How To Measure The Neutron Absorption Cross Section On Rock Samples

Igielski, A. (Henryk Niewodniczaalski Institute of Nuclear Physics) | Woinicka, U. (Henryk Niewodniczaalski Institute of Nuclear Physics) | Czubek, Jan A. (Henryk Niewodniczaalski Institute of Nuclear Physics) | Krynicka-Drozdowicz, E. (Henryk Niewodniczaalski Institute of Nuclear Physics)


The knowledge of the true absorption cross section of thermal neutrons for different rock materials and brines is an important factor in the quantitative interpretation of the neutron lifetime logs. This absorption cross section is directly related to the elemental composition of these materials. Geological materials, however, very often contain some admixtues of highly absorbing isotopes in quantities which are still too low to be detected in the usual elemental analysis (like boron, rare earths, etc. ). The only way to know that cross section is to measure it on geological samples. Due to the particular features of rock material (heterogeneities, impossibility to reproduce the sample of exactly the same composition and bulk density and with different dimensions, etc. ) this is not an easy task using the experimental methods known so far. A new approach to this problem is presented in the paper. In consecutive measurements the rock sample (having a fixed and well known shape - in our case it is a sphere and the sample is powdered or liquid) is enveloped in shells of a Plexiglas moderator (the neutron parameters of which are known) of variable thickness and irradiated with the pulsed beam of fast neutrons. The die-away rate of thermal neutrons escaping from the whole system is measured. The absorption cross section of the sample is found as the intersection of the experimental curve (i. e. die-away rate vs. thickness of the moderator) with the theoretical curve. The theoretical curve is calculated for a given moderator under the assumption of a constant value of the neutron flux inside the sample. This method is independent of the transport cross section of the sample. The method has been checked on artificial materials with a well known elemental composition (liquid or solid) and on the natural brine. For the liquid sample the one standard deviation of the measurement is of the order of 2 or 3 capture units, whereas for the solid sample it is less than 5 capture -units. The volume of the sample needed here is of the order of 1000 ccm.