Production of helium and neon in the lunar regolith by solar cosmic ray protons
Walton, James Richmond (b. 1947)
Master of Science
In three sets of irradiations at Texas A and M's Variable Energy Cyclotron, stacks of thin Mg, Al, Si, Ca, Ti, and Fe foils were bombarded with 1 to 1 intermediate energy protons (15 to 45 MeV) in order to determine He, Ne, and Ar production cross sections. This energy range was selected because it represents a significant portion of the solar cosmic ray (SCR) spectrum. Certain radioactivities were measured by Dr. Rowe and coworkers, but these will be reported elsewhere. The He and Ne isotopes produced in the Mg, Al, and Si targets have been measured mass spectrometrically and cross sections calculated, using the Na activity in the Mg targets as a monitor. Six determinations of the He, He-, Ne, Ne, and Ne cross sections from Mg and five for Al and Si are reported. The He and He cross sections from the Mg, Al, and Si targets are all fairly comparable at similar proton energies. The He cross sections increase with energy up to a maximum value of about 2 mb at 4 to 45 MeV. The Ne and Ne cross sections from Mg are significantly greater than those from Al and Si at similar energies. The maximum Ne and Nel cross sections measured for Mg were 46.5 mb at 24.6 MeV and 12. mb at 15.3 MeV respectively. The Ne cross sections from all three elements up to 45 MeV are generally less than 1 mb and negligible in comparison to the Na2 cross sections. A major portion of the Ne measured is attributed to Na2 decay in the targets since the end of the irradiations. Combining these cross sections with those reported in literature, He, He, Ne, Ne, Ne, and Na cross section functions for the target elements Mg, A1 , and Si were derived up to 2 MeV. The literature cross sections from A1 and the Na2 cross sections from Na were also considered. By integrating the product of these functions and differential solar cosmic ray spectra, (for normal and isotropic incidence), He, (including H), He, Ne®, Ne, and Ne (including Na) production rates from the individual target elements, Na, Mg, Al, and Si were computed. These are reported as a function of depth for the top 1 to 15 centimeters of a model lunar regolith with Na, Mg, Al, and Si contents of 1. mass percent each. Applying these results to the known average chemical composition of the lunar soils, total production rates were computed for the Apollo 11, 12, 14 and 15 soils. The production rates reported are based on a solar cosmic ray spectrum derived from data observed in the two high years, 1959, and 196, in solar cycle 19 (1954-1964). In general, all of the production rates are at least comparable to the galactic cosmic ray (GCR) production rates down to a depth of about 1. gm/cm or approximately in the top 5 mm of the lunar regolith. With increasing depth beyond 1. gm/cm, all production rates decrease rapidly, becoming negligible compared to the galactic. The direct SCR production of Ne in the Apollo soils is generally less than 5 percent of the total Ne and Na production at any depth. The SCR-produced elemental (He/tîe)scR ratio and the isotopic (NeVNe)gcR and (Ne/NegcR ratios have also been calculated as a function of depth in the Apollo soils. These ratios appear to be at least moderately dependent not only on depth but also on chemical composition. At almost all depths down to 1. gm/cm, these ratios, especially (Ne/Ne), are significantly different from those characteristic of galactic spallation. Considering the maximum range in the SCR produced neon isotopic ratios in any of the four Apollo soils, SCR production is thus concluded to be an isotopically distinct component in the neon 3-isotope diagram. Using the absolute production rate values, example speculations on the two independent and unknown phenomena, past solar activity and the "gardening" process in the lunar regolith have been made. Also an example of how a "solar cosmic ray exposure age" may be derived is discussed. Several reported measurements of Apollo 11 and 12 samples along with the results of these calculations support the possibility that solar cosmic ray proton production may not only be present, but also detectable in lunar soil samples.