An innovative system has been designed for the reliable calibration and response qualification of neutron logging instruments. The new calibrator produces selectable and precisely repeatable environments that represent an extensive range of formation porosities. The environments created are longitudinally homogeneous and sufficiently massive to serve as secondary standards instead of uniform formations surrounding a cylindrical borehole. Direct traceability to generally accepted primary porosity standards, comprising the API water-filled limestone "test pit" blocks at the University of Houston, Texas, and a volume of water sufficiently large to represent 100 porosity units has been established. Selecting different physical configurations for the inner environment of the calibrator accomplishes representation of secondary porosity standards covering a range from as low as 0 porosity units to as high as 70 porosity units. This span practically encompasses the whole range of porosities normally encountered during neutron logging. The new calibrator is engineered for precision, stability, compatibility with anticipated operating conditions, and for safety and simplicity of operation. It has been tested on a global basis. Field-worthiness of the new calibration system is demonstrated by its performance under field test and during continual research and engineering evaluation. To evaluate precision, performance studies have included calibration and response qualification comparisons among several instruments, with reference to laboratory "test pit" and in situ "test well" logs; direct comparisons among several calibrators; and observations of environmental temperatures during calibration and response qualification.

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