An input-capacity-insensitive, charge-sensitive preamplifier for simultaneous use in vacuum with fast amplifiers for charged-particle studies with semiconductor detectors

Abstract

A low-noise, charge-sensitive preamplifier was designed for service in a system used for experimental studies of charged particles with the Oak Ridge Isochronous Cyclotron 60-MeV proton beam 1). In these studies all charged particles from 1 to 60 MeV produced in targets bombarded by 60-MeV protons were examined and identified by an on-line computer using two methods simultaneously: time-of-flight mass identification, and E by d E/d x mass and charge identification. The first method requires a detector and preamplifier system that can resolve the ∼3-nsec flight-time difference between 10-MeV 3He and alpha particles. The second method requires a system with an energy resolution of 100 keV for 60-MeV protons. The charge-sensitive preamplifier described herein meets these requirements with good versatility and without sacrificing other performance characteristics. The preamplifier consists of a charge-sensitive loop only. Its charge sensitivity is from 54 mV/MeV (in Ge) to as much as 2 V/MeV with from one to three FET's in parallel at the input without the addition of parasitic suppressors. For 1.6-μsec RC- RC pulse shaping, the measured noise of this preamplifier at room temperature is 1.1 keV fwhm (referred to Ge), measured with zero detector capacity but with all other connections made, including a 1-pF test pulse capacity. An input pulse of either polarity is accepted; the maximum output is either ±3.0 V into 93 ω with better than 0.05% linearity. The temperature stability is better than 0.05% from −10° to 50°C, and as measured is determined by the temperature stability of both the charge injection capacitor and the feedback capacitor. The gain stability under prolonged vacuum operation inside a scattering chamber is better than 0.05% for 5 h. To increase the signal received by the fast amplifiers used simultaneously with this charge-sensitive preamplifier and to minimize the loop closure time, no bandwidth suppression is used even for the larger feedback ratios with three FET's. Under these conditions the preamplifier is stable against parasitic oscillations with input cable impedances from 50 to 125 ω, cable lengths continuously variable from 0 to 4 ft, and input capacities at the ends of these cables from zero to several hundred picofarad. One of the necessary design considerations achieved with this preamplifier is that the dc bias conditions are adequate to ensure linear operation over the large dynamic range of energies expected. This includes energy depositions of ∼16 MeV in a 100-μ diode, whose collection time is faster than the response time of the charge-sensitive loop. The design also includes an internal feedback network which when adjusted permits the preamplifier to operate with a detector capacitance from 0 to 100-pF with no detectable change in the output pulse amplitude.