Three types of photon detectors for in situ measurements

The authors have been involved in the calibration and use of three types of gamma- and X-ray detectors for in situ measurements of soil contamination. These three detectors are an N-type, thin-window Ge semiconductor detector (5.0 cm diam. x 2.0 cm deep), a plastic scintillator (30.5 cm x 30.5 cm x 3.8 cm thick), and an array of six CaF2 detectors (each 7.6 cm x 7.6 cm x 0 .15 cm thick). The latter two detectors have been used with scanning system s that allow significant areas (say, > 100 m(2)) to be surveyed completely with the aid of either laser-based triangulation or a global positioning sy stem (GPS) to record the precise position for each measurement. Typically, these systems scan at a rate of 15-30 cm/s which allows an area of 100 m(2) to covered with the plastic scintillator in about 15 min. The data are tel emetered or transferred via RS232 protocol to a computer, providing operato rs with real-time mapping of the area surveyed and of the measured detector count rate. The "effciencies" of these detectors have been determined by a combination of measurements of calibrated planar sources and Monte Carlo t ransport calculations for a variety of source sizes and depths in soil, as well as by comparing these field measurements with independent laboratory s ample analysis. (C) 1999 Elsevier Science B.V. All rights reserved.