SOME PROBLEMS CONCERNING THE MIGRATION AND DISTRIBUTION OF HELIUM-4 AND RADON-222 IN THE UPPER SEDIMENTS OF THE CRUST - A THEORETICAL MODEL; AND THE DEVELOPMENT OF A QUADRUPOLE ION FILTER FOR MEASURING HELIUM AT THE SOIL-AIR INTERFACE
PEREIRA, ENIO BUENO
Doctor of Philosophy
The generation, distribution, and migration process of helium-4 and radon-222 in the upper lithosphere and their fluxes at the soil-air interface were extensively reviewed. Global baseline concentration values were obtained calculated from more than 11,000 natural gas analyses of helium and 1,600 for radon from different places reported in the literature. These values are compatible with the lognormal hypothesis for the distribution of trace elements in the crust. The large volume of helium data in natural gases permitted an estimate of the global scale helium production rate from sediments to be between 1 x 10('5) to 6.7 x 10('5) atoms per cubic meter of sediment per second with an average helium concentration in sediments of about 67 ppm. These values are in close agreement with existing estimates for uranium and thorium concentrations in rather average sediments. The theoretical analysis and modeling of helium and radon production in subsurface radioactive deposits have shown that radon is unlikely to produce superficial anomalies of analytical significance unless some special mechanisms exist that are capable of providing fluid transport velocities larger than about 10('-7) meter per second. Some of the more probable mechanisms are discussed. The use of helium as a geophysical tool to locate uranium deposits is shown to be hampered by the very high atmospheric background and the lack of appropriate instrumentation for its measurement. The development of a quadrupole mass filter especially designed to monitor helium at the soil-air interface is described. Preliminary laboratory tests using author's prepared helium standards with several concentrations down to 12 ppm are also reported.