Phoswich detector

A Hybrid Detector Phoswich detectors are used extensively in hard x-ray/ gamma ray astronomy, but suffer from poor performance at low energies due to the small number of visible photons produced by the Nal crystal. The hybrid instrument [14, optically couples a 40 cm x 40 cm phoswich, currently... 

Urine None Gamma-ray spectrometry (phoswich detector) 0.08 nCi/200 cm3 No data Ide et al. 1985... 

Feces None Phoswich detector 0.02-0.09 nCi/200g No data Kramer et al. 1989... 

Kramer GH, Diebert T, Leon JW. 1989. Calibration of a double phoswich detector assembly for estimation of homogeneously or in homogeneously distributed 241 Am and/or 239Pu in untreated fecal samples. Health Phys 56(3) 361-365. 

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 ... 

Figure 6.18 A Phoswich detector (from Harshaw Chemical Company).

A phoswich detector consists of a 1-mm-thick Nal(Tl) scintillator coupled to a 25-mm-thick CsI(Tl) scintillator. A 0.1-mm-thick beryllium window protects the NaKTI) crystal. If the detector is exposed to a thin parallel beam of 150-keV X-rays and 1.5-MeV y rays, what are the fractions of interactions of each type of photon in each scintillator ... 

Fig,4 shows the schematic diagram of a proposed hard X-ray modulation telescope HXMT (Li T.P. and Li Q.B., 1994). The telescope consists of 6 phoswich detectors (HXD, for 10 — 600 keV, total area 5400 cm ) and two proportional coimters (MXD, 2-30 keV, 1120 cm ). The field of view of each HXD module is defined by a slat collimator and is 5° X 0.5° (FWHM) oriented with the planes of the slats inclined by 30° to the neighbouring collimator. The predicted continuum and line sensitivities of HXMT are shown in Fig.5. 

The addition of the gas detector offers significant improvements over the bare phoswich - approximately a factor of two in energy resolution and improved spatial resolution. For single events, around 1mm is achievable at 25 keV, compared with 16 mm for the bare crystal. For fluorescent pairs (section 2.1) simulations show that the very high light yield from the chamber still permits the true position of each event to be deconvolved from the overlapping light distributions despite the coarse 7x7 photomultiplier array [13]. 

With phoswich type rejection in the scintillator, escape-gating rejection in the gas detector together with risetime discrimination and the possibility of mutual rejection between the two sections, the hybrid offers broad-band high-sensitivity coverage which greatly improves the standard phoswich for little added complexity. A 1/2 scale prototype is under construction to resolve remaining design questions [16]. 

The high energy instruments onboard SAX are respectively a 5 atm Xenon filled Gas Scintillation Proportional Counter (High Pressure GSPC) and a Nal/Csl phoswich scintillator (Phoswich Detector System, PDS). 

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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