Thermoluminescence Studies of the Thermal and Radiation Histories of Chondritic Meteorites.
The thermoluminescence properties of chondritic meteorites are investigated to understand the ways in which the stored TL reflects the thermal and radiation histories of these objects. Differences in TL levels measured in recent falls are attributed to small differences in orbital temperatures. In addition, a correlation between TL level and terrestrial age is observed in meteorites of known terrestrial age. The thermoluminescence in chondrites is produced primarily by ionization from galactic cosmic rays with a much smaller contribution from the decay of natural radionuclides (U, Th, K, Rb). The production of most of the TL occurs after the break up of the large parent bodies into meter -size objects which are thus exposed to the ionizing effects of the cosmic rays. Measurements indicate that the low temperature TL represents a dynamic equilibrium between build up from ionizing radiation and thermal draining. The high temperature TL is near saturation. The orbital TL equilibrium level begins to decay once a meteorite falls to earth because it is then shielded from cosmic rays by the atmosphere. The characteristics of this decay are studied by means of isothermal decay experiments at elevated temperatures and fractional glow determination of the depths of the electron traps. The decay of the TL provides a useful estimate of a meteorites's terrestrial age although the technique is limited by the relatively large scatter in TL levels in meteorites of the same terrestrial age. The terrestrial ages currently of greatest interest are those of the recently discovered meteorites in Antarctica. TL measurements were made on 11 of these meteorites and compared with the activities of ('14)C, ('26)Al, and ('36)Cl measured by other workers in terrestrial age studies. A good correlation was found between the TL levels and the activities of cosmogenic radionuclides in these meteorites. Since the TL measurements can be made more rapidly and require much smaller samples ((TURN)10 mg) than the radionuclide measurements, TL is most useful as a screening process to select potentially interesting samples for further study by more precise techniques. The TL levels measured in recent falls vary by about a factor of ten. The range of perihelian temperature necessary to produce this range of TL levels is calculated to be (TURN)24(DEGREES)K. This range of temperature will result from perihelia similar to the perihelia of the three meteorites whose orbits have been photographically determined (Pribram, Lost City, and Innisfree) and albedos measured by other workers. Thus, 43 of the 45 ordinary chondrites studied apparently have perihelia similar to the perihelia of the three known orbits. The results suggest that unusual orbits are rare and that the three known orbits are representative of the vast majority of ordinary chondrites. Two unusual cases are discussed. Calculations show that thermal gradients of (TURN)2(DEGREES)K/cm can exist in stony meteorites while in orbit. Such a thermal gradient is required to explain the unusually large gradient in the nature TL in the Farmville meteorite. The natural TL in the Malakal meteorite is about two orders of magnitude lower than that in other ordinary chondrites. The best explanation of this low TL level is draining of the TL by solar heating in an orbit with a perihelion of 0.5 to 0.6 AU. A modification of the apparatus for measurement of thermoluminescence is described in which a microcomputer is interfaced to the existing light detection and heating control units. The computer allows photons to be counted directly and the numbers stored automatically for later manipulation. This results in more precise as well as more efficient data acquisition and reduction.
- Pub Date:
- Physics: Condensed Matter