Total Absorption Gamma-Ray Spectrometers Utilizing Anticoincidence Shielding

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Abstract
Two total absorption gamma‐ray spectrometers utilizing anticoincidence shielding are described. One of these consists of a 5‐in. diam by 5‐in. thick NaI(Tl) well crystal enclosed in a 26‐in. diam by 30‐in. high plastic phosphor. The plastic phosphor serves as the anticoincidence guard. The second spectrometer consists of a 3‐in. diam by 3‐in. thick NaI(Tl) crystal with a 9⅜‐in. diam by 8½‐in. thick NaI(Tl) well crystal as its anticoincidence guard. With the anticoincidence shielding, and a point source in the well of the 5×5‐in. crystal, the Compton portion of the Zn65 spectrum is reduced by a factor of about 5, while the Compton portion of the Co60 spectrum (below 1 Mev) is reduced by factors of 20 to 25. With these point sources on top of the 3×3‐in. solid crystal, these Compton regions are reduced by factors of about 4 and 14, respectively. These anticoincidence shielding arrangements also provide background reductions of a factor of 2 to 5 in the energy region below 3 Mev. The response of these spectrometers to other gamma‐ray energies is included. The photomultiplier mounting arrangements for the large NaI(Tl) and plastic phosphors, and the resolutions of the plastic and NaI(Tl) crystal are discussed.

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