Model 177
Technical Manual
Section 6
Ludlum Measurements, Inc.
Page 6-2
October 2018
Establishing an Operating Point
The operating point for the instrument and detectors is established by
setting the detector voltage and instrument sensitivity (
HV
and
DIS
). 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 significant difference in the
contribution of these three sources of count.
The purpose of setting the operating point 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. The pulses above the discrimination level are counted
by the instrument, while those below are not.
The total system gain is controlled by adjusting the instrument gain or the
high voltage. Voltage affects the output of the detector. Amplifier gain is
controlled by the
DIS
(discriminator) control.
In special cases 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 detector.
The operating point for each detector is set at a compromise point between
sensitivity, stability, and background contribution. These operating points are
best for general monitoring. In application, these arbitrarily selected points
may not be a better operating point. The following guidelines are presented:
GM Detectors
: The output pulse height of the GM detector is not
proportional to the energy of the detected radiation. Adjusting
DIS
will
have minimal effect on the observed count rate unless the setting is so
low that the instrument double pulses.
For most GM detectors, set
DIS
for 30-40 millivolts and adjust
HV
to the
GM tube recommended high voltage. Most GM detectors operate at
900 volts; however, some miniature detectors operate at 400-600 volts.
If a recommended setting is unavailable, run a plateau of
HV
setting vs.
count rate. Then set the high voltage on the low side of “center.”
