Monte Carlo Simulation Software Research Articles

A fast, angle‐dependent, analytical model of CsI detector response for optimization of 3D x‐ray breast imaging systems

Accurate models of detector blur are crucial for performing meaningful optimizations of three-dimensional (3D) x-ray breast imaging systems as well as for developing reconstruction algorithms that faithfully reproduce the imaged object anatomy. So far, x-ray detector blur has either been ignored or modeled as a shift-invariant symmetric function for these applications. The recent development of a Monte Carlo simulation package called MANTIS has allowed detailed modeling of these detector blur functions and demonstrated the magnitude of the anisotropy for both tomosynthesis and breast CT imaging systems. Despite the detailed results that MANTIS produces, the long simulation times required make inclusion of these results impractical in rigorous optimization and reconstruction algorithms. As a result, there is a need for detector blur models that can be rapidly generated. In this study, the authors have derived an analytical model for deterministic detector blur functions, referred to here as point response functions (PRFs), of columnar CsI phosphor screens. The analytical model is x-ray energy and incidence angle dependent and draws on results from MANTIS to indirectly include complicated interactions that are not explicitly included in the mathematical model. Once the mathematical expression is derived, values of the coefficients are determined by a two-dimensional (2D) fit to MANTIS-generated results based on a figure-of-merit (FOM) that measures the normalized differences between the MANTIS and analytical model results averaged over a region of interest. A smaller FOM indicates a better fit. This analysis was performed for a monochromatic x-ray energy of 25 keV, a CsI scintillator thickness of 150 microm, and four incidence angles (0 degrees, 15 degrees, 30 degrees, and 45 degrees). The FOMs comparing the analytical model to MANTIS for these parameters were 0.1951 +/- 0.0011, 0.1915 +/- 0.0014, 0.2266 +/- 0.0021, and 0.2416 +/- 0.0074 for 0 degrees, 15 degrees, 30 degrees, and 45 degrees, respectively. As a comparison, the same FOMs comparing MANTIS to 2D symmetric Gaussian fits to the zero-angle PRF were 0.6234 +/- 0.0020, 0.9058 +/- 0.0029, 1.491 +/- 0.012, and 2.757 +/- 0.039 for the same set of incidence angles. Therefore, the analytical model matches MANTIS results much better than a 2D symmetric Gaussian function. A comparison was also made against experimental data for a 170 microm thick CsI screen and an x-ray energy of 25.6 keV. The corresponding FOMs were 0.3457 +/- 0.0036, 0.3281 +/- 0.0057, 0.3422 +/- 0.0023, and 0.3677 +/- 0.0041 for 0 degrees, 15 degrees, 30 degrees, and 45 degrees, respectively. In a previous study, FOMs comparing the same experimental data to MANTIS PRFs were found to be 0.2944 +/- 0.0027, 0.2387 +/- 0.0039, 0.2816 +/- 0.0025, and 0.2665 +/- 0.0032 for the same set of incidence angles. The two sets of derived FOMs, comparing MANTIS-generated PRFs and experimental data to the analytical model, demonstrate that the analytical model is able to reproduce experimental data with a FOM of less than two times that comparing MANTIs and experimental data. This performance is achieved in less than one millionth the computation time required to generate a comparable PRF with MANTIS. Such small computation times will allow for the inclusion of detailed detector physics in rigorous optimization and reconstruction algorithms for 3D x-ray breast imaging systems.

Differential dose contributions on total dose distribution of 125I brachytherapy source

This work provides an improvement of the approach using Monte Carlo simulation for the Amersham Model 6711 125I brachytherapy seed source, which is well known by many theoretical and experimental studies. The source which has simple geometry was researched with respect to criteria of AAPM Tg-43 Report. The approach offered by this study involves determination of differential dose contributions that come from virtual partitions of a massive radioactive element of the studied source to a total dose at analytical calculation point. Some brachytherapy seeds contain multi-radioactive elements so the dose at any point is a total of separate doses from each element. It is momentous to know well the angular and radial dose distributions around the source that is located in cancerous tissue for clinical treatments. Interior geometry of a source is effective on dose characteristics of a distribution. Dose information of inner geometrical structure of a brachytherapy source cannot be acquired by experimental methods because of limits of physical material and geometry in the healthy tissue, so Monte Carlo simulation is a required approach of the study. EGSnrc Monte Carlo simulation software was used. In the design of a simulation, the radioactive source was divided into 10 rings, partitioned but not separate from each other. All differential sources were simulated for dose calculation, and the shape of dose distribution was determined comparatively distribution of a single-complete source. In this work anisotropy function was examined also mathematically.

Silicon planar detectors adapted to slow neutron detection

AbstractMost neutron detectors use some type of conversion material to convert the incident neutrons into secondary charged particles, which can be detected inside the detector bulk afterwards. In these reactions, charged particles and recoil nuclei are emitted, with energies high enough to be distinguished from the gamma ray background. In this way, semiconductor detectors incorporating a layer of a neutron reactive material on top of the substrate can be used as neutron detectors. Based on their experience in the fabrication of radiation detectors, CNM‐IMB (National Center of Microelectronics in Barcelona, Spain) has started a new research line in solid‐state neutron detectors for imaging or dosimetry. The Geant4 Monte–Carlo simulation package has been used to predict the detection efficiency of planar silicon detectors with a neutron converter layer. At the same time, preliminary tests of silicon pad detectors covered with neutron converters have been carried out in the Autonomous University of Barcelona with a neutron source (241Am‐Be). Copyright © 2010 John Wiley & Sons, Ltd.

Monte Carlo calibration of the response of the University of Chicago’s Cosmic Ray Nuclei Experiment (CRNE) on IMP-8 to electrons above 0.5 MeV

Modern instrument-simulation techniques offer the possibility of increasing the scientific yield from archival space datasets. In this paper, we report on a simulation of the electron response of the University of Chicago’s Cosmic Ray Nuclei Experiment (CRNE) instrument on the IMP-8 satellite. IMP-8/CRNE returned data from 1973 to 2006. The CRNE particle telescope was designed to measure the isotopic composition of Galactic cosmic-ray (GCR) nuclei and has also been used in many studies of protons and ions above 10 MeV/nucleon from solar energetic particle (SEP) events. But CRNE also functions as a highly-capable detector for solar electrons above 0.5 MeV, an energy range that has not been extensively studied. Utilization of the CRNE electron data has heretofore been limited by the fact that CRNE was never calibrated for electrons. We have therefore used the GEANT4 Monte Carlo simulation package to model the CRNE response to electrons and (separately) protons for multiple energies and incident angles. The results were used to compute the energy- and angle-dependence of the effective area and the energy-dependence of the geometric factor. The response to protons, which was already well understood, was used to verify the mass model, the simulation settings, and the post-processing software. Our simulation of the IMP-8/CRNE electron response now allows analysis of hundreds of relativistic solar electron events observed by CRNE over the years, including studies of evolution of electron energy spectra with high time resolution. We show examples of these results and briefly discuss potential applications to future scientific investigations.

Design of the neutral K0L beamline for the KOTO experiment

The J-PARC E14 KOTO experiment aims at the first observation of the KL0→π0νν¯ decay. This experiment requires a well-collimated neutral beam with the small amount of halo neutrons. Collimation lines and beamline components in the beamline have been optimized to suppress halo neutrons by utilizing the GEANT-3 Monte-Carlo simulation package. We have achieved the design with sharp cross-sectional profiles of the beam and the halo-to-core neutron flux ratio of 10−5 level.

Simulation study of spatial resolution and sensitivity for the tapered depth of interaction PET detectors for small animal imaging

Improvements to current small animal PET scanners can be made by improving the sensitivity and the spatial resolution of the scanner. In the past, efforts have been made to minimize the crystal dimensions in the axial and transaxial directions to improve the spatial resolution and to increase the crystal length to improve the sensitivity of the scanner. We have designed tapered PET detectors with the purpose of reducing the gaps between detector modules and optimizing the sensitivity of a future-generation small animal PET scanner. In this work, we investigate spatial resolution and sensitivity of a scanner based on tapered detector elements using Monte Carlo simulations. For tapered detector elements more scintillation material is used per detector resulting in a higher sensitivity of the scanner. However, since the detector elements are not uniform in size, degradation in spatial resolution is also expected. To investigate characteristics of tapered PET detectors, the spatial resolution and sensitivity of a one-ring scanner were simulated for a system based on traditional cuboid detectors and a scanner based on tapered detectors. Additionally, the effect of depth of interaction (DOI) resolution on the spatial resolution for the traditional and tapered detectors was evaluated. All simulations were performed using the Monte Carlo simulation package GATE. Using the tapered arrays, a 64% improvement in the sensitivity across the field of view was found compared with traditional detectors for the same ring diameter. The level of DOI encoding was found to be the dominating factor in determining the radial spatial resolution and not the detector shape. For all levels of DOI encoding, no significant difference was found for the spatial resolution when comparing the tapered and the cuboid detectors. Detectors employing the tapered crystal design along with excellent DOI resolution will lead to PET scanners with higher sensitivity and uniform spatial resolution across the field of view.

Comparing Actual to Estimated Base Rates of "Abnormal" Scores on Neuropsychological Test Batteries: Implications for Interpretation

Clinicians can use the prevalence of low scores to help interpret test performance. However, this information is limited for most test batteries. In 2007, Crawford, Garthwaite, and Gault presented Monte Carlo simulation software for estimating the base rates of low scores for any battery of tests. The purpose of this study is to examine the accuracy of a Monte Carlo simulation program for estimating the base rates of low scores. Base rates of low scores were: (a) calculated from large normative samples (actual base rates) for the Neuropsychological Assessment Battery and the Wechsler Adult Intelligence Scale-III/Wechsler Memory Scale-Third Edition and compared to (b) Monte Carlo estimations (estimated base rates). Monte Carlo estimations of the base rates of low scores had good accuracy when compared with the actual base rates of low scores for the two batteries. However, estimated base rates lose considerable accuracy in those with low or high intelligence. Monte Carlo simulation software is a potential option for clinicians to compute the base rates of low scores for any battery with published intercorrelations. However, the Monte Carlo program underestimates the base rates for those with low intelligence and overestimates the base rates for those with high intelligence.

Theoretical and Numerical Study of MLEM and OSEM Reconstruction Algorithms for Motion Correction in Emission Tomography

Patient body-motion and respiratory-motion impacts the image quality of cardiac SPECT and PET perfusion images. Several algorithms exist in the literature to correct for motion within the iterative maximum-likelihood reconstruction framework. In this work, three algorithms are derived starting with Poisson statistics to correct for patient motion. The first one is a motion compensated MLEM algorithm (MC-MLEM). The next two algorithms called MGEM-1 and MGEM-2 (short for Motion Gated OSEM, 1 and 2) use the motion states as subsets, in two different ways. Experiments were performed with NCAT phantoms (with exactly known motion) as the source and attenuation distributions. Experiments were also performed on an anthropomorphic phantom and a patient study. The SIMIND Monte Carlo simulation software was used to create SPECT projection images of the NCAT phantoms. The projection images were then modified to have Poisson noise levels equivalent to that of clinical acquisition. We investigated application of these algorithms to correction of (1) a large body-motion of 2 cm in Superior-Inferior (SI) and Anterior-Posterior (AP) directions each and (2) respiratory motion of 2 cm in SI and 0.6 cm in AP. We determined the bias with respect to the NCAT phantom activity for noiseless reconstructions as well as the bias-variance for noisy reconstructions. The MGEM-1 advanced along the bias-variance curve faster than the MC-MLEM with iterations. The MGEM-1 also lowered the noiseless bias (with respect to NCAT truth) faster with iterations, compared to the MC-MLEM algorithms, as expected with subset algorithms. For the body motion correction with two motion states, after the 9th iteration the bias was close to that of MC-MLEM at iteration 17, reducing the number of iterations by a factor of 1.89. For the respiratory motion correction with 9 motion states, based on the noiseless bias, the iteration reduction factor was approximately 7. For the MGEM-2, however, bias-plot or the bias-variance-plot saturated with iteration because of successive interpolation error. SPECT data was acquired simulating respiratory motion of 2 cm amplitude with an anthropomorphic phantom. A patient study acquired with body motion in a second rest was also acquired. The motion correction was applied to these acquisitions with the anthropomorphic phantom and the patient study, showing marked improvements of image quality with the estimated motion correction.

Simulations of a Scintillator Compton Gamma Imager for Safety and Security

We are designing an all-scintillator Compton gamma imager for use in security investigations and remediation actions involving radioactive threat material. To satisfy requirements for a rugged and portable instrument, we have chosen solid scintillator for the active volumes of both the scatter and absorber detectors. Using the BEAMnrc/EGSnrc Monte Carlo simulation package, we have constructed models using four different materials for the scatter detector: LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , NaI, CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and PVT. We have compared the detector performances using angular resolution, efficiency, and image resolution. We find that while PVT provides worse performance than that of the detectors based entirely on inorganic scintillators, all of the materials investigated for the scatter detector have the potential to provide performance adequate for our purposes.

WE-C-303A-06: Design of a PET Scanner for in Situ Dose Verification in Proton Beam Therapy

Introduction — During proton therapy,positron emitting radio‐isotopes are produced along the trace where incident protonbeam interacts with human tissue. The isotope activity profile is related to the proton dose distribution. Such isotope activities can be measured and imaged using a PET scanner. Therefore an accurate and sensitive PETimage of the isotope activities can be used for verification of proton dose. Our work is to design an efficient PET system for in situproton dose measurement in order to measure the activities of short‐lived isotopes.Method — A non‐rotating partial PET ring configuration is advantageous for such measurements as the decay of short‐lived isotopes can be measured almost immediately after the stop of proton irradiation. The PET design is optimized by varying 1)crystal cross‐section that is related to spatial resolution; 2) crystal types that affect intrinsic sensitivity and Time of Flight information; 3) angular coverage of the PET ring that is related to system sensitivity and reconstructed image quality. This study uses EGS4 based Monte Carlo simulation software that has been developed at Penn for PET scanner design studies. Geant4 software is used to produce the protonisotope activity. This isotope distribution is parameterized and used as the source distribution in EGS4. It is placed in a cylindrical water phantom initiating from phantom surface inward to mimic the patient treatment configuration. Iterative image reconstruction, together with data corrections for scatter, and attenuation, are incorporated to achieve high‐quality images and quantitative data. Results — The reconstructedisotope source from PETimages agrees well with the source activity for several design configurations. The distal falloff region of the isotope trace can be clearly identified. Summary — The design of PET system can be optimized so that the configuration yields an accurate and efficient reconstruction of the isotope activities.

A new simulation package to model detector systems with fragmentation reactions and ion separators: Application to the LYCCA-0 system

A Monte-Carlo simulation package has been developed to model the response of a detector system for ion identification used in conjunction with ion separators following nuclear reactions. The simulation is written predominantly using the GEANT4 framework but utilises the ion transport code MOCADI for accurate separator and reaction modelling. A novel MOCADI–GEANT4 interface has been developed to utilise the parameter file output option of MOCADI as an event generator for the GEANT4 detector simulation. Test simulation results have been compared with experimental data and excellent agreement was observed. The simulation has successfully been used to model a new particle detection system prototype (Lund-York-Cologne CAlorimeter (LYCCA)-0) and validate a method of ion identification using energy and time-of-flight with this system.

A feature parameter of optical diffuse reflectance on normal and malignant human breast tissue

Light that is delivered to the tissue surface undergoes multiple elastic scattering and absorption; part of it returns to the surface as optical diffuse reflectance, carrying quantitative information on the structure and composition of the measured tissue. In this paper, we utilized a well tested and publicly available Monte Carlo simulation software to simulate optical diffuse reflectance on normal and malignant human breast tissue in the visible wavelength region 450–650 nm. Based on the Monte Carlo simulation results, we discovered a feature parameter of optical diffuse reflectance on the simulated tissue surface. Normal and malignant human breast tissue may be discriminated by the value of the feature parameter. The values of the feature parameter are shown for normal and malignant human breast tissue in the visible wavelength region 450–650 nm. Experiments with tissue phantoms showed that the feature parameter is correlated to the component of the phantom. So the feature parameter is useful for the non-invasive optical diagnosis of biological tissue.

Exploring monte carlo simulation applications for project management

Monte Carlo simulation is a useful technique for modeling and analyzing real-world systems and situations. This paper is a conceptual paper that explores the applications of Monte Carlo simulation for managing project risks and uncertainties. The benefits of Monte Carlo simulation are using quantified data, allowing project managers to better justify and communicate their arguments when senior management is pushing for unrealistic project expectations. Proper risk management education, training, and advancements in computing technology combined with Monte Carlo simulation software allow project managers to implement the method easily. In the field of project management, Monte Carlo simulation can quantify the effects of risk and uncertainty in project schedules and budgets, giving the project manager a statistical indicator of project performance such as target project completion date and budget.

Design of neutron beamline for fundamental physics at J-PARC BL05

A new beamline for a fundamental physics experiment is under construction at BL05 port in the Materials and Life Science Facility (MLF) at Japan Proton Accelerator Research Complex (J-PARC); this beamline is designed using novel techniques of neutron optics and it is termed “Neutron Optics and Physics.” The beam from the moderator is deflected by multi-channel supermirrors and split into three branches for individual experiments. In this study, we have optimized the design of the beam optics and shields using the Monte Carlo simulation package PHITS. The neutron fluxes of beams are expected to be 9.2×10 5 /cm 2/μstr/s/MW, 1.2×10 9 /cm 2/s/MW, and 4.0×10 8 /cm 2/s/MW, with polarization of 99.8%.

Improved radioxenon gamma-spectrometry counting system and its efficiency calibration: Monte Carlo simulation and experimental results at enriched xenon counting environment

To give satisfactory efficiency both for X- and gamma-ray photon, an improved counting system has been developed in CTBT Canadian radioxenon laboratory. The counting system consists of a BEGe detector coupled with a thin carbon fiber window counting cell, that can perform a reliable and efficient radioxenon measurement. A semi-empirical calibration procedure was adopted, which is a combination of experimental measurement and mathematical simulation. Mathematical calibration tool is Monte Carlo simulation software named VGSL. Advanced gamma-spectrum analysis software, named Aatami, was used for gamma-ray peak shape fitting and X-ray multiplets deconvolution. The calculated full energy peak efficiency curve covers from 30 to 700 keV and agrees well with experimental data points within 2%. The efficiency curve can provide radioxenon analysis both for X-rays and gamma-rays with high quality. The efficiency distortion near xenon k-absorption edge of 35 keV, which is caused by high concentrated xenon in the counting cell, is also discussed.

SU‐GG‐T‐321: Validation Study of the GEANT 4 Monte‐Carlo Simulation of the Proton Dose Distribution From Passively Scattered Proton Beam Nozzles at the Proton Therapy Center at Houston

Purpose: To validate the accuracy of the representation of the hardware components and the process of Monte‐Carlo simulation of the dose distributions from the passively scattered proton beam nozzles of the Hitachi ProBeat system at the Proton Therapy Center at Houston (PTCH) using the GEANT4 software package. Method and Materials: GEANT4 is a widely used software package for Monte‐Carlo simulation (MCS) of ionizing radiation transport, which we plan to use extensively, (a) to check the accuracy of both the measured beam data of our proton therapy beams and the patient dose distributions calculated by the treatment planning system, (b) to obtain the dose distribution in many situations where accurate measurement would not be possible and (c) to compare the results with other MCS packages used at our institution. As the first step, we have carried out simulations of the passively scattered nozzles for selected energies by properly including the material and geometry of the first and second scatterer, range modulating wheels, dose and beam profile monitors, and calculated the dose distribution in a water phantom. The depth dose curves for the pristine Bragg peaks (PBP), spread out Bragg peaks (SOBP) and profiles at selected depths are compared with those obtained from ion chamber measurements. Results: The ranges obtained from the GEANT4 MCS agreed within 0.5 mm both for the PBP and SOBP. The overall agreement of the depth dose curves is within 2%. The profiles agree within 3% in most of the regions, with some points differing by as much as 5%. The disagreement for profiles is expected to be reduced by increasing the number simulation histories. Conclusion: We have accurately implemented the passively scattered proton beam nozzle at PTCH for MCS of proton dose distributions using the GEANT4 software package.

WE-D-AUD B-08: An Easy Method to Implement the Portal Dosimetry for IMRT Validation

Purpose: The aim of this study was to develop a convenient method for IMRT validation with an amorphous silicon electronic portal imaging device (EPID). Instead of using Monte Carlo simulation or custom-built proprietary software, a commercial treatment planning software(TPS) was utilized to calculate the dose response of the imager. Method and Materials: To calculate the dose response of the EPID, a series of images were acquired for the commissioning of TPS. A new machine was created in the TPS for modeling the scattering properties of the detectors. Each pixel of the portal image was first converted to dose and followed by the off-center correction. The pixel value at field center was used to project the absolute dose delivered to the EPID. Results: The accuracy of this method was investigated with several IMRT plans for different sites(prostate, head & neck and breast). The corrected portal images and calculated dose maps were compared by isodose lines, percent dose difference and gamma (γ) function. Both the absolute dose level and relative dose distribution show excellent agreement. For all the test cases, the passing rates of γ function with 3% and 3 mm criteria are more than 90% of the area enclosed by 20% isodose lines, and the passing rates with 5% and 5 mm criteria are higher than 99%. Conclusion: There are many challenging problems when the EPID is used for dosimetric applications. The proposed method applied necessary corrections to the portal images and the they showed excellent agreement with the calculated dose maps. The pencil-beam algorithm in the TPS successfully modeled the dose kernels of the detectors. Most importantly, the simplicity of this method make it very easy to be implemented for most of the users of EPID.

Realistic PET Monte Carlo Simulation With Pixelated Block Detectors, Light Sharing, Random Coincidences and Dead-Time Modeling

We have developed and validated a realistic simulation of random coincidences, pixelated block detectors, light sharing among crystal elements and dead-time in 2D and 3D positron emission tomography (PET) imaging based on the SimSET Monte Carlo simulation software. Our simulation was validated by comparison to a Monte Carlo transport code widely used for PET modeling, GATE, and to measurements made on a PET scanner. METHODS: We have modified the SimSET software to allow independent tracking of single photons in the object and septa while taking advantage of existing voxel based attenuation and activity distributions and validated importance sampling techniques implemented in SimSET. For each single photon interacting in the detector, the energy-weighted average of interaction points was computed, a blurring model applied to account for light sharing and the associated crystal identified. Detector dead-time was modeled in every block as a function of the local single rate using a variance reduction technique. Electronic dead-time was modeled for the whole scanner as a function of the prompt coincidences rate. Energy spectra predicted by our simulation were compared to GATE. NEMA NU-2 2001 performance tests were simulated with the new simulation as well as with SimSET and compared to measurements made on a Discovery ST (DST) camera. RESULTS: Errors in simulated spatial resolution (full width at half maximum, FWHM) were 5.5% (6.1%) in 2D (3D) with the new simulation, compared with 42.5% (38.2%) with SimSET. Simulated (measured) scatter fractions were 17.8% (21.3%) in 2D and 45.8% (45.2%) in 3D. Simulated and measured sensitivities agreed within 2.3 % in 2D and 3D for all planes and simulated and acquired count rate curves (including NEC) were within 12.7% in 2D in the [0: 80 kBq/cc] range and in 3D in the [0: 35 kBq/cc] range. The new simulation yielded significantly more realistic singles' and coincidences' spectra, spatial resolution, global sensitivity and lesion contrasts than the SimSET software.

Commissioning a fast Monte Carlo dose calculation algorithm for lung cancer treatment planning

A commercial Monte Carlo simulation package, NXEGS 1.12 (NumeriX LLC, New York, NY), was commissioned for photon‐beam dose calculations. The same sets of measured data from 6‐MV and 18‐MV beams were used to commission NXEGS and Pinnacle 6.2b (Philips Medical Systems, Andover, MA). Accuracy and efficiency were compared against the collapsed cone convolution algorithm implemented in Pinnacle 6.2b, together with BEAM simulation (BEAMnrc 2001: National Research Council of Canada, Ottawa, ON). We investigated a number of options in NXEGS: the accuracy of fast Monte Carlo, the re‐implementation of EGS4, post‐processing technique (dose de‐noising algorithm), and dose calculation time. Dose distributions were calculated with NXEGS, Pinnacle, and BEAM in water, lung‐slab, and air‐cylinder phantoms and in a lung patient plan. We compared the dose distributions calculated by NXEGS, Pinnacle, and BEAM. In a selected region of interest (7725 voxels) in the lung phantom, all but 1 voxel had a γ (3% and 3 mm thresholds) of 1 or less for the dose difference between the NXEGS re‐implementation of EGS4 and BEAM, and 99% of the voxels had a γ of 1 or less for the dose difference between NXEGS fast Monte Carlo and BEAM. Fast Monte Carlo with post‐processing was up to 100 times faster than the NXEGS re‐implementation of EGS4, while maintaining ±2% statistical uncertainty. With air inhomogeneities larger than 1 cm, post‐processing preserves the dose perturbations from the air cylinder. When 3 or more beams were used, fast Monte Carlo with post‐processing was comparable to or faster than Pinnacle 6.2b collapsed cone convolution.PACS numbers: 87.18.Bb, 87.53.Wz

A portable energy-sensitive cosmic neutron detection instrument

The construction and testing of a portable energy-sensitive neutron instrument are described. This instrument has been designed and constructed for the primary purpose of characterizing cosmic-ray neutron fields in the upper atmosphere and in cosmic reference field facilities. The instrument comprises a helium-3 proportional counter surrounded by 15 mm of lead and 140 mm of polyethylene creating a spherical structure with a diameter of 34 cm. The instrument also incorporates 12 boron-coated diodes, six on the outside of the polyethylene layer with six placed within the structure. The dimensions, materials, and arrangement of these in the instrument have previously been optimized with the MCNPX Monte Carlo simulation software to provide a compromise between the requirements of portability and spectral response. Testing took place at several locations and experimental data from the instrument's operation at the high-altitude Jungfraujoch laboratory in the Swiss alps are presented.