Development of an ultracompact neutron spectrometer for identifying near-surface water on mars. Page: 3 of 3
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
NEUTRON SPECTROMETER FOR IDENTIFYING NEAR-SURFACE WATER ON MARS: D. J. Lawrence et al.
called MCNPX has been developed at Los Alamos
National Laboratory that models both high-energy
charged particle interactions as well as neutron and
gamma-ray transport interactions within materials .
Here we use the MCNPX code for exploring how a
rover based neutron detector is expected to operate.
Figure 1 shows a schematic diagram of a neutron
detector that could be easily built using existing tech-
nology. The instrument consists of two 3He gas pro-
portional counters (10 cm x 1 cm diameter). One of
the 3He counters is wrapped in Sn and measures
thermal plus epithermal neutrons and one 3He counter
is wrapped in Cd and only measures epithermal neu-
trons. Between the two counters is an electronics
board that can house all the analog and digital elec-
tronics as well as the high voltage supply needed to
power the detector.
3He Neutron T
Detectors 10 cm long x
1 cm dia.
Figure 1: Schematic diagram of a rover based neu-
For our model, we set up two test cases. Case 1 is
the situation with only 3He tubes on a Martian sur-
face. For a Martian surface composition, we used the
average Pathfinder soil composition of . To see
how sensitive the 3He detectors are to subsurface wa-
ter, we kept the top 30 cm of the surface dry, and then
added varying amounts of water (from 1 to 40 wt.%
H20) to the soil below 30 cm. For Case 2, we at-
0 10 20
Wt % H0
Figure 2: Epithermal neutron count rates for bare
and rover bases 3He as a function of H20 content.
tached the 3He neutron detectors to an electronics
board (Figure 1) and rover mass. We simulated the
electronics board material as being roughly 60% fi-
berglass and 40% epoxy. For the rover, we assumed
the total mass was 150 kg, after the MER design (D.
Sevilla, JPL, pers. comm.). For simplicity, we di-
vided this mass into 90% aluminum and 10% elec-
tronics board material. We then spread the material
over a 50cm x 50cm x 50 cm cube. As our study
progresses, we will use more accurate estimations of
the rover body.
Results: Figures 2 and 3 show results from the
initial modeling of the two cases. Figure 2 shows the
modeled epithermal neutron counting rate (in arbi-
trary units) as a function of water content. In both
cases, we see that the epithermal counting rate de-
creases as the water content increases, with a clear
difference between non-hydrated soil and soil with
even only 1% hydration. In addition, we see that the
counting rate for the rover based tubes is about a fac-
tor of two higher compared to the bare tube case.
Figure 3 shows that the counting rate normalized to
the dry soil is very similar in both cases, with the
rover-attached tube showing a somewhat lower per-
centage effect for increasing hydrogen abundances.
While additional work needs to be done in carry-
ing out more realistic models and benchmark meas-
urements, these results show that attaching a 3He neu-
tron detector to a rover does not substantially degrade
its ability to detect water on the Martian surface.
References:  Bridges and Grady, Earth Plan. Sci Lett.,
176, 267, 2000;  Malin and Edgett, Science, 288, 2330, 2000;
 Christensen et al., JGR, 105, 9623, 2000;  Zent and McKay,
Icarus, 108, 146, 1994;  Feldman et al., 1998, Science, 1489,
1998;  Feldman et al., JGR, in press, 2001;  Waters,
MCNPX Users Manual, LA-UR 99-6058, 1999;  Bruckner et
al., 32nd LPSC, #1293, 2001.
0 Bare Tube
A Tube attached to roves
0 10 20
Wt % H_0
Figure 3: Epithermal neutron count rates normalized
to a dry soil for bare and rover bases 3He as a func-
tion of H20 content.
O Bare Tube d
A Tube attached to rover
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Lawrence, David J. (David Jeffery); Wiens, R. C. (Roger C.); Moore, K. R. (Kurt R.) & Prettyman, T. H. (Thomas H.). Development of an ultracompact neutron spectrometer for identifying near-surface water on mars., article, January 1, 2001; United States. (digital.library.unt.edu/ark:/67531/metadc934132/m1/3/: accessed February 15, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.