Snap-off constant fraction timing discriminators

A snap-off constant fraction timing discriminator has been designed, involving no critical adjustments and free of shape limitations. The instrument accepts both short pulses from photo-multipliers (using fast or slow scintillators) and tail pulses from low-noise preamplifiers. Its performance (without rejecting events close to the detector junction) was checked with the aid of fast and slow scintillators and Ge(Li) detectors. Typical time resolutions (measured at a dynamic range of 100:1) were 0.41 nsec FWHM or 0.92 nsec FWTM for Co
⁶⁰
with a pair of fast scintillators, and 3.85 nsec FWHM or 8.10 nsec FWTM for Na
²²
with a 7% efficiency closed-end Ge(Li) detector in the stop channel. The spectral time distribution symmetry obtained is excellent. The advantages of the snap-off type over a tunnel diode, or a voltage-sensitive discriminator, for timing zero crossing signals are discussed. Experimental results are reported in detail.