584 Constant-Fraction Discriminator

• Good time resolution over a wide range of pulse amplitudes with scintillation and semiconductor detectors
• 50-MHz count-rate capability
• 5 mV minimum threshold
• Time walk £100 ps for 100:1 dynamic range
• Constant-fraction, leading-edge, and slow-rise-time reject modes

The ORTEC Model 584 Constant-Fraction Discriminator allows good time resolution to be obtained from all commonly used detectors such as HPGe, silicon charged-particle, fast plastic, NaI(Tl), and photomultiplier tubes. Three timing modes are provided in the Model 584: constant-fraction, constant-fraction with slow-rise-time reject, and leading-edge. This economical unit has a minimum threshold of –5 mV, allowing good timing measurements to very low energies. The maximum input signal acceptable without saturation is –5 V, which provides a 1000:1 input dynamic range. The Model 584 is useful in high-count-rate applications to 50 MHz with £20 ns pulse-pair resolving time. The time walk of the Model 584 is £±100 ps for a 100:1 input dynamic range.

A variety of controls is provided, allowing optimization of the Model 584 in various applications. A precision 10-turn potentiometer sets the threshold from –5 mV to –1 V. The blocking time set by the Blocking Output Width is continuously adjustable from £10 to ³1000 ns. This feature is useful for preventing multiple triggering on pulses from scintillators having long decay time, e.g., NaI (Tl). A front-panel LED indicates that the discriminator has been triggered and can therefore be used to set the threshold just above the noise. Walk is adjusted by a front-panel 20-turn potentiometer. The Constant-Fraction Monitor on the front panel can be used to optimize walk adjustment. Since the constant-fraction shaping delay is selected by external cable, the optimum delay for a specific detector application is easily selected.

Four NIM-standard output signals are available from the Model 584. The positive output signal is continuously variable from £0.5 to ³2.5 µs by means of a printed wiring board (PWB) potentiometer. The polarity of the positive output is PWB selectable to be either a NIM-standard positive output signal or the complement signal. The two timing output signals are NIM-standard fast negative logic signals, each having a 2-ns rise time and a 5-ns width FWHM. The blocking output signal is a NIM-standard fast negative logic signal whose width is adjustable from £10 to ³1000 ns.

The Model 584 can be gated externally. A rear-panel locking toggle switch selects either Gated or Ungated operation. In the Gated Mode, a printed wiring board jumper selects the Bin Gate line in the NIM bin, a NIM-standard positive signal via the rear-panel BNC connector, or a NIM-standard negative signal via the rear-panel BNC connector.

Logic current for the Model 584 is selected from either the –6 V or –12 V NIM supply by means of a rear-panel locking toggle switch. The Model 584 is within the allotment of current for a single-width NIM module for a NIM Class V power supply when the logic current is obtained from –6 V.

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