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The kinetics of the pressure dependent O + SO2 + Ar reaction have been investigated using laser photolysis resonance fluorescence at temperatures of 289 K, 399 K, 581 K, 699 K, 842 K and 1040 K and at pressures from 30-665 torr. Falloff was observed for the first time in the pressure dependence. Application of Lindemann theory yielded an Arrhenius expression of k(T) = 3.3 x 10-32exp(-992/T) cm6 molecule-1 s-1 for the low pressure limit and k(T) = 8.47 x 10-14exp(-468/T) cm3 molecule-1 s-1 for the high pressure limit at temperatures between 289 and 842 K. The reaction is unusual as it possesses a positive activation energy at low temperature, yet at higher temperatures the activation energy is negative, illustrating a reaction barrier.

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.