Application of positron annihilation spectroscopy to the characterization of rocks

Abstract

This work is a first comprehensive study on rocks by use of the positron annihilation methods: Doppler Broadening Spectroscopy (DBS) and the lifetime measurements (LT). The complementary methods like Scanning Electron Microscopy (SEM), the residual gas analysis method (RGAS) and gas absorption (BET) methods were used also. Several rock samples with the known composition and porosity, mainly natural sandstones, carbonates but also anhydrite, opals and synthetic sandstones have been studied. Rocks have been investigated in different states, dry versus soaked (in water, brine or hydrocarbons) and heated (20¿220¿C) versus non-heated For the rocks with similar structure and porosity, the positron annihilation (PA) signal has been found to depend primarily on the rock composition (S-parameter and the mean lifetime values were higher for sandstones than for carbonates) and on the rock state, but to a lesser extent. We found also how the structural differences affect the PA signal by studying powderized versus non-powderized samples and opals versus sandstones. Our measurements have proved that rocks can be segregated according to their PA-parameter values. The positron annihilation parameters, like S,SW,W-parameters and the mean lifetime values have been found to be characteristic for each rock. Physical models explaining how positron annihilation response is sensitive to the rock composition, structure and state have been developed. We also propose a new analysis method to get more insight into the rock microstructure. In the long perspective, the understanding of the physics of positron interactions in rocks can lead to the development of a new nuclear well-logging tool using the positron annihilation technique, which would characterize the rock and its state uniquely.