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 [7].
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.
Electronics
Board
3He Neutron                       T
Detectors                       10 cm long x
1 cm dia.
11
Figure 1: Schematic diagram of a rover based neu-
tron detector.
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 [8]. 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

30         40

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: [1] Bridges and Grady, Earth Plan. Sci Lett.,
176, 267, 2000; [2] Malin and Edgett, Science, 288, 2330, 2000;
[3] Christensen et al., JGR, 105, 9623, 2000; [4] Zent and McKay,
Icarus, 108, 146, 1994; [5] Feldman et al., 1998, Science, 1489,
1998; [6] Feldman et al., JGR, in press, 2001; [7] Waters,
MCNPX Users Manual, LA-UR 99-6058, 1999; [8] Bruckner et
al., 32nd LPSC, #1293, 2001.

0  Bare Tube
A Tube attached to roves
06
04
E02

001
0         10         20
Wt % H_0

30         40

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

z