Luminescence-based retrospective dosimetry using Al2O3 from mobile phones: a simulation approach to determine the effects of position.

Monte Carlo modelling has been performed in support of efforts to establish emergency dosimetry services based on optically or thermally stimulated luminescence (OSL/TL) of the Al(2)O(3) substrate present on the resistors found in mobile phones, which can act as fortuitous retrospective dosemeters for photon exposures. Specifically, a range of exposure conditions has been modelled to assess the dependence of the dosimetry on factors such as the position of resistors within a phone, the orientation of the phone relative to the source, and the location of the phone relative to its owner. Variations due to the resistors' positions and the phone's orientation were generally found to contribute just a few percent to the uncertainty on the dose assessments, though the electrical contacts surrounding the resistors could potentially enhance these by several 10s of percent. But, the location of the phone was found to impact dosimetry greatly. The largest discrepancies in the results were found for low-energy exposures: for (192)Ir, differences of up to an order-of-magnitude were found between resistor and whole body doses. The outcome of the work was to derive correction / calibration factors that can be applied to estimate whole body doses from OSL/TL readings, the accurate application of which would depend on the knowledge of the exposure geometry and the degree of conservatism acceptable for the dose assessment.

Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling.

A carbon-rich black layer, dating to approximately 12.9 ka, has been previously identified at approximately 50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at approximately equal 12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at approximately 12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.