A Large Well Crystal With Anticoincidence Shielding

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Abstract
An anticoincidence shielded gamma-ray spectrometer has been fabricated using a 9-3/8-in. diameter by 8-13/16-in. thick Nal(Tl) crystal with a well of sufficient size to accommodate a 500 ml polyethylene bottle. The crystal is shielded from its phototubes by a 5-in. thick pure Nal light pipe. The crystal is enclosed in a 36-in. diameter by 36-in. thick plastic phosphor which serves as an anticoincidence annulus. A 4-in. thick boron-paraffin shield located between the plastic phosphor and the massive lead shield serves to reduce the background contribution from cosmic-ray-produced neutrons. The background in the energy region of 0.1 to 4 MeV is reduced by five- to thirtyfold by the combined shielding techniques. The Compton response for a 65Zn point source is reduced by two- to threefold while that of 60Co Compton response below the 1.17 MeV photopeak is reduced by six- to sevenfold. For our standard geometry 500 ml aqueous samples the photopeak counting efficiency of 65Zn in the large well crystal is forty times higher than with a 3-in. x 3-in. crystal yet the background of a large well crystal is only two times higher.

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