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MODEL 2000 Technical Manual
Section 2
Ludlum Measurements, Inc. Page 2-3 November 2020
For proportional detectors: Set
THR
on 0.20 (approximate 2 mV input
threshold).
Operating Point
The instrument and detector operating point is established by setting the
probe voltage (
HV
) and instrument sensitivity (
THR
). Efficiency, background
and noise for a given detector system are fixed by the physical makeup of
the 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 the singular case of the GM detector, a minimum operating voltage is
required to establish the GM operating region. (At lower voltages, the
detector operates as a very insensitive proportional counter.) This detector is
not capable of energy discrimination (pulse height discrimination).
For gain sensitive detectors (proportional or scintillation), the most
straightforward method of selecting the operating point is to develop a
graph, relating count rate to system gain. This relationship is commonly
referred to as a plateau or instrument plateau curve. System gain may be
changed by adjusting detector high voltage (
HV
).
Determining Instrument Plateau
Set the time multiplier toggle switch to ×0.1 and the
MINUTES
thumbwheel
switch to “001.” This gives a 0.1 min (6 second) count.
Set the
THR
control to 1.0 for a scintillation detector or to 0.2 for a
proportional detector.
With the detector shielded from the source, adjust the high-voltage control
by 50 V increments, and take a plot of
HV
-versus-background count rate
until the detector voltage rating is reached. Return the
HV
control to zero.
Expose detector to source and make another count-versus-voltage plot.
Plot both sets of data and select the operating point to correspond with the
maximum source count and minimum background count. Avoid areas of
very fast count rate changes with small changes in detector voltage. The
optimum operating point for low background detectors is just above the
inflection point (or break-over point) of the plateau curve. If the background
count is irrelevant, shift the operating point to the plateau center for greater
stability.