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A light-tight, hermetically sealed module for use in high resolution positron emission tomography systems is described. The module has external dimensions 3.8Ã13.2Ã33 mm and contains two 3Ã5Ã20 Bismuth Germanate (BGO) scintillators, each with its own 3Ã3 mm silicon avalanche photodiode. When stacked, the vertical packing fraction is 80%. As measured with a 137Cs (662 keV) source, the typical energy resolution is 20% at 22Â°C, reducing to 16% at 0Â°C. The single detector time resolution for the 22Na gamma at 511 keV is typically less 20 ns at 22Â°C, reducing to less than 15 ns at 0Â°C. Further cooling does not improve the performance since the emission time of light from BGO increases at lower temperature. Preliminary results with Gadolinium Orthosilicate show similar energy resolution, better timing resolution (under 10 ns), but as is known, a slightly poorer photofraction and stopping power.