Continuous Phoswich™ detector for molecular imaging

We report on the development of a novel scintillator in which the decay time of its light emission varies continuously with the depth of an interaction in the crystal. The depth-of-interaction (DOI) information is thus encoded in the signal timing, which can be used to localize the position of the gamma interaction within the scintillator with high accuracy. This concept relies on the fact that decay times in certain scintillators vary considerably with the amount of dopant concentration. We are exploiting this property to create scintillators in which dopant concentration varies continuously and monotonically with depth in the crystals. Synthesis of such structures is accomplished using a specialized vapor deposition technique, which provides us with the control to vary the dopant concentration in the crystal during growth. Our technique also provides a reliable and cost-effective means to synthesize this seemingly complex structure in the large physical volumes required to provide the high absorption efficiency and large sensor areas required for PET and SPECT imaging, respectively. To date we have produced Continuous Phoswich™ scintillator (CPS™) structures measuring up to 7 cm in diameter and approaching 1 cm in thickness using cerium-doped lanthanum chloride (LaCl₃:Ce). Controlled vapor deposition is used to create a Ce³⁺ concentration gradient of 1% to 30% over the specimen thickness. This paper discusses the fabrication and characterization of CPS LaCl₃:Ce scintillators and Continuous Phoswich detectors (CPD™), and illustrates the continuous DOI capability of the CPS LaCl₃:Ce/PMT detector.