1. Observation of Fast Scintillation of Cryogenic PbI 2 with VLPCs William W. Moses, 1* W.- Seng Choong, 1 Stephen E. Derenzo, 1 Alan D. Bross, 2 Robert Dysert, 2 Victor Rykalin, 3 Kanai S. Shah, 4 and Misha Klugerman 4 Lawrence Berkeley National Laboratory, Berkeley, CA, 1 Fermi National Accelerator Laboratory, Batavia, IL, 2 NICADD, Northern Illinois University, Dekalb, IL, 3 RMD, Inc., Watertown, MA 4 PbI 2 Scintillation Properties at 11 K 50% @ 0.5 ns 50% @ 1.8 ns PbI 2 Decay Lifetime PbI 2 BGO Density (g/cc)6.27.1 Atten. Len. (cm)1.41.1 Photofraction40%43% Wavelength (nm)510480 Decay Time (ns)0.5, 1.8300 Luminosity (ph/MeV)33008200 Like BGO, but VERY Fast... VLPC Photodetectors at 8.5 K Gain40 k – 60 k Quantum Efficiency~80% Wavelength Range400–750 nm Rise Time1 ns Bias Voltage6–8 V Operating Temp.7–9 K Noise Floor<1 p.e. MaterialSilicon VLPC / PbI 2 Time-of-Flight PET? PbI 2 / VLPC Timing Similar to BaF 2 / PMT Spatial Resolution Similar to BGO & LSO TOF Reduces Noise Variance 5x PbI 2 / VLPC Timing Similar to BaF 2 / PMT Spatial Resolution Similar to BGO & LSO TOF Reduces Noise Variance 5x PbI 2 Crystals Coincidence Timing Spectrum (using “Large” 2.5 mm Cube Crystal) Averaged Oscilloscope Traces Experimental Setup Discussion & Conclusions Pulse Height Spectra (using “Small” 1 mm Crystal) Identifiable Photopeak (55% fwhm) for 57 Co Photopeak Not Identified for 68 Ge, 137 Cs, or 60 Co Inhomogeneous Light Collection Efficiency? Identifiable Photopeak (55% fwhm) for 57 Co Photopeak Not Identified for 68 Ge, 137 Cs, or 60 Co Inhomogeneous Light Collection Efficiency? VLPC Visible Light Photon Counter Combines Advantages of PMTs and APD Arrays Visible Light Photon Counter Combines Advantages of PMTs and APD Arrays “Small” Crystal (1 mm tall, 1 mm dia., right circular cylinder) “Large” Crystals (2.5 mm cube, 2x2x4 mm 3 block) PbI 2 has an orange color, but is transparent to it’s emissions. Micaceous - cleaves very easily! Mounted into plastic cassette. Surrounded with reflector (TiO 2 and Epotek 301-2) 8 Element VLPC Array shroud cassette 2x4 array of VLPC pixels. Each pixel 1 mm dia. Shroud surrounds 8 pixels. Cassette w/ PbI 2 crystal plugs into shroud. 0.001˝ air gap between PbI 2 and VLPC. 5.8 V bias, Gain = 40,000 Cryostat Radioactive Source Fast PMT 1 cm Cube of BaF 2 0.125˝ Window (Stainless Steel) PbI 2 Crystal & VLPC (behind window) Lots of Material  Lots of Compton Scatter ~1 ns Rise Time Pulse Height ~ Proportional to Energy ~1 ns Rise Time Pulse Height ~ Proportional to Energy 137 Cs (662 keV) 60 Co (1225 keV) Timed Against BaF 2 Scintillator 100 cm Wire (not Coax) Between VLPC & Amp Timed Against BaF 2 Scintillator 100 cm Wire (not Coax) Between VLPC & Amp Good (1.2 ns fwhm) Timing Resolution Achieved The theoretically achievable resolution is ~200 ps fwhm. Our timing resolution was degraded by a 100 cm long wire (instead of a coaxial cable) that was used to bring the VLPC signal out of the cryostat. Poor Energy Resolution Observed The 511 keV emissions of 68 Ge produce signals of ~250 p.e. This should produce a photopeak of ~15% fwhm. However, a broad shoulder was observed rather than a photopeak. This is probably due to poor optical quality of the PbI 2 crystal. The material is micaceous and cleaves easily, so it is likely that internal structure and/or flaws make the light collection efficiency non-uniform, thus preventing a clear photopeak from being observed. This Is a Long Way From a Viable PET Detector! This work was supported 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 Time-of-Flight PET TOF Constrains Position of Source Predicted PET Spatial Resolution Time-of-flight can localize the source along the line of flight. Time-of-flight information reduces noise in images. Time of flight tomographs were built in the 80’s with BaF 2 and CsF.