GB1515132A - Scintillation counting apparatus - Google Patents

Abstract

1515132 Scintillation counters J E NOAKES 25 June 1975 26901/75 Heading G1A Scintillation counting apparatus comprising an inorganic phosphor crystal having a well to receive a sample cuvette 44, the well being lined with a highly transparent material 46 e.g. quartz, a photodetector 40 coupled to the crystal, optical sealing means 49, 47 coupling the crystal to the detector and to the liner, respectively, a photon reflective coating 50 covering all surfaces of the crystal, except those interfacing the detector and the liner, a metal casing 52, enclosing the crystal and detector, the refractive indices of the liner, optical sealing means, and crystal being similar to provide efficient optical coupling particularly in the range 3350 to 4200 Š and the reflector having a high reflectivity in this range. The detector output is analyzed in respect of pulse shape and/or pulse height and the apparatus is designed to operate in two modes. When operating using liquid scintillation samples, counts of beta activity may be carried out, compensating for background, e.g. cosmic, or 7-radiation in the beta count by anticoincidence gating. When the γ-activity of a sample, not contained in a liquid scintillator, is to be measured, this anti-coincidence circuit is disconnected, the crystal acting as the 7-detector. In the beta-counting mode, 7-counts are also taken, but these include the background. The background compensation of the #-count relies on pulses developed within the liquid scintillator and within the crystal having different respective shape and intensity characteristics, and being essentially representative of the #- and γ-pulses, respectively. Thus pulse height discrimination may be used, but in the embodiments described pulse shape analysis is more important. In the circuit arrangement of Fig. 5, in which a two detector structure is used (construction detailed in Fig. 1 (not shown)), pulse shape analyzers PSA5, PSA 6 are used, operating on the rise time and/or decay time. One set of outputs from these analyzers are height discriminated in SCA 9 and SCA10 and summed in amplifier 12 before application to γ-scaler 16. These gammarepresentative pulses are also applied to amplitude converter TAC 11 to prevent an output in the case of a γ-pulse transmitted by this channel. TAC 11 acts to receive the output of height analyzer 7 and delayed output of height analyzer 8, and its output is representative of the difference in the arrival times. It resets anyway, in the absence of stop pulse, after a pre-set interval. Further height analyzer 13 feeds #-scale 14. An equivalent arrangement for a single detector counting arrangement is described with reference to Fig. 8 (not shown). Another single detector construction, with a well transverse to the detector axis is described, Fig. 3 (not shown). The detector(s) may be a photo-multiplier, (e.g. having a bialkali cathode, gallium phosphide dynode) photodiode. The crystal may be NaI(T1), CsI(T1). The reflective coating may be an oxide of Mg, Ca, Ba, Ti. The sealing means may be epoxy glue, silicone grease, lucite, polymethylmethacrylate, clear liquid polymer. The housing may be Al, Mg, Be.