here to study in terms of point spread function and efficiency the theoretical feasibility of the emission points reconstruction with our set-up in the case of a photon point source in air. First we analyze the nature of all the interactions which are likely to produce an energy deposit in the three detectors of the camera. It is underlined that upper energy thresholds in both scatter detectors are required in order to select mainly Compton events (one Compton interaction in each scatter detector and one interaction in the absorber detector). Then, we study the influence of various parameters such as the photon energy and the inter-detector distances on the Compton camera response. These studies are carried out by means of Geant4 simulations. We use a source with a spectrum corresponding to the prompt-\gamma spectrum emitted during the carbon ion irradiation of a water phantom. In the current configuration, the spatial resolution of the Compton camera is about 6 mm (Full Width at Half Maximum) and the detection efficiency 10^-5. Finally, provided the detection efficiency is increased, the clinical applicability of our system is considered. [ABSTRACT FROM AUTHOR]

A method based on the Compton coincidence technique was applied to study the energy resolution of Compton electrons in wide energy range. The experimental setup was comprised of a High Purity Germanium (HPGe) detector and a LaBr3:Ce scintillator coupled to a photomultiplier. The detectors were set in a face-to-face geometry and the source was placed between them. Thus gating on events backscattered in the scintillator and detected in HPGe allowed one to measure the energy resolution of Compton scattered electrons at an energy corresponding to the Compton edge for a given source. This study confirms the importance of the electron scattering (δ-rays) as the main component of the intrinsic resolution in scintillators. The results are particularly important for discussion of the light yield non-proportionality contribution to the energy resolution of scintillators. [ABSTRACT FROM AUTHOR]

The integration of a high resolution PET insert into a conventional PET system can significantly improve the resolution and the contrast of its images within a reduced imaging field of view. For the rest of the scanner imaging field of view, the insert is a highly attenuating and scattering media. In order to use all available coincidence events (including coincidences between 2 detectors in the original scanner, namely the scanner-scanner coincidences), appropriate scatter and attenuation corrections have to be implemented. In this work, we conducted a series of Monte Carlo simulations to estimate the composition of the scattering background and the importance of the scatter correction. We implemented and tested the Single Scatter Simulation (SSS) algorithm for a hypothetical system and show good agreement between the estimated scatter using SSS and Monte Carlo simulated scatter contribution. We further applied the SSS to estimate scatter contribution from an existing prototype PET insert for a clinical PET/CT scanner. The results demonstrated the applicability of SSS to estimate the scatter contribution within a clinical PET/CT system even when there is a high resolution half ring PET insert device in its imaging field of view. [ABSTRACT FROM AUTHOR]

We present a model to describe the origin of non-proportional dependence of scintillator light yield on the energy of an ionizing particle. The non-proportionality is discussed in terms of energy relaxation channels and their linear and non-linear dependences on the deposited energy. In this approach, the scintillation response is described as a function of the deposited energy deposition and the kinetic rates of each relaxation channel. This mathematical framework allows both a qualitative interpretation and a quantitative fitting representation of scintillation non-proportionality response as function of kinetic rates. This method was successfully applied to thallium doped sodium iodide measured with SLYNCI, a new facility using the Compton coincidence technique. Finally, attention is given to the physical meaning of the dominant relaxation channels, and to the potential causes responsible for the scintillation non-proportionality. We find that thallium doped sodium iodide behaves as if non-proportionality is due to competition between radiative recombinations and non-radiative Auger processes. [ABSTRACT FROM AUTHOR]

The evaluation of electromagnetic transients in overhead power lines due to nearby lightning return strokes requires accurate models for the calculation of both the incident lightning electromagnetic pulse (LEMP) and the effects of coupling of this field to the line conductors. Considering also the complexity of distribution networks in terms of their topology and the presence of power system components and protection devices, the implementation of the LEMP-to-transmission-line coupling models into software tools used to represent the transient behavior of the entire network is of crucial importance. This paper reviews the most significant results obtained by the authors concerning the calculation of lightning-induced voltages. First, the theoretical basis of advanced models for the calculation of LEMP-originated transients in overhead power lines is illustrated; then, the relevant experimental validation using: 1) reduced-scale setups with LEMP and nuclear electromagnetic pulse (NEMP) simulators and 2) full-scale setups illuminated by artificially initiated lightning are reported. Finally, the paper presents comparisons between simulations and new experimental data consisting of measured natural lightning- induced voltages on a real distribution network in northern Italy, correlated with data from lightning location systems. [ABSTRACT FROM AUTHOR]

Based on experimental evidence of multiple reflections and modeling, we infer that the so-called compact intra- cloud lightning discharge (CID) is essentially a bouncing-wave phenomenon. Some tens of reflections may occur at both radiating- channel ends. The reflections have little influence on the overall CID electric field signature (narrow bipolar pulse (NBP) waveform), but are responsible for its fine structure, "noisiness" of dE/dt wave- forms, and accompanying HF-VHF radiation bursts. [ABSTRACT FROM AUTHOR]

Compton imaging is a gamma ray imaging technique that has many possible applications including homeland security and medical imaging. Using the Compton scattering formula the origin of a scattered γ-ray can be localized to a point on the surface of a cone using a minimum of two position and energy measurements. List-mode Maximum likelihood expectation maximization (MLEM) is an iterative statistical algorithm that makes successive approximations to the most probable source distribution that would have led to the observed data. Conventional MLEM is often performed in two dimensions, which still requires knowledge of the source-to-detector distance. We propose iteration over all three spatial dimensions in order to maximize the probability in three-dimensional space. This paper reports on the development of a three-dimensional MLEM (3DMLEM) algorithm to reconstruct images from a Compton imager in three dimensions for single, multiple and line sources. Several techniques for reducing convergence times are also discussed. [ABSTRACT FROM AUTHOR]

The Nuclear Compton Telescope (NCT) is a bal- loon-borne telescope designed to study astrophysical sources of nuclear line emission and polarization at soft gamma-ray (0.2-10 MeV) energies. NCT uses high-purity germanium strip detectors for 3D tracking of photon interactions. Compton imaging enables effective background rejection, resulting in a compact but highly efficient instrument. The NCT prototype completed a successful flight from Fort Sumner, New Mexico in 2005. We have since integrated additional detectors, updated the readout electronics, and improved other flight systems. Two flights of the full instrument are upcoming: a conventional flight in New Mexico and a long duration flight from Australia. We give an overview of the instrument and its status prior to the planned balloon flights. [ABSTRACT FROM AUTHOR]

Monte Carlo simulations have been performed to design a prompt gamma detector system for use as a quality assurance device in proton therapy. The system measures prompt gammas emitted in the direction normal to the incident therapeutic protons to correlate with the location of the distal dose falloff. The system consists of a drift chamber to measure the energies and directions of recoil electrons and a scintillator array to measure the positions and energies of scattered gammas in coincidence with the electrons. To ensure the use of the system in a therapeutic environment, the detection rate of Compton-scattered prompt gammas was simulated. The simulation predicts that the reconstruction uncertainty is around 1 cm for a practical configuration of the detector system. The resolution of the reconstruction was evaluated for varying dimensions and energy resolutions assumed for the detector. An actual system is being constructed based on this work. [ABSTRACT FROM AUTHOR]

A Compton camera is usually composed of a scatterer detector and an absorber detector. The interaction position resolution of the absorber detector is the critical or limiting detector parameter that most seriously affects the imaging resolution of the table-top Compton camera currently under development in our group. Currently, it is assumed that all interactions occur at the centers of the detector segments of the absorber detector (25-segmented germanium detector), which is a simple but obviously not the best practice, considering that the interaction depths of the photons in the absorber detector, especially of low-energy photons, are not distributed uniformly. More photons have interaction in the front region of the detector due to the limited penetrating depth of the photons. Therefore, the present study developed a novel, "average interaction depth (AID)" method, which improves the imaging resolution of Compton camera by additionally considering the average interaction depth of the incident photon in the absorber detector. Simulations with Geant4 showed that the imaging resolution of the Compton camera can be significantly improved by using the AID method. For example, the resolution for a 511 keV γ point source was improved from 9.9 mm to 7.6 mm in full width at half maximum (FWHM). For a 364 keV γ point source, the improvement was even more significant from 11.4 mm to 7.2 mm. [ABSTRACT FROM AUTHOR]

Using Canadian National (CN) Tower lightning data acquired during the summer of 2005, the performance characteristics of the North American Lightning Detection Network (NALDN) were evaluated, including the flash detection efficiency, stroke detection efficiency, absolute location error, location accuracy model error (50%, 90%, and 99% error ellipses), and peak current estimation. The NALDN detected seven out of the seven flashes recorded at the CN tower, resulting in a 100% flash detection efficiency. Furthermore, the NALDN detected 21 out of the 38 return strokes recorded at the tower, resulting in a stroke detection efficiency of 55%. Relative to the CN tower, the NALDN was found to have a median and mean absolute stroke location error of 0.358 and 0.395 km, respectively, for the 21 detected strokes. It was also demonstrated that the NALDN stroke location error seems to have a clear bias towards the north of the CN tower and a slight bias toward the east, with 18 of the 21 strokes predicted to be northeast of the tower. The 50%, 90%, and 99% error ellipses provided by the NALDN were also evaluated. It was found that 71% of the detected strokes (15 out of 21) were enclosed by the 50% error ellipse, 90% of the detected strokes (19 out of 21) were enclosed by the 90% error ellipse, and 95% of the detected strokes (20 out of 21) were enclosed by the 99% error ellipse. The minimum value for the 50% error ellipse axis is set at 0.4 km by Vaisala, Inc., and 20 of the 21 detected strokes had a semimajor axis length of 0.4 km, suggesting that the median location error for strokes hitting the CN tower is 0.4 km or less. The 0.358 km median location error obtained for the 21 detected strokes appears to support this. The dependence of stroke detection efficiency and location error on the characteristics of the current measured at the CN tower is evaluated. The NALDN is found to overestimate the current peak, which is possible to explain for tall-structure lightning. The dependence of stroke detection efficiency and location error on the characteristics of the CN tower lightning-generated electromagnetic pulse, measured 2 km north of the tower, is also evaluated. [ABSTRACT FROM AUTHOR]

Simultaneous measurements of the lightning return-stroke current derivative at the Canadian National (CN) Tower and the corresponding lightning electromagnetic pulse (LEMP) 2 km north of the tower have been performed since 1991 using broadband, high-resolution measurement systems. The recent installation of global positioning systems for time synchronization of CN Tower lightning measurement stations has enabled the correlation of the wavefront characteristics (peak, maximum derivative, and 10-90% rise time) of the return-stroke current with those of its LEMP. The characteristics of the tower's lightning electric and magnetic fields have also been correlated with each other. The current and field correlation analysis is based on August 19, 2005, CN Tower lightning return-stroke data. In addition to the strong linear correlations of the wavefront characteristics of the electric and magnetic field waveforms, an excellent linear correlation has been established between the magnetic field and current wavefront peaks. Also, a good linear correlation has been found between the magnetic field wavefront maximum derivative and that of the current. These findings are of interest in lightning detection methodology. For example, the current wavefront maximum derivative can be estimated from the detected magnetic field after taking into consideration the propagation effect on its derivative. Although weakly correlated, a general trend of increase in the magnetic field peak and maximum rate of rise has been observed as the current maximum rate of rise and peak increase, respectively. It is hoped that the field-current relationships developed in this paper, using the recent CN Tower lightning return-stroke data, will contribute to solving the inverse-source problem, one of the challenging problems in lightning research, where lightning current characteristics are estimated based on the characteristics of the measured LEMP. [ABSTRACT FROM AUTHOR]

This paper reviews the currently used minimum phase algorithm approach for the prediction of induced current as a result of the incidence of a time-domain electromagnetic environment such as an electromagnetic pulse. An alternative to this approach is proposed that enables the user to identify a lower bound on the minimum phase prediction. The technique has been applied to over 500 transfer functions with three different high-power electromagnetic environments and the results are presented and discussed within this paper. The paper concludes that the proposed method is a useful technique in providing an alternative method of predicting the induced current. [ABSTRACT FROM AUTHOR]

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 HPD's 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. [ABSTRACT FROM PUBLISHER]

A first prostate probe prototype based on the Compton imaging technique has been developed, with a scatter detector composed of a stack of five thick silicon pad detectors, and a scintillator as absorption detector. The silicon sensors are 4 cm times 1 cm, 1 mm thick, with 1.4 mm times 1.4 mm pad dimensions. The scatter detector performance has been optimized, and an energy resolution about 1.4 keV FWHM is obtained. The absorption detector consists of three Nal(Tl) scintillators that are placed around the scatter detector. A spatial resolution of 5 mm FWHM has been measured reconstructing an image of a point-like 133 Ba source placed at 11 cm distance from the scatter detector. Additional studies show the improvement of the detector resolution with increasing incident photon energy and relative distance from the scatter to the absorption detector. Simulations have also been performed for a complete understanding of the data, and to predict the expected resolution at near field operation of the device. The prototype construction is the first step in the development of a Compton probe for prostate imaging. [ABSTRACT FROM PUBLISHER]

A hybrid approach was developed to compute the dose deposited in cancerous and healthy tissues during stereotactic synchrotron radiation therapy treatment. The proposed approach divides the computation into two parts: (1) the dose deposited by primary radiation (coming directly from the incident X-ray beam) is calculated using a deterministic algorithm based on ray casting, optimized for transport in voxelized geometries; (2) the dose deposited by secondary radiation (Rayleigh and Compton scattering, fluorescence) is computed using a hybrid algorithm combining Monte Carlo and deterministic calculations. In the Monte Carlo part, a set of scattering and fluorescence events occurring in the patient is determined. These events are further processed in a deterministic way, which considerably improves the statistics of the final dose map. The results obtained in test cases are compared to those obtained with the Monte Carlo method alone (Geant4 and MCNPX codes) and found to be in excellent agreement. The proposed simulation scheme makes it possible to simulate dose maps with a single PC, featuring computation time and statistical fluctuations substantially reduced in comparison with full Monte Carlo simulations. [ABSTRACT FROM PUBLISHER]

An 8 × 8 pixellated NaI(TI) detector (1.7 cm × 1.7 cm × 1.0 cm) coupled to a multi anode PMT read out system was modeled to study the impact of light yield nonproportionality due to multiple photon scatterings and assess the contribution of delta rays in NaI(Tl) energy resolution. Geant4 simulation tool with modified nonproportional optical photons sampling routine was used in the study. A deconvolution technique was imple- mented to allow decoupling of the multiple photon scatterings contribution from the total energy resolution. Results from the simulation demonstrated that the effect of nonproportionality due to multiple photon scatterings is too small to be observed. However, the contribution of the delta rays was found to be a significant component of the evaluated NaI(TI) energy resolution. [ABSTRACT FROM AUTHOR]

Electromagnetic pulses radiated by parabolic antennas and similar structures are needed in many applications like air and ground penetrating radar or high power microwaves (HPM) weapons. In this paper, an approach based on Skulkin and Turchin's work and on physical optics approximation in the time domain is developed to determine the radiated fields. Closed-form time-domain expressions of the electromagnetic step and impulse responses, along the axis of a classical parabolic reflector antenna, are derived. A closed-form expression of the impulse response duration is also given. The obtained E-field and H-field formula are valid along the axis, both near and far from the reflector. Using these closed-form expressions, the radiated fields are computed by means of a convolution product of the primary source excitation and the impulse response. Numerical results have been obtained in the case of a causal sine and a generalized Gaussian impulse excitations to illustrate some specific transients effects which occur with such an antenna. [ABSTRACT FROM AUTHOR]

Respiratory motion remains a significant source of errors in treatment planning for the thorax and upper abdomen. Recently, we proposed a method to estimate two-dimensional (2-D) object motion from a sequence of slowly rotating X-ray projection views, which we called deformation from orbiting views (DOVs). In this method, we model the motion as a time varying deformation of a static prior of the anatomy. We then optimize the parameters of the motion model by maximizing the similarity between the modeled and actual projection views. This paper extends the method to full three-dimensional (3-D) motion and cone-beam projection views. We address several practical issues for using a cone-beam computed tomography (CBCT) scanner that is integrated in a radiotherapy system, such as the effects of Compton scatter and the limited gantry rotation for one breathing cycle. We also present simulation and phantom results to illustrate the performance of this method. [ABSTRACT FROM AUTHOR]

Gamma-ray attenuation is used to measure cryogenic mixture density. Parameters are optimized for specific space application. A simple error formula is established. The feasibility of the measurement is demonstrated. The effect of container's walls is measured. An automatic measurement device is built. A theoretical calculation is made for the geometry of Ariane V launcher. [ABSTRACT FROM AUTHOR]

Recent developments of large-area Si(Li) orthogonal-strip detectors have revealed their capability for applications in Compton-effect-based instruments. Some inherent advantages of silicon such as the dominance of Compton scattering in photon interactions and operation at room or somewhat lower temperature combined with the availability of large-volume Si(Li) detectors could stimulate the development of powerful Compton instruments. Several diodes 10 mm in thickness with a diameter of 102 mm were fabricated. Two 10 mm thick diodes were cut to form a 74 mmtimes74 mm square with slightly rounded corners. The same position-sensitive structure, 32 strips with a pitch of 2 mm, was produced on the thin Li-diffused n-contact and boron-implanted p+-contact by means of photolithography and plasma etched grooves. The position-sensitive area of 64 mmtimes64 mm is surrounded by a 5 mm wide guard-ring. One of these 10 mm thick Si(Li) orthogonal-strip detectors has been mounted in a cryostat prepared at Lawrence Livermore National Laboratory (LLNL). The detector will be extensively tested there with the goal of being integrated into the Compact Si+Ge Compton camera system consisting of this Si(Li) and a HPGe orthogonal-strip detector [ABSTRACT FROM PUBLISHER]

Today we face serious threats from radiological dispersion devices, improvised nuclear devices, and unsecured nuclear materials. Detectors which are currently used to detect and characterize these radioactive materials (or the radioactive materials within larger assemblies) suffer from large background rates. In addition, these detectors have only minimal ability to localize the position of the source without the use of mechanical collimators - which reduce efficiency. Imaging detectors using the Compton scattering process have the potential to provide greatly improved sensitivity through their ability to reject off-source background by localizing the source. We are developing a prototype device using readily available detector systems to explore Compton imaging technology. Our aim is to build a proof-of-concept device to study the Compton technology and to benchmark simulation efforts that will guide development of larger, more efficient devices that would be needed for deployment in the field. Here we discuss the concept of our detector design and results from Monte Carlo simulations of our prototype detector. We present an extension of an imaging technique used for gamma-ray astronomy to near-field sources [ABSTRACT FROM PUBLISHER]

The soft gamma-ray detector (SGD) onboard the Japanese future high energy astrophysics mission (NeXT) is a Compton telescope with narrow field of view, which utilizes Compton kinematics to enhance its background rejection capabilities. It is realized as a hybrid semiconductor gamma-ray detector which consists of silicon and cadmium telluride (CdTe) detectors. It can detect photons in a wide energy band (0.05-1 MeV) at a background level of 5 times10-7 counts/s/cm2/keV; the silicon layers are required to improve the performance at a lower energy band (<0.3 MeV). Excellent energy resolution is the key feature of the SGD, allowing it to achieve both high angular resolution and good background rejection capability. An additional capability of the SGD, its ability to measure gamma-ray polarization, opens up a new window to study properties of astronomical objects. We will present the development of key technologies to realize the SGD: high quality CdTe, low noise front-end application-specific integrated circuit, and bump bonding technology. Energy resolutions of 1.7 keV (full-width at half-maximum) for CdTe pixel detectors and 1.1 keV for Si strip detectors have been measured. We also present the validation of Monte Carlo simulation used to evaluate the performance of the SGD [ABSTRACT FROM PUBLISHER]

Impulse radiating antennas (IRAs) are an emerging class of antenna that are designed to radiate extremely short electromagnetic pulses with multiple decades of instantaneous band-width. The most common IRAs are made with a transverse electromagnetic (TEM) transmission line feeding a paraboloidal reflector. The IRA is usually constructed so that the electrical feed point of the TEM transmission line coincides with the focal point of the paraboloid. The paraboloidal reflector converts the spherical wave emanating from the feed point into a plane wave (i.e., a spherical wave centered at - ∞). In many practical cases, the feed point and focal points are not exactly aligned, producing some defocus of the reflector. In this paper, we model the case of hyperbolic defocus because of its analytic form and compare this model with experimental measurements. A hyperboloidal reflector fed from one focus converts the expanding spherical wave into a second expanding wave which appears to emanate from the second focal point of the hyperboloid (which is behind the reflector). Hyperboloidal defocus is roughly equivalent to moving the electrical feed closer to the reflector than the optical focal point. Previous theoretical results from in-focus IRAs predicted that the E- and H-plane temporal responses should be symmetric with respect to the temporal center of the response. The results shown here demonstrate that the defocusing causes these responses to become asymmetric. The new results are in better agreement with experimental measurements of IRAs and provide a physical explanation for experimental results that differ from the original theory. [ABSTRACT FROM AUTHOR]

Usually the position resolution in imaging applications with gamma rays is limited due to the range of secondary reaction products to a few mm. Here a new approach will be presented using the vertex of the gamma interaction as the quantity for position measurement. Because the dominant interaction is the Compton effect the vertex detection method as used in particle physics needs a number of essential modifications. Therefore a detector system is investigated consisting of a stack of Si-drift detectors for the vertex detection of the first Compton interaction and a secondary absorption detector where the position of the scattered photon is detected. A number of effects and possible solutions will be discussed including depth of interaction measurement and track reconstruction of the Compton electron which yields useful information. Another very promising approach of measuring the track projection as in a TPC (Time Projection Chamber) will be presented including first successful measurements in a gas detector. Because the expected rate in the first and second detector is of the order of 1 MHz and 10 MHz respectively the concept of read out electronics and data processing based on VLSI custom chips for signal shaping and first buffering and digital pipe-line processors based on FPGAs is presented. Finally a full system for small animal imaging based on the principle of a Compton Camera will be discussed in terms of achievable resolution and sensitivity. [ABSTRACT FROM AUTHOR]

In this paper, we describe a 4π Compton imager composed of a single 15 mm × 15 mm × 10 mm CdZnTe detector. Full 4π images are reconstructed via list-mode maximum likelihood (ML). A new weighting method for ML reconstruction is proposed in which the contributions of small-uncertainty sequences are enhanced relative to sequences with large uncertainties. The new reconstruction method is compared with traditional ML techniques for measured imaging data. The 4π Compton imager has a measured intrinsic imaging efficiency of nearly 2% and an imaging resolution using the weighted ML reconstruction method of 17° at 662 keV after 10 iterations. [ABSTRACT FROM AUTHOR]

Evaluates the impact of compton scatter, lead X-rays and high-energy contamination and their compensation on tumor detection and estimation tasks in Ga-67 imaging. Materials and methods; Spectrum of detected photons for Monte Carlo simulation of the anterior view of digital torso phantom; Overview of difference-of-Gaussian channels used for the channelized hotelling observer.

Presents a modeling of an electromagnetic pulse (EMP) simulator using a three-dimensional FDTD code. Background on EMP; Details of the model; Information on the electric field inside the transverse electromagnetic structure.

Presents a study that measured the electron and photon response of an aluminum canned LaCl[sub3]:10% Ce&sup3+; crystal using the Compton Coincidence Technique (CTT). Overview of the theoretical photon response calculation; Features of the CTT; Description of the response.

Provides information on a study which simulated the three-plane Compton-imaging technique using a prototype detector geometry for monoenergetic gamma sources. Methods; Results; Discussion.

Presents information on a study which described a technique for validating electron energy resolution results measured using the modified Compton coincidence technique. Components of scintillator energy resolution; Experimental techniques; Results and discussion; Conclusions.

Discusses the performance and simulations of the multiple Compton telescope. Calculation of the incident energy based on the Compton scattering formula; Impact of detectors on the accuracy of energy reconstruction; Limitations to the efficiency of the three-Compton telescope.

Proposes a high-energy resolution Compton camera for multitracer research. Principle and spatial resolution of the proposed Compton camera; Examination of the efficiency and resolution of the camera using the Monte Carlo simulation method; Conclusions.