A common problem in experiments where the scattering angles are large, necessitating long scintillators, is the difficulty in the extraction of precise timing information. Since a scintillator of several meters length is not uncommon at many facilities, the time of an output pulse emerging from a phototube attached to one end of the scintillator may vary by up to tens of nanoseconds depending upon where the particle struck the detector.

The Model 624 Octal Meantimer was designed to equalize the photon transit time by providing an output pulse at always a fixed time independent of where the impact occurred. Used in conjunction with a phototube and one discriminator at each end of the scintillator, the 624 accepts the two discriminator outputs, feeds them through a delay line element in opposite directions, and gives an output pulse at a time when the pulses overlap. The delay line element, socketed and field-changeable, has 17 pickoff points giving a time resolution of 0.5 nsec.

The three bridged outputs of each 624 channel are driven from a 45 mA differential stage current source, each output giving an amplitude of -750 mV into 50 Ohm if all are terminated. Conveniently, for applications where long cables must be driven, one or more outputs may be left unterminated to give a larger output amplitude to compensate for attenuation through the cable. The duration of the outputs is approximately equal to the total overlap time of the two input signals.

Packaged in a standard NIM #1 width module, the Model 624 may also be provided by special factory option at increased cost with six outputs per channel at the expense of additional current usage (Model 624M).

      SPECIFICATIONS
      NIM Model 624
      OCTAL MEANTIMER

 INPUT CHARACTERISTICS

 Number of Channels:   8, all identical.

 Logic Inputs:    Two, 50 Ohm direct-coupled; reflections <7% for standard NIM fast logic signals
      (-600 mV minimum) of FWHM of +/-7 nsec with 2 nsec risetime.


 OUTPUT CHARACTERISTICS

 Number and Type:   Three, bridged, driven from 45 mA current source; quiescently, 0 mA; 45 mA
      during output (-800 mV nominal with all 3 terminated into 50 Ohm). Maximum
      output amplitude, -2 volts.

 Duration:    Input pulse duration less approximately 4 nsec.

 Risetime-Falltime:   2.5 nsec maximum, with all outputs terminated.


 GENERAL

 Delay Line Elements:   16 nsec, 17 pickoff points' (15 taps, plus ends).

 Time Resolution:   0.5 nsec.*

 Input-Output Delay:   5 nsec PLUS td + (D - td)/2, where td = time delay between the two input
      pulses and D = value of delay line element.

 Total Meantime Calculation:  With reference to the actual time the particle passes through the scintillator,
      the time of output is 5 nsec plus one-half the sum of all the delays through
      the loop, including total scintillator transit time, delay line element value,
      all cable and discriminator delays.
      NOTE: This total time is constant with respect to the scintillator impact
      time, regardless of impact position.

 Packaging:    In RF-shielded AEC/NIM #1 module, (AEC Report TID-20893); Lemo-type
      connectors.

 Approximate Current
 Requirements:    + 6V at 240 mA
      -6V at 285 mA
      + 12V at 50 mA
      -12V at 150 mA
      - 24V at 20 mA

 *32 nsec and 40 nsec scales available upon request.