Ludlum Measurements 12, Техническое руководство

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Model 12 Technical Manual
Section 6
Ludlum Measurements, Inc. Page 6-3 July 2017
Establishing an Operating Point
The operating point for the instrument and probes is established by setting
the probe 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 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.
Adjusting either the instrument gain or the high voltage controls the total
system gain. Voltage affects control in the probe;
DIS
(Discriminator)
controls the amplifier gain.
In the special case of GM detectors, a minimum voltage must be applied to
establish the Geiger-Mueller (GM) characteristic. Further changes in gain
will not affect this type of probe.
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. In application, these arbitrarily selected points
may not be a better operating point. The following guides 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
DIS
setting is so low that the instrument will double-pulse.
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, although,some miniature detectors operate at 400-500 volts. If
a recommended setting is unavailable, plot a
HV
versus count rate curve
to produce a plateau graph similar to the one on the following page.
Then set
DIS
on the low side of center. For mixed detector use, both
sensitivity and high voltage may be adjusted for the other detectors