Image Reconstruction Techniques in Neutron and Gamma Ray Spectroscopy: Improving Lunar Prospector Data

Assessing the chemical composition of the Moon is a key part of its investigation. Elements can be grouped according to their condensation and geochemical behavior, and thus chemical abundances of key elements can be used to asses both the origin and the evolution of the body. The relative abundance of the surface (down to a few &#181;m) major elements with atomic number &#163;20 can be estimated using X-ray fluorescence, using solar X-rays to excite element&#8217;s atoms (both quiescent Sun X-ray emission, in particular during solar maxima, and solar flares). Only an handle of successful X-ray fluorescence experiments have been flown so far, including Apollo15 XRFS, Chandrayaan-1 C1XS, SMART-1 D-C1XS at the Moon, Messanger-XRS, BepiColombo-MIXS at Mercury, and NEAR-XRS at EROS, Hayabusa-XRS at Itokawa, OSIRIS-REX/REXIS at Bennu (Allen et al. 2013, arXiv:1309.6665 and refs. therein). The main goal of fluorescent X-ray spectroscopy of asteroids is to determine the intensity of Fe-L, Fe-K, Al-K, Mg-K, Si-K complexes and S-Ka, S-Kb fluorescent lines. From these lines mass abundance ratios can be determined [Mg/Si], [Fe/Si]. The comparison of these abundance ratio to those of meteorites can understand whether the surface of the Moon resembles a particular chondritic meteorite. Gamma-ray spectroscopy of nuclear lines can also be used to assess abundances of elements in the planetary surface (down to 10-20cm, e.g. Reedy et al. 1978, Proc. Lunar Planet Sci. Conf. 9th 1978). The main source of gamma-ray lines are: decay of natural radionuclides, reaction induced by energetic cosmic rays, capture of low-energy neutrons, and solar-proton induced radioactivity. Gamma-ray spectroscopy can provide information on many elements, including those with high atomic number, hardly accessible to X-ray fluorescence spectroscopy. Several successful gamma-ray experiment have been performed so far, including early spectroscopy of the Moon (Luna10, Apollo15, 16), Mars (Mars-5). More recently, Kayuga gamma-ray spectrometer obtained a detailed mapping of radioactive elements on the Moon (K, Th, U), as well major elements, O, Al, Mg, Si, Ca, Ti, Fe (Yamashita et al. EPSC Abstracts Vol. 5, EPSC210-580, 2010, Hasebe et al. Proc. Int. Workshop Advances in Cosmic Ray Science J. Phys. Soc. Jpn. 78, 2009). Other lunar experiments include Lunar prospector and Chang&#8217;E. Other experiments include more Mars spectroscopy (MARS Odyssey) Mercury (Messenger) and the asteroids EROS (NEAR) and 4 Vesta (Dawn). The K/Th ratio is a diagnostic of the body bulk composition and provide information about where and when the body was formed. The collecting area of most X-ray and gamma-ray spectrometers used for planetary observations is quite small, a few cm2. The REXIS instrument working today at Bennu as a collecting area of 25cm2. The largest instrument flown so far is the Kaguya-XRS with a collecting are of 100cm2. A new miniaturize payload is here proposed, currently under development for astrophysics goals and to be installed on a 3U cubesat platform; the sensors uses GAGG scintillator crystals and Silicon Drift Detectors to both detect direct X-ray photons and optical photons produced in the scintillators by interaction with gamma-rays, and can cover a unique wide band, from a few keV to several MeV, providing at the same time both X-ray fluorescent spectroscopy and gamma-ray spectroscopy. The collecting area of a compact instrument (~3-4kg) can be conservatively ~100cm2. A passive collimator can shield the SDD from most Cosmic X-ray Background, reducing the background at energies <20keV, where it is largely dominated by the CXB. Laboratory calibration in the next few months will provide more precise numbers on the resolution achievable. The gamma-ray line sensitivity of this instruments with collecting area ~ 100 cm2 and active anticoincidences (efficiency &#160;&#187; twice that of Kayuga spectrometer) should therefore be a factor several better than that of other planetary gamma-ray spectrometers. The paper shows the proposed scientific mission goals around which the miniaturized payload is designed, and its possible embarking on a cubesat like space segments, 6U large; the whole feasibility study for a smallsat scientific mission is presented, showing the generality of the approach and its applicability also for small bodies exploration as well.

Download Full-text