Heterogeneous scintillation γ-ray detector with adjustable spectral sensitivity

Abstract

Analysis has shown that the configuration of a heterogeneous scintillation detector [1] makes it possible to combine the advantages of crystalline inorganic scintillators with atomic number Z > 50 (such as NaI, CsI, and others) with high γ-ray detection efficiency, but as a rule comparatively small size (entry window area <400 cm 2 ), and plastic scintillators with comparatively low detection efficiency (Z < 10) but with record large size (entry window area ~1 m 2 [2]).The present work is a continuation of the Monte Carlo computational studies [1] of the measurement system ( Fig. 1) with a heterogeneous scintillation detector consisting of flat converting layers K i of thickness Δ c made of lead (Pb, ρ = 11.3 g/cm 3 ) and flat sensor layers D i of thickness Δ s made of polystyrene (CH 0.99 , ρ = 1.05 g/cm 3 ), i = 1, 2, ..., M. Here S(E) is the γ-ray source, and E is the energy of the γ-ray striking the entry window of the detector.The calculations took account of all processes where γ-rays interact with the working body of the detector, the transport of the fast electrons (positrons) with energy E e ≥ 0.03 MeV formed in these processes, as well as the formation of annihilation radiation and Bremsstrahlung. The computed energy dependences W D (E) of the γ-ray detection efficiency with threshold total absorbed energy in the sensor layers ε ≥ ε thr = 0.1 MeV are presented in Fig. 2 for some models of a heterogeneous scintillation detector (see Table 1) with a 30 × 40 cm entry window and the indicated composition.Data analysis showed that the spatial-functional structure studied here differs in important ways from conventional scintillation detectors: 1) the detection efficiency of a heterogeneous scintillation for high-energy γ-rays (E > 1 MeV, W D ≅ 0.2-0.5) is comparable to the corresponding NaI(Tl) detector but the absence of strict limits on the aperture (P~ 1 m 2 ) opens a possibility for creating a heterogeneous scintillation detector with record high sensitivity I = W D P~ 2·10 3 counts/(photon·cm -2 ) at comparatively low cost;2) the energy dependence of the detection efficiency W D (E) can be changed purposefully by selecting an appropriate material and thickness of the converting and sensor layers;3) the high operating speed is determined by the short de-excitation time of the scintillations in the sensor layers (~3 and ~50 nsec for sensors made of scintillating polystyrene and glass fiber, respectively); 4) oriented high-sensitivity (W D~ 0.2-0.5) γ-ray detectors with aspect ratio M(Δ c + Δ s )L -1 < 0.1 for M(Δ c + Δ s ) ≅ ≅ 3-4 cm, L ≅ P 1/2 ≅ 50-100 cm, can be created; and