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Physics
12-1-2012
The first cosmic ray albedo proton map of the
Moon
Jody K. Wilson
University of New Hampshire
Harlan E. Spence
University of New Hampshire, [email protected]
Justin Kasper
Michael Golightly
University of New Hampshire
J. B. Blake
See next page for additional authors
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Recommended Citation
Wilson, J. K., et al. (2012), The first cosmic ray albedo proton map of the Moon, J. Geophys. Res.,117, E00H23, doi:10.1029/
2011JE003921.
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Authors
Jody K. Wilson, Harlan E. Spence, Justin Kasper, Michael Golightly, J. B. Blake, J. E. Mazur, L. W. Townsend,
A. W. Case, M. D. Looper, C. Zeitlin, and Nathan A. Schwadron
This article is available at University of New Hampshire Scholars' Repository: http://scholars.unh.edu/physics_facpub/170
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E00H23, doi:10.1029/2011JE003921, 2012
The first cosmic ray albedo proton map of the Moon
Jody K. Wilson,1 Harlan E. Spence,2 Justin Kasper,3 Michael Golightly,1 J. Bern Blake,4
Joe E. Mazur,5 Lawrence W. Townsend,6 Anthony W. Case,3 Mark Dixon Looper,4
Cary Zeitlin,7 and Nathan A. Schwadron1
Received 4 August 2011; revised 1 February 2012; accepted 3 April 2012; published 5 June 2012.
[1] Neutrons emitted from the Moon are produced by the impact of galactic cosmic
rays (GCRs) within the regolith. GCRs are high-energy particles capable of smashing
atomic nuclei in the lunar regolith and producing a shower of energetic protons, neutrons
and other subatomic particles. Secondary particles that are ejected out of the regolith
become “albedo” particles. The neutron albedo has been used to study the hydrogen
content of the lunar regolith, which motivates our study of albedo protons. In principle,
the albedo protons should vary as a function of the input GCR source and possibly as a
result of surface composition and properties. During the LRO mission, the total detection
rate of albedo protons between 60 MeV and 150 MeV has been declining since 2009 in
parallel with the decline in the galactic cosmic ray flux, which validates the concept of an
albedo proton source. On the other hand, the average yield of albedo protons has been
increasing as the galactic cosmic ray spectrum has been hardening, consistent with a
disproportionately stronger modulation of lower energy GCRs as solar activity increases.
We construct the first map of the normalized albedo proton emission rate from the lunar
surface to look for any albedo variation that correlates with surface features. The map
is consistent with a spatially uniform albedo proton yield to within statistical uncertainties.
Citation: Wilson, J. K., et al. (2012), The first cosmic ray albedo proton map of the Moon, J. Geophys. Res., 117, E00H23,
doi:10.1029/2011JE003921.
1. Introduction
[2] The Lunar Reconnaissance Orbiter (LRO) has a comprehensive suite of remote sensing instruments for measuring mineral and elemental variations over the lunar surface
[Chin et al., 2007]. Instruments on LRO have made compositional maps using infrared [Greenhagen et al., 2010],
visible [Gustafson et al., 2010] and ultraviolet spectra
[Hendrix et al., 2010; Denevi et al., 2011] as well as radar
1
Space Science Center, University of New Hampshire, Durham, New
Hampshire, USA.
2
Institute for Earth, Oceans and Space, University of New Hampshire,
Durham, New Hampshire, USA.
3
High Energy Astrophysics Division, Smithsonian Astrophysical
Observatory, Harvard-Smithsonian Center for Astrophysics, Cambridge,
Massachusetts, USA.
4
Space Sciences Department, Aerospace Corporation, Los Angeles,
California, USA.
5
Space Sciences Department-Chantilly, Aerospace Corporation, Chantilly,
Virginia, USA.
6
Department of Nuclear Engineering, University of Tennessee, Knoxville,
Tennessee, USA.
7
Southwest Research Institute, Boulder, Colorado, USA.
Corresponding author: J. K. Wilson, Space Science Center, University
of New Hampshire, Morse Hall Room 353, 8 College Rd., Durham
NH 03824, USA. ([email protected])
Copyright 2012 by the American Geophysical Union.
0148-0227/12/2011JE003921
data [McAdam et al., 2011]. The LEND instrument has also
produced lunar surface compositional maps using the energy
spectra of lunar neutrons [Mitrofanov et al., 2010; Crites
et al., 2011]. The purpose of this paper is to present the
first map of high energy protons emitted from the surface of
the Moon as observed by the CRaTER instrument on LRO.
[3] Neutrons emitted from the Moon are produced by the
impact of galactic cosmic rays (GCRs), extremely energetic
ions from outside the solar system which are capable of
fragmenting atomic nuclei (spallation) in the lunar regolith.
Fragments from these collisions can themselves collide with
and break apart additional nuclei, thus producing a shower
of energetic protons, neutrons and other subatomic particles
radiating away from the incident path of the parent ion. This
phenomenon is analogous to galactic cosmic ray air showers
which occur in the Earth’s atmosphere, albeit with smaller
length scales due to the higher density of regolith. Some of
the secondary particles produced by these collision cascades
are ejected upwards out of the regolith, becoming “albedo”
particles [Ulmer, 1994; Dorman, 2004; Spence et al., 2010].
[4] The LEND instrument on LRO and the Neutron Spectrometer on the earlier Lunar Prospector mission both used
albedo neutrons to produce maps of compositional variations
of the lunar surface. Both instruments detected a reduction in
the neutron flux at certain energies near the lunar polar regions,
indicating a higher abundance of hydrogen in the soil there
than at lower latitudes [Feldman et al., 1998; Lawrence et al.,
2006; Mitrofanov et al., 2010]. This hydrogen presumably
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WILSON ET AL.: FIRST ALBEDO PROTON MAP OF THE MOON
Figure 1. Diagram of CRaTER instrument showing crosssectional cutaway view of the stack of six detectors (D1–D6)
and pieces of tissue equivalent plastic (TEP). Example particle trajectories are shown for a high-energy galactic cosmic
ray from the zenith passing completely through the instrument (red line) and for an albedo proton (blue line) coming
up from the lunar surface and passing through four detectors
before being stopped in one of the blocks of TEP. (Adapted
from Spence et al. [2010].)
exists in the form of ancient water ice in or near cold permanently shadowed craters [Feldman et al., 1997, 2001; Elphic
et al., 2004; Lawrence et al., 2011a]. LRO and Lunar prospector also measured variations in albedo neutron fluxes that
are indicative of rough compositional differences in the regolith between the maria and highland regions [Feldman et al.,
1998; Gasnault et al., 2001; Maurice et al., 2004; Crites et al.,
2011; Lawrence et al., 2011b]. These spatial variations in
albedo particle fluxes demonstrate that lunar-impacting GCRs
and the upward-moving secondaries that they produce constitute a type of natural sounding experiment which can provide information on the elemental abundances in the top
meter of lunar regolith. This paper reports on a search for an
analogous effect in albedo protons, which are also emitted
from the Moon as a result of GCR spallation.
2. CRaTER Instrument
[5] The Cosmic Ray Telescope for the Effects of Radiation
(CRaTER) [Spence et al., 2010] is a cosmic ray telescope on
LRO composed of six silicon semiconductor detectors
arranged in three pairs, which are in turn separated by two
large blocks of tissue equivalent plastic (TEP) (see Figure 1).
When the instruments on LRO are pointed at the lunar nadir,
detectors D1 and D2 on CRaTER face outer space, and D5
and D6 face the Moon, while D3 and D4 lie in the middle
between the two blocks of TEP. Each of the three detector
pairs has one thin (149 microns, odd numbered) and one
thick (1 mm, even numbered) detector with low and high
amplifier gains, respectively, allowing for measurements of
galactic cosmic rays over a wide range of deposited energies.
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A GCR that passes through two detector pairs must also traverse the TEP that lies between them, and because the plastic
absorbs some of the GCR’s energy, the detector pairs on
either end of the path will register unequal amounts of energy
deposited in the detectors; this makes it possible to determine
which direction the GCR was traveling. (The microsecond
time resolution of the detector measurement electronics is
much too slow to observe a difference in arrival times at
different detectors.)
[6] For this study we choose to look at GCRs which passed through the thick detectors of the Moon-facing and
central detector pairs, designated as D6 and D4, respectively.
The thick detectors are more sensitive to protons than the
thin ones, and coincident detections in D4 + D6 cover the
widest field of view of any two thick detectors. The D4 + D6
coincidence pair allows us to differentiate energetic protons
coming from zenith (primary GCR from deep space) and
from nadir (secondary albedo protons from the Moon). Since
the larger piece of TEP partially shields D4 from GCRs
arriving from the zenith direction, the sensitivity to primary
GCRs just above the threshold energy of 60 MeV is somewhat reduced relative to the secondaries from the Moon,
which have nearly unobstructed access to D6.
3. Data Preparation
[7] Several culling steps are required to extract D4 + D6
proton detections from the raw CRaTER data stream.
CRaTER detects on average about six million events per
day, of which only about 200,000 per day qualify as
simultaneous detections by D4 and D6 that we need for this
study. Second, LRO is not always oriented with its instrument suite pointed at lunar nadir (D6 pointed at the Moon),
and data gathered at orientations greater than one degree
from nadir pointing are rejected. Third, the CRaTER instrument performs periodic calibration sequences during which
artificial GCR detections are registered, so we reject these
periods of time. Finally, we must determine the fraction of
valid D4 + D6 events that are known with high certainty to be
protons. This final identification step is the most complex
one, as discussed in the remainder of this section.
[8] A 2-dimensional histogram of valid D4 + D6 events
from June 2009 to January 2011 (Figure 2) shows the major
features in the data set. The brightness of each pixel reflects
the number of particles registering in each pair of (D4, D6)
energy values, with darker pixels signifying more events.
Each detector has 4096 energy channels, and Figure 2 shows
only the lowest 800 channels in each detector, which corresponds to the lightest GCR nuclei (protons and alpha
particles) and the fastest of the next heavier nuclei such as
carbon and nitrogen. The highest-energy GCRs are slowed
so little as they pass through matter that they deposit
essentially equal amounts of energy in D4 and D6, and
fall along the diagonal that runs lower-left to upper-right of
the histogram. Slower ions deposit unequal amounts of
energy in D4 and D6 because they lose a significant fraction
of their energy to the intervening TEP; these ions form
the “branches” extending off of the diagonals. Slower GCRs
arriving from deep space pass through D4, are slowed in
the TEP, and then deposit more energy in D6 than D4; this
population is represented by the vertical branches of protons
and alpha particles labeled in Figure 2. On the other hand,
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Figure 2. Two-dimensional histogram of D4 + D6 coincident detection events. Axes are labeled with the energy
deposited in the central thick detector (D4) and lunar-facing
thick detector (D6). Events lying along the diagonal extending from the lower-left to the upper-right are from particles
with the highest incident energy and correspondingly lowest
energy deposition in the detectors. Branches extending
upwards from the diagonal are lower-energy particles arriving from space, while the single horizontal branch extending
to the right represents protons arriving from the Moon. A
diffuse background source which is spread over a wide area
of channel pairs is also indicated.
slower protons arriving from the Moon impact D6 first, are
slowed in the TEP, and then deposit more energy in D4 than
D6; this population is the horizontal branch labeled in
Figure 2. This process by which energetic particles lose
energy as a function of their charge, energy, and speed, as
they pass through matter is given by the Bethe-Bloch
equation (see Section 3.1.2 of Spence et al. [2010], for the
application of this principle to the CRaTER instrument).
[9] Also visible in Figure 2 are events which fall on D4 +
D6 energy channel pairs in-between those of GCR species.
These events are more uniformly distributed across channels,
E00H23
but are also concentrated near both axes. Preliminary analysis
indicates that these events are caused by ions that miss
D4 and/or D6 but produce electromagnetic and/or nuclear
showers in the material of the sensor. Given the gradual
falloff in this diffuse background source with increasing
deposited energy, it follows that some of the events in the
(D4, D6) channel pairs where protons are expected to register
are not necessarily caused by protons. Since there is no way
to distinguish a normal proton detection event from an event
in the diffuse background population that registers at the
same (D4, D6) channel pair, we must calculate the probability that each D4 + D6 event is a proton that passed through
both detectors normally.
[10] To calculate the probability that a D4 + D6 detection
in a given channel pair is a proton, we statistically subtract
the diffuse background source described in the previous
paragraph from the data and then divide the resulting 2-D
histogram by the original 2-D histogram. We first fit a 2-D
surface to the regions to either side of the proton branches in
order to characterize the shape of the diffuse source. We then
subtract that fitted surface from the entire 2-D histogram
plot, leaving an image of the proton branch which more
accurately shows the number of proton events at each
channel pair. Then, dividing the “proton only” histogram by
the original uncorrected histogram yields a 2-D image of the
probability that an event at a given channel pair is in fact a
proton. A value of “0” on this image corresponds to (D4,
D6) pairs where zero protons were recorded; all events there
are from the diffuse source. A value of “1.0” would correspond to a (D4, D6) channel pair where only protons were
recorded with no background events, but the diffuse source
exists throughout (D4, D6) space, so there are no channel
pairs with a proton probability of exactly 1.0. Figure 3
shows the region of the D4 + D6 proton probability plot
where the lunar proton branch lies; the plot for the GCR
proton branch looks similar but with somewhat higher
probabilities.
[11] The procedure of determining the magnitude of the
diffuse background source introduces a significant systematic uncertainty into this study. We performed hundreds of
trials of fitting functions to the diffuse source histogram, each
with slightly different parameters, in order to estimate the
systematic uncertainty introduced by not knowing exactly
how much of the diffuse source contributes to the signal in the
proton “branches” in the D4 + D6 histogram. Note that any
over-estimation or under-estimation of the diffuse background
source results in a corresponding under-estimation or overestimation of both GCR protons and lunar albedo protons at
Figure 3. Color-coded plot of the probability that D4 + D6 coincident events are lunar protons which
passed through both detectors.
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of arrival. Thus our best available option with the data at
hand is to calculate the ratio of lunar protons to GCR protons
for the energy ranges that we have access to. Because solar
wind modulation causes the GCR spectrum to rise and fall at
all energies, albeit with energy-dependent magnitudes [e.g.,
Parker, 1958] this calculated ratio will increase or decrease
in the same sense as the actual yield of albedo protons to
GCRs, but not proportionately.
Figure 4. Cylindrical projection albedo proton maps of the
Moon at two spatial resolutions: (top) 15 degrees and (bottom) 10 degrees. Colors represent the ratio of lunar protons
to GCR protons, from red (high) to blue/purple (low).
all times and at all locations on the Moon, so while this
uncertainty applies to the absolute yield of albedo protons per
GCR proton, it does not affect the uncertainties in the relative
yields from month-to-month or from location-to-location on
the Moon.
[12] The protons which we have isolated here have
incident energies of between 60 MeV and about 150 MeV.
Protons with energies below 60 MeV are unable to pass
completely through the TEP that lies between D4 and D6,
and thus do not pass the requirement of coincident detections
in D4 and D6. On the other hand, protons with energies
greater than about 150 MeV deposit too little energy in D4
and D6, and register in channel pairs below 1 MeV of
deposited energy where lunar protons, GCR protons and
GCR alpha particles all overlap.
4.2. Yield Map
[14] Since CRaTER is simultaneously detecting upwardmoving albedo protons and downward-moving GCR protons at all times, the most straightforward representation of
the relative yield is the ratio of these two detected populations. To make a map, we define latitude/longitude pixels or
areas of interest, sum the GCR and lunar protons detected
over each pixel or area, and then take the ratio of the lunar to
GCR protons to represent the relative yield. This method
conveniently corrects for temporal changes in the GCR flux
[e.g., Owens and Jokipii, 1973; Mulligan et al., 2009;
Jordan et al., 2009; Schwadron et al., 2010] which would
have modified the albedo proton flux detected by LRO.
Figure 4 shows the resulting map of this ratio for spatial
resolutions of 10 degrees and 15 degrees.
[15] Since the actual and measured albedo proton yields
are expected to change over time as solar activity modulates
the GCR proton spectrum, there is the potential problem that
a temporal change in the yield will manifest itself as a spatial
feature in the yield map. However, as the Moon rotates
under LRO’s polar orbit, LRO sweeps over all lunar longitudes in only two weeks, so that the modulations in the
background GCR rate are slow relative to the mapping time
scale. In addition, theory [Huang et al., 2009] and data
[Case et al., 2010] both indicate that lunar phase modulation
of GCRs is not significant.
4. Albedo Proton Yield
4.1. Yield Magnitude
[13] The spallation yield of protons from GCRs impacting
the lunar surface is the average number of protons which
escape from the lunar surface for each impacting GCR. All
albedo particles necessarily have lower energies than their
parent impacting GCRs, so a determination of the absolute
albedo proton yield from the Moon would require simultaneous measurements of GCRs and albedo particles covering
several orders of magnitude of energy. Since we only have
access to a limited portion of the GCR and albedo proton
energy spectra in this data set (60 MeV to 150 MeV) we
cannot compute an absolute yield. The GCR protons that we
detect produce albedo particles with energies well below
60 MeV, which is too low for a D4 + D6 detection, and most
of the albedo protons that we do detect were produced by
GCR protons with energies well in excess of 150 MeV,
which is too high for our method to distinguish the direction
Figure 5. Histogram of albedo proton/GCR proton ratio for
10-degree binning of cylindrical projection map, compared
to 100 simulated flat maps. The simulations randomly distribute 330,000 lunar protons (the same number as in the
data) over the lunar map to mimic what CRaTER would be
expected to measure for a perfectly uniform lunar/galactic
proton ratio.
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Table 1. Ratios of Regional Albedo Proton Yields
Region
Maria/Highlands
Poles/Low-Lat
Relative yield
1.006 0.006
1.001 0.007
[16] The maps of the ratio of albedo protons to GCR protons in Figure 4 are consistent with a spatially uniform albedo
proton yield, meaning regions of apparently high and low
yield are merely artifacts of the counting statistics. A brief
inspection of the two resolutions in Figure 4 reveals that high
and low yield pixels in one map are not necessarily pronounced in the other. Indeed, the histogram of yield values is
entirely consistent the counting statistics of 330,000 lunar
protons produced by a surface with a spatially uniform yield,
as demonstrated by the simulated measurements of 100 flat
maps shown in Figure 5.
[17] Dividing the lunar surface into a few large regions gives
similarly uniform results. As shown in Table 1, the yield at the
lunar poles is essentially identical to the average yield over
the rest of the surface, while the difference in yields between the
maria regions and highlands is larger but still within statistical
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uncertainty. These results are consistent with preliminary
Geant4 (See Allison et al. [2006] for a description of Geant4)
simulations that show systematic but extremely small differences between the albedo proton spectra coming from maria,
highlands, or pure water ice at energies detected by CRaTER.
4.3. Time-Dependent Variations
[18] Figure 6 shows the monthly averaged time variation
in the GCR rate, albedo proton rate, and spallation yield.
Detection rates decreased significantly over the 19 months
of this study, as expected given the increase in solar activity
which occurred over this span of time [e.g., Schwadron et al.,
2010]. The relative spallation yield jumped about 8% in just
two months (February to April, 2010) and has gradually
increased since then. The most likely explanation for this is
that as solar activity has increased, the lowest-energy GCRs
have been preferentially excluded from the inner solar system, meaning only the more energetic GCRs (which produce
more albedo protons particle-for-particle within the range of
energies that we’re measuring) are still reaching the Moon at
the same rate [e.g., Webber and Lezniak, 1974]. As CRaTER
sees only a portion of the GCR spectrum, we cannot directly
verify that the average GCR energy has increased, though
Figure 6. Time variation of (top) cosmic ray protons, (middle) lunar protons, and (bottom) the ratio of
lunar to cosmic ray protons. Error bars for the month-to-month statistical uncertainty are shown separately
from the systematic calibration uncertainty which does not affect the shapes of the plots.
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preliminary studies do indicate that the linear energy transfer
spectrum has changed over this time period.
5. Discussion
[19] Within statistical uncertainty, we have found no
significant difference in the proton spallation yields from
different regions on the Moon. This is consistent with
predictions from preliminary modeling results which suggest that any real yield variations are simply too small to
be detected in this data set.
[20] We calculate a global average ratio of lunar protons
to galactic cosmic ray protons of 0.38 0.02 for D4 + D6
detections with incident energies between 60 MeV and
150 MeV; this ratio is up to three times larger than balloon-borne measurements of “splash albedo” protons made
in the Earth’s upper atmosphere. Murayama [1967] calculated ratios at Earth of between 0.3 and 0.4 for albedo particles with energies above 75 MeV, but this included all
charged albedo particles, meaning the ratio for protons alone
was smaller. McDonald’s [1958] measurements of primary
and albedo protons at Earth suggest a ratio of 0.13 for
protons with energies between 100 and 300 MeV, while
Verma [1967] derived albedo-to-primary proton ratios of
0.15 at Earth for energies between 90 and 200 MeV. The
ratio calculated in this work for the Moon must inevitably be
higher than the real ratio, since the larger of the two pieces of
TEP partially shields both D4 and D6 from the lowest
energy GCRs arriving from space, thus reducing the detection efficiency of GCRs coming from the zenith and
increasing the measured ratio of albedo-to-source protons.
Correcting for this partial shielding would not be trivial, as
the GCR spectrum is not known precisely, and the modification of the spectrum depends on both the angle of incidence and location of impact of each galactic cosmic ray on
the CRaTER instrument. Regardless, given that the necessary correction would increase the GCR flux from zenith and
decrease the ratio, such a correction would reduce the lunar
ratio to a value closer to those measured at Earth.
[21] The total flux of GCRs and albedo protons has been
decreasing since 2009, as expected for a period of increasing
solar activity. If the Sun is entering a multidecade “grand
minimum” period as suggested by some studies [Feulner
and Rahmstorf, 2010; Miyahara et al., 2010], then the
solar cycle-averaged flux of GCRs and of all albedo particles
from the Moon should be higher for the foreseeable future
than it was during the previous five decades. This may have
implications for any future human colony on the Moon, both
in terms of radiation dose for astronauts [e.g., Schwadron
et al., 2010] and decade-scale space weathering of materials [Schwadron et al., 2012].
[22] The average global spallation yield from the Moon
jumped significantly in March and April of 2010, and has
been increasing more gradually since then, indicative of the
increase in solar activity and corresponding reduction in
lower energy galactic cosmic rays relative to higher energy
ones. This is an expected result, as higher energy GCRs
should result in higher spallation yields from unprotected
planetary surfaces than lower energy GCRs.
[23] We conclude that galactic cosmic rays generate a
proton albedo that depends primarily on the intensity and
energy spectrum of the incident high energy particles. The
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map of lunar albedo protons reveals apparent uniformity
over the lunar surface with little or no variability due to the
target composition and surface properties. This is consistent
with early modeling results that predict finite but small differences in albedo proton yield at these energies owing to
GCR interactions with different surface minerals; accumulation of additional observations may improve statistics
enough to reveal an expected but weak albedo proton signal.
[24] Acknowledgments. This work is supported by the NASA
CRaTER contract NNG11PA03C. We thank Alexander Boyd for assistance with data reduction.
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