The Phoswich Detector

The phoswich detector is used for the detection of low-level radiation in the presence of considerable background. It consists of two different scintillators coupled together and mounted on a single photomultiplier tube. By utilizing the difference in the decay constants of the two phosphors, differentiation between events taking place in the two detectors is possible. The combination of crystals used depends on the types of particles present in the radiation field under investigation.  

The basic structure of a phoswich detector is shown in Fig. 6.18. A thin scintillator (scintillator A) is coupled to a larger crystal (scintillator B), which in turn is coupled to the cathode of a single phototube. Two examples of scintillators used are these  

Nal(Tl) is the thin scintillator (A) and CsI(Tl) is the thick one (B). Pulses originating in the two crystals are differentiated based on the difference between the 0.25-/rs decay constant of the NaI(Tl) and the l-/is decay constant of the CsI(Tl). Slow pulses come from particles losing energy in the CsI(Tl) or in both crystals simultaneously. In a mixed low-energy-high-energy photon field, the relatively fast pulses of the Nal(Tl) will come from the soft 

1 If the dead time of a detection system using a scintillator is 1 pts, what is the gross counting rate that will result in a loss of 2 percent of the counts  

2 A typical dead time for a scintillation detector is 5 fis. For a gas counter, the corresponding number is 200 /ns. If a sample counted with a gas counter results in 8 percent loss of gross counts due to dead time, what is the corresponding loss in a scintillation counter that records the same gross counting rate  

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The Phoswich Detector System (PDS) consists of a square array of four independent NaI(Tl)/CsI(Na) phoswich scintillation detectors. Each of the four detectors is made of two crystals of Nal(Tl) and CsI(Na) optically coupled and forming what is known as PHOSWICH (acronym of PHOsphor and semdWICH). The scintillation light produced in each phoswich is viewed, through a light guide of quartz, by a photomultiplier tube (PMT). The Nal(Tl) acts as X-ray detector, while the CsI(Na) scintillator acts as an active shield. 

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