Some elements in the Moon can be resources, such as hydrogen and oxygen. Other elements, like Ti or the minerals in which they occur, such as ilmenite, could be used in processing lunar materials. Certain elements can also be used as tracers for other elements or lunar processes, such as hydrogen for mature regoliths with other solar-wind-implanted elements like helium, carbon, and nitrogen. A complete knowledge of the elemental composition of a lunar region is desirable both in identifying lunar resources and in lunar geochemical studies, which also helps in identifying and using lunar resources. The use of gamma ray and neutron spectroscopy together to determine abundances of many elements in the top few tens of centimeters of the lunar surface is discussed. To date, very few discussions of elemental mapping of planetary surfaces considered measurements of both gamma rays and the full range of neutron energies. The theories for gamma ray and neutron spectroscopy of the Moon and calculations of leakage fluxes are presented here with emphasis on why combined gamma ray/neutron spectroscopy is much more powerful than measuring either radiation alone

Byrd, R. C.

Drake, D. M.

Feldman, W. C.

Masarik, J.

Moss, C. E.

Reedy, R. C.

English

NASA Technical Reports Server

LP1 Technical Report 92-06 45
could penetrate to depths of about 1 m. Sample volume of a few
cubic centimeters from depth intervals of a few centimeters would
be obtained. The auger would be housed in a tube such that samples
from a given depth remain isolated from regolith material at other
depths.
A physical properties payload would be intended to study the
geotechnical properties of the surface and subsurface. Those data
would be aimed primarily at engineering objectives but would also
be applicable to geoscience problems. Parameters of interest
include particle size and shape, density and porosity, compressi-
bility, shear strength, bearing capacity, trafficability, electrical
conductivity, and charging/discharging of the surface. The com-
plete payload is under study and has not yet been determined.
However, it is anticipated that an electron-magnetic sounding
system would be used to determine the thickness, stratigraphy,
and boulder content of the regolith. This system would be mounted
on the rover body such that long distance or areally extensive
traverses could be obtained.
Later Artemis landers have a larger payload capability, about
200 kg or larger. Additional studies are underway that consider
the possibility of building larger rovers that have more capability,
particularly with respect to lifetime, range, and drilling. Alterna-
tively, the larger payload capacity could be used to deploy more
of the_small rovers outlined above.
uyto-73
COMBINED GAMMA R£Y/5lEUTRON SPECTROSCOPY
FOR MAPPING LUNAR RESOURCES. R. C. Reedy, R. C.
Byrd, D. M. Drake, W. C. Feldman, }. Masarik, and C. E. Moss,
Space Science and Technology Division, Los Alamos National
Laboratory, Los Alamos, NM 87545.
Some elements in the Moon can be resources, such as hydrogen
and oxygen. Other elements, like Ti or the minerals in which
they occur, such as ilmenite, could be used in processing lunar
materials. Certain elements can also be used as tracers for other
elements or lunar processes, such as hydrogen for mature regoliths
with other solar-wind-implanted elements like helium, carbon, and
nitrogen. A complete knowledge of the elemental composition of
a lunar region is desirable both in identifying lunar resources and
in lunar geochemical studies, which aiso helps in identifying and
using lunar resources. Discussed here is the use of gamma ray and
neutron spectroscopy together to determine abundances of many
elements in the top few tens of centimeters of the lunar surface.
To date, very few discussions of elemental mapping of planetary
surfaces considered measurements of both gamma rays and the
full range of neutron energies.
The concepts of using gamma rays or neutrons escaping from.
the Moon to determine lunar elemental composition date back
over 30 years (e.g., [ 1 1). In 1 97 1 and 1 97 2, gamma ray spectrometers
(GRS) on Apollos 15 and 16 mapped =22% of the lunar surface
but only could determine the abundances of several elements (Th,
Fe, Ti, K, and Mg) because of the use of low-resolution Nal(Tl)
scintillator detectors and the short durations of the missions. A
high-resolution germanium GRS is on the Mars Observer sche-
duled to be launched in late September 1992 [2). Such a spec-
trometer is much more sensitive for determining lunar elemental
abundances and for identifying surface components than was the
Apollo GRS [3]. Neutron spectroscopy of another planet has not
been done yet, although the Mars Observer GRS is capable of
detecting thermal andepithermal neutrons [2]. A germanium GRS
can also detect neutrons >0.6 MeV by triangular-shaped peaks
in the spectrum made by neutron-induced inelastic-scattering
reactions in the detector [4]. The inclusion of a detector specifically
designed to measure fast (E,, ~ 0.1-10 MeV) neutrons improves
the sensitivity of detecting hydrogen in the lunar surface [5).
Spectrometers to measure the fluxes of garnma rays and neutrons
escaping from the Moon are discussed elsewhere in this volume
[6]. A gamma ray spectrometer can also be used to detect thermal
and epithermal neutrons using coatings of thermal-neutron-
absorbing materials [3], although not with the sensitivity of detec-
tors designed specifically for such neutrons. The theories for
gamma ray and neutron spectroscopy of the Moon and calculations
of leakage fluxes are presented here with emphasis on why com-
bined gamma ray/neutron spectroscopy is much more powerful
than measuring either radiation alone.
Sources of Neutrons and Gamma Rays: Most gamma ray
lines made by the decay of excited states in nuclei are produced
by several types of nuclear reactions, mainly neutron nonelastic
scattering and neutron capture [7-9]. The main exceptions are
the gamma rays made by the decay of the naturally radioactive
elements (K, Th, and U). The neutrons are made by the interaction
of the energetic particles in the galactic cosmic rays (GCR) with
the lunar surface [10]. Neutrons are produced mainly with energies
of —0.1-10 MeV, with some made with higher energies. The rates
for the production of these fast neutrons depends on the intensity
of the GCR particles, which can vary by a factor of =3 over an
11-year solar cycle [11] , and is slightly (~5%) dependent on the
surface composition [12]. The transport and interactions of these
fast neutrons are also dependent on the surface composition.
Neutrons in the Moon can be slowed by scattering reactions to
epithermal (£„ ~ 0.5-103 eV) and thermal (E,, - 0.01-0.5 eVj
energies. The flux of epithermal neutrons is mainly dependent
on the hydrogen content of the surface, while the flux of thermal
neutrons depends both on the amounts of neutron moderators
like H and of thermal-neutron absorbers like Fe, Ti, Sm, and Gd
[ 1,5,12,13). Many neutrons escape from the lunar surface and can
be detected in orbit. The gravitational field of the Moon affects
slightly the flux at orbit of the lowest-energy neutrons [14].
The reaction of these neutrons with atomic nuclei in the lunar
surface produces most of the gamma rays used for elemental
mapping. Fast neutrons make gamma rays by a large variety of
nonelastic-scattering reactions. Many elements (e.g., O, Mg, Si,
and Fe) are mapped by neutron inelastic-scattering reactions mak-
ing excited states in the target nucleus, such as 24Mg(n,n y)'4Mg
exciting the 1.369-MeV state of 24Mg (which almost immediately,
~1 ps, decays to the ground state). Neutrons with energies of
~0.5-10MeVmake most of these inelastic-scattering gamma rays.
Neutrons with higher energies can induce many types of reactions,
such as the 28Si(n,n or y)24Mg reaction, which also can make the
1.369-MeV gamma ray. Many thermal and some epithermal neu-
trons produce gamma rays by neutron-capture reactions, such as
the 28Si(n,y)29Si reaction, which makes a cascade of gamma rays.
The cross sections for neutron-capture reactions vary by orders
of magnitude, and the elements mapped by gamma rays made by
such reactions (e.g., Ti, Fe, and possibly Cl, Sm, and Gd) are those
with high cross sections for (n,y) reactions and that emit one
or more gamma ray with a high yield per captured neutron.
Calculations of Neutron and Gamma Ray Fluxes: The ulti-
mate sources of lunar neutrons and most lunar gamma rays are
from interactions of the high-energy (E ~ 0.1-10 GeV) particles
in the galactic cosmic rays. The Los Alamos high-energy transport
https://ntrs.nasa.gov/search.jsp?R=19930008074 2020-03-24T07:24:07+00:00Z
46 New Technologies for Lunar Resource Assessment
code LAHET is used to calculate the transport and interactions
of the GCR particles and the production of secondary particles,
especially neutrons. The LAHET code is based on the HETC code
used by [-12J but with additional physics included in the code,
such as the production of neutrons by preequilibrium pro-
cesses in excited nuclei. Neutrons with energies below 20 MeV
are transported with the MCNP or ONEDANT codes. The
ONEDANT code has been modified |14) to transport low-energy
neutrons to orbital altitudes with consideration of gravity and the
neutron's half-life. This series of codes is used to calculate the
fluxes of neutrons escaping from the Moon. Preliminary results
for neutrons escaping from the Moon were calculated by |5] using
the ONEDANT code. As in [5], a range of lunar compositions
and hydrogen contents has been examined with the LAHET/
MCNP codes. The results of our calculations are consistent with
those in [5] and show that taking ratios of the fast/epithermal
and thermal/epithermal neutron fluxes is the best for determining
both lunar hydrogen contents and the effective 1/u macroscopic
cross section for the absorption of thermal neutrons in a lunar
region.
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-
tron fluxes of [10] and cross sections for producing specific gamma
rays as a function of energy. The gamma ray fluxes from neutron-
capture reactions were calculated with neutron-capture rates
derived from 113] and evaluated nuclear data for gamma ray yields.
Elemental detection sensitivities for a number of elements using
both the Apollo Gamma Ray Spectrometer system and a high-
resolution germanium spectrometer are given in (3). Hydrogen is
best measured using thermal, epithermaj, and fast neutron spec-
trometers together with elemental abundances determined from
a gamma ray spectrometer [5|. We are now in the process of
coupling the neutron fluxes calculated with the codes discussed
above with codes for the production and transport of gamma rays
and will soon have new sets of elemental detection sensitivities
using gamma ray and neutron spectrometers. For now, the cal-
culated gamma ray fluxes of (8) are being used, as in the recent
study by (3), which showed the great superiority of using high-
resolution gamma ray spectrometers for lunar mapping.
Conclusions: The calculations for fluxes of neutrons escaping
from the Moon have been extended and support the conclusions
of 15] for using ratios of thermal, epithermal, and fast neutrons
for lunar elemental studies. The fluxes of fast neutrons can vary
by —10% depending on the surface composition independent of
the major variations expected from GCR-flux changes over a solar
cycle. The direct measurement of the fluxes of fast neutrons can
be used to help determine elemental abundances from nonelastic-
scattering gamma rays, and the use of ground truths (as with the
Apollo gamma ray spectra) or normalizing the sum of all abun-
dances to uni ty may not needed. Similarly, direct measurements
of the fluxes of thermal (and epithermal) neutrons can be used
to determine abundances from fluxes of neutron-capture gamma
rays. The elemental abundances derived from the gamma ray data
are needed in interpreting the neutron measurements. The ther-
mal/epithermal ratio can be used to get an independent measure
of the effective 1/u macroscopic cross section for thermal neutrons.
Differences of this macroscopic cross section determined from the
major elements mapped with gamma rays from that inferred from
low-energy neutrons could indicate the presence of other elements
that strongly absorb thermal neutrons, such as Sm and Gd. Thus
in many ways the complementary nature of gamma rays and
neutrons make a combined gamma ray/neutron spectrometer more
powerful than each technique by itself.
This work was supported by NASA and done under the auspices
oftheU.5. DOE.
References: [ 1 ] Lingenfelter R. E. et al. (196DJGR, 66, 2665-
267 1 .[ 2 1 Boynton W. V . et al. ( 1 992) JGR, 97, in press. [ 3) Metzger
A. E. and Drake D. M. (1991) JGR, 96, 449-460. [4] Bruckner
J. et al. (1987) Proc. LPSC 17 ih, in JGR, 92, E603-E616.
[5] Feldman W. C. et al. (1991) GRL, 18, 2157-2160. [6] Moss
C. E. et al., this volume. [7J Reedy R. C. et al. (1973) JGR, 78,
5847-5866. [8] Reedy R. C. (1978) Proc. LPSC 9th, 2961-2984.
[9] Evans L. G. et al. (1992) In Remote Ceochemical Analysis
(C. Pieters and P. ]. Englert, eds.), Cambridge, in press. (10) Reedy
R. C. and Arnold J. R. (1972) JGR, 77, 537-555. [1 1] Reedy R. C.
(1987) Proc. LPSC 17th, in JGR, 92, E697-E702. |12] Drake D. M.
et al. (1988) JGR, 93, 6353-6368. [ 13] Lingenfelter R. E. et al.
(1972) EPSL, 16. 355-369. [14] Feldman W. C. et al. (1989) JGR,
94,513-525.
COMPAS: COMPOSITIONAL MINERALOGY WITH A
PHOTOACOUSTIC SPECTROMETER. W. Hayden Smith,
Washington University, St. Louis MO 63130, USA.
There is an important need for an in siiu method of mineral
and rock identification and quantification that provides true
absorption spectra for a wide spectral range for lunar lander/rover
missions.
Many common minerals, e.g., some feldspars, magnetite, ilmen-
ite, and amorphous fine solids or glasses, can exhibit flat spectral
reflectances in the 400-2500-nm spectral region that render inac-
curate or difficult their spectral detection and quantitative anal-
ysis. Ideal rock and mineral spectra are, of course, pure absorption
spectra that are independent of the spectral effects of scattering,
particle size, and distribution that can result in a suppression or
distortion of their spectral features. This ideal seldom pertains to
real samples. Since sample preparation is difficult and may fun-
damentally alter the observed diffuse spectral reflectance, an in
situ spectral measurement method for rocks and minerals on the
Moon, insensitive to the sample morphology, would be invaluable.
Photoacoustic spectroscopy is a, well-established technique
appropriate for this task that has been widely applied in condensed-
phase spectral studies of complex, highly light scattering, unpre-
pared samples of everything from coal to whote blood, including
rock and mineral characterization.
A Compositional Mineralogy Photoacoustic Spectrometer, or
COMPAS, can enable in situ spectral measurement of rocks and
minerals, bypassing the major limitations of diffuse reflectance
spectroscopy. COMPAS has the following features: (1) it is designed
for in situ spectral characterization of rocks and minerals and their
surface weathering species; (2) it does not require modifying or
altering the sample or its surroundings; (3) it provides spatial
resolution on a submillimeter scale, functions at the ambient (high
or low) temperature, and requires no coolants; (4) spatial and
spectral data are acquired in a serial mode at a modest data rate;
(5) it has no internal moving components, which gives it high
reliability; and (6) it is physically very small with low weight and
power requirements.


Combined Gamma Ray/neutron Spectroscopy for Mapping Lunar Resources

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