Detecting small low emission radiating sources

The article addresses the possibility of robust detection of geometrically small, low emission sources on a significantly stronger background. This problem is important for homeland security. A technique of detecting such sources using Compton type cameras is developed, which is shown on numerical examples to have high sensitivity and specificity and also allows to assign confidence probabilities of the detection. 2D case is considered in detail

Paleomagnetism of the Newcastle Range, northern Queensland : Eastern Gondwana in the Late Paleozoic

The Newcastle Range is an extensive (2500 km²) and well-exposed caldera system erupted on the trailing edge of Eastern Gondwana between 325 and 295 Ma. Paleomagnetic samples were collected from ignimbrites and associated microgranitoid intrusions from the central, northern and southern calderas from which three components of magnetization are recognized. Component 1 is considered to be a viscous magnetization acquired during the Brunhes Chron. A presumed Permian component, C2, is found in seven paleomagnetic sites with a mean pole at 30.9°S, 139.7°E (K = 13.9, A95 = 16.8°, ASD = 21.7°), agreeing with previously reported Permian data from Australia. Carboniferous units have a well-defined characteristic component, C3, distinguished by dual polarity (predominantly reversed) and moderate to steep inclination directions. Paleomagnetic polarities in the Newcastle Range Volcanics are formation dependent and new constraints on the timing of Carboniferous volcanism (~325–317 Ma) are consistent with recent reanalysis of the base of the Permo-Carboniferous Reversed Superchron (PCRS). A mean paleomagnetic pole, calculated from 15 VGPs, lies at 63.4°S, 125°E (K = 26.22, A95 = 7.6°, ASD = 15.8°), suggesting that Australia remained at midlatitudes into the Middle Carboniferous. This paleomagnetic pole is consistent with similarly aged poles from Western Gondwana, the conformity of which indicates contributions from nondipole components of the Earth's paleofield were probably not significant in the time immediately preceding the PCRS.16 page(s

Return to Black Mountain : palaeomagnetic reassessment of the Chatsworth and Ninmaroo formations, western Queensland, Australia

Palaeomagnetic results from late Middle Cambrian–Early Ordovician carbonate sequences sampled at Black Mountain (Mt Unbunmaroo), Mt Datson and near Chatsworth Station (southeastern Georgina Basin) are presented. A palaeomagnetic reassessment of these carbonates was designed in an effort to constrain regional magnetization ages as results from an earlier study, conducted at Mt Unbunmaroo, play a pivotal role in a proposed Cambrian inertial interchange true polar wander (IITPW) event. Remanent magnetizations within these carbonates were found to be variably developed with most specimens displaying two of the five isolated components. Component PF, for which goethite is the identified remanence carrier, is thought to reflect a chemical remanent magnetization of recent origin. Component TR, held by haematite, has a palaeomagnetic pole consistent with the Tertiary segment of Australia's apparent polar wander path (APWP) and most probably was acquired as a consequence of prolonged weathering during this period. The A component has a palaeomagnetic pole at 54.7°S, 262.3°E (dp= 2.3°, dm= 4.5°) after unfolding. This direction, constrained by positive fold and reversal test statistics, is consistent with Australia's Early Devonian APWP, perhaps reflecting a remagnetization event associated with the intracratonic Alice Springs Orogeny. A Late Ordovician–Early Silurian remanence, component B, is also described; with 100 per cent unfolding the associated palaeopole lies at 8.0°S, 216.8°E (dp= 2.6°, dm= 5.1°) . A third Palaeozoic, and presumed primary or early diagenetic, component, C, also passes applied fold and reversal tests and has a palaeomagnetic pole at 48.6°N, 186.0°E (dp= 2.1°S, dm= 4.0°) . This palaeopole is dissimilar from younger magnetizations, is consistent with Cambrian poles from other parts of cratonic Australia and falls within a cluster of Middle–Late Cambrian (515–500 Ma) palaeopoles from other Gondwanan continents. The age attributed to the palaeopole associated with the C component, ~510 Ma, provides a tight constraint on the younger boundary of the proposed Cambrian IITPW event and its agreement with other Gondwanan palaeopoles is incompatible with the IITPW hypothesis. Components A, B and C are analogous to palaeomagnetic results reported in the earlier investigation of this region, and a comparison of results from the two studies, coupled with rigorous statistical analyses of the new findings, is presented.18 page(s

A directional algorithm for an electronically-collimated gamma-ray detector

We are developing an electronically-collimated gamma-ray detector for safety and security applications; localization is achieved using Compton-scatter coincidence detection. Electronic collimation allows the device to operate without physical collimation, providing high sensitivity while also allowing directional information to be determined. We report on the directional algorithm implemented for this system. Two approaches to direction reconstruction were evaluated; the first is filtered backprojection on a sphere using stereographic projection to implement filtering (Gunter, NSS-MIC2005). The second technique also backprojects events onto the sphere, then determines source location by the intersection of circumscribed rectangles for the backprojected cones. Algorithm performance was evaluated using randomly generated ideal Compton-scatter events from point sources for our detector geometry, as well as data from concurrent Monte Carlo simulations of a prototype system. Direction angles are calculated within 5% accuracy for source positions up to 45° off-axis for the filtering approach and ∼30° for the circumscription approach. Error in calculated direction angles depends on the arbitrary diameter of the sphere; optimally, the sphere should intersect the source. The circumscription technique converges to an estimate of direction angles in ∼50 events; the filtering approach requires ∼1000 events. The two methods complement each other in speed and field-of-view. ©2006 IEEE

An electronically-coilimated portable gamma-ray detector for locating environmental radiation sources

We are developing a detector system for locating environmental radiation sources. The design emphasizes compact size (ideally hand-held), wide field of view and high detection efficiency, and uses cadmium-zinc-telluride (CZT) detectors and electronic collimation via Compton-scatter detection. The detector design is a 6-sided box with a primary scatter detector on one end. GEANT4 simulations, allowing variations of detector parameters and source energies/locations, provided performance estimates. A partial prototype, using 16×16-pixel 38×38×5-mm3 CZT detectors, was developed and tested. Two methods to calculate source direction in real-time from the Compton scatter data were evaluated: (1) filtered backprojection of cones onto a sphere; (2) intersection with the sphere of bounding boxes circumscribed around the cones. Simulation results of the 6-sided box with the current CZT modules indicated 1-5% of incident gamma rays produce valid direction angles, with an angular resolution of ∼15°. The directional algorithms allowed a FOV (directional error \u3c10°) of approximately ±60°. The direction algorithms converge on a source direction estimate in as few as 100 detected events. With improvements in detector energy and spatial resolution, reasonable performance seems achievable for a range of radioisotopes, e.g., from Am-241 through Co-60