Delay Line Pulse Shaping

Delay-Line Pulse Shaping

Amplifiers employing delay-line pulse shaping are well suited to the pulse processing requirements of scintillation detectors. The propagation delay of distributed or lumped delay lines can be combined into suitable circuits to produce an essentially rectangular output pulse from each step-function input pulse. For pulse pile-up prevention, this shaping method is close to ideal because an immediate return to baseline is obtained. With scintillation detectors, the signal-to-noise ratio of the preamplifier and amplifier combination is seldom a limitation on the energy resolution. As a result of the high gain of the photomultiplier tube, the energy resolution is determined by the statistics of light production in the scintillator and the conversion to photoelectrons at the cathode. However, for detectors having no internal gain, delay-line shaping is seldom appropriate, because the signal-to-noise ratio for preamplifier noise with delay-line shaping is inferior to that obtained with simple CR-RC or semi-Gaussian shaping.

There are many circuits that can be used for delay-line shaping, and the circuit shown in Fig. 5 is typical of one that is very tolerant of delay-line imperfections. The step pulse from the preamplifier is inverted, delayed, and added back to the original step pulse. The result is a rectangular output pulse with a width equal to the delay time of the delay line. In practice, the value of the resistor labeled 2R_D is made adjustable over a small portion of its nominal value to allow compensation for the exponential decay of the input pulse. With proper adjustment, the output pulse will return to baseline promptly without undershoot. The main advantage of delay-line shaping is a rectangular output pulse with fast rise and fall times. In fact, the falling edge of the pulse is a delayed mirror image of the rising edge. These characteristics make delay-line pulse shaping ideal for timing and pulse-shape discrimination applications with scintillation detectors at low or high counting rates.

By following one delay-line shaper with a second, a doubly- differentiated delay-line shape is obtained, as illustrated in Fig. 6. The result is an output pulse shape that has a positive rectangular lobe followed by a negative rectangular lobe with equal amplitude and duration. The double-delay-line shaping is ideal for use with scintillation detectors in systems incorporating ac-coupling. The baseline shift caused by changing counting rates in ac-coupled systems is virtually eliminated by the two lobes having equal area above and below the baseline. This benefit is gained at the expense of doubling the pulse width. Double-delay-line shaping is often useful for simple zero-crossover timing with scintillation detectors at low or high counting rates. Double-delay-line shaping is not a good choice for detectors having a substantial preamplifier noise. Its signal-to-noise ratio is worse than single-delay-line shaping, and much worse than semi-Gaussian shaping.

Figure 5. Single-Delay-Line Pulse Shaping.

Figure 6. Double-Delay-Line Pulse Shaping.