The neutron absorption cross section (SIGMA) of a sample is a measure of the moderating power of the sample in response to its bombardment by neutrons. This physical property is measured automatically by the geochemical logging tool (GLT) and is used in the subsequent land-based processing of the GLT log data. This contribution reports part of a project to measure accurately the SIGMA of lithologies commonly drilled in the Ocean Drilling Program for both lithological characterization and the calibration and modeling of the nuclear measurements. SIGMA was measured on 13 samples obtained during Leg 149. These samples, from Holes 897C, 899B, and 900A, were chosen to include a range of both sediment and basement lithologies. Measurements of SIGMA on two altered basalts from Hole 900A gave an average of 0.00688 ± 0.00014 cm²/g. These figures are higher than, but still generally similar to, values obtained on basalts from Hole 896A (Leg 148) in the East Equatorial Pacific, which suggests that oceanic basalts could be characterized by a reasonably consistent range of SIGMA (0.0055 ± 0.00011 to 0.0075 ± 0.00015 cm²/g). Three serpentinized peridotites and eight sediment samples measured gave higher values of SIGMA, averaging 0.01217 ± 0.00025 and 0.01478 ± 0.00030 cm²/g, respectively. Comparison of the measured values of SIGMA with estimates calculated from the routine chemical analyses of the samples confirms that the use of calculated SIGMA values is unreliable unless the chemical analysis is particularly complete.

Date of initial receipt: 30 November 1994
Date of acceptance: 8 June 1995

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