From: st17a@judy.uh.edu (University Space Society)
Subject: More Lunar Resource Mapper Information
News-Software: VAX/VMS VNEWS 1.41    
Organization: University of Houston
Date: Fri, 24 Jul 1992 03:12:00 GMT

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 1, 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

Dennis, University of Alabama in Huntsville


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