ARCHIVING HIGH-RESOLUTION LUNAR GAMMA RAY SPECTRA. N. Yamashita1 and T. H. Prettyman1, 1 Planetary Science Institute (1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, yamashita@psi.edu).

Introduction: The lunar regolith preserves a rec-

ord of the early conditions of the evolutionary stage of the terrestrial planets (e.g. [1]). By examining the composition of such materials, detailed information about the origin and the evolution of the Moon can be retrieved [2]. Knowledge of surface elemental compo-sition is needed for the exploration of planetary bodies and the Solar System.

Gamma-ray spectroscopy is sensitive to the ele-mental composition of planetary surfaces (e.g. [3]). A high-energy resolution remote-sensing geochemical survey of the Moon was executed by JAXA’s Kaguya Gamma-Ray Spectrometer (KGRS) from 2007 to 2009 using a high-purity Ge (HPGe) gamma-ray detector [4,5]. This sensor type has the highest energy resolu-tion currently achievable, enabling the identification and quantification of a plethora of elements (Fig. 1).

We will archive fully calibrated and corrected gamma-ray spectra obtained by KGRS at Planetary Data System (PDS). This will give the planetary and lunar science community access to a high fidelity, global lunar chemistry data set that complements simi-lar data sets acquired by Lunar Prospector (LP) [6,7] and other lunar missions (e.g. [8]).

Dataset: With permission from JAXA, we have obtained the raw data of the KGRS time-series spectra, counters, and ancillary information. The SPICE ker-nels needed to determine the ephemeris data for the Kaguya satellite are available to the public and are distributed by the Data Archives and Transmission System at JAXA [9].

The complete, fully-calibrated and corrected time-series spectra of gamma rays will be archived at PDS as reduced data records (level 1B). The records will include both high-gain (0.1-3 MeV, 8192ch) and low-gain (0.1-12 MeV, 8192ch) spectra. In addition to the science data, we will also archive ancillary data needed to interpret the spectrum, such as live time, subsatellite latitudes and longitudes, the solid angle subtended by the Moon at the spacecraft, pointing information, and correction factors for galactic cosmic rays. We will also generate, validate, and document the response function of the detector system needed for absolute calibration of the counting rates to determine elemental abundances. This will enable more accurate determina-tion of the concentration of key rock-forming elements such as O, Na, Mg, Al, Si, Ca, Ti, Fe, K, Th, and U and volatiles such as H (e.g. [10,11]).

Data Reduction: Data reduction processes similar to those successfully applied to several missions in-cluding LP, Mars Odyssey (MO), and Dawn [12-15]

will be applied to the raw time-series of KGRS. The descriptions of the data and methods used to derive the resulting PDS product will be documented. Data re-duction steps include the following.

Elimination of invalid events. Each time-series spectrum will be evaluated for its validity for use in geochemical studies. Invalid spectra include those rec-orded when the spacecraft was not pointing towards nadir, during solar energetic particle events, and while the instrument was not configured to proper settings or was receiving interference from other instruments on board, and the reaction wheels were unloading. These unsuitable events will be filtered or flagged.

Correction of analog-to-digital converter differen-tial non-linearity. Periodic artifacts in spectra can be caused by the differential nonlinearity (DNL) of the analog to digital converter (ADC). DNL produces a recognizable pattern in the spectrum at frequencies higher than gamma-ray peaks, which can be identified and corrected.

Gain correction and energy calibration. Because gamma-ray counting rates are low, accumulation of many spectra is required to achieve ample precision for

Figure 1. Gamma-ray spectra acquired in the lunar orbit by Kaguya (this work), Lunar Prospector [6,7], and Chang’e-2 (priv. comm. with M.-H. Zhu, [8]) mis-sions with various sensors. Some of the signals from key elements are labeled. The counts are scaled by an arbitrary power law for visualization. The LaBr3 spec-trum contains contributions from radioactive contami-nants in the detector itself at below ~3 MeV.

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geochemical studies. Variations in gain of each spec-trum broadens the widths of peaks in accumulated spectra, resulting in reduced precision for elemental analyses. Gain corrections will be applied to put each spectrum in the time series on the same pulse height scale, with the goal of maximizing the apparent energy resolution of the cumulative spectra. The centroids of high-intensity, isolated peaks and corresponding gam-ma-ray energies will be used to determine the gain and offset for each spectrum. As a result, this process also simultaneously calibrates the ADC output histogram to gamma-ray energy.

Ancillary data. Because of the broad spatial reso-lution, the gamma-ray counting rate varies with the solid angle subtended by the Moon at the detector, which is a function of altitude. The solid angle for each observation will be calculated, given spacecraft posi-tions determined by the SPICE toolkit [16]. A correc-tion factor will be derived to remove altitude variations from spectra. Spacecraft ephemerides and pointing needed to create distribution maps of elements will be derived and included in the reduced data set.

Corrections for variations in galactic cosmic ray intensity. Gamma rays from rock-forming elements and H are induced by neutrons produced in the lunar surface by high-energy galactic cosmic rays (GCR). The natural radioelements K, Th, and U emit gamma rays spontaneously. Accurate determination of ele-mental composition requires the removal of variations in GCR intensity from the time series data set.

Deriving instrument response function. Conver-sion of gamma-ray counting rates into elemental abun-dances requires the knowledge of how the spectrome-ter responds to gamma rays. The response function depends on the energy of the incident gamma rays, their direction, and detected pulse-height.

The response function for any selected combination of energy and direction can be calculated using Monte Carlo, as was similarly done by [12-15] in modeling response of the LP-GRS, MO Neutron Spectrometer, and Dawn Gamma Ray and Neutron Detector. For validation, simulations of spectra will be carried out for representative lunar compositions and compared to spectra acquired in representative surface regions.

Results: At this meeting, we will report detailed data reduction methods applied to the KGRS spectra as well as some of resulting elemental maps of the Moon to demonstrate the quality of the data and implications for lunar science. Figure 2 shows example maps by KGRS of Ca, Al, and Ti [11] using the low altitude data. The reduced data set is planned to be archived at PDS in 2018.

References: [1] Hartmann W.K. et al. (1986) (Eds), Origin of the Moon. [2] Taylor S.R. et al. (2006) GCA

70, 5904. [3] Prettyman T.H. (2014) In Spohn T. et al. (Eds), Encyclopedia of the Solar System, 1161. [4] Hasebe N. et al. (2008) ASR 42, 323. [5] Yamashita N. et al. (2009) J. Phys. Soc. Jpn. Supp. 78, 153. [6] Feldman W.C. et al. (2004) JGR 109, E07S06. [7] Prettyman T.H. et al. (2006) JGR 111, E12007. [8] Zhu et al. (2013) Sci. Reports 3, 1611. [9] http://darts.isas.jaxa.jp/pub/spice/SELENE/kernels/. [10] Yamashita N. et al. (2010) GRL 37, L10201. [11] Yamashita N. et al. (2012) EPSL 353-354, 93. [12] Lawrence D.J. et al. (2004) JGR 109, E07S05. [13] Prettyman T.H. et al. (2004) JGR 109, E05001. [14] Prettyman et al. (2011) SSR 163, 371. [15] Yama-shita N, Prettyman T.H. (2016), NASA PDS, DAWN-A-GRAND-3-RDR-CERES-COUNTS-V1.0. [16] Ac-ton, C.H. (1996) PSS, 44, 65.

Acknowledgement: Portions of this material are based upon work supported by the NASA under Grant No. NNX16AG54G issued through the Planetary Data Archiving, Restoration, and Tools Program.

Figure 2. Elemental maps of the Moon for Ca (top), Al (middle), and Ti (bottom) by Kaguya GRS [11] displayed on a relative scale.

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