One of the early Space Exploration Initiatives will be a lunar orbiter to map the elemental composition of the Moon. This mission will support further lunar exploration and habitation and will provide a valuable dataset for understanding lunar geological processes. The proposed payload will consist of the gamma ray and neutron spectrometers which are discussed, an x ray fluorescence imager, and possibly one or two other instruments

Byrd, R. C.

Drake, D. M.

Feldman, W. C.

Martin, R. A.

Merrigan, M. A.

Moss, C. E.

Reedy, R. C.

English

NASA Technical Reports Server

40 New Technologies for Lunar Resource Assessment
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ls/FeO (Arb.)
Fig. 3.
which is the concentration of fine-grained metal determined by
ferromagnetic resonance (Is) normalized to the total iron content
(as FeO) [ 1 ]. Is/FeO has been measured for virtually every soil
returned by the Apollo and Luna missions to the Moon; values
for surface soils are compiled by [2).
Because the technique is sensitive to both oxidation state and
mineralogy, iron Mossbauer spectroscopy (FeMS) is a viable tech-
nique for in situ lunar resource assessment. Its u t i l i t y for mineralogy
is apparent from examination of published FeMS data for lunar
samples (e.g., 3-5]. From the data published by |5], we can infer
that FeMS data can also be used to determine soil maturi ty. We
discuss here (1) the use of FeMS to determine mineralogy and
matur i ty and (2) progress on development of a FeMS instrument
for lunar surface use.
Mineralogy and Maturity: Figure 1 shows FeMS spectra for
two Apollo 1 7 soils, 72131 and 73131. Even by visual inspection,
the two soils have very different proportions of the ferrous-bearing
minerals pyroxene, olivine, and ilmenite. From peak areas, the
percentages of total iron associated with each of those mineralogies
are 66, 15, and 16 for 72131 and 57, 39, and 4 for 73131. Thus,
a Mossbauer spectrometer on a lunar rover could be used to
prospect for ilmenite-rich soil horizons (deposits).
Backscarter Mossbauer Spectrometer (BaMS)
SC H£MATtC DRAWIMG
SPECIFICATIONS
SIZE
Shape: Cylinder. 6 cm 1t2 cm long,
volume: 3OOcm3.
Wass: SOO g.
ELECTRICAL POWER
Total: < 2-0 W.
High Voltage: None required
hem spacecraft.
DATA
Experiment Duraten: 60 min.
Type: Digital.
rGJGSits cf Data: 6O kB pet spectrum.
Data Rale: Data stored by instrument
until requested Dy spacecraft bus.
Data Product Mosstauer spectra(counts per channel vs. channel
number ot a <nu!ti-chaenel analyzer)
A reason for finding ilmenite deposits is shown in Fig. 2, which
presents FeMS spectra for a crushed lunar rock (70035) both before
and after reduction by hydrogen. The only two iron-bearing
mineralogies present in the untreated rock are ilmenite and pyr-
oxene, with 2 3% of the total Fe in the ilmenite. After H? reduction,
there is no evidence in the FeMS spectra for ilmenite. Only metallic
iron and pyroxene are present, which implies that reduction of
ferrous iron in ilmenite is kinetically favored over that in pyroxene.
The 36% area for Fe metal does indicate, however, that some
of the ferrous iron in pyroxene has been reduced to metal.
In Fig. 3, we plot the excess area near zero velocity, which is
a measure of the concentration of superparamagnetic Fe metal
(sp-Fe°), as a function of the maturity index Is/FeO for nine lunar
samples. The area for sp-Fe was calculated using the procedure
of [5] except that no attempt was made to correct for saturation
effects. A linear correlation is present, which demonstrates that
maturity data can be derived from Mossbauer measurements.
Instrument Development: The current status of develop-
ment of our backscatter Mossbauer spectrometer (BaMS) is de-
scribed by (6). A schematic of the instrument, together with
specifications, is given in Fig. 4. The instrument has low mass,
low volume, low power consumption, and low data transfer rate.
We have built and fully tested the velocity drive. Operation in
backscatter mode means that no sample preparation is required.
The instrument can be placed on a lander or rover or used at
a lunar base.
References: [11 Morris (1976) Proc. LSC 7th, 315. (2) Morris
(1978) Proc. LPSC 9th, 2287. [3] Herzenberg et al. (1971) Proc.
LSC 2nd, 2103. [4] Gibbet al . ( 1972) Proc. LSC 3rd, 2479. [5] Hous-
ley et al. (1974) Proc. LSC 5th, 2623. [6] Agresti et al. (1992)
Hyperfine Inter., in press.
Fig. 4.
GAMMA RAY AND NEUTRON SPECTROMETER FOR
THE LUNAR RESOURCE MAPPER. C. E. Moss, R. C. Byrd,
D. M. Drake, W. C. Feldman, R. A. Martin, M. A. Merrigan,
and R. C. Reedy, Los Alamos National Laboratory, Los Alamos
NM 87545, USA.
One of the early Space Exploration Initiatives will be a lunar
orbiter to map the elemental composition of the Moon. This
mission will support further lunar exploration and habitation and
will provide a valuable dataset for understanding lunar geological
processes. The proposed payload will consist of the gamma ray
and neutron spectrometer discussed here, an X-ray fluorescence
imager [ 1 ], and possibly one or two other instruments. The space-
craft will have a mass of about 1000 kg including fuel, be built
on a fast schedule (about three years), and have alow cost (about
$100 M including launch). Launch is tentatively scheduled for
April 1995.
Most gamma rays used to map lunar elements are in the energy
range of 0.2-8 MeV. The proposed gamma ray detector will contain
an n-type germanium crystal that is =70% efficient [relative to
a 7.62-cm-diameter X 7.62-cm-length Nal(Tl) scintillator). N-type
is used because it is much less susceptible to radiation damage
than p-type germanium. Because a Stirling cycle cooler will be
used, the crystal will be mounted using techniques commercially
developed in recent years for operating germanium detectors on
vibrating platforms. A segmented cesium iodide (Csl(Na)] anti-
coincidence shield on the sides and back of the germanium crystal
https://ntrs.nasa.gov/search.jsp?R=19930008070 2020-03-24T07:24:13+00:00Z
LPI Technical Report 92-06 41
GERMANIUM GAMMA-RAY SENSOR
LECTRONICS / / ANTICOINCIDENCE
COMPRESSOR DETECTOR/ SHIELD
PHOTOMULTIPLIERTUBE'
NEUTRON SENSORS
EPITHERMAL
Fig. 1. Schematics of a generic germanium detector with a split-cycle
Stirling cooler (adopted from [5]) and the neutron sensors for thermal,
epithermal, and fast neutrons.
will eliminate most events due to charged particles, gamma rays
produced by cosmic rays incident on the spacecraft, and Compton-
scattered events in the crystal. A plastic scintillator over the nadir-
pointing surface of the germanium crystal will provide a similar
capability in the forward direction without significantly a t tenuat-
ing the gamma ray flux from the Moon. The gamma ray detector
will be on a short cantilever to further reduce the background
from the spacecraft. One side segment of the cesium iodide shield
will serve as a back-up for the germanium crystal.
The critical issue for operating a germanium detector in space
is the method of cooling. For short missions, stored cryogens such
as liquid nitrogen, solid methane, or solid argon have been pro-
posed. For longer missions a passive radiator, such as used on the
Mars Observer, or an active device, such as a Stirling cycle cooler,
is required. We propose not to use a passive radiator because of
complications in shielding the radiator from the Sun, Earth, and
Moon when the spacecraft is in a polar orbit and, instead, propose
to use the British Aerospace Stirling cycle cooler based on the
Oxford design. This closed-cycle mechanical cooler is designed for
a 10-year lifetime and has operated successfully in the laboratory
without maintenance for over 2/j years. Two of these miniature
cryocoolers were launched on 12 September 1991 as part of the
Improved Stratospheric and Mesospheric Sounder (1SAMS) mul-
tichannel infrared radiometer on the Upper Atmosphere Research
Satellite and are still operating successfully. Because the germa-
nium detector energy resolution can be degraded by vibration,
we plan to use a pair of these coolers with two compressors and
two expanders mounted back to back to minimize vibration. In
addition, we will use a flexible vibration decoupler between the
expander cold tips and the germanium crystal. Microphonics
should not be a problem because a germanium detector cooled
by a cryocooler with a higher vibration level gave 3.6-keV reso-
lution at 1332 keV, which is ~2 times worse than with a good
laboratory Ge detector, but is more than adequate for planetary
elemental mapping.
A neutron detector is required because it provides maximum
sensitivity for hydrogen and hence water. Data from the gamma
ray detector and the neutron detector are complementary because
the neutron flux, which produces most gamma rays, can be used
to normalize the gamma ray line intensities; in turn, the gamma
ray data are needed to determine the composition of the lunar
surface and hence the moderation of neutrons by elements other
than hydrogen [2]. Three different sensors are used to measure
the neutrons in three energy ranges. Thermal neutrons (£„ ~
0.010.4 eV) are measured with a bare 3He proportional counter,
epithermal neutrons (£„ ~ 0.4-103 eV) with a 3He proportional
counter wrapped with cadmium (which strongly absorbs thermal
neutrons), and fast neutrons (£„ ~ 0.5-10 MeV) with the backup
Army Background Experiment (ABE) instrument, which contains
four boron-loaded plastic scintillators [3]. Thermal/epithermal and
fast/epithermal counting ratios are very sensitive to the amount
of hydrogen in the lunar surface (4).
The gamma ray and neutron spectrometer will provide data
on many elements over the entire lunar surface. Published esti-
mates of the detection limits for similar detectors range from 0.016
ppm for uranium to 1.3% for calcium (5). We estimate a hydrogen
detection limit of 100 ppm based on this neutron detector system
[4]. The spatial resolution is about 140 km X 140 km, which is
determined by the orbit altitude of 100 km [6]. Both gamma rays
and neutrons sense the elemental composition of the lunar surface
to depths of tens of centimeters. The results from this instrument
will complement the measurements from elemental and/or mine-
ralogical remote sensing instruments with higher spatial resolution.
This work was supported by NASA and done under the auspices
of the U.S. DOE.
References: [1] Edwards B. C. et al., this volume. [2] Reedy
R. C. et al., this volume. (3) Feldman W. C. et al. (1991) Nitcl.
lustrum. Met/i. Pro's. Res., A306, 350-365. [41 Feldman W. C. et al.
(1991) GRL, /8, 2157-2160. [5) Metzger A. E. and Drake D. M.
(1991UGR, 96, 449-460. |6) Reedy R. C. et al. (1973);GR, 78,
5847-5866.
MAGNETIC SUSCEPTIBILITY MEASUREMENTS FOR
IN SITU CHARACTERIZATION OF LUNAR SOIL. R. R.
Oder, EXPORTech Company, Inc., New Kensington PA, USA.
Magnetic separation is a viable method for concentration of
components of lunar soils and rocks for use as feedstocks for
manufacture of metals, oxygen, and for recovery of volatiles such
as 3He (1 ].
Work with lunar materials indicates that immature soils are
the best candidates for magnetic beneficiation [2]. The magnetic
susceptibility at which selected soil components such as anorthite,
ilmenite, or metallic iron are separated is not affected by soil
matur i ty , but the recovery of the concentrated components is.
Increasing soil maturity lowers recovery [3]. This is illustrated in
Fig. 1.
Mature soils contain significant amounts of glass-encased metal-
lic iron. Magnetic susceptibility, which is sensitive to metallic iron
content, can be used to measure soil maturity. The relationship
between the ratio of magnetic susceptibility and iron oxide and
the conventional maturity parameter, Is/FeO, ferromagnetic
resonant intensity divided by iron oxide content, is given in Fig. 2.
The magnetic susceptibilities were determined using apparatus
designed for magnetic separation of the lunar soils.
Magnetic susceptibility should be incorporated in the instru-
mentation packages carried by lunar landers and rovers for in situ
identification of candidate soils best suited for rhagnetic benefi-


Gamma Ray and Neutron Spectrometer for the Lunar Resource Mapper

Gamma Ray and Neutron Spectrometer for the Lunar Resource Mapper

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