Theoretical principles of a method for measuring the absorption cross-section for thermal neutrons are presented in a one-velocity approach. In consecutive measurements the sample investigated is enveloped in shells of a known moderator of varying thickness and irradiated with a pulsed beam of fast neutrons. The die-away rate of thermal neutrons escaping from such a system is measured. The absorption cross-section of the unknown sample is found as the intersection of the experimental curve (die-away rate versus thickness of the moderator) with the theoretical one, calculated for the case when material buckling of the sample is zero.

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The physical principles of a new method for uranium detection based on use of a pulsed source of neutrons are presented. The method employs measurement of the neutron time distributions in rock media following a burst of fast neutrons. A time‐ and energy‐dependent (but space‐independent) theory is developed for the four principal time distributions that follow production of fission neutrons by each of the two possible reactions: 1. Prompt thermal fission of [Formula: see text]: a. Epithermal neutron time distribution, b. Thermal neutron time distribution. 2. Delayed fission of uranium: a. Neutron time distribution due to delayed fission in [Formula: see text], b. Neutron time distribution due to delayed fast fission in [Formula: see text]. Theoretical analysis of these phenomena leads to the conclusion that the detection of uranium in rocks is possible using either the epithermal neutron time distribution from the prompt fission of [Formula: see text] by thermal neutrons or the delayed neutron distribution from fast neutron fission of [Formula: see text] or thermal neutron fission of [Formula: see text]. Experiments performed on laboratory models of uranium ore show the feasibility of detecting fission neutrons and the validity of the theory presented in the paper. For normal borehole‐logging measurements, a pulsed neutron source with an average output of about [Formula: see text] n/sec is needed.

A new method of determination of the thermal neutron macroscopic absorption cross-section Sigma a for small samples has been checked experimentally. The measurements have been performed with spherical geometry by the pulsed method. Two samples (aqueous solutions of boric acid) with known values of the absorption cross-section have been used. Good agreement of the measured values of Sigma a with the known ones has been observed.

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