Source Collimator Research Articles

Response Functions for Neutron Skyshine Analysis

Neutron and associated secondary photon line-beam response functions (LBRFs) for point monodirectional neutron sources are generated using the MCNP Monte Carlo code for use in neutron skyshine analysis employing the integral line-beam method. The LBRFs are evaluated at 14 neutron source energies ranging from 0.01 to 14 MeV and at 18 emission angles from 1 to 170 deg, as measured from the source-to-detector axis. The neutron and associated secondary photon conical-beam response functions (CBRFs) for azimuthally symmetric neutron sources are also evaluated at 13 neutron source energies in the same energy range and at 13 polar angles of source collimation from 1 to 89 deg. The response functions are approximated by an empirical three-parameter function of the source-to-detector distance. These response function approximations are available for a source-to-detector distance up to 2500 m and, for the first time, give dose equivalent responses that are required for modern radiological assessments. For the CBRFs, ground correction factors for neutrons and secondary photons are calculated and also approximated by empirical formulas for use in air-over-ground neutron skyshine problems with azimuthal symmetry. In addition, simple procedures are proposed for humidity and atmospheric density corrections.

Nondestructive Inspection of Thin, Low-Z Samples using Multiplexed Compton Scatter Tomography

ABSTRACTThe viability of a Compton scattering tomography system for nondestructively inspecting thin, low Z samples for corrosion is examined. This technique differs from conventional x-ray backscatter NDI because it does not rely on narrow collimation of source and detectors to examine small volumes in the sample. Instead, photons of a single energy are backscattered from the sample and their scattered energy spectra are measured at multiple detector locations, and these spectra are then used to reconstruct an image of the object. This multiplexed Compton scatter tomography technique interrogates multiple volume elements simultaneously. Thin samples less than 1 cm thick and made of low Z materials are best imaged with gamma rays at or below 100 keV energy. At this energy, Compton line broadening becomes an important resolution limitation. An analytical model has been developed to simulate the signals collected in a demonstration system consisting of an array of planar high-purity germanium detectors. A technique for deconvolving the effects of Compton broadening and detector energy resolution from signals with additive noise is also presented. A filtered backprojection image reconstruction algorithm with similarities to that used in conventional transmission computed tomography is developed. A simulation of a 360–degree inspection gives distortion-free results. In a simulation of a single-sided inspection, a 5 mm × 5 mm corrosion flaw with 50% density is readily identified in 1-cm thick aluminum phantom when the signal to noise ratio in the data exceeds 28.

Imaging focused x-rays from glass capillaries

Recent developments of glass capillaries allow x-ray probes to be focused down to micrometer sizes. These probes show promise for microanalysis applications, if they can reliably deliver high intensity, well formed x-ray beams to a specimen. We have developed an optical bench for viewing and evaluating the focused x-rays emitted from these capillaries. The optical bench allows pulse counting, direct viewing of the x-ray probe spatial distributions, and energy pass band by using exchangeable detectors. Our results are used to determine the probe forming capabilities of the lenses and to indicate problems and directions for improving the manufacturing of the x-ray optics.The x-ray optical bench is shown in a schematic diagram in figure A. It consists of a conventional CuKα x-ray diffraction tube joined to an optical table. An optical rail extends across them. A shielded box with a hinged side covers the source and the optical rail. The x-ray optic and exchangeable detector system are located on computer controlled stages to allow fine adjustments in all necessary translation and tilt directions. This optical bench permits measurements to be devised, performed, and changed with a minimum of difficulty. The optical bench allows for maximum flexibility in sample positioning, detector positioning, source size, and source collimation. The quantities that we evaluate in each optical element include focal spot size, spot uniformity, total spot flux, spot aberrations, diffuse scattered radiation, and depth of field or convergence.

Determination of the morphology of deposited islands by grazing-incidence small-angle scattering

It is shown that grazing-incidence small-angle X-ray scattering (GISAXS) is a suitable technique to study the morphology of gold islands deposited on a silicon wafer covered by a carbon sublayer. New possibilities of this technique are realized when using a synchrotron source (flux and collimation) and when patterns are recorded with image plates (IPs). It is possible to study the anisotropic shape of the scattering pattern and to deduce the dimensions of the deposited clusters which are formed by the Volmer-Weber mechanism of film growth. Quantitative measurements can be made with a reference sample, leading to an absolute value of the scattered intensity.

The geometric modulation transfer function of a transmission imaging system that uses a SPECT scintillation camera and parallel hole collimation

An analytic expression has been derived to calculate the geometric modulation transfer function of a transmission imaging system that uses parallel hole collimation for both the source and the SPECT camera. This expression describes the resolution of the transmission imaging system and replaces the need to use computer intensive Monte Carlo simulations for the system design. The geometric modulation transfer function, denoted as MTFg(rho) = [A2sc(Ssc rho)**A2cc(Scc rho)]D(rho), where ** denotes two-dimensional convolution; Asc(rho) and Acc(rho) are the Fourier transforms (FT) of the aperture functions for the parallel hole source collimator (SC) and the camera collimator (CC) holes, respectively; D(rho) is the FT of the camera response; and ssc and scc are scaling constants that depend on the respective collimator dimensions, the system dimensions, the object distance above camera collimator and whether MTFg(rho) is calculated for the object or image plane. The theoretical MTFg(rho) was verified with Monte Carlo simulations and experimental results. The formalism shows that the system resolution is characterized by the camera resolution and a combination of the resolutions of the source and camera collimators. This expression can be used to optimize the design of transmission imaging systems to be used in nuclear medicine.

The fraction of photons undergoing head scatter in X-ray beams

The output factor in air for a high-energy X-ray beam varies with the collimator setting. Collimator backscatter and obscuring of the source in the target contribute to this variation, but the main component is photon scatter in structures in the accelerator head. To determine the scatter-to-primary ratio, SPR, between the air kerma from such scattered photons and from those that originate directly from the source, it is essential to know the shape of this function SPR(c) of the square collimator opening c, especially for small c. To determine this, simulated head scatter was generated in lead blocks inside the head. By taking the ratio of the output factors for two such blocks of slightly different thickness, the effects of source obscuring and collimator backscatter were eliminated. Using the measured difference in transmission through the two lead blocks, the limiting value for c to 0 could be determined and hence the scatter-to-primary ratios SPR(c). The results could be fitted well with an error function, which then was applied to data measured for clinical beams of 6 and 25 MV. For c=20 cm, SPR for one 6 MV beam was determined to be 0.016 without a flattening filter, 0.06 for the open beam, and 0.16 with the built-in wedge. At 25 MV, the SPR was higher, 0.09 and 0.20 for the open and wedged beams, respectively.

The Naval Research Laboratory’s new hard x-ray beamline at the National Synchrotron Light Source

The Naval Research Laboratory is currently rebuilding its hard x-ray beamline at the National Synchrotron Light Source (NSLS, X-23B) to increase the flux and energy range. The optical and mechanical design of the new configuration is described and its performance predicted. The new design contains a fixed-exit pair of mirrors for harmonic rejection and source collimation and a fixed-exit two-crystal monochromator with sagittally focusing second crystal. In addition, the horizontal acceptance of the line will be increased from 6 to 18 mrad. The line is expected to focus radiation to a spot 0.5 mm vertical by 1.0 mm horizontal in the energy range 2 to 30 keV. At 8.5 keV a throughput of 5.7×1012 photons/s/200 mA is expected with an E/ΔE of 5×103 with Si (111) crystals, a factor of 10 to 30 over present beamline throughput. The increased flux is due to improved mirror surfaces, decreased optical aberrations from conical sagittal focusing, and the increased horizontal acceptance.

Information analysis for 3D SPECT imaging of radionuclide distributions in attenuating objects

The existing communication model of SPECT studies is extended by including the attenuating medium in the communication channel. Based on the extended model two fundamental theorems are presented to demonstrate the significance of the mutual information in system design and imaging reconstruction. In addition, the authors present a mutual information formula for SPECT systems to image radionuclide distributions in attenuating objects using single photon emission computed tomography (SPECT) collimators, The formula is, in turn, expressed in terms of the source distribution, attenuation coefficients, and collimator parameters such as collimator thickness, hole size, septal thickness, hole shape factor, and focal length. The polyhedral collimator is employed for the authors' mutual information analysis in this report. Based on imaging surface decomposition, the analysis of the polyhedral collimator is reduced to the analysis of a single-headed collimator for planar imaging. Conventional collimators including parallel and cone beam collimators are used for the analysis of the single-headed collimator. The information loss due to attenuation is evaluated for myocardial imaging using technetium-99m and thalium-201.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Monte Carlo modelling of neutron depth dose distributions in optimizing prompt gamma in vivo neutron activation analysis systems

Optimization of prompt gamma in vivo neutron activation analysis systems is best achieved using Monte Carlo simulation. In this study the modelling of the dimensions and materials for source holders and collimators is described and compared with experimentally derived results where feasible. Results show that valid depth doses are obtained by modelling only the central part of an IVNAA system and that the use of borated paraffin as a reflector provides acceptable thermal fluence and depth dose.

Attenuation correction in PET using single photon transmission measurement.

The use of single photon transmission measurement with a rotating rod source has been evaluated to measure the attenuation correction factors in positron emission tomography (PET). The singles projections are resampled into the coincidence geometry using the detector positions and the rod source location. A nonparalyzable dead time correction algorithm was developed for the block detectors used in the McMaster PET scanner. This enables accurate attenuation correction factors (ACFs) to be computed using a wide range of source strengths for transmission scanning. Transaxial resolution is approximately 6 mm, which is comparable to emission scanning performance. Axial resolution is about 25 mm, with only crude source collimation. ACFs are underestimated by as much as 10% due to increased cross-plane scatter, compared to coincidence transmission accuracy. The response of the correction factors to object density is within 15%, when comparing singles transmission measurement to current coincidence transmission measurement. The major advantage of using singles transmission measurement is a dramatically increased count rate. A factor of 7 increase in count rate over coincidence scanning is possible with a 2-mCi transmission rod source. Uniformity of 2% in the transmission images is possible with this source strength and a 2-min acquisition. There are no randoms counted in singles transmission scans, which makes the measured count rate vary linearly with source activity. Singles detector dead time losses are approximately 6% in the detectors opposite a 2-mCi rod source.

Optimization of a pinhole collimator in a SPECT scintillating fiber detector system: A Monte Carlo analysis

Monte Carlo simulations were used to optimize the dimensions of a lead pinhole collimator in a SPECT system consisting of a line of equally spaced Tc-99m point sources and a plastic scintillating fiber detector. The optimization was performed by evaluating the spatial resolution and scanner sensitivity for each source distribution location and collimator parameter variation. An optimal spatial resolution of 0.43 cm FWHM was observed for a source distribution positioned 2.0 cm from the collimated scintillating fiber detection system with a pinhole radius of 1.0 mm and a collimator thickness of 3.0 cm for a 10,000 emission photon simulation. The optimal sensitivity of 257 cps/MBq (7704 cps/mCi) occurred for a source distance of 2.0 cm, a radius of 3.0 mm and a thickness of 3.0 cm.

Optimization of a polarized source for in vivo X-ray fluorescence analysis of platinum and other heavy metals

The Monte Carlo method was used to optimize a polarized photon source for the X-ray fluorescence analysis of platinum and other heavy metals in vivo. The source consisted of a 140 kVp, 25 mA X-ray tube with the photons plane-polarized by 90 degrees scattering. The use of plane-polarized photons results in a significant reduction in background when the fluorescent radiation is measured along the direction of polarization. A Monte Carlo computer programme was written to simulate the production and interaction of polarized photons in order to determine the optimal polarizing material and dimensions, together with beam width and geometrical arrangement of source, polarizer and beam collimators. Calculated photon energy distributions are compared with experimental data to test the validity of the model. The best configuration of the polarization system for the in vivo analysis of platinum consisted of a 20 mm Cu polarizing block with a secondary collimator subtending a 0.1 radian angle at the polarizer.

Concentration and collimation of diffuse linear light sources

We present an approach for concentration and collimation of a diffuse linear light source to achieve extremely high concentration in the long direction at the expense of the short direction. The approach is based on free-space optics and involves either arrays of prisms or holographic optical elements that couple between the two spatial directions. The results for a specific arrangement that concentrates a 40 mm by 0.4 mm source, with diffusive angle of about 0.1 rad, into a 1 mm by 1 mm isotropic spot are presented.

Photon absorption measurements of bone density in the presence of scattered radiation

Bone density measurements are frequently carried out using photon absorptiometry techniques. The analysis of data collected in this way depends upon scatter-free detector signals. In practice the detector and source collimation lead to high levels of scattered radiation being detected. An analysis of the problem using photon transport computer models has shown that the current systems are reasonably insensitive to the scatter contribution except where patient changes occur over a long series of measurements or where new results taken with DEXA are compared with those taken on older systems such as DPA. This is frequently the case since the major body of data related to serial bone density measurements has been achieved with DPA. Inclusion of scattered radiation in the bone density calculations can lead to a 0.5-3.5% reduction between system types. Patient size changes could lead to a reduction in bone density of 0.5% (femoral neck measurement) to a 1% (lumbar spine measurement).

Performance characteristics of transmission imaging using a uniform sheet source with parallel-hole collimation.

Transmission imaging is receiving increasing attention in SPECT due to the need to compensate for nonuniform attenuation in cardiac-chest SPECT. The quality of a transmission image has an important effect on the measured attenuation distribution. To improve image quality, knowledge of the performance characteristics of a transmission imaging system is essential. The characteristics, spatial resolution, detection efficiency, photon flux, and exposure to the object, of a transmission imaging system consisting of a gamma camera and a uniform sheet source have been studied. The results demonstrate that spatial resolution of a transmission imaging system can be improved by use of a high-resolution source collimator at the price of a moderate decrease in detection efficiency, in comparison to the uncollimated case. Also, the source collimator significantly reduces the photon flux and exposure to the object. This investigation suggests that a high-resolution collimator be used with an intense sheet source to improve spatial resolution and reduce statistical noise with low exposure to the patient. This research further suggests that the amount of source activity is determined by the requirement of image quality, detection geometry, and allowed absorbed dose to the patient.

Depth of investigation of density tools

The depth of investigation is an important parameter for all tools used in well logging. The authors examine the most appropriate method for characterizing the depth of investigation, using the results of Monte Carlo calculations by the MCNP code. A common measure of the depth of investigation called S/sub 1/(h) characterizes the detector response to a limited thickness of formation with a vacuum beyond. Alternative parameters which characterize average or median values of depth have been calculated. The variation of penetration depth as a function of the formation density, source collimation angle, and short spacing detector collimation angle has been characterized. It is shown that all these factors have a significant effect on the penetration depth of a tool. An alternative function S/sub 2/(h), based on the effect of a small density, source collimation angle and short spacing detector collimation angle has been characterized. It is shown that all these factors have a significant effect on the penetration depth of a tool. An alternative function S/sub 2/(h) curve, which is directly relevant to cases of invasion by a low-Z mudcake, is very different from S/sub 1/(h) and predicts substantially lower penetration depths.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Design of an instrument for far-infrared microspectroscopy using a synchrotron radiation source

A design of a microspectrophotometric system using a synchrotron radiation (SR) source is described. The system covers a wide spectral range of 50–13000 cm−1, being under construction at the UVSOR BL6B beamline in the Institute for Molecular Science. Preliminary experiments in the mid-infrared region (500–5000 cm−1) have qualitatively confirmed the theoretical calculation that the synchrotron radiation is more intense than a blackbody (T=1200 K) when a microspectrophotomeric technique is applied, which is due to natural collimation and high brilliance of SR source. The SR as an infrared source exhibits its advantage on measuring the spectra of small single crystals especially in the far-infrared region.

Improvements in the precision of in vivo bone lead measurements

The measurement of Pb in bone by X-ray fluorescence using 109Cd gamma -ray to excite K-shell X-rays is considered. Improvements to the basic system performance were made by changing the design of the source collimator and by including data from the Pb K beta 1,3 photopeaks, having used pile-up rejection to reduce background in their region of the spectrum. The summed variances for eight sample measurements are given: two each of four plaster-of-Paris phantoms embedded in wax.

Diffraction of Thermally Scattered X-Rays in Mosaic Crystal

A white line was observed across the 002 X-ray thermal diffuse spot of a pentaerythritol crystal. Cause of the line was attributed to the Bragg reflection of thermally scattered X-rays. The contrast and the width of the line obtained by means of photographic method in the Bragg case as well as in the Laue case were compared successfully with those calculated on the basis of the theory given by Bushuev et at .: Sov. Phys.- Solid State 25 (1983) 228. The application limit of the theory is discussed. In the analysis, a correction of the contrast and the width of the line for the source size and collimator system was introduced. This gives a new interpretation about the fact that the line becomes clear as the film stands at a long distance from the specimen.

Stimulated scanning infrared imaging system

A scanning infrared display system capable of variable magnification and high spatial resolution measures the emitted infrared radiation caused by the scanning of a highly collimated energy source beam on the surface to be imaged. The resulting infrared emissions are collected with a high sensivity stationary detector. The display image is formed on a CRT by scanning the object with a raster pattern in synchronization with the CRT and modulating the Z axis CRT potential (brightness) with the infrared detection signal.