NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/sci/space/10767

Text File | 1992-07-24 | 7.3 KB | 142 lines

Newsgroups: sci.space
Path: sparky!uunet!gatech!paladin.american.edu!darwin.sura.net!tulane!rouge!srl01!pgf
From: [e-mail] (Phil G. Fraering)
Subject: Re: More Lunar Resource Mapper Information
Message-ID: <pgf.711998024@srl01>
Sender: [e-mail] (Anonymous NNTP Posting)
Organization: Univ. of Southwestern Louisiana
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Date: Fri, 24 Jul 1992 17:13:44 GMT
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And now, reformatted!

Here is some more Lunar Resource Mapper information that I have gleaned from
publications and proceedings. I hope this is of some interest.

    Gamma-Ray and Neutron Spectrometer for the Lunar Resource Mapper

One of the early Space Exploration Initiatives will be a lunar orbiter
to map the composition of the Moon. This mission is needed to support
further lunar exploration and habitation and will provide a valuable
dataset for understanding lunar geological processes. The payload will
consist of the gamma-ray and neutron spectrometer discussed here, an
X-ray fluorescence imager, and possibly one or two other instruments.
The spacecraft will be small (<100 kg), built on a fast schedule
(about three years), and have a low cost (about $100M including
launch). Launch is tentatively scheduled for April 1995. The program
will be similar to the ALEXIS (Array of Low- Energy X-ray Imaging
Sensors) program at Los Alamos, which is scheduled to be launched as a
small satellite in April 1992.

    Most gamma rays used to map lunar elements are in the energy
range of 0.2-8 MeV. The gamma-ray detector will contain a ~70%
efficient [relative to a 7.62-cm- diameter x 7.62-cm-length NaI(Tl)
scintillator] n-type germanium crystal. N-type is used because it is
much less susceptible to radiation damage than p-type germanium. No
annealing is planned because the radiation damage accumulated in the
one-year mission will not seriously degrade the energy resolution if
the crystal remains below 100 K. Because a Stirling cycle cooler will
be used, the crystal will be mounted ustechniques commercially
developed in recent years for operating germanium detectors on
vibrating platforms. A bismuth germanate (BGO) anticoincidence shield
on the sides and back of the germanium crystal 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 attenuating the gamma-ray flux from
the Moon. The gamma-ray detector will be on a short boom to further
reduce the background from the spacecraft.

    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 proposed. For
longer missions a passive radiator, as used on the Mars Observer, or
an active device, such as a Stirling cycle cooler, is required.

    We have chosen not to use a passive radiator because of the
complications in shielding the radiator from the Sun, Earth, and Moon
when the spacecraft is in a polar orbit and instead have chosen 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 three years. Two of these miniature cryocoolers
were launched on 12 September 1991 as part of the ISAMS multichannel
infrared radiometer on the Upper Atmosphere Research Satellite, and
they are still operating successfully. Research is being done on these
coolers (1) concerning vibration, thermal performance, and
reliability.

    Because the germanium detector energy resolution is degraded
by vibration, we also will 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 low-distortion electronic
feedback system to minimize harmonics and a flexible vibration
decoupler between the expander cold tips and the germanium crystal.

    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, is needed to normalize
the gamma-ray line intensities; in turn, the gamma-ray dattermine 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 (E(sub)n ~ 0.01-0.4 eV) neutrons are
measured with a bare ^3He proportional counter, epithermal (E(sub)n ~
0.4-10^3 eV) neutrons with a ^3He proportional counter wrapped with
thermal-neutron-absorbing cadmium, and fast (E(sub)n ~ 0.5-10 MeV)
neutrons with a plastic scintillator and ^3He proportional counter
operated in coincidence (3). The thermal se the epithermal sensor will
be mounted on a short boom opposite the gamma-ray detector boom to
reduce neutron backgrounds. Ratios of the three count rates are very
sensitive to the amount of hydrogen in the lunar surface (4).

    The gamma-ray and neutron spectrometer will provide data on
almost all elements over all of the lunar surface. Published estimates
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 the neutron detector (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 data from this instrument will complement the data
from the X-ray fluorescence imager (7), which has a resolution of 1 km
x 1 km for six elements.

[Figure w1, which appears in the hard copy here, shows the schematics
of a genegermanium detector with a split cycle Stirling cooler
(adopted from 5) and the neutron sensors for thermal, epithermal, and
fast neutrons.]  Work supported by NASA and done under the auspices of
the US DOE.

References

(1) Ross R. G. et al. (1991) Advances in Cryogenic Engineering, 37, in press.
(2) Reedy R. C. et al. (1992), this workshop.
(3)Jenkins R. W. et al. (1970) J. Geophys. Res., 75, 4197-4204.
(4))Feldman W. C. et al. (1991) Geophys. Res. Lett., 18, 2157-2160.
(5) Metzger(6) Reedy R. C. et al. (1973) J. Geophys. Res., 78, 5847-5866.
(7) Edwards B. C. et al. (1992), this workshop. Edwards B. C.*   Ameduri F.   Bloch J. J.
Priedhorsky W. C.   Roussel-Dupre D.Smith B. W.

Sorry about the slight overrun. I am still learning how to do this stuff. If
someone can grab this and reformat, go for it. Thank you

Me: Maybe if they installed a good copy of emacs at the site,...
Anyway, it took all of 20 seconds to reformat using M-q.

Dennis, University of Alabama in Huntsville


Phil


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