IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 5, OCTOBER 2004 2533 Observation of Fast Scintillation of Cryogenic PbI With VLPCs W. W. Moses, Senior Member, IEEE, W.-S. Choong, S. E. Derenzo, Fellow, IEEE, A. D. Bross, Member, IEEE, R. Dysert, V. V. Rykalin, K. S. Shah, Member, IEEE, and M. Klugerman Abstract—At cryogenic temperatures ( K), undoped lead io- dide (PbI ) has material and scintillation properties that are very attractive for positron emission tomorgraphy (PET). However, the PbI emissions are quenched at temperatures K, so close op- tical coupling between the scintillator and photodetector requires a photodetector that also operates at cryogenic temperatures. This suggests the visible light photon counter (VLPC), which operates at similar temperatures and combines high gain and high quantum efficiency. We proximity couple (0.001 in air gap) PbI crystals with 1.0–2.5 mm dimensions to a 1 mm diameter VLPC and cool the system to 8.5 K. Signals with short ( ns) duration are observed. When excited with 511 keV photons, a coincidence timing reso- lution of 1.3 ns full-width at half-maximum is measured. While a clear photopeak is observed for 122 keV excitation, no clear photo- peak is seen under 511 keV excitation (possibly due to the poor op- tical quality of the PbI crystals). While the present configuration must be scaled-up considerably, a cryogenic PbI /VLPC combina- tion may eventually become the basis for a practical time-of-flight PET camera. Index Terms—Time of flight, positron emission tomography (PET), cryogenic lead iodide (PbI ), visible light photon counter (VLPC). I. INTRODUCTION F OR MANY many years, positron emission tomography (PET) detector modules have used bismuth germanate (BGO) [1] or lutetium oxyorthosilicate (LSO) [2] scintillators read out with photomultiplier tubes (PMTs). While their per- formance meets the PET requirements well, there is room for improvement. For example, scintillators with shorter decay lifetime would reduce the dead time and possibly enable time-of-flight PET (which would reduce statistical noise and have other benefits [3]). Photodetectors with higher quantum Manuscript received November 14, 2003; revised April 7, 2004. This work was supported in part by the Director, Office of Science, Office of Biological and Environmental Research, Medical Science Division, U.S. Department of Energy under Contracts DE-AC03-76SF00098 and DE-AC02-76CH03000 and in part by the National Institutes of Health, National Cancer Institute, and National In- stitute of Biomedical Imaging and Bioengineering under Grants R01-CA48002 and R43-EB001 921. Reference to a company or product name does not imply approval or recommendation by the University of California or the U.S. Depart- ment of Energy to the exclusion of others that may be suitable. W. W. Moses, W.-S. Choong, and S. E. Derenzo are with Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e-mail: wwmoses@lbl.gov). A. D. Bross and R. Dysert are with Fermi National Accelerator Laboratory, Batavia, IL 60510 USA USA. V. V. Rykalin is with Northern Illinois Center for Accelerator and Detector Development, National Illinois, DeKalb, University, IL 60115 USA. K. S. Shah and M. Klugerman are with RMD, Inc., Watertown, MA 02472 USA. Digital Object Identifier 10.1109/TNS.2004.835772 efficiency could improve the energy resolution (reducing back- ground from Compton scatter events in the patient), and easily pixellated photodetectors could improve the spatial resolution. The combination of a cryogenic lead iodide (PbI ) scintil- lator read out with a visible light photon counter (VLPC) is an intriguing possibility. Cryogenic lead iodide has material and scintillation properties that are similar to BGO, but with con- siderably faster decay lifetime [4]. VLPCs combine high gain quantum efficiency that is much higher than that of PMTs, and the ability to be pixellated [5]. Thus, we have explored the per- formance of a VLPC coupled to a PbI crystal in order to de- termine whether this combination satisfies the PET energy and timing resolution requirements. II. MATERIALS A. Lead Iodide Scintillator The scintillation properties of undoped lead iodide are de- scribed in [4]. Its density is 6.16 g/cc, its attenuation length (for 511 keV photons) is 1.4 cm, its photoelectric fraction (at 511 keV) is 40%, and its light output (at 11 K) is 3300 photons/MeV. Approximately half of this light is emitted with a 0.5 ns exponential decay time constant and half with a 1.8 ns time constant. The scintillation wavelength is 510 nm. Most of its material and scintillation properties are similar to (albeit slightly worse than) those of room temperature BGO (7.1 g/cc density, 1.1 cm attenuation length, 43% photoelectric fraction, 485 nm wavelength, and 8200 photons/MeV light output). However, its scintillation decay time is much shorter than the 300 ns of BGO and is comparable to that of BaF (whose fast component is 0.6 ns). Because the number of scintillation pho- tons contained in the fast component of BaF is relatively low (1800 photons/MeV), the initial photon rate for PbI is higher than for BaF ( photons/MeV/ns for PbI vs. 2300 pho- tons/MeV/ns for BaF ). As this initial photon rate determines the achievable timing accuracy, we expect exceptional timing accuracy from PbI . Thus, cryogenic PbI has material and scintillation properties that are attractive for conventional and for time-of-flight PET. However, these properties only exist at cryogenic temperatures, as the scintillation of PbI is thermally quenched at temperatures above 40 K. Room-temperature PbI has been used to detect both X-rays [6]–[10] and optical photons [11]. However, it has only been used as a solid-state detector (in which electron/hole pairs cre- ated by the ionizing radiation are collected) rather than a scin- tillation detector (in which fluorescent photons created by the ionizing radiation are collected). These applications have used 0018-9499/04$20.00 © 2004 IEEE