Tritium Loss from Iron Meteorites by Solar Wind Hydrogen

An estimate of the diffusion loss of tritium from iron meteorites in space cannot be based, in a direct manner, on the results of diffusion measurements in the laboratory, because the hydrogen content of the samples in the laboratory is always higher by many orders of magnitude than is the tritium content of iron meteorites in space, which results f rom bombardement by cosmic rays. This is true because the kinetics of gas loss from a metal change with concentration and temperature, especially the kinetics of hydrogen loss from meteoritic iron, where the amounts of cosmic rayproduced tritium are sufficiently small to be trapped completely by chemisorption of nickel. Estimates based on diffusion data for hydrogen in the laboratory lead to much higher rates of hydrogen loss than can be expected for cosmic ray-produced tritium in meteorites. In this connection, however, the fact has to be considered that meteoritic iron in space may not only contain hydrogen isotopes derived f rom cosmic ray-induced spallation processes, but also derived from solar wind bombardement. The steady-state hydrogen content of an iron meteorite, resulting from such bombardement in outer space, depends, of course, on the size and the structure of the meteoritic body. In general, the body will have to be assumed to possess a grain structure and this complicates the situation considerably. In any case, it can be shown that the steady-state hydrogen content is much higher than the tritium content and this makes it possible to use laboratory diffusion data in a calculation of the diffusion losses of tritium. By using the laboratory data of FESTAG, FECHTIG and SCHULTES, it can be shown that cosmogenic tritium will be lost almost completely whenever the temperature is close to 0 ° C , or higher. Only meteorites consisting of grains of a size exceeding several millimeters retain tritium at such temperatures.