UV luminescence characterisation of organics in Mars-analogue substrates

This project was supported by a Leverhulme Trust Research Project Grant (RPG-2015-071). C Cousins also wishes to acknowledge funding by the Royal Society of Edinburgh.Detection of organic matter is one of the core objectives of future Mars exploration. The ability to probe rocks, soils, and other geological substrates for organic targets is a high priority for in situ investigation, sample caching, and sample return. UV luminescence – the emission of visible light following UV irradiation – is a tool that is beginning to be harnessed for planetary exploration. We conducted  UV photoluminescence analyses of (i) Mars analogue sediments doped with polyaromatic hydrocarbons (PAHs; <15 ppm), (ii) carbonaceous CM chondrites and terrestrial kerogen (Type IV), and (iii) synthetic salt crystals doped with PAHs (2 ppm). We show that that detection of PAHs is possible within synthetic and natural gypsum, and synthetic halite. These substrates show the most apparent spectral modifications, suggesting that the most transparent minerals are more conducive to UV photoluminescence detection of trapped organic matter. Iron oxide, ubiquitously present on Mars surface, hampers but does not completely quench the UV luminescence emission. Finally, the maturity of organic carbonaceous material influences the luminescence response, resulting in a reduced signal for UV excitation wavelengths down to 225 nm. This study demonstrates the utility of UV luminescence spectroscopy for the analysis of mixed organic-inorganic materials applicable to Mars exploration.PostprintPeer reviewe

The velocity peaks in the cold dark matter spectrum on Earth

The cold dark matter spectrum on earth is expected to have peaks in velocity space. We obtain estimates for the sizes and locations of these peaks. To this end we have generalized the secondary infall model of galactic halo formation to include angular momentum of the dark matter particles. This new model is still spherically symmetric and it has self-similar solutions. Our results are relevant to direct dark matter search experiments.Comment: 12 pages including 1 table and 4 figures, LaTeX, REVTEX 3.0 versio

Signal Intensity Analysis and Optimization for in Vivo Imaging of Cherenkov and Excited Luminescence.

During external beam radiotherapy (EBRT), in vivo Cherenkov optical emissions can be used as a dosimetry tool or to excite luminescence, termed Cherenkov-excited luminescence (CEL) with microsecond-level time-gated cameras. The goal of this work was to develop a complete theoretical foundation for the detectable signal strength, in order to provide guidance on optimization of the limits of detection and how to optimize near real time imaging. The key parameters affecting photon production, propagation and detection were considered and experimental validation with both tissue phantoms and a murine model are shown. Both the theoretical analysis and experimental data indicate that the detection level is near a single photon-per-pixel for the detection geometry and frame rates commonly used, with the strongest factor being the signal decrease with the square of distance from tissue to camera. Experimental data demonstrates how the SNR improves with increasing integration time, but only up to the point where the dominance of camera read noise is overcome by stray photon noise that cannot be suppressed. For the current camera in a fixed geometry, the signal to background ratio limits the detection of light signals, and the observed in vivo Cherenkov emission is on the order of 100×  stronger than CEL signals. As a result, imaging signals from depths  \u3c 15 mm is reasonable for Cherenkov light, and depths  \u3c 3 mm is reasonable for CEL imaging. The current investigation modeled Cherenkov and CEL imaging of two oxygen sensing phosphorescent compounds, but the modularity of the code allows for easy comparison of different agents or alternative cameras, geometries or tissues

Mapping the Galactic Halo I. The `Spaghetti' Survey

We describe a major survey of the Milky Way halo designed to test for kinematic substructure caused by destruction of accreted satellites. We use the Washington photometric system to identify halo stars efficiently for spectroscopic followup. Tracers include halo giants (detectable out to more than 100 kpc), blue horizontal branch stars, halo stars near the main sequence turnoff, and the ``blue metal-poor stars'' of Preston et al (1994). We demonstrate the success of our survey by showing spectra of stars we have identified in all these categories, including giants as distant as 75 kpc. We discuss the problem of identifying the most distant halo giants. In particular, extremely metal-poor halo K dwarfs are present in approximately equal numbers to the distant giants for V fainter than 18, and we show that our method will distinguish reliably between these two groups of metal-poor stars. We plan to survey 100 square degrees at high galactic latitude, and expect to increase the numbers of known halo giants, BHB stars and turnoff stars by more than an order of magnitude. In addition to the strong test that this large sample will provide for the question `was the Milky Way halo accreted from satellite galaxies?', we will improve the accuracy of mass measurements of the Milky Way beyond 50 kpc via the kinematics of the many distant giants and BHB stars we will find. We show that one of our first datasets constrains the halo density law over galactocentric radii of 5-20 kpc and z heights of 2-15 kpc. The data support a flattened power-law halo with b/a of 0.6 and exponent -3.0. More complex models with a varying axial ratio may be needed with a larger dataset.Comment: 55 pages, 22 figures, to appear in the Astronomical Journa

Strategies for equivalent dose determination without heating, suitable for portable luminescence readers

This work was supported by the UK Space Agency CREST3 program under grant ST/P001998/1. Research in Next Generation Luminescence methods in Aberystwyth is supported by NERC grant CC003, and by HEFCW infrastructure funding for SPARCL.In recent years a number of portable instruments have been built for measuring the optically stimulated luminescence (OSL) signal from naturally occurring minerals. Some of these instruments have incorporated ionising radiation sources, giving the possibility of determining an equivalent dose (De), but little use has been made of these. One challenge has been that heating samples in this type of equipment is a major engineering challenge, yet methods for De determination use thermal pretreatments to remove charge from unstable traps, making signals arising from irradiation in nature and the laboratory comparable. This paper explores three strategies for obtaining accurate estimates of the De of samples in situations where thermal treatments are not possible: (1) deriving a correction factor based on comparing De values obtained using protocols with and without heating; (2) removing the contribution from the 110 °C TL peak and other unstable defects by component fitting the unheated OSL signal; and (3) adding a small beta dose to the sample prior to measurement of the natural luminescence signal so that the 110 °C TL peak is filled, making this measurement comparable with regeneration measurements where this peak is also populated. All three methods are promising when applied to quartz that has been physically separated from samples using standard laboratory procedures. The next step in this work will be to explore whether such methods can be applied to mixed mineral assemblages as would be encountered in the field.PostprintPeer reviewe

Determination of the bond percolation threshold for the Kagome lattice

The hull-gradient method is used to determine the critical threshold for bond percolation on the two-dimensional Kagome lattice (and its dual, the dice lattice). For this system, the hull walk is represented as a self-avoiding trail, or mirror-model trajectory, on the (3,4,6,4)-Archimedean tiling lattice. The result pc = 0.524 405 3(3) (one standard deviation of error) is not consistent with the previously conjectured values.Comment: 10 pages, TeX, Style file iopppt.tex, to be published in J. Phys. A. in August, 199

Are superflares on solar analogues caused by extra-solar planets?

Stellar flares with ${10^2-10^7}$ times more energy than the largest solar flare have been detected from 9 normal F and G main sequence stars (Schaefer, King & Deliyannis 1999). These superflares have durations of hours to days and are visible from at least x-ray to optical frequencies. The absence of world-spanning aurorae in historical records and of anomalous extinctions in the geological record indicate that our Sun likely does not suffer superflares. In seeking to explain this new phenomenon, we are struck by its similarity to large stellar flares on RS Canum Venaticorum binary systems, which are caused by magnetic reconnection events associated with the tangling of magnetic fields between the two stars. The superflare stars are certainly not of this class, although we propose a similar flare mechanism. That is, superflares are caused by magnetic reconnection between fields of the primary star and a close-in Jovian planet. Thus, by only invoking known planetary properties and reconnection scenarios, we can explain the energies, durations, and spectra of superflares, as well as explain why our Sun does not have such events.Comment: 13 pages, Accepted for publication in Ap