![]() A steep gradient of track densities over a distance of much less than 0.1 cm is a sign that the tracks were made by SCR nuclei. Although such very surface layers of meteorites are removed by ablation, we are still very much interested in tracks of SCR nuclei in meteorites because they occur in grains now inside a meteorite that were once exposed at the very surface of the meteorite's parent body. However, we do know that the tracks from heavy nuclei in the SCR dominate over those from GCR nuclei for depths less than about 0.1 cm. ![]() 1 for the production rates of tracks at depths shallower than 0.1 cm reflects these uncertainties in interpreting such profiles in lunar rocks or in measuring the spectrum of heavy SCR nuclei in space. As the tracks from SCR nuclei mainly occur in the top millimeter, the observed track densities very near the surface depend critically on the erosion rate. Typical erosion rates for lunar rocks are of the order of a millimeter per million years. The problem here is that the very surface layers in which the SCR nuclei produce tracks are continuously removed by micrometeoroids and solar-wind ions. Lunar rocks with simple exposure histories of short durations have been used by several research groups to infer the spectrum of SCR nuclei with Z < 20 averaged over various time periods. Each event has its own energy distribution for the SCR particles, so it is hard to determine the long-term-averaged spectrum of heavy SCR nuclei. Another problem is that the low energies of the SCR heavy nuclei make them hard to detect using instruments on satellites. Thus, the production-rate profiles for tracks of GCR nuclei are predicted fairly well.Īs the solar cosmic rays are only emitted irregularly from the Sun, there have been only a few chances to directly observe their spectra in space. ![]() The energy spectra of heavy ( Z > 20) nuclei in the galactic cosmic rays have been measured by detectors on satellites and inferred from track profiles in meteorites and lunar samples with simple exposure histories. The relations for the ranges in meteoritic material of heavy nuclei as a function of their energy are well known. These observations, plus a knowledge of the particle fluxes and processes involved in track formation, have resulted in several operational curves for the production rates of tracks as a function of depth in meteoroids of various radii, such as the profile shown for a large-radius object in Fig. The densities of the tracks made by heavy cosmic-ray nuclei have been measured in a number of extraterrestrial objects, such as meteorites, lunar samples, and artificial materials exposed in space. Reedy, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 III.A Tracks
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |