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Managing Space Radiation Risk in the New Era of Space Exploration
Radiation Measurements on or near the Lunar Surface
Explorer 35 (1967-1973) had a combination of an ionization chamber and Geiger counters. The ionization chamber responded to electrons above 0.7 MeV and protons above 12 MeV. One of the Geiger counters was used for low-energy electrons. The second responded to electrons and protons above 22 keV and 300 keV, respectively.
Clementine was launched in January 1994 and orbited the Moon between February and April 1994. Among its instruments were a charged particle telescope and solid-state dosimeters. The charged particle telescope on Clementine measured the flux and spectra of energetic protons (3 MeV to 80 MeV) and electrons (25 keV to 500 keV). The dosimeters were proton-sensitive static random access memory chips sensitive to protons with energies from a few to more than 20 MeV.
Lunar Prospector collected data in lunar orbit from January 1998 until July 1999. It contained a gamma ray spectrometer (with a fast neutron spectrometer with sensitivity to albedo neutrons with energy up to 8 MeV), a neutron spectrometer (with sensitivity to neutrons with energy less than 1 keV), and an alpha particle spectrometer (detecting alpha particle decay products with energy of a few MeV). Each instrument was optimized to provide information about the lunar surface composition, not the lunar surface radiation environment.
Apollo Measurements The most relevant of the Apollo measurements were made with the Cosmic Ray Detector Experiment, a set of passive glass detectors with sensitivity from 100 keV to 150 MeV per nucleon. The exposure time was limited. Other indirect surface measurements during the Apollo program included the following:
The filter glass of Surveyor III (brought back by Apollo 12) was analyzed;
The window of the Apollo 12 spacecraft was analyzed for cosmic ray tracks;
One helmet on Apollo 8 and three worn on Apollo 12 were used as heavy-particle dosimeters;
A control helmet was also exposed to cosmic rays at a balloon altitude of 41 km; and
Lunar regolith was analyzed for upper limits on high-energy exposure over the half-lives of long-lived isotopes (thousands to millions of years).
The Lunar Reconnaissance Orbiter, scheduled for launch in late 2008, will have two radiation monitoring instruments: Cosmic Ray Telescope for the Effects of Radiation (CRaTER) and Low Energy Neutron Detector (LEND). The CRaTER telescope consists of three ion-implanted silicon detectors separated by two pieces of tissue-equivalent plastic. It will be sensitive to protons with energy above 20 MeV and energetic high-Z particles (iron nuclei with energy greater than 90 MeV per nucleon, for example). The LEND instrument will provide data similar to the Neutron Spectrometer on Lunar Prospector.
GALACTIC COSMIC RADIATION
Interplanetary space is bathed by a low flux1 (particles per square centimeter per second or particles per square centimeter per steradian per second) of essentially uniformly distributed, highly energetic, and extremely penetrating ions that are believed to be accelerated by supernova shocks in our Galaxy. These ions make up the GCR. The highest-intensity GCR is found between a few tenths and a few tens of GeV per nucleon, where the particles can penetrate tens to hundreds of centimeters of shielding. Every naturally occurring element in the periodic table is present in the GCR: nearly 90 percent are protons (hydrogen), close to 10 percent are helium, and the remaining percentage are elements heavier than helium, with a relative abundance roughly similar to that found in our solar system (Figure 2-1). These subtle compositional differences were a key factor in understanding the origin of GCR and provided the original impetus for the development of heavy-ion transport codes and heavy-ion cross-section libraries that we take for granted today. Galactic cosmic rays also include electrons and positrons, but their intensities are too low to be of practical concern.
The GCR flux outside the solar system is presumed to be constant, at least on timescales of tens of millions of years related to the solar system’s motion through the Galaxy (Lieberman and Melott, 2007) and barring a nearby
“Flux” is an older, but not preferred term, which has been replaced by “fluence rate.” However, since “flux” is more common within the space weather and radiation protection communities, it is used in this report.