22nd Annual NASA Space Radiation Investigators' Workshop (2011)
7090.pdf
Flight Measurements of GCR Shielding by TEP with the CRaTER Instrument
C. Zeitlin1, H. E. Spence2, N. A. Schwadron2, M. J. Golightly2, J. B. Blake3, M. D. Looper3, .J. E. Mazur3,
A. W. Case4, J. C. Kasper4, L. W. Townsend5
1
Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302 zeitlin@boulder.swri.edu, 2University of New
Hampshire, 3The Aerospace Company, Los Angeles, CA., 4Harvard-Smithsonian Astrophysical Observatory,
5
University of Tennessee, Knoxville
THE CRaTER INSTRUMENT
The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) has been flying in lunar orbit aboard the LRO
spacecraft since 2009. The unique design of CRaTER, shown below in Figure 1, allows for measurements of dose
and dose equivalent at depths approximately corresponding to the skin, BFO, and deep organ sites. The instrument
consists of three pairs of silicon detectors, each pair being sandwiched around a depth of tissue-equivalent plastic
(TEP). The first detector pair, D1 and D2, are nearly unshielded and point in the zenith direction. They are followed
by the first piece of plastic, TEP1, which is about 6.1 g cm -2 deep, approximating the depth of shielding of the BFO.
The second detector pair, D3 and D4, are downstream of TEP1 from the
perspective of a particle entering the telescope. Downstream of D4 is the
second piece of TEP, TEP2, about 3 g cm-2 deep, and then a final detector
pair, D5 and D6. These last two detectors measure at a total depth of about
9 g cm-2, and in addition measure upward-going particles coming from the
lunar surface.
Fig. 1. Schematic drawing of CRaTER.
The CRaTER detectors are arranged in pairs. The odd-numbered detectors
are thin, about 150 m deep, and the even-numbered detectors are thicker,
1 mm deep. The “thins” measure the high-LET particles while the “thicks”
measure in the LET range from protons to high-energy silicon ions
(deposited energy E from about 0.2 MeV to about 90 MeV). Different
coincidence requirements correspond to different geometry factors, e.g., the
D2-D4 coincidence geometry factor is 1.9 cm2 sr (with a field of view halfangle of 28º), about three times larger than the D2-D6 geometry factor
(half-angle of 16.5º). The E spectra recorded in a given thick/thin detector
pair can be combined into a single E/ x spectrum. This can then, with
reasonable accuracy, be converted to the LET spectrum in water, which can
in turn be used to calculate the dose equivalent. We will present the
justification for this method, which is largely based on Monte Carlo
simulation, along with results for dose and dose equivalent at three depths.
Figure 2 shows the deposited energy in the D3-D4 pair (y-axis) vs. deposited energy in the D1-D2 pair. The band of
well-correlated events with its origin around (10,10) is due to highenergy heavy ions that fully penetrate D1, D2, TEP1, D3, and D4.
Bands that turn up (above the 45º line) can be seen that are due to
B (faint), C, N (also faint), O, Ne, Mg, Si, and Fe. The even-Z ions
are more abundant than odd-Z ions and those bands are therefore
more heavily populated. These are lower-energy ions that lose
significant fractions of their energy in TEP1 and are therefore more
ionizing in D3-D4. The events below the 45º line are due to nuclear
interactions of heavy ions in TEP1. These interactions reduce the
dose and dose equivalent at D3-D4 compared to that at D1-D2. The
two effects – increased ionization due to particles slowing down,
vs. decreased ionization due to fragmentation – compete with and
tend to offset each other. In beam experiments at high energy,
fragmentation is the more important effect and dose is reduced by
TEP and similar materials. GCR ions are sufficiently energetic that
this same result is expected on theoretical grounds, and (as will be
shown) is also seen in the data.
Figure 2. dE/dx in D3-D4 vs. dE/dx in D1-D2.