An improved cooled FET assembly for ultra-low-background germanium spectrometers

The cooled FET and associated feedback resistor and capacitor (the "cooled FET assembly") are part of resistive feedback preamplifiers often used with germanium spectrometers. This cooled FET assembly is placed close to the germanium crystal, usually in thermal contact with the crystal housing. In low-background applications, the remaining stages of the preamplifier are placed 10-100 cm away, isolating the trace radioactivity in the preamplifier from the germanium crystal. Its proximity to the crystal makes the cooled FET assembly a critical source of radioactive contamination in ultra-low background germanium spectrometers. The elimination of all materials not specifically checked for radiopurity leaves a very limited palette with which to construct the cooled FET assembly. This has lead, in the past, to serious compromises in the mechanical and thermal ruggedness of research ultra-low background germanium spectrometers. Additionally, with poor thermal design the FET operating temperature can be too high or, more likely, too low, increasing noise. The design goals were to create an assembly that was capable of withstanding repeated thermal cycles, was radiologically clean, had good noise performance, and supported the fast rise-times needed for pulse shape analysis applications for large ultra-low-background germanium spectrometer systems.