Investigation of Narrow Well in NaI (Tl) for Reduction of Iodine K X-Ray Escape when Measuring Continuous X-Ray Spectra with Collimated Photon Beams

Abstract

A technique described previously by Hettinger and Starfelt for the reduction of iodine K x-ray escape when measuring narrow collimated beams of continuous x rays with a NaI(Tl) scintillation spectrometer consisted of confining the photon beam to a small well in the crystal to reduce the solid angle through which iodie K x-ray escape is possible. This technique was evaluated for tungsten characteristic radiation (∼60 kev) and was reported to provide a very substantial reduction in the escape peak. The present paper describes an investigation of this technique at lower photon energies (10 to 60 kev). A tail was observed to appear on the low energy side of the photopeak of magnitude showing inverse dependence on photon energy. This distortion of pulse-height response is large enough to indicate that the well crystal technique is unsatisfactory when making measurements of continuous x-ray spectra in the energy region where the escape of iodine K x rays is an important factor. The likely explanation is that light losses associated with the well itself are responsible for the observed tails since reflecting liners were not present. Introduction of a reflecting liner into the well was found to reduce the tail effect considerably.