POINT-SOURCE CALIBRATION OF A SEGMENTED GAMMA-RAY SCANNER*

For a conventional segmented gamma-ray scanner (SGS) in which the sample is rotated continuously within a fixed detector field of view, the data will not support alternatives to the assumption that the gamma-emitting nuclides and the matrix in which they reside are uniformly distributed. This homogeneity assumption permits the geometry of samples and calibration standards to be approximated by that of a nonattenuating line source on the axis of rotation. Other common SGS assumptions are that the detector is perfectly collimated, that its response is flat over its field of view, and that it can be approximated adequately by a line. All of these assumptions have led to a preference for homogeneous calibration standards. Preparation and certification of such calibration standards are usually difficult and expensive. Storage and transportation of SGS standards can be inconvenient or even quite troublesome. We have proposed and tested an alternative method of SGS calibration that only requires a point-source standard. The proposed technique relies on the empirical determination of a normalized two-dimensional detector response and the measurement of the count rate from a point-source standard located at the response apex. With these data, the system's response to a distributed, homogeneous sample can be predicted using numerical integration. Typical biases measured using a commercially available SGS calibrated with a point source have been less than 2%.