Small Sample Thermal Neutron Cross Section Determination From Reactivity Measurements

Exploration for hydrocarbons and other minerals can frequently be aided through the use of neutrons as a probe into a mineral-bearing formation. The analysis of these nuclear logs usually requires thermal neutron absorption data. The thermal macroscopic absorption cross section can be obtained using a variety of measurements conducted on samples from the formation. The technique described in this paper requires about 30 minutes to predict the macroscopic absorption cross section of a 100-gram specimen from the formation. Samples of this size can usually be obtained from a small-bore core, whereas samples of 10 kilograms required by some of the alternative techniques require larger cores. The thermal absorption cross section is derived from reactivity measurements in a thermal nuclear reactor. Cross sections calculated from data taken at the Advanced Reactivity Measurement Facility (ARMF) at the Idaho National Engineering Laboratory (INEL) are compared to those calculated from elemental analysis by Prompt Gamma Activation Analysis (PGAA) to verify the accuracy of the reactivity technique. Results of this comparison show that the thermal absorption cross section can be predicted to an accuracy of about + 4 ×10−4 cm2/g or + 10% of the predicted value, whichever is greater.