Optimization of the Neutron Absorption Steady Source Measurement Method --by using an 124Sb-Be Source
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Date
2002-12-16
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Abstract
In the mineral exploration and oil well logging industries where logging tools are used based on neutron moderation and absorption principles, one needs to have an accurate knowledge of the neutron absorption cross-sections of the sample of interest for calibration purposes. This is difficult to ascertain by ordinary chemical analysis since some elements that are present in trace amounts (like boron, samarium, and gadolinium) have very large neutron absorption cross-sections. In oil well logging applications, chlorine appears in larger than trace amounts and often dominates the absorption cross-sections. For this reason a sensitive measurement is obtained by using the response to a source of thermal neutrons when a small sample is placed within a good moderating medium. Tittle and Crawford (1983) described and demonstrated such an approach. This method has evolved into one that uses a 252Cf source of neutrons with a 3He proportional counter. Attempts to optimize the sensitivity of this technique by Monte Carlo simulation with both MCNP and a specific purpose Monte Carlo code (Mickael, 1988) involving different detector positions relative to the sample, variable source-to-sample distances, and different moderators (Bussian, Jetter, and Supernaw, 1991, Talavarjula, 1995, and Sood, Gardner, and Gray, 2000) have had only limited success. The present approach involves the use of a 124Sb-Be neutron source with an average neutron energy of about 12 Kev compared to the 3 to 4 MeV average of the 252Cf source. Monte Carlo calculations with MCNP and subsequent benchmark experiment indicate a much improved sensitivity and accuracy with the 124Sb-Be source.
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Keywords
Absorption cross-section, Sensitivity, MCNP, Steady neutron source
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Degree
MS
Discipline
Nuclear Engineering
