Demonstration of energy-coded Compton scatter tomography with fan beams for one-sided inspection

An instrument is demonstrated whereby radiographic images of a sample's electron density are compiled from the information encoded in the energy spectra of gamma rays backscattered from one side of the sample. It is assumed that access is restricted to only one surface of the object under inspection. Use of energy coding allows imaging in a fan beam rather than independent interrogation of individual volume elements. The Multiplexed Compton Scatter Tomograph instrument consists of an array of high-energy-resolution detectors and fan beam collimators. Instrument signals are converted to electron density images using a penalized weighted least squares image reconstruction algorithm coupled with a deterministic system model that includes effects of Doppler broadening. The proof-of-principle instrument is demonstrated on aluminum samples. In an 8 mm thick sample with a 4 mm void in its center, contrast recovery of 90% is achieved. In a 10 mm thick sample with a 3 mm void at the back about 85% of the contrast is recovered.     

Design of a new vacuum Compton gamma ray detector with clad metal electron converter plate

A newly designed vacuum Compton gamma-ray detector with Ta-Al clad-metal electron converter plate is described. The detecting efficiency for 1.25MeV gamma-ray is 7.85×10-3 electron/, which is 2.5 times higher than that with Fe converter plate. The designed detector has the merits of well processed and static vacuum keeping and can be used for intense pulsed gamma ray detecting.     

复合金属发射极真空康普顿探测器结构设计

采用Ta—Al复合金属电子转换靶设计了一种厚窗真空型康普顿探测器,其对1．25MeV1射线的探测效率达到7．85×10^-3e／γ,比Fe发射极探测器的探测效率高出约2．5倍。探测器具有良好的封装加工和静态真空保持特性,可用于强流脉冲γ射线测量场合。     

Hard X-ray polarization measured with a Compton polarimeter at synchrotron radiation facility

A hard X-ray polarimeter with CdTe detectors has been developed for measurement of the degree of X-ray polarization at synchrotron radiation facilities. It utilizes 90° Compton scattering from the low Z targets. Measurements were performed at both facilities of the beamline BL38B1 in SPring-8 and the beamline BL14A in KEK-PF. The degrees of X-ray polarization for 20 keV X-rays are 99% and 82% at the BL38B1 in SPring-8 and BL14A in KEK-PF, respectively. The polarization degrees in the energy range of 15 and 40 keV correspond to 99.6±0.2% and 96.1±0.2% at the beamline BL38B1 in SPring-8. The analyzing power of the polarimeter was estimated by the Monte Carlo simulation with EGS4. The synchrotron radiation facilities provide highly polarized X-ray beams at energies above 15 keV.     

Monte Carlo simulation of grating-based neutron phase contrast imaging at CPHS

Since the launching of the Compact Pulsed Hadron Source (CPHS) project of Tsinghua University in 2009, works have begun on the design and engineering of an imaging/radiography instrument for the neutron source provided by CPHS. The instrument will perform basic tasks such as transmission imaging and computerized tomography. Additionally, we include in the design the utilization of coded-aperture and grating-based phase contrast methodology, as well as the options of prompt gamma-ray analysis and neutron-energy selective imaging. Previously, we had implemented the hardware and data-analysis software for grating-based X-ray phase contrast imaging. Here, we investigate Geant4-based Monte Carlo simulations of neutron refraction phenomena and then model the grating-based neutron phase contrast imaging system according to the classic-optics-based method. The simulated experimental results of the retrieving phase shift gradient information by five-step phase-stepping approach indicate the feasibility of grating-based neutron phase contrast imaging as an option for the cold neutron imaging instrument at the CPHS.     

A kinetic Monte Carlo study of mixed 1D/3D defect migration

Defect clusters form readily in collision cascades in metals, and some of the self-interstitial atom clusters form as crowdion clusters that diffuse by one-dimensional migration along a close-packed direction. Defect interactions and thermal fluctuations can cause the direction of one-dimensional migration to change, resulting in a mixed one-dimensional/ three-dimensional migration. Kinetic Monte Carlo computer simulations applied to model systems are used to investigate the effects of one-dimensional, three-dimensional and mixed one-dimensional/ three-dimensional migration on defect reaction kinetics. The functional relationships between the sink strength, the size of sinks and the average distance between direction changes during mixed one-dimensional/three-dimensional migration are explored. This revised version was published online in July 2006 with corrections to the Cover Date.     

3D calculation of absorbed dose for 131I-targeted radiotherapy: a Monte Carlo study

Various methods, such as those developed by the Medical Internal Radiation Dosimetry (MIRD) Committee of the Society of Nuclear Medicine or employing dose point kernels, have been applied to the radiation dosimetry of (131)I radionuclide therapy. However, studies have not shown a strong relationship between tumour absorbed dose and its overall therapeutic response, probably due in part to inaccuracies in activity and dose estimation. In the current study, the GATE Monte Carlo computer code was used to facilitate voxel-level radiation dosimetry for organ activities measured in an (131)I-treated thyroid cancer patient. This approach allows incorporation of the size, shape and composition of organs (in the current study, in the Zubal anthropomorphic phantom) and intra-organ and intra-tumour inhomogeneities in the activity distributions. The total activities of the tumours and their heterogeneous distributions were measured from the SPECT images to calculate the dose maps. For investigating the effect of activity distribution on dose distribution, a hypothetical homogeneous distribution of the same total activity was considered in the tumours. It was observed that the tumour mean absorbed dose rates per unit cumulated activity were 0.65E-5 and 0.61E-5 mGY MBq(-1) s(-1) for the uniform and non-uniform distributions in the tumour, respectively, which do not differ considerably. However, the dose-volume histograms (DVH) show that the tumour non-uniform activity distribution decreases the absorbed dose to portions of the tumour volume. In such a case, it can be misleading to quote the mean or maximum absorbed dose, because overall response is likely limited by the tumour volume that receives low (i.e. non-cytocidal) doses. Three-dimensional radiation dosimetry, and calculation of tumour DVHs, may lead to the derivation of clinically reliable dose-response relationships and therefore may ultimately improve treatment planning as well as response assessment for radionuclide therapy.     

Broadband holography applied to eddy current imaging by using signals with multiplied phases

In this paper an eddy current imaging method for nondestructive testing purposes is presented which utilizes the concept of broadband holography. An eddy current coil which is used simultaneously as an antenna for eddy current generation and as a probe for detection of response of interaction between eddy currents and flaws, respectively, is moved along a synthetic aperture during the imaging procedure generating synthetic eddy current pulses by scanning a certain frequency range. In terms of wave propagation phenomena the penetration depth (range) of eddy currents in conducting media is small compared to the equivalent wavelength of this type of fields. Therefore, adequate resolution can only be obtained in the reconstructed cross-sectional images by phase multiplication of received multifrequency signals, which is equivalent to a fictitious reduction of wavelengths. Experimental results verify the imaging capability of this method with improved resolution compared to conventional eddy current testing methods.     

GEANT4 used for neutron beam design of a neutron imaging facility at TRIGA reactor in Morocco

Neutron imaging has a broad scope of applications and has played a pivotal role in visualizing and quantifying hydrogenous masses in metallic matrices. The field continues to expand into new applications with the installation of new neutron imaging facilities.In this scope, a neutron imaging facility for computed tomography and real-time neutron radiography is currently being developed around 2.0MW TRIGA MARK-II reactor at Maamora Nuclear Research Center in Morocco (Reuscher et al., 1990 [1]; de Menezes et al., 2003 [2]; Deinert et al., 2005 [3]).The neutron imaging facility consists of neutron collimator, real-time neutron imaging system and imaging process systems. In order to reduce the gamma-ray content in the neutron beam, the tangential channel was selected. For power of 250 kW, the corresponding thermal neutron flux measured at the inlet of the tangential channel is around 3×10¹¹ ncm²/s.This facility will be based on a conical neutron collimator with two circular diaphragms with diameters of 4 and 2 cm corresponding to L/D-ratio of 165 and 325, respectively. These diaphragms' sizes allow reaching a compromise between good flux and efficient L/D-ratio. Convergent-divergent collimator geometry has been adopted.The beam line consists of a gamma filter, fast neutrons filter, neutron moderator, neutron and gamma shutters, biological shielding around the collimator and several stages of neutron collimator. Monte Carlo calculations by a fully 3D numerical code GEANT4 were used to design the neutron beam line (http://www.info.cern.ch/asd/geant4/geant4.html[4]).To enhance the neutron thermal beam in terms of quality, several materials, mainly bismuth (Bi) and sapphire (Al₂O₃) were examined as gamma and neutron filters respectively. The GEANT4 simulations showed that the gamma and epithermal and fast neutron could be filtered using the bismuth (Bi) and sapphire (Al₂O₃) filters, respectively.To get a good cadmium ratio, GEANT 4 simulations were used to define the design of the moderator in the inlet of the radiation channel. A graphite block of 22 cm thickness seems to be the optimal neutron moderator.The results showed that the combination of 5 cm of bismuth with 5 cm of sapphire permits the filtration of gamma-rays, epithermal neutrons as well as fast neutrons in a considerable way without affecting the neutron thermal flux.     

Detection unit optimization of a neutron searching detector using Monte Carlo Simulations

The NSD is a portable Neutron Searching Detector developed at Rotem Industries Ltd. with a high efficiency for counting fast and thermal neutrons employing improved gamma rejection. The NSD detection-unit consists of two ³He detectors installed within a polypropylene moderator. The latest international standards for detection of illicit trafficking of radioactive materials require high sensitivity, relatively small dimensions, and light mass. In order for it to meet these standards, the NSD detection-unit was optimized using Monte Carlo N-Particle transport code (MCNP). The moderator mass and dimensions were reduced without deterioration, even improving the instrument's sensitivity. The purposed moderator improvements covered in this paper work well for traditional hand-held neutron search detectors based on ³He tubes as well as for new neutron detection technologies due to the severe worldwide shortage of ³He.Three geometrical moderator configurations were examined using the MCNP code—a rectangular box, a circular cylinder, and an elliptical base cylinder. The optimization results showed that both the rectangular box moderator and the elliptical base cylinder moderators achieve the appropriate sensitivity required by the standards with about 30% reduced mass. A prototype was fabricated with the rectangular box moderator configuration, and its response was successfully validated by comparing empirical measurements against the results of the MCNP code.Performance examination of the optimal detection unit prototype was made regarding the latest international standards. The results showed a 17% improvement in detection limit for radioactive materials along with a 14% to 17% increased neutron detection response, while keeping the false alarm rate below the required threshold, and maintaining a 26% mass reduction.     

Monte Carlo simulation of energy balance in a pure 3He migma of 4 MeV

The energy balance in a migma of self-colliding orbits of 4 MeV ³He is studied using a two-dimensional Monte Carlo simulation supplemented with results from a three-dimensional Monte Carlo simulation and a self-consistent calculation of diamagnetic effects. Electron drag is balanced against synchrotron radiation and bremsstrahlung and losses of hot electrons. Synchrotron radiation was assumed to be reflected and reabsorbed, but bremsstrahlung was not. Reabsorption was taken to be a function of position, but not of angle, which should be an underestimate. Electrons were treated relativistically. The results are presented as a function of density from just below to well into the diamagnetic regime. We find that diamagnetism can reduce synchroton radiation losses to manageable levels.     

Microdosimetry of alpha particles for simple and 3D voxelised geometries using MCNPX and Geant4 Monte Carlo codes

Microdosimetry using Monte Carlo simulation is a suitable technique to describe the stochastic nature of energy deposition by alpha particle at cellular level. Because of its short range, the energy imparted by this particle to the targets is highly non-uniform. Thus, to achieve accurate dosimetric results, the modelling of the geometry should be as realistic as possible. The objectives of the present study were to validate the use of the MCNPX and Geant4 Monte Carlo codes for microdosimetric studies using simple and three-dimensional voxelised geometry and to study their limit of validity in this last case. To that aim, the specific energy (z) deposited in the cell nucleus, the single-hit density of specific energy f(1)(z) and the mean-specific energy were calculated. Results show a good agreement when compared with the literature using simple geometry. The maximum percentage difference found is <6 %. For voxelised phantom, the study of the voxel size highlighted that the shape of the curve f(1)(z) obtained with MCNPX for <1 μm voxel size presents a significant difference with the shape of non-voxelised geometry. When using Geant4, little differences are observed whatever the voxel size is. Below 1 μm, the use of Geant4 is required. However, the calculation time is 10 times higher with Geant4 than MCNPX code in the same conditions.     

Diffusion Monte Carlo Study of the Small Mixed 3He-4He Clusters

The ground state properties of helium mixed clusters ³He_2,3,4 ⁴He₂, ³He_2,3,4 ⁴He₃, ³He_2,3,4 ⁴He₄, ³He_2,3 ⁴He₅, consisting of up to eight helium atoms are studied using variational and diffusion Monte Carlo calculations. For clusters with three and four ³He atoms released-node diffusion Monte Carlo method is used. Our calculations show that, within errorbars, clusters ³He_3,4 ⁴He₂ have the same binding energy as the cluster ³He₂ ⁴He₂, and that ³He₃ ⁴He₃ has the same binding energy as ³He₂ ⁴He₃. The clusters ³He_3,4 ⁴He₂ and ³He₃ ⁴He₃ are in states in which one or two ³He atoms are far away from the rest of the system. Other considered clusters are bound. In particular, we have shown the stability of the cluster ³He₄ ⁴He₃, which was previously considered unstable. The calculations are performed using several different interatomic potentials and the conclusions concerning stability are insensitive to the particular form of the interaction potential. We compare our results with the recent experiment and other theoretical calculations.     

Comparison of wurtzite and zinc blende III–V nitrides field effect transistors: a 2D Monte Carlo device simulation

An ensemble Monte Carlo method is used to compare the potentialities of zinc blende and wurtzite GaN for field effect transistor applications. First, bulk material electron transport properties are compared and we find that mobility, steady state velocity and velocity overshoot are at the advantage of zinc blende GaN. Then, zinc blende GaN and wurtzite GaN MESFET with very short gate length (L_g=0.12 μm) are investigated using a 2D Monte Carlo device simulation. A 50% gain in performance is obtained for the zinc blende GaN MESFET as compared with the wurtzite one. A zinc blende AlGaN/GaN HEMT is also simulated and exhibits a current density of 900 mA mm⁻¹, a transconductance of 480 mS mm⁻¹ and a cut-off frequency of 180 GHz.     

Coloured computational imaging with single-pixel detectors based on a 2D discrete cosine transform

We propose and demonstrate a computational imaging technique that uses structured illumination based on a two-dimensional discrete cosine transform to perform imaging with a single-pixel detector. A scene is illuminated by a projector with two sets of orthogonal patterns, then by applying an inverse cosine transform to the spectra obtained from the single-pixel detector a full-colour image is retrieved. This technique can retrieve an image from sub-Nyquist measurements, and the background noise is easily cancelled to give excellent image quality. Moreover, the experimental set-up is very simple.     

A 4D Monte Carlo laser-Compton scattering simulation code for the characterization of the future energy-tunable SLEGS

For the full determination of the characteristics of laser-Compton X/γ-ray source, a four-dimensional (three-dimensional time and frequency-domain) Monte Carlo laser-Compton scattering simulation code has been developed with Geant4 toolkit. The code has the capability to calculate the spatial, spectral and temporal characteristics of the LCS X/γ-ray source with slanting collision configuration. This capability is crucial for both the design of successful LCS X/γ-ray source, as well as future experiments and applications utilizing this kind of sources. In this paper, the algorithm of the code and its benchmark processes are presented in detail. To benchmark the code, the X/γ-ray data taken from the worldwide and typical LCS facilities, such as HIγS at Duke University and PLEIADES at LLNL are compared to the simulated results. The benchmark processes show that the simulated results are consistent with the measurements. Utilizing the code, partial characteristics of Shanghai laser electron gamma source (SLEGS) are studied as well. The SLEGS is expected to be an energy tunable γ-ray source with its maximum energy adjustable from tens MeV to hundreds MeV and its photon flux of 10⁵-10⁶ photons/s if a 1-mJ/pulse, 1-kHz Ti:Sa laser system is employed.     

Monte Carlo study of the ion-induced electron current tunneling through a metal-insulator-metal junction

A Monte Carlo program is developed to investigate the kinetically excited electrons passing through a realistic Ag-Al₂O₃-Al junction when Ar⁺ ions impact on the top Ag layer. The program includes excitation of the target electrons (by projectile ions, recoiling target atoms and fast primary electrons) and subsequent transport of these excited electrons from Ag to bottom Al layer of the metal-insulator-metal (MIM) junction. The calculated tunneling electron yield is consistent with the recently reported experimental results. The simulation, however, enables the calculation of partial tunneling electron yields of the electrons excited by the projectile ions, recoil atoms and cascade electrons, the depth distribution of the electron excitation points in the MIM junction and energy distribution of the tunneling electrons. Our calculation showed that the electrons excited by fast cascade electrons are the major contributor to the tunneling electron yield while the direct contribution of projectile ions to tunneling electron yield is evident only at the projectile energies greater than 10 keV. The tunneling electrons have their origin close to the bottom end of the Ag layer and bulk of the tunneling electrons have energies around 2 eV.     

Charge collection efficiency characterization of a HgI2 Frisch collar spectrometer with collimated high energy gamma rays

In this study, a 2.1×2.1×4.1 mm³ HgI₂ Frisch collar device was characterized through probing the device with a highly collimated 662 keV gamma rays (¹³⁷Cs check source) along the length and width of the device. In a systematic series of experiments, the detector was probed along its central line under different operating voltages of 1600, 1300, 1000, 800, 600 and 500 V. The experimental results were confirmed through a simulation of the charge collection for a device with the same size and operating conditions. It is shown that the HgI₂ Frisch collar device has a uniform response to gamma rays over two-thirds of the detector volume. The HgI₂ crystals and the Frisch collar detectors were grown and fabricated within the S.M.A.R.T Laboratory at Kansas State University.     

Path Integral Hybrid Monte Carlo Study on Structure of Small Helium-4 Clusters Doped with a Linear Molecule

Microscopic structure of small helium-4 clusters doped with a carbonyl sulfide molecule, OCS(⁴He) _N , at 0.37 K is studied by the path integral hybrid Monte Carlo method; the size of the cluster N ranges from N=2 to N=5. In all the cases examined in the present study, the helium atoms are localized around the carbon atom of the OCS molecule, forming a doughnut-type structure around the molecular axis. Bosonic exchange among the helium atoms is found to be promoted in the doughnut region, showing an anisotropic “superfluid” response of the clusters.     

Preliminary results in the construction of a 2D imaging device with metastable superconducting grains

We have built and tested a 16×16 channel 2D matrix detector based on metastable superconducting grains. The camera was constructed from different thicknesses and size distributions of tin grains in paraffin, placed between two orthogonal planes of pickup coils and operated at 1.5 K. The homogeneity of the response curves of the 100 central pixels was better than 1.3%. Test objectives were provided by short range electron sources of³⁵S and¹⁴C, of varying geometries, placed in close proximity to the suspension. Each pixel was 700×700 µm², and was separated from its nearest neighbour by 2000 microns. Preliminary images have been obtained without collimating optics or maskings, and an image reconstruction limited to analysis of the transition slopes of the nucleated grains. Analysis suggests a single pixel resolution. Prospects for the construction of a 3D tracking detector are considered.