Lunar surface outgassing and alpha particle measurements Page: 2 of 2
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LUNAR SURFACE OUTGASSING AND ALPHA PARTICLE MEASUREMENTS. S. L. Lawson',
W. C. Feldman', D. J. Lawrence', K. R. Moore', R. C. Elphic', S. Maurice2, R. D. Belian', and A. B. Binder.
'Los Alamos National Laboratory, Los Alamos, NM; 2Observatoire Midi-Pyr6n6es, Toulouse, France; 'Lunar
Research Institute, Tucson, AZ. (email@example.com)
Introduction: The Lunar Prospector Alpha Particle
Spectrometer (LP APS) searched for lunar surface gas
release events and mapped their distribution by
detecting alpha particles produced by the decay of
gaseous radon-222 (5.5 MeV, 3.8 day half-life), solid
polonium-218 (6.0 MeV, 3 minute half-life), and solid
polonium-210 (5.3 MeV, 138 day half-life, but held
up in production by the 21 year half-life of lead-210).
These three nuclides are radioactive daughters from the
decay of uranium-238. Radon reaches the lunar surface
either at areas of high soil porosity or where fissures
release the trapped gases in which radon is entrained.
Once released, the radon spreads out by "bouncing"
across the surface on ballistic trajectories in a random-
walk process. The half-life of radon-222 allows the gas
to spread out by several 100 km before it decays
(depositing approximately half of the polonium-218
recoil nuclides on the lunar surface) and allows the
APS to detect gas release events up to several days
after they occur. The long residence time of the lead-
210 precursor to polonium-210 allows the mapping of
gas vents which have been active over the last
approximately 60 years. Because radon and polonium
are daughter products of the decay of uranium, the
background level of alpha particle activity is a function
of the lunar crustal uranium distribution.
Using radioactive radon and polonium as tracers,
the Apollo 15 and 16 Command Module orbital alpha
particle experiments obtained evidence for the release
of gases at several sites beneath the orbit tracks,
especially over the Aristarchus Plateau and Mare
Fecunditatis . Aristarchus crater had previously been
identified by ground-based observers as the site of
transient optical events . The Apollo 17 surface
mass spectrometer showed that argon-40 is released
from the lunar interior every few months, apparently in
concert with some of the shallow moonquakes that are
believed to be of tectonic origin . The latter tectonic
events could be associated with very young scarps
identified in the lunar highlands  and are believed to
indicate continued global contraction. Such quakes
could open fissures leading to the release of gases that
are trapped below the surface. A primary goal of the
APS was to map gas-release events, thus allowing
both an appraisal of the current level of tectonic
activity on the Moon and providing a probe of
subsurface uranium concentrations.
Analysis: The APS consisted of five pairs of
silicon ion-implanted detectors which were collimated
to a 450 half-angle aperture. The detectors were
positioned on the five outward-pointing faces of a cube
which was mounted at the tip of one of the spacecraft
booms. The five analog signals sent to the electronics
box were digitized using a common 8 bit ADC,
spanning the nominal energy range between 4.5 MeV
and 6.6 MeV (corresponding to 0.02 MeV per
channel). The detector resolution was approximately
0.1 MeV. During data acquisition, LP spun at 12
RPM with its spin axis approximately perpendicular to
the ecliptic. Thus, three of the APS faces swept
through nadir when the spacecraft was near the equator,
while the other two faces were either nadir- or zenith-
pointing when the spacecraft was near the poles.
The primary lunar alpha-particle signal is from
reflected interplanetary alpha particles. Therefore, we
have examined only APS data acquired during periods
of low interplanetary alpha particle flux. To minimize
statistical uncertainties, spectra were summed over all
LP mapping cycles when the instrument was turned on
(approximately 229 days over 16 months).
Conclusions: The LP APS data were studied to
map sites of radon release on the lunar surface. We
found only a faint indication of alpha particles
resulting from the decay of polonium-218. However,
our radon-222 alpha particle map indicates that radon
gas is presently emanating from the vicinity of craters
Aristarchus and Kepler. The LP gamma-ray
spectrometer, which effectively has significantly higher
spatial resolution than the APS, identified thorium
enrichments at these two craters . Thorium and
uranium are both incompatible elements whose lunar
surface abundances are highly correlated; thus, it is
likely that the radon-222 alpha particles measured
using the LP APS originate from Kepler and
Aristarchus. Our detection of radon over Aristarchus is
consistent with one of the results of the Apollo 15
APS. The polonium-210 distribution mapped by the
APS indicates a variability in time and space of lunar
gas release events.
References:  Gorenstein P. (1993) in Remote
Geochemical Analysis, C.M. Pieters and P.A.J.
Englert, eds., 235.  Middlehurst B. (1967) Rev.
Geophys., 5, 173.  Hodges R. and J. Hofian
(1975) Proc. Lunar Sci. Conf 6th, 3039.  Schultz
P. (1976) Moon Morphology, Univ. of Texas, Austin.
 Lawrence D.J. et al. (2000) JGR, 105, 20307.
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Lawson, S. L. (Stefanie L.); Feldman, W. C. (William C.); Lawrence, David J. (David Jeffery),; Moore, K. R. (Kurt R.); Elphic, R. C. (Richard C.); Maurice, S. (Sylvestre) et al. Lunar surface outgassing and alpha particle measurements, article, January 1, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc934215/m1/2/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.