Ludlum Measurements 239605, Manual

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Model 3-IS & 3-IS-1 Technical Manual
Section 6
Ludlum Measurements, Inc. Page 6-4 October 2010
the discrimination level and are therefore not counted. The system gain is
controlled by adjusting the HV (high voltage).
Measure the HV 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 HV.
Note:
G-M 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 GM detector is not proportional to the energy
of the detected radiation. The GM detectors for this unit operate at 900
volts
.
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
the knee or start of the plateau. This provides the most stable operating
point for the detector.
Note:
If more than one detector is to be used with the instrument and the
operating voltages are different, the
HV
will have to be readjusted
for each detector substitution.