Ludlum Measurements 16, Technical Manual

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Model 16 Analyzer Technical Manual
Section 6
Ludlum Measurements, Inc. Page 6-1 July 2011
Calibration and Maintenance
Establishing an Operating Point
The operating point for the instrument and probes is established by
setting the probe voltage and instrument sensitivity (
HV
and
THR
). 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 the discrimination level, and are therefore, not
counted.
Total system gain is controlled by adjusting either the instrument
threshold or high voltage. Voltage affects control in the probe;
THR
(threshold) controls the amplifier gain.
The operating point for each detector is set at a compromise point of
sensitivity, stability, and background contribution. These operating
points are best for general monitoring. However, the factory set point
may be adjusted for your specific operation.
In the special case of GM detectors, a minimum voltage must be applied
to establish the Geiger-Mueller characteristic. Further changes in gain
will not affect this type of probe.
Calibration
To access the calibration controls, remove the front-panel calibration
cover. In performing the following, use a Ludlum Model 500 Pulser or
equivalent for generation of pulse height, HV testing, and count rate
generation.
Section
6