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Cosmic-ray tracks in plastics - The Apollo helmet dosimetry experiment
Heavy cosmic ray nuclei track counts in plastics, examining Apollo mission 8 and 12 helmets
The particle track record of the Ocean of Storms
Solar and galactic iron group cosmic ray track distributions in Apollo 12 lunar rocks, investigating surface residence times
The particle track record of lunar soil
Primary cosmic ray and spallation track density distribution in Apollo 12 deep core soil samples
Investigations of lunar materials
The investigations were directed at determining the radiation history and surface chronology of lunar materials using the etched particle track technique. The major lunar materials studied are the igneous rocks and double core from Apollo 12, the breccia and soil samples from Apollo 14, and the core samples from Luna 16. In the course of this work two new and potentially important observations were made: (1) Cosmic ray-induced spallation-recoil tracks were identified. The density of such tracks, when compared with the density of tracks induced by a known flux of accelerator protons, yields the time of exposure of a sample within the top meter or two of moon's surface. (2) Natural, fine scale plastic deformation was found to have fragmented pre-existing charged particle tracks, allowing the dating of the mechanical event causing the deformation.
Very heavy solar cosmic rays: Energy spectrum and implications for lunar erosion
Particle tracks were investigated in the glass plate of a neutral density (clear flint) optical filter housed in the Surveyor 3 TV camera but exposed directly to space. The track density vs depth curve was determined and descends sharply from approximately 2.6 million tracks/sq cm at a depth of 3.6 mg/sq cm to about 35/sq cm at 700 mg/sq cm. Several tracks were of V-shapes characteristic of high energy induced fission. The erosion rate on the moon due to solar wind ions was determined from the energy spectrum, and was found to be low (0 to 2 x 10 to the minus 8th power cm/yr).
The particle track record of the Sea of Plenty.
We have measured the particle track densities in 36 grains taken from two levels of the soil column returned from the Sea of Plenty by Luna 16. One sample is from near the surface, the other is from about 30 cm depth. All but one of the grains contained very high track densities. We conclude that all of the Luna 16 soil has been irradiated very close to the surface, that there has been little or no 'recent' admixture of previously shielded material from below 30 cm, and that the regolith is both unusually thin at the Luna 16 site and extremely old (about 3 b.y. and more).
Cosmic-ray source and local interstellar spectra deduced from the isotopes of hydrogen and helium.
A self-consistent model for cosmic-ray hydrogen and helium propagation from the sources in the Galaxy to the orbit of earth is obtained, taking into account experimental information now available on the isotopes H-1, H-2, He-3, and He-4. The only adjustable parameters include the shape of the energy spectra of H-1 and He-4 at the time of source injection, the distribution of particle path lengths in interstellar space, and the solar modulation parameters. It is found that the allowed form of the source differential spectra of the H-1 and He-4 nuclei is dominated by a power law in total energy.
The particle track record of the lunar surface.
Information about lunar surface history revealed by fossil particle tracks is summarized. Such tracks are the result of damage left in dielectric materials by highly ionizing charged particles including heavy solar and galactic cosmic ray nuclei, heavy nuclei recoiling from cosmic ray induced spallation reactions, and induced- and spontaneous-fission fragments. From the distribution of cosmic ray and spallation tracks in the lunar rock, surface residence times of 1 to 30 million years and rock erosion rates of 1 to 10 A/yr have been determined. Particle tracks also record surface orientation and depth history of the rocks and contain information about ancient solar activity.
The particle track record of Fra Mauro.
Apollo 14 breccias show a mixture of high and low track densities at most interior positions, indicating that the majority of the tracks have been inherited from the parent ingredients of the breccias. Using the lowest of these track densities as indicative of maximum postbrecciation surface residence times, we find a median 1.35 m.y., much younger than the less friable Apollo 11 and 12 igneous rocks. The igneous rock 14310 is studied as a part of a consortium, the results indicating a complex irradiation history. Soils are extremely variable, median track densities ranging over at least a factor of 200. Individual high density soil grains yield track density gradients having variable slopes, most of which are lower than expected from the Surveyor III filter glass results.
Particle track record of the Luna missions.
Measurements are reported of particle-track densities in 100 to 200-micron crystalline grains taken from one level of the soil column returned from the lunar highlands between Mare Fecunditatis and Mare Crisium by Luna 20 and from two levels in the soil column from Mare Fecunditatis by Luna 16. Ninety-three percent of the grains from Luna 16 have very high densities, greater than 10 to the 8th power per cu cm and the lower-track density grains are all in the deeper soil level. In contrast, most Luna 20 grains show densities less than 10 to the 8th power per cu cm. Track density gradients and exposure times have been measured for six Luna 16 grains with a wide spread in absolute track densities. The more extensive track counts in crystals strengthens an earlier conclusion that the Luna 16 soil has received long irradiations very close to the surface. Two possible histories are that the highly irradiated soil blanket at the Luna 16 site is either well mixed and thin, or else has accumulated by transport from surrounding higher regions.
Apollo 14 and Apollo 16 heavy-particle dosimetry experiments.
Doses of heavy particles at positions inside the command modules of Apollo missions 8, 12, 14, and 16 correlate well with the calculated effects of solar modulation of the primary cosmic radiation. Differences in doses at different stowage positions indicate that the redistribution of mass within the spacecraft could enhance safety from the biological damage that would otherwise be expected on manned, deep-space missions.
The simulated depth history of dust grains in the lunar regolith
Trajectories giving the individual depth variations of lunar dust grains with time are randomly generated by a Monte Carlo code, where the variables are the mass and speed distribution of meteorites at the lunar surface and the geometrical shape of impact craters. A statistical analysis of a great number of such trajectories is then used to define the 'average' depth history of lunar dust grains for two grain radii: 1 and 50 microns. This yields: (1) estimates for time constants involved during the dynamic evolution of the regolith, (2) a model for the layering of the regolith, and (3) a better understanding of the basic dust-grain mechanisms responsible for the formation of the most mature lunar soil samples. The validity of various soil models proposed for the dynamic evolution of the regolith is discussed in terms of experimental constraints on the models.
New lunar microcrater evidence against a time varying meteoroid flux
An attempt to measure the relative times of formation of 25 microcraters on lunar rock 60015 is discussed which was carried out as part of a study of the flux of interplanetary dust and possible time variations of that flux over periods of 1000 to 1 million yr. The data obtained could indicate that the formation rate of 100-micron-sized pits was lower in the past as compared with the formation rate of micron-sized pits, that the flux of particles forming 100-micron-sized pits may have varied relative to that of particles forming micron-sized pits, or that 100-micron-sized pits are forming now at a higher rate, relative to the formation rate of accretionary disks, than in the past. The last suggestion is shown to be almost untenable, and the overall results are found to lead to a logical paradox. It is concluded that the attempt probably failed and that the reason for failure raises serious questions regarding the use of lunar rock surfaces as meteoroid detectors.
Miniregoliths. I - Dusty lunar rocks and lunar soil layers
A detailed Monte-Carlo model for rock surface evolution shows that erosion processes alone cannot account for the shapes of the solar flare particle track profiles generally observed at depths of about 100 microns and less in rocks. The observed profiles are easily explained by a steady accumulation of fine dust at a rate of 0.3 to 3 mm per m.y., depending on the micrometeoroid impact rate which controls the dust cover and results in maximum dust thicknesses on the order of 100 microns to 1 mm. The commonly used lunar soil track parameters are derived in terms of parameters characterizing the exposure of soil grains in the few-millimeter-thick surface mixing and maturation zone which is one form of miniregolith. Correlation plots permit determining the degree of mixing in soil samples and the amount of processing (maturation) in surface miniregoliths. It is shown that the sampling process often artificially mixes together finer distinct layers, and that ancient miniregolith layers on the order of a millimeter thick are probably common in the lunar soil.
Space environment and lunar surface processes
The development of a general rock/soil model capable of simulating in a self consistent manner the mechanical and exposure history of an assemblage of solid and loose material from submicron to planetary size scales, applicable to lunar and other space exposed planetary surfaces is discussed. The model was incorporated into a computer code called MESS.2 (model for the evolution of space exposed surfaces). MESS.2, which represents a considerable increase in sophistication and scope over previous soil and rock surface models, is described. The capabilities of previous models for near surface soil and rock surfaces are compared with the rock/soil model, MESS.2.
Optical search for lightning on Venus
Electrical signals attributed to lightning events on Venus have been observed by instruments aboard Veneras 11 and 12 and aboard the Pioneer Venus Orbiter. This paper reports the results of a search for optical pulses from the dark side of Venus made with the star sensor aboard the Pioneer Venus Orbiter. A comparison of both the frequency and amplitude distributions of received pulses with those of control experiments in which the star sensor observed deep space showed no statistically significant differences. Based on observations made during orbits 300 through 345, an upper limit of 30 flashes/sq km/yr has been derived for the average lightning frequency on the dark side of Venus. This value assumes that the amplitude distribution of lightning and the attenuation of the optical pulse by clouds on Venus are not substantially different from that occurring on earth.
Interpretation of cosmic ray composition - The path length distribution
The chemical composition of cosmic ray nuclei, the value of Z varying between 3 and 28, and being between a few hundred MeV/nucleon and a few hundred GeV/nucleon, is compared with a consistent set of propagation calculations. These include the effects of spallation (energy-dependent cross sections are used), escape, ionization loss in the interstellar medium, and deceleration in the solar cavity. The amount of matter traversed by cosmic rays is found to be approximately 7 g/sq cm, independent of energy between 100 MeV/nucleon and 2 GeV/nucleon. Above 2 GeV/nucleon, the escape length varies as the -0.4 + or 0.1 power of the energy. In addition, a procedure has been developed to measure the shape of the cosmic ray path length distribution. Utilizing the ratio of Fe secondaries to Fe in the cosmic rays, presently available data are found to be consistent with an exponential distribution and they eliminate models in which the path length distribution is severely truncated. To tie down the shape of the distribution more precisely, new measurements of the cosmic ray composition, presently becoming available from experiments on the HEAO 3 satellite, will have to be coupled with improved measurements of the energy dependence of partial and total cross sections.