Originally, "Phoswich detectors were developed to detect low-intensity, low-energy gamma rays, X-rays, as well as alpha and beta particles efficiently in a higher-energy ambient background. Some detector designs can measure and separately identify all energies simultaneously."[1]
Scintillator manufacturer Saint-Gobain Crystals & Detectors (SGC; Hiram, OH) provides a good introduction to this topic. SGC's two-page document states "A phoswich ('phosphor sandwich') is a combination of scintillators with dissimilar pulse shape characteristics optically coupled to each other and to a common PMT (or PMTs). Pulse shape analysis distinguishes the signals from the two scintillators, identifying in which scintillator the event occurred." The document also describes the operating principles and applications of phoswich detectors and includes two detailed drawings.
However, the technology described by SGC lags behind the needs of many modern medical imaging, pre-clinical (small animal) imaging, security imaging and other radiation detection, measurement and imaging applications, which require economical scintillators and detectors that can provide enhanced spatial information regarding gamma-ray interactions within scintillation materials, primarily to reduce blur in acquired and reconstructed images, achieve improved spatial and contrast resolution, simplify data acquisition devices and reduce system costs.
Research and development firm Radiation Monitoring Devices (RMD; www.rmdinc.com; Watertown, MA) in 2010 announced its development of a monolithic phoswich sensor technology, departing from the discrete designs (with their serious limitations) described by SGC. RMD's novel development, termed the Continuous PhoswichTM scintillator (CPSTM) and detector (CPDTM), provides a number of bits of depth-of-interaction (DOI) information, a significant improvement over the 1 to 2 bits available previously with discrete phoswich scintillators (as described by SGC).
RMD's web site provides an excellent description of the characteristics and advantages of their CPS and CPD technology [2].