Ludlum Measurements 239605, Manual

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Model 3-IS & 3-IS-1 Technical Manual
Section 6
Ludlum Measurements, Inc. Page 6-3 October 2010
Caution!
The instrument input operates at a high potential. Connect the
pulse generator through a 0.01µF, 3,000-volt capacitor, unless the
pulse generator is already protected
Adjust the
HV
control for the proper operating voltage of the detector to be
used. Adjust the Pulser negative pulse frequency to provide a meter deflection
of approximately 80% of full-scale on the ×100 range. Adjust the ×100
calibration control for the proper reading.
Check the 20% scale indication of the Model 3-IS by reducing the Pulser count
rate by a factor of 4. The Model 3-IS should read within 10% of the actual
pulse rate. Decrease the pulse rate of the Model 500 by one decade and turn
the Model 3-IS range selector to the next lower range. Repeat the above
procedure for the remaining lower ranges.
Note:
In the event that any reading is not within 10% of the true value on
any scale after any of the above calibration methods is performed, a
reading within 20% of true value shall be acceptable- if a calibration
graph or chart is provided with the instrument. Instruments that
cannot meet these criteria are defective and require repair.
Establishing an Operating Point
The operating point for the instrument and detector is established by setting
the instrument high voltage (
HV
). The proper selection of this point is the key
to instrument performance. Efficiency, background sensitivity and noise are
fixed by the physical makeup of the given detector and rarely vary from unit
to unit. However, the selection of the operating point makes a marked
difference in the apparent contribution of these three sources of count.
In setting the operating point, the final result of the adjustment is to establish
the system gain so that the desirable signal pulses (including background) are
above the discrimination level and the unwanted pulses from noise are below