Model 2241-2RK
Technical Manual
Section 8
Ludlum Measurements, Inc.
Page 8-9
July 2012
For most GM detectors, set
DISC
for 30-40 millivolts and adjust
HV
to
the GM detector recommended high voltage. Most GM detectors
operate at 900 volts, although some miniature detectors operate at
450-550 volts. If a recommended setting is unavailable, plot count rate
versus HV to produce a plateau graph. Adjust the
HV
for 25-50 volts
above the knee or start of the plateau. For mixed detector use, both
sensitivity and high voltage may be tailored for other detectors as long as
the GM detector is operated within the recommended voltage range.
Caution must be observed in lowering the input sensitivity to ensure that
the counter does not double or multi-pulse.
Alpha Air-Proportional Detectors
: For air proportional alpha detectors,
set the
DISC
for 2-millivolt discrimination. Adjust
HV
until the detector
just breaks down (shown by a rapid increase of count rate without a
source present). Measure the HV output; then decrease the
HV
setting to
operate 100 volts below breakdown.
Proportional Detectors
: For proportional detectors, set the
DISC
control for 2-millivolt discrimination (near maximum clockwise).
Expose the detector to a check source and plot count rate versus HV,
similar to the one in the figure below. Refine the
HV
adjustment for
optimum source efficiency with a minimum acceptable background
count.
Scintillators
: Set the
DISC
for 10 millivolts. Plot background
and source counts versus HV to produce a plateau graph
similar to the one in the figure. Adjust the
HV
to 25-50 volts
above the knee or start of the plateau. This provides the most
stable operating point for the detector.
C
OUNTS PER MINUTE
(C/
M
)
C
ALIBRATION
This procedure will setup the Model 2241-2 for the Counts/minute (C/m)
mode of operation. Refer to Section 8, (Page 8-2 and following) for more
information on setup parameter variables.
A Ludlum Model 500 Pulser or equivalent is required. If the Pulser does not
have a high-voltage display, use a high-impedance voltmeter with at least
1000 megohms input resistance to measure the detector high voltage.
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