Design of Low-Noise Vacuum-Tube Pulse Amplifiers for Semiconductor Radiation-Detector Spectroscopy

The pulse-height resolution of semiconductor nuclear-radiation detectors for particle energies below 200 kev is primarily limited by detector and amplifier noise. Amplifier noise may be large compared to that contributed by cooled, deep-depletionlayer diodes, and minimum noise is achieved by optimizing amplifier design for specific diode parameters. Vacuum-tube performance is realistically evaluated by measuring equivalent noise resistances of both plate-current shot noise and grid-current noise as a function of plate current and by measuring the tube input capacitance under actual operating conditions. Nomographs were developed to solve the equations relating noise performance to all diode and amplifier parameters except flicker noise, which is negligible for low transconductance tubes in this application. Graphical noise calculation facilitates optimum amplifier design. These nomographs may also be used to calculate noise from field-effect transistors. By use of these nomographs, a vacuum-tube amplifier was designed which has a noise performance of 175 electronic charges rms (1.5 kev fwhm Si) for zero external capacitance and 225 electronic charges rms (2 kev Si) for 10 pf external capacitance.