Uncollimated Detector Research Articles

An assessment of contamination pickup on ground robotic vehicles for nuclear surveying application

Ground robotic vehicles are often deployed to inspect areas where radioactive floor contamination is a prominent risk. However, the accuracy of detection could be adversely affected by enhanced radiation signal through self-contamination of the robot occurring over the course of the inspection. In this work, it was hypothesised that a six-legged robot could offer advantages over the more conventional ground robotic devices such as wheeled and tracked rovers. To investigate this, experimental contamination testing and computational Monte Carlo simulation techniques (GEANT4) were employed to understand how radioactive contamination pick-up on three different robotic vehicles would affect their detection accuracy. Two robotic vehicles were selected for comparison with the hexapod robot based on their type of locomotion; a wheeled rover and a tracked rover. With the aid of a non-toxic fluorescent tracer dust, the contamination received by the all three vehicles when traversing a contaminated area was initially compared through physical inspection using high definition cameras. The parametric results from these tests where used in the computational study carried out in GEANT4. A cadmium zinc telluride detector was simulated at heights ranging from 10 to 50 cm above each contaminated vehicle, as if it were mounted on a plinth. Assuming a uniform activity of 60 Bq cm−2 on all contaminated surfaces, the results suggested that due to the hexapod’s small ground-contacting surface area and geometry, radiation detection rates using an uncollimated detector are likely to be overestimated by between only 0.07%–0.12%, compared with 3.95%–8.43% and 1.75%–14.53% for the wheeled and tracked robot alternatives, respectively.

Analytic Inversion of a Radon Transform on Double Circular Arcs With Applications in Compton Scattering Tomography

In this work we introduce a new Radon transform which arises from a new modality of Compton Scattering Tomography (CST). This new system is made of a single detector rotating around a fixed source. Unlike some previous CST, no collimator is used at the detector. Such a system allows us to collect scattered photons coming from two opposite sides of the source-detector segment, hence the manifold of the associated Radon transform is a family of double circular arcs. As first main theoretical result, an analytic inversion formula is established for this new Radon transform. This is achieved through the formulation of the transform in terms of circular harmonic expansion satisfying the consistency conditions in Cormack's sense. Moreover, a fast and efficient numerical implementation via an alternative formulation based on Hilbert transform is carried out. Simulation results illustrate the theoretical feasibility of the new system. From a practical point of view, an uncollimated detector system considerably increases the amount of collected data, which is particularly significant in a scatter imaging system.

Un-collimated single-photon imaging system for high-sensitivity small animal and plant imaging

In preclinical single-photon emission computed tomography (SPECT) system development the primary objective has been to improve spatial resolution by using novel parallel-hole or multi-pinhole collimator geometries. However, such high-resolution systems have relatively poor sensitivity (typically 0.01–0.1%). In contrast, a system that does not use collimators can achieve very high-sensitivity. Here we present a high-sensitivity un-collimated detector single-photon imaging (UCD-SPI) system for the imaging of both small animals and plants. This scanner consists of two thin, closely spaced, pixelated scintillator detectors that use NaI(Tl), CsI(Na), or BGO. The performance of the system has been characterized by measuring sensitivity, spatial resolution, linearity, detection limits, and uniformity. With 99mTc (140 keV) at the center of the field of view (20 mm scintillator separation), the sensitivity was measured to be 31.8% using the NaI(Tl) detectors and 40.2% with CsI(Na). The best spatial resolution (FWHM when the image formed as the geometric mean of the two detector heads, 20 mm scintillator separation) was 19.0 mm for NaI(Tl) and 11.9 mm for CsI(Na) at 140 keV, and 19.5 mm for BGO at 1116 keV, which is somewhat degraded compared to the cm-scale resolution obtained with only one detector head and a close source. The quantitative accuracy of the system’s linearity is better than 2% with detection down to activity levels of 100 nCi. Two in vivo animal studies (a renal scan using 99mTc MAG-3 and a thyroid scan with 123I) and one plant study (a 99mTcO4− xylem transport study) highlight the unique capabilities of this UCD-SPI system. From the renal scan, we observe approximately a one thousand-fold increase in sensitivity compared to the Siemens Inveon SPECT/CT scanner. UCD-SPI is useful for many imaging tasks that do not require excellent spatial resolution, such as high-throughput screening applications, simple radiotracer uptake studies in tumor xenografts, dynamic studies where very good temporal resolution is critical, or in planta imaging of radioisotopes at low concentrations.

Optimising in situ gamma measurements to identify the presence of radioactive particles in land areas

High-coverage in situ surveys with gamma detectors are the best means of identifying small hotspots of activity, such as radioactive particles, in land areas. Scanning surveys can produce rapid results, but the probabilities of obtaining false positive or false negative errors are often unknown, and they may not satisfy other criteria such as estimation of mass activity concentrations. An alternative is to use portable gamma-detectors that are set up at a series of locations in a systematic sampling pattern, where any positive measurements are subsequently followed up in order to determine the exact location, extent and nature of the target source. The preliminary survey is typically designed using settings of detector height, measurement spacing and counting time that are based on convenience, rather than using settings that have been calculated to meet requirements. This paper introduces the basis of a repeatable method of setting these parameters at the outset of a survey, for pre-defined probabilities of false positive and false negative errors in locating spatially small radioactive particles in land areas. It is shown that an un-collimated detector is more effective than a collimated detector that might typically be used in the field.

A Dual System for Monitoring the Positions of Multiple Radioactive Tracer Pebbles in Scaled Pebble Bed Reactors

A dual measurement system for monitoring the simultaneous positions of multiple radioactive tracer pebbles in scaled pebble bed reactors (PBRs) has been developed and benchmarked to the prototype stage. The first system (the collimated system) is an updated version of a previously developed system that is now a completely automatic system that uses three collimated directionally variable NaI detectors that are programed to continuously search for a maximum counting rate from a single radioactive pebble. This system can be used by itself when a single radioactive tracer pebble is of interest and the pebble is relatively slow moving. In the present case, its primary use is to provide an independent measurement of the position of a stationary tracer pebble that is used to provide a point for calibration of the second system. The second system (the uncollimated system) is a modified version of a multiple uncollimated NaI detector system commonly called CARPT. The modified version involves those changes necessary to allow for use of the entire gamma-ray spectra for the inverse problem instead of only the gamma-ray full energy peaks. This allows one to use multiple radioisotopes each in a different tracer pebble so that up to ten individual tracer pebbles can be followed simultaneously with the best possible accuracy. The inverse problem is treated with the Monte Carlo library least-squares approach in which Monte Carlo–generated library spectra for each radioisotope are made available for a complete range of reference positions within the scaled PBR. Then, any unknown total gamma-ray spectra can be analyzed in an iterative fashion with the radioisotope library spectra to yield the position of all the radioisotope tracer pebbles.The scaled PBR used was a 30-cm-high and 30-cm-diam circular cylindrical section on the top and a cone with a 25-deg angle on the bottom. The pebbles are 1.2-cm glass marbles. Results have been obtained with both single tracer radioisotope marbles and multiple tracer radioisotope marbles, simultaneously.

Dose rates in nuclear medicine and the effectiveness of lead aprons

Answers to common nuclear medicine radiation safety questions often involve the consideration of dose rates from injected patients and the inverse square law. For staff, lead aprons are available as an option, although they are not routinely used and their effectiveness varies depending on the isotope. New tests and procedures have been introduced at this hospital, including PET and Y microsphere implantation, which have required a review and investigation of their potential impact on staff doses. To answer these questions and to account for the recently introduced technologies and procedures, a study was conducted to measure and demonstrate the level of effectiveness of the department's lead aprons and to simulate patient dose rate measurements and estimations by obtaining measurements from water phantoms filled with these isotopes. A calibrated survey meter was used to measure dose rates at varying distances from water phantoms filled with Tc, Ga, I, F and Y. Thermoluminescence dosimeters attached to an anthropomorphic phantom with a lead apron were used to assess the effectiveness of the lead aprons available within the department. An uncollimated detector from a gamma camera was used to observe the changes to the energy spectrum in the presence of the lead apron. The results from the dose rate measurements demonstrated an overestimation by the inverse square law at close distances. This overestimation can be in excess of four times the measurements made within this study. The use of a lead apron was shown to reduce doses by varying degrees depending on the isotope used. A 64.5% dose reduction was observed when shielding against Tc with diminishing effectiveness against the remaining isotopes. The results for Y suggest that using a lead apron could result in dose escalation at shallow depths. A table of conversion factors, independent of the isotope, was generated for the estimation of dose rates from injected patients at various distances. An isotope-specific conversion table was also generated. The effectiveness of the lead aprons within the department was also successfully measured and assessed and recommendations were passed on to staff regarding their use.

Characterization of the count rate performance of modern gamma cameras

Evaluation of count rate performance (CRP) is an integral component of gamma camera quality assurance and system deadtime (τ) may be utilized for image correction in quantitative studies. This work characterizes the CRP of three modern gamma cameras and estimates τ using two established methods (decay and dual source) under a variety of experimental conditions. For the decay method, uncollimated detectors were exposed to a Tc-99m source of relatively high activity and count rates were sampled regularly over 48 h. Input count rate at each time point was based on the lowest observed count rate data point. The input count rate was plotted against the observed count rate and fit via least-squares to the paralyzable detector model (PDM) to estimate τ (rates method). A novel expression for observed counts as a function of measurement time interval was derived, taking into account the PDM and the presence of background but making no assumption regarding input count rate. The observed counts were fit via least-squares to this novel expression to estimate τ (counts method). Correlation and Bland-Altman analyses were performed to assess agreement in estimates of τ between the rates and counts methods. The dependence of τ on energy window definition and incident energy spectrum were characterized. The dual source method was also used to estimate τ and its agreement with estimates from the decay method under identical conditions was also investigated. The dependences of τ on the total activity and the ratio of source activities were characterized. The observed CRP curves for each gamma camera agreed with the PDM at low count rates but deviated substantially from it at high count rates. The estimates of τ determined from the paralyzable portion of the CPR curve using the rates method and the counts method were found to be highly correlated (r = 0.999) but with a small (~6%) difference. No statistically significant difference was observed between the estimates of τ using the decay or dual source methods under identical experimental conditions (p = 0.13). Estimates of τ increased as a power-law function with decreasing ratio of counts in the photopeak to the total counts. Also, estimates of τ increased linearly as spectral effective energy decreased. No significant difference was observed between the dependences of τ on energy window definition or incident spectrum between the decay and dual source methods. Estimates of τ using the dual source method varied as a quadratic on the ratio of the single source to combined source activities and linearly with total activity. The CRP curves for three modern gamma camera models have been characterized, demonstrating unexpected behavior that necessitates the determination of both τ and maximum count rate to fully characterize the CRP curve. τ was estimated under a variety of experimental conditions, based on which guidelines for the performance of CRP testing in a clinical setting have been proposed.

Deconvolving gamma-ray logs by adaptive zone refinement

Uranium exploration and mining make widespread use of gamma-ray logs to determine ore-grade distribution in situ. Attenuation, scattering, and uncollimated detectors all degrade the signal, hampering accuracy. We deconvolve their effects from the measured logs by space-domain processing with the aid of a function that takes account of detector length and uranium ore-zone thickness. The output quantifies uranium grade by zones of specified location and thickness. With this method, thin zones of very high grade can be used for calibration. Illustrative data come from model pits in Australia, Canada, and the USA.

Application of a remote-controlled collimated dosimetric system for surveying spent MR fuel storage facilities

The results of using a specially developed collimated dosimetric system, placed on a Brokk-90 remote-controlled machine, to survey a spent fuel storage facility are presented. The measuring block of the system consists of collimated and uncollimated γ-ray detectors, video cameras, and headlights. The range of the dose-rate measurements of the γ-ray detectors varies from 0.4 mSv/h to 8.5 Sv/h. The collimation angle of the collimated detector is 12° and the lead shielding is 30 mm thick. The system made it possible to perform this work in large radiation fields by remote means. The dose rate distribution along extended elements, extracted from the spent fuel storage facility in the MR hall, is obtained. The contents of the storage facility were visualized in all details. Using the results of the survey, the contents of the storage facility were repacked and removed from the MR room.

The Burst and Transient Source Experiment Earth Occultation Technique

An Earth orbiting detector sensitive to gamma-ray photons will see steplike occultation features in its count rate when a gamma-ray point source crosses the Earth's limb. This is due to the change in atmospheric attenuation of the gamma rays along the line of sight. In an uncollimated detector, these occultation features can be used to locate and monitor astrophysical sources provided their signals can be individually separated from the detector background. We show that the Earth occultation technique applied to the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) is a viable and flexible all-sky monitor in the low-energy gamma-ray and hard X-ray energy range (20 keV-1 MeV). The method is an alternative to more sophisticated photon imaging devices for astronomy and can serve well as a cost-effective science capability for monitoring the high-energy sky. Here we describe the Earth occultation technique for locating new sources and for measuring source intensity and spectra without the use of complex background models. Examples of transform imaging, step searches, spectra, and light curves are presented. Systematic uncertainties due to source confusion, detector response, and contamination from rapid background fluctuations are discussed and analyzed for their effect on intensity measurements. A sky location-dependent average systematic error is derived as a function of Galactic coordinates. The sensitivity of the technique is derived as a function of incident photon energy and also as a function of angle between the source and the normal to the detector entrance window. Occultations of the Crab Nebula by the Moon are used to calibrate Earth occultation flux measurements independent of possible atmospheric scattering effects.

Lateral migration radiography application to land mine detection, confirmation and classification

Lateral migration radiography (LMR) employs scattered photons to acquire detailed images of covered objects. images of plastic encased real mines buried in soil using LMR have shown dramatic differences compared to images generated using simulated mines. The major characteristic that enables the discernibility of land mines to the degree of actual type identification is the presence of voids (air volumes) required for the operation of the fuse assembly or for blast direction control. Air volumes greatly modify the detected field of both once and multiple-scattered photons. The LMR system consists of an x-ray generator and two uncollimated detectors positioned to detect once- scattered photons and two collimated detectors designed to detect primarily multiple-scattered photons. The x-ray generator is located in the gap between symmetrically arranged detectors; the collimated x-ray beam typically has a spot size of 1.5x 1.5 cm with perpendicular incidence on the soil surface. The optimal x-ray spectra for land mine detection with the LMR system range from 130 to 180 kVp with mean x-ray energies of from around 40 to 60 keV. Air volumes modify both exit paths and the position of first-scatter events; they also modify the migration paths of multiple-scattered photons, thus producing different images in the two detector types. The burial mode (below surface or laid on the surface) of the land mine can also be discerned by LMR due to a shadowing effect seen for surfaced-laid land mines. The presence of even a minute amount of metal in the land mine also aids in discerning the mine, because metal produces a signal decrease in both types of detectors. Monte Carlo calculations are performed with the MCNp code to obtain an understanding of the details of the photon lateral migration process. Images generated from these Monte Carlo calculations are in agreement with the experimental measurements. The real mine images confirm that LMR is capable not only of mine detection, but also of mine identification.

In-situ measurements of 137Cs in soil by unfolding method.

An improved in-situ spectrometry measurement of 137Cs concentration in soil is introduced. The method uses the information contained in the pulse spectrum in order to forego the need for soil sampling. The approach is based on the unfolding of responses of a collimated and uncollimated HPGe detector to primary 0.662 MeV photons and to photons scattered forward in the soil. The calibration of the in situ equipment has been performed by Monte Carlo calculations and by experiments. For unfolding of experimental detector responses the code SAND II has been found reliable and capable of calculating distribution of 137Cs in soil profile with adequate accuracy for environmental monitoring purposes. The analysis of the spectra indicates that 137Cs concentration in soil 10 y after Chernobyl accident would be measurable using a middle HPGe detector (20-30% relative efficiency) and a counting time on the order of 1 h. Even with smaller detectors, 137Cs concentrations of 5 kBq m(-2) are measurable, and the depth distribution of 137Cs activities above 10 kBq m(-2) in the soil can be estimated by the presented method when a counting time on the order of 3 h is used.

An enhanced sensitivity K-shell X-ray fluorescence technique for tibial lead determination

A novel irradiation-detection geometry capable of enhancing sensitivity for the measurement of tibial lead content by K-shell X-ray fluorescence (XRF) is described. The high-count-rate system comprised a small-area high-specific-activity (0.147 GBq mm-2) 109Cd source and a large-area (nominally 20 cm-2) uncollimated detector, forming an axially symmetric back-scattering arrangement. Precisions in the range+or-4.9 to+or-14.2 mu g Pb (g bone mineral)-1 have been obtained in a study of cohort of 63 controls and 73 workers industrially exposed to lead. These precisions are comparable with those obtained in results using earlier systems, but at reduced source activities (less than 50% of the activity of other systems) and with significant reduction in measurement time (some 30% less than the measurement times of other systems).

COMPTON PROFILE MEASUREMENT OF TWO-PHASE FLOW DISTRIBUTION

Abstract To improve the proposed technique of Compton profile measurements and develop an accurate density distribution measurement technique, the detected scattered spectrum in a system of collimated source and uncollimated detector is expressed in terms of the density distribution and a multiple-scattering factor. A particular experimental approach to this technique is outlined. Multiple-scattering is carefully considered in the analytical-numerical interpretation of the problem. The proposed technique is shown to be clearly valuable for stochastically steady flows in which relatively low frame rates for local time average two-phase flow parameters are acceptable.

ASSESSMENT OF DETECTOR SENSITIVITY AND UNIFORMITY DURING SPECT ROTATION.

The purpose of this research was to: a) evaluate variations in sensitivity and uniformity of SPECT detectors during 360/sup 0/ rotation, b) explore the causes of the variations and c) to discuss a correction procedure. A flood source consisting of a lucite disc 47.7 cm in diameter containing 3 microcuries of Co-57 was constructed. This source can be firmly attached to the uncollimated detector guaranteeing no detector-source geometry change during rotation. Four different SPECT cameras were tested. Measurements were obtained at 45/sup 0/ intervals throughout a 360/sup 0/ rotation, and repeated in 3 different orientations with respect to the earth's magnetic field. In one camera the effects of the direction and strength of induced magnetic fields were studied. All detectors showed cyclic rotational variations in sensitivity; in three cameras 6%, in one 3%. The amplitude and phase of the sensitivity variation curves and the variations in uniformity were unique for each camera. The detector's degree of variation did not change with regard to the earth's magnetic field orientation, but did change with regard to direction and strength of B fields. By measuring the gain of each PM tube at different detector positions, with and without B field influence, it wasmore » found that the gains shift were the cause of rotational changes in sensitivity and uniformity. By recording the variations of the detector during rotation at the clinical setting, a correction factor (either using software or hardware) can be applied to each angular image to improve SPECT resolution.« less

Diaschisis Resulting From Acute Unilateral Cerebral Infarction

HOEDT-RASMUSSEN and Skinhoj 1,2 were the first to report bilateral reduction of hemispheric blood flow (HBF) in patients with unilateral cerebral infarction. They measured HBF by means of intracarotid injection of a bolus of radioactive inert gas and an uncollimated detector placed over each hemisphere and found the HBF to be reduced on the healthy side as well as in the diseased hemisphere. They did not measure cerebral metabolism but hypothesized that unilateral infarction reduced metabolism, and consequently, resulted in reduced blood flow in the contralateral hemisphere due to transneural depression. There were only six patients with unilateral cerebral infarction in their series, three of whom had thrombosis or embolic occlusion of the internal carotid artery. The smallness of their series was complicated further by the fact that one of the requirements of their method is that the bolus be injected into the internal carotid artery to avoid extracranial