A Compton telescope for remote location and identification of radioactive material

The spare detectors from NASA Compton Gamma-Ray Observatory COMPTEL instrument have been reconfigured to demonstrate the capability at ground level to remotely locate and identify sources of g radiation or the movement of material that might shield γ-ray sources. The Gamma-Ray Experimental Telescope Assembly (GRETA) employs two 28 cm diameter scintillation detectors separated by 81 cm: one 8.5 cm thick liquid scintillator detector and one 7.5 cm thick NaI(Tl) detector. The assembly electronics and real-time data acquisition system measures the energy deposits and time-of- flight for each coincident detection and compiles histograms of total energy and incident angle as computed using the kinematics of Compton scattering. The GRETA field of view is a cone with full angle approximately 120°. The sensitive energy range is 0.3 to 2.6 MeV. Energy resolution is ~10% FWHM. The angular resolution, ~19° in the simplified configuration tested, will improve to better than 5° with well-defined enhancements to the data acquisition hardware and data analysis routines. When operated in the mode that was used in space, the instrument is capable of measuring and imaging up to 30 MeV with an angular resolution of 1.5°. The response of the instrument was mapped in the laboratory with 14 Ci 22Na source 3 m from the instrument. Later, we conducted demonstrations under two measurement scenarios. In one, the remotely located instrument observed an increase of background radiation counts at 1.4 MeV when a large amount of lead was removed from a building and a corresponding decrease when the lead was replaced. In the other scenario, the location and isotope-identifying energy spectrum of a 500 μCi137Cs source 3-5 m from the instrument with two intervening walls was determined in less than one minute. We report details of the instrument design and these measurements

El asiento de avería y el origen de la compañía privilegiada en España

El presente artículo analiza cómo la autonomía patrimonial del capital destinado a la organización de las flotas con América acabó adquiriendo la forma de una verdadera compañía privilegiada de comercio, antecedente directo de la actual sociedad anónima, cuando la Corona dejó por entero a los particulares la administración de la avería a través del sistema de asientos. Los paralelismos de los asientos con las compañías llegaron a ser tan grandes que los funcionarios a quienes el conde-duque de Olivares encomendó el estudio de las compañías extranjeras, con objeto de introducirlas en España, vieron en los asientos una verdadera compañía privilegiada.This paper analyses how the progressive autonomy of the capital used to arrange the treasure fleets from Seville to America derived in a real chartered company, a precedent of today’s joint-stock company, when the Crown finally privatised it through the asientos de avería. The parallelism among the asientos and the companies were so noticeable that the public officers appointed by the Count-Duke of Olivares to study the foreign companies couldn’t detect any difference between both institutions.Publicad

The economics of debt clearing mechanisms

We examine the evolution of decentralized clearinghouse mechanisms from the 13th to the 18th century; in particular, we explore the clearing of non- or limitedtradable debts like bills of exchange. We construct a theoretical model of these clearinghouse mechanisms, similar to the models in the theoretical matching literature, and show that specific decentralized multilateral clearing algorithms known as rescontre, skontrieren or virement des parties used by merchants were efficient in specific historical contexts. We can explain both the evolutionary self-organizing emergence of late medieval and early modern fairs, and its robustness during the 17th and 18th century

Ocean surface features and currents measured with synthetic aperture radar interferometry and HF radar

This paper describes the results of the first quantitative comparison between high‐resolution ocean surface current fields extracted from interferometric synthetic aperture radar (INSAR) measurements and those from a high‐frequency (HF) ocean surface current radar (OSCR) system. Data from each of these radar systems along with supporting measurements from shipboard and buoy‐mounted sensors were collected during the High‐Resolution Remote Sensing Experiment (High‐Res) on June 20, 1993, on the continental shelf off the coast of Cape Hatteras, North Carolina. Both components of the surface current were obtained from the INSAR system at roughly 10‐m resolution from two orthogonal flight legs over the region separated in time by about 30 min. The OSCR system measured two‐dimensional surface current vectors at about 1‐km resolution over this same region, while the USNS Bartlett was collecting hydrographic samples and near‐surface current measurements. Two‐dimensional wave spectra as well as meteorological and additional current measurements were collected at two buoys within the experimental area. We discuss in the paper two techniques for eliminating the effect of surface wave motion on the INSAR current estimates. One method relies on some knowledge of the local wind and wave field and the use of a microwave scattering model. The other method makes use of a few in situ current measurements spaced at different range locations across the INSAR image. Using either of these techniques, we find that the agreement between the INSAR and OSCR current estimates is generally very good. Furthermore, the INSAR current and magnitude imagery show the presence of undulating surface features where abrupt changes in the current speed and direction occurred. The ship surveys indicate that these features were caused by the collision of water masses of different density. We show for the first time in this paper a high‐resolution, area‐extensive vector surface current map derived from the INSAR of the two‐dimensional flow in the vicinity of these features. Our results demonstrate convincingly that high‐resolution oceanic surface current vectors can be derived from INSAR current measurements and that these measurements may be very beneficial for detailed studies of the dynamics of small‐scale surface features in regions of strong current divergences or shears

Space-Based Measurements of Sheet Flow Characteristics in the Everglades, Wetland, Florida

New space-based observations of South Florida with interferometric SAR (InSAR) reveal spatially detailed, quantitative images of water levels in the Everglades. The new data capture dynamic water level topography, providing the first detailed picture of wetland sheet flow. We observe localized radial sheet flow in addition to well-known southward unidirectional sheet flow, modelled as a linear diffusive flow. We obtain quantitative estimates of flow diffusivity (23–91 m2/s), the first space-based estimates of such hydrologic parameter for the Everglades. Space-based hydrologic observations can provide critical information for monitoring, understanding and managing wetland sheet flow, and contribute to wetland restoration

Space Geodetic Imaging of Rapid Ground Subsidence in Mexico City

Since the late 1950s, several areas of Mexico City have undergone accelerated ground subsidence and have developed associated fracturing and faulting. New interferometric synthetic aperture radar (InSAR) and global positioning system (GPS) data indicate that rates of current land subsidence in Mexico City exceed 350 mm/yr. These rates are close to historical maximum levels of the mid-twentieth century, when mitigation efforts were first undertaken to reduce damage to urban infrastructure. The locus of maximum subsidence has shifted from its historical location in the old city center to the east. Correlation of our InSAR results with seismically mapped stratigraphic units suggests that subsidence is primarily controlled by compaction of Quaternary lacustrine clays and silts. We also evaluate spatial gradients in subsidence and suggest that this, rather than subsidence magnitude, is the key factor in risk assessment. Subsidence represents a major geologic risk for Mexico City and imposes serious constraints to any further urban development