LP1 Technical Report 92-06 45 could penetrate to depths of about 1 m. Sample volume of a few can also detect neutrons >0.6 MeV by triangular-shaped peaks cubic centimeters from depth intervals of a few centimeters would in the spectrum made by neutron-induced inelastic-scattering be obtained. The auger would be housed in a tube such that samples reactions in the detector [4]. The inclusion of a detector specifically from a given depth remain isolated from regolith material at other designed to measure fast (E,, ~ 0.1-10 MeV) neutrons improves depths. the sensitivity of detecting hydrogen in the lunar surface [5). A physical properties payload would be intended to study the Spectrometers to measure the fluxes of garnma rays and neutrons geotechnical properties of the surface and subsurface. Those data escaping from the Moon are discussed elsewhere in this volume would be aimed primarily at engineering objectives but would also [6]. A gamma ray spectrometer can also be used to detect thermal be applicable to geoscience problems. Parameters of interest and epithermal neutrons using coatings of thermal-neutron- include particle size and shape, density and porosity, compressi- absorbing materials [3], although not with the sensitivity of detec- bility, shear strength, bearing capacity, trafficability, electrical tors designed specifically for such neutrons. The theories for conductivity, and charging/discharging of the surface. The com- gamma ray and neutron spectroscopy of the Moon and calculations plete payload is under study and has not yet been determined. of leakage fluxes are presented here with emphasis on why com- However, it is anticipated that an electron-magnetic sounding bined gamma ray/neutron spectroscopy is much more powerful system would be used to determine the thickness, stratigraphy, than measuring either radiation alone. and boulder content of the regolith. This system would be mounted Sources of Neutrons and Gamma Rays: Most gamma ray on the rover body such that long distance or areally extensive lines made by the decay of excited states in nuclei are produced traverses could be obtained. by several types of nuclear reactions, mainly neutron nonelastic Later Artemis landers have a larger payload capability, about scattering and neutron capture [7-9]. The main exceptions are 200 kg or larger. Additional studies are underway that consider the gamma rays made by the decay of the naturally radioactive the possibility of building larger rovers that have more capability, elements (K, Th, and U). The neutrons are made by the interaction particularly with respect to lifetime, range, and drilling. Alterna- of the energetic particles in the galactic cosmic rays (GCR) with tively, the larger payload capacity could be used to deploy more the lunar surface [10]. Neutrons are produced mainly with energies of the_small rovers outlined above. of —0.1-10 MeV, with some made with higher energies. The rates for the production of these fast neutrons depends on the intensity uyto-73 of the GCR particles, which can vary by a factor of =3 over an COMBINED GAMMA R£Y/5lEUTRON SPECTROSCOPY 11-year solar cycle [11], and is slightly (~5%) dependent on the FOR MAPPING LUNAR RESOURCES. R. C. Reedy, R. C. surface composition [12]. The transport and interactions of these Byrd, D. M. Drake, W. C. Feldman, }. Masarik, and C. E. Moss, fast neutrons are also dependent on the surface composition. Space Science and Technology Division, Los Alamos National Neutrons in the Moon can be slowed by scattering reactions to Laboratory, Los Alamos, NM 87545. epithermal (£„ ~ 0.5-103 eV) and thermal (E,, - 0.01-0.5 eVj energies. The flux of epithermal neutrons is mainly dependent Some elements in the Moon can be resources, such as hydrogen on the hydrogen content of the surface, while the flux of thermal and oxygen. Other elements, like Ti or the minerals in which neutrons depends both on the amounts of neutron moderators they occur, such as ilmenite, could be used in processing lunar like H and of thermal-neutron absorbers like Fe, Ti, Sm, and Gd materials. Certain elements can also be used as tracers for other [ 1,5,12,13). Many neutrons escape from the lunar surface and can elements or lunar processes, such as hydrogen for mature regoliths be detected in orbit. The gravitational field of the Moon affects with other solar-wind-implanted elements like helium, carbon, and slightly the flux at orbit of the lowest-energy neutrons [14]. nitrogen. A complete knowledge of the elemental composition of The reaction of these neutrons with atomic nuclei in the lunar a lunar region is desirable both in identifying lunar resources and surface produces most of the gamma rays used for elemental in lunar geochemical studies, which aiso helps in identifying and mapping. Fast neutrons make gamma rays by a large variety of using lunar resources. Discussed here is the use of gamma ray and nonelastic-scattering reactions. Many elements (e.g., O, Mg, Si, neutron spectroscopy together to determine abundances of many and Fe) are mapped by neutron inelastic-scattering reactions mak- elements in the top few tens of centimeters of the lunar surface. ing excited states in the target nucleus, such as 24Mg(n,n y)'4Mg To date, very few discussions of elemental mapping of planetary exciting the 1.369-MeV state of 24Mg (which almost immediately, surfaces considered measurements of both gamma rays and the ~1 ps, decays to the ground state). Neutrons with energies of full range of neutron energies. ~0.5-10MeVmake most of these inelastic-scattering gamma rays. The concepts of using gamma rays or neutrons escaping from. Neutrons with higher energies can induce many types of reactions, the Moon to determine lunar elemental composition date back such as the 28Si(n,n or y)24Mg reaction, which also can make the over 30 years (e.g., [ 1 1). In 1 97 1 and 1 97 2, gamma ray spectrometers 1.369-MeV gamma ray. Many thermal and some epithermal neu- (GRS) on Apollos 15 and 16 mapped =22% of the lunar surface trons produce gamma rays by neutron-capture reactions, such as but only could determine the abundances of several elements (Th, the 28Si(n,y)29Si reaction, which makes a cascade of gamma rays. Fe, Ti, K, and Mg) because of the use of low-resolution Nal(Tl) The cross sections for neutron-capture reactions vary by orders scintillator detectors and the short durations of the missions. A of magnitude, and the elements mapped by gamma rays made by high-resolution germanium GRS is on the Mars Observer sche- such reactions (e.g., Ti, Fe, and possibly Cl, Sm, and Gd) are those duled to be launched in late September 1992 [2). Such a spec- with high cross sections for (n,y) reactions and that emit one trometer is much more sensitive for determining lunar elemental or more gamma ray with a high yield per captured neutron. abundances and for identifying surface components than was the Calculations of Neutron and Gamma Ray Fluxes: The ulti- Apollo GRS [3]. Neutron spectroscopy of another planet has not mate sources of lunar neutrons and most lunar gamma rays are been done yet, although the Mars Observer GRS is capable of from interactions of the high-energy (E ~ 0.1-10 GeV) particles detecting thermal andepithermal neutrons [2]. A germanium GRS in the galactic cosmic rays. The Los Alamos high-energy transport 46 New Technologies for Lunar Resource Assessment code LAHET is used to calculate the transport and interactions that strongly absorb thermal neutrons, such as Sm and Gd. Thus of the GCR particles and the production of secondary particles, in many ways the complementary nature of gamma rays and especially neutrons. The LAHET code is based on the HETC code neutrons make a combined gamma ray/neutron spectrometer more used by [-12J but with additional physics included in the code, powerful than each technique by itself. such as the production of neutrons by preequilibrium pro- This work was supported by NASA and done under the auspices cesses in excited nuclei. Neutrons with energies below 20 MeV oftheU.5. DOE. are transported with the MCNP or ONEDANT codes. The References: [ 1 ] Lingenfelter R. E. et al. (196DJGR, 66, 2665- ONEDANT code has been modified |14) to transport low-energy 267 1 .[ 2 1 Boynton W. V . et al. ( 1 992) JGR, 97, in press. [ 3) Metzger neutrons to orbital altitudes with consideration of gravity and the A. E. and Drake D. M. (1991) JGR, 96, 449-460. [4] Bruckner neutron's half-life. This series of codes is used to calculate the J. et al. (1987) Proc. LPSC 17 ih, in JGR, 92, E603-E616. fluxes of neutrons escaping from the Moon. Preliminary results [5] Feldman W. C. et al. (1991) GRL, 18, 2157-2160. [6] Moss for neutrons escaping from the Moon were calculated by |5] using C. E. et al., this volume. [7J Reedy R. C. et al. (1973) JGR, 78, the ONEDANT code. As in [5], a range of lunar compositions 5847-5866. [8] Reedy R. C. (1978) Proc. LPSC 9th, 2961-2984. and hydrogen contents has been examined with the LAHET/ [9] Evans L. G. et al. (1992) In Remote Ceochemical Analysis MCNP codes. The results of our calculations are consistent with (C. Pieters and P. ]. Englert, eds.), Cambridge, in press. (10) Reedy those in [5] and show that taking ratios of the fast/epithermal R. C. and Arnold J. R. (1972) JGR, 77, 537-555. [1 1] Reedy R. C. and thermal/epithermal neutron fluxes is the best for determining (1987) Proc. LPSC 17th, in JGR, 92, E697-E702. |12] Drake D. M. both lunar hydrogen contents and the effective 1/u macroscopic et al. (1988) JGR, 93, 6353-6368. [ 13] Lingenfelter R. E. et al. cross section for the absorption of thermal neutrons in a lunar (1972) EPSL, 16. 355-369. [14] Feldman W. C. et al. (1989) JGR, region. 94,513-525. To date, most studies of lunar gamma ray fluxes have used the values in [7,8). In these works, the gamma ray fluxes from nonelastic-scattering reactions were calculated using the fast neu- COMPAS: COMPOSITIONAL MINERALOGY WITH A tron fluxes of [10] and cross sections for producing specific gamma PHOTOACOUSTIC SPECTROMETER. W. Hayden Smith, rays as a function of energy. The gamma ray fluxes from neutron- Washington University, St. Louis MO 63130, USA. capture reactions were calculated with neutron-capture rates derived from 113] and evaluated nuclear data for gamma ray yields. There is an important need for an in siiu method of mineral Elemental detection sensitivities for a number of elements using and rock identification and quantification that provides true both the Apollo Gamma Ray Spectrometer system and a high- absorption spectra for a wide spectral range for lunar lander/rover resolution germanium spectrometer are given in (3). Hydrogen is missions. best measured using thermal, epithermaj, and fast neutron spec- Many common minerals, e.g., some feldspars, magnetite, ilmen- trometers together with elemental abundances determined from ite, and amorphous fine solids or glasses, can exhibit flat spectral a gamma ray spectrometer [5|. We are now in the process of reflectances in the 400-2500-nm spectral region that render inac- coupling the neutron fluxes calculated with the codes discussed curate or difficult their spectral detection and quantitative anal- above with codes for the production and transport of gamma rays ysis. Ideal rock and mineral spectra are, of course, pure absorption and will soon have new sets of elemental detection sensitivities spectra that are independent of the spectral effects of scattering, using gamma ray and neutron spectrometers. For now, the cal- particle size, and distribution that can result in a suppression or culated gamma ray fluxes of (8) are being used, as in the recent distortion of their spectral features. This ideal seldom pertains to study by (3), which showed the great superiority of using high- real samples. Since sample preparation is difficult and may fun- resolution gamma ray spectrometers for lunar mapping. damentally alter the observed diffuse spectral reflectance, an in Conclusions: The calculations for fluxes of neutrons escaping situ spectral measurement method for rocks and minerals on the from the Moon have been extended and support the conclusions Moon, insensitive to the sample morphology, would be invaluable. of 15] for using ratios of thermal, epithermal, and fast neutrons Photoacoustic spectroscopy is a, well-established technique for lunar elemental studies. The fluxes of fast neutrons can vary appropriate for this task that has been widely applied in condensed- by —10% depending on the surface composition independent of phase spectral studies of complex, highly light scattering, unpre- the major variations expected from GCR-flux changes over a solar pared samples of everything from coal to whote blood, including cycle. The direct measurement of the fluxes of fast neutrons can rock and mineral characterization. be used to help determine elemental abundances from nonelastic- A Compositional Mineralogy Photoacoustic Spectrometer, or scattering gamma rays, and the use of ground truths (as with the COMPAS, can enable in situ spectral measurement of rocks and Apollo gamma ray spectra) or normalizing the sum of all abun- minerals, bypassing the major limitations of diffuse reflectance dances to unity may not needed. Similarly, direct measurements spectroscopy. COMPAS has the following features: (1) it is designed of the fluxes of thermal (and epithermal) neutrons can be used for in situ spectral characterization of rocks and minerals and their to determine abundances from fluxes of neutron-capture gamma surface weathering species; (2) it does not require modifying or rays. The elemental abundances derived from the gamma ray data altering the sample or its surroundings; (3) it provides spatial are needed in interpreting the neutron measurements. The ther- resolution on a submillimeter scale, functions at the ambient (high mal/epithermal ratio can be used to get an independent measure or low) temperature, and requires no coolants; (4) spatial and of the effective 1/u macroscopic cross section for thermal neutrons. spectral data are acquired in a serial mode at a modest data rate; Differences of this macroscopic cross section determined from the (5) it has no internal moving components, which gives it high major elements mapped with gamma rays from that inferred from reliability; and (6) it is physically very small with low weight and low-energy neutrons could indicate the presence of other elements power requirements.