A transfer standard for fast neutron fluence rate measurement

Neutron Flux meters and Neutron Rem Counters, which are calibrated in the standard neutron fields, are generally used as reference standards for measurement of the neutron field at various locations in Reactors and Accelerators. These instruments are made with a thermal neutron detector and a characteristically shaped moderator-absorber assembly. Dimensions of these instruments are very large as the diameter of the moderating assembly itself is 20-25 cms. This limits the usefulness of these instruments in a narrow space. A small portable transfer standard has been designed and established for in-situ measurement of fast neutron fluence rate using a small polythene sphere of diameter 3 inches covered with 1 mm thick cadmium sheet. Thin gold foils are used as the thermal neutron detector. The traceability of this standard has been established with the primary standard through a standardized 252Cf neutron source. This paper also describes the use of this transfer standard in measuring neutron fluence rate and hence airkerma rate of a 252Cf neutron source in an enclosure at defense laboratory Jodhpur which was difficult to measure using conventional instruments. (author)