Access Tube Characteristics and Neutron Meter Calibrationby Richard G. Allen, Utah State Univ, Logan, UT, USA,
Daniel Segura, Utah State Univ, Logan, UT, USA,
Abstract: Neutron meter calibration was found to be influenced by material type, thickness and size of access tube. Meter counts were decreased by an average of 15% for a 20% increase in tube diameter for polyvinyl chloride (PVC) tubes and by an average of 11% for a 33% increase in tube diameter for aluminum tubes. Meter counts were decreased by an average of 10% for a 20% increase in tube wall thickness for PVC access tubes due to increased neutron capture by chloride ions in the PVC material. The presence of air gaps around tubes decreased counts similar to increased tube diameter. Positioning of the neutron meter had a significant effect on standard shield counts. Positioning of the meter within 0.3 m of the ground surface increased standard shield counts by as much as 5% due to influence of surface soil moisture and back scattering.
Subject Headings: Soil water | Thickness | Calibration | Chloride | Walls
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