Model 3 Technical Manual Section 6
Ludlum Measurements, Inc. Page 6-4 April 2016
Establishing an Operating Point
The operating point for the instrument and detector is established by setting
the instrument high voltage (HV). 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. The system gain is controlled by adjusting the high voltage.
Note:
Measure the high voltage with a Ludlum Model 500 Pulser. If
the pulser does not have a high-voltage readout, use a high-
impedance voltmeter with at least 1000 megohm input
resistance to measure the high voltage.
GM Detectors: In the special case of GM detectors, a minimum voltage
must be applied to establish the Geiger-Mueller characteristic. The output
pulse height of the G-M detector is not proportional to the energy of the
detected radiation. Most GM detectors operate at 900 volts, although some
miniature detectors operate at 400-500 volts. Refer to the detector operating
manual for specific recommendations. If a recommended setting is
unavailable, plot an HV-versus-count-rate curve to produce a plateau graph
similar to the one displayed below. Adjust the HV for 25-50 volts above the
knee or start of the plateau. For mixed detector use, the high voltage may be
tallied for both, as long as the GM detector is operated within the
recommended voltage range.
Scintillators: Scintillation type detectors have a wide gain spectrum, typically
1000:1 at a single operating point. An operating voltage versus count rate
curve (plateau) must be established to determine the proper operating
voltage. The operating voltage is typically set above the knee of the plateau.
Plot the
HV-versus-background and source count to produce a plateau graph
similar to the one in the figure below. Adjust the
HV to 25-50 volts above
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