DESIGN AND EVALUATION OF A PORTABLE 252CF SOURCES FOR MEASUREMENT OF CHLORINE CONCENTRATION IN REINFORCED CONCRETE. H H Saleh, R. A. Livingston. Federal Highway Administration, McLean, VA.
A portable system for the rapid nondestructive determination of chloride concentration in reinforced concrete structures is being developed. The system consists of a high-purity germanium detector (HPGe) for gamma ray detection and a portable 252Cf neutron source. Tradeoffs in the design of the neutron source include data quality, operational efficiency and radiation safety. A review of the chlorine corrosion literature indicated a minimum level to initiate corrosion is 800 ppm chlorine by weight of concrete. Precision was specified as better than 10% for a 10 minute counting period. For use in the field, the maximum weight was set at 23 kg (50 Ibs) and its greatest dimension was set at 30 cm. The maximum occupational radiation dose rate is set by the Nuclear Regulatory Commission at 2.5 mrem/hr. Iterative design calculation using the Monte Carlo Neutron Photon (MCNP) computer codes until a configuration satisfying these specification was achieved. Physical mockup of concrete slabs with known chloride concentration are being cast to evaluate the system.
Calibration of the gamma detector was done at the Cold Neutron Research Facility operated by the National Institute of Standards and Technology at Gaithersburg MD. The prompt gamma neutron activation station at this facility provided a well characterized monoenergetic cold (77 K) neutron flux with a very low gamma ray background. Mortar (Portland cement and fine sand aggregate) specimens were cast using water containing known chloride concentrations. These concentrations ranged from 200 to 4000 ppm including the threshold value for reinforcement corrosion and the chloride concentration of seawater. The calibration curve suggests excellent linearity between chlorine concentration and count rate (R2=0.999) with a precision of 1.2%. The curve suggests a minimum level of detection of 84 ppm.