Ludlum Measurements 18, Manual

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Model 18 Analyzer Technical Manual
Section 6
Ludlum Measurements, Inc. Page 6-4 March 2012
4. Expose the source to the detector. Slowly increase the high
voltage. The source count should increase, then decrease
with increasing high voltage.
5. Reduce and refine the high voltage until the source count
peaks. When the source is peaked, the unit may be
recalibrated for another energy using a pulse generator.
Example:
With
THR
at 10 millivolts and
WIN
at 15 millivolts, high
voltage adjusted for a 137 Cs peak, then the center of the
peak would be in the center of the window, or 12.5
millivolts would be equivalent to 662 KeV for 137
To re-peak for 1.25 MeV with a window width of 300
KeV, (i.e. threshold at 1.1 MeV max, window at 1.4 MeV
max) reset
THR
to 21 millivolts and
WIN
to 26 millivolts.
Cs. This
gives 53 KeV per millivolt. Ten millivolts is equivalent to
53 KeV and the 5 millivolt wide window (window setting
minus threshold setting) is equivalent to 265 KeV, or the
window is between 530 KeV and 795 KeV.
HV C
ALIBRATION
High voltage may be adjusted by the
HV
control. Select the desired
position on the
HV
selector switch and press the
HV
pushbutton to
observe high-voltage reading on the meter.
Note:
Measure high voltage with a Model 500 Pulser or a high-
impedance voltmeter with a high-meg probe. If one of
these instruments is not available, use a voltmeter with a
minimum of 1000 megohm input resistance.
High-voltage readout may be calibrated with an internal control.
Remove instrument can. Connect the Model 18 to the Model 500
Pulser. Observe the high-voltage reading on the Model 500. Press the
HV
pushbutton. Adjust R423 on the main circuit board for the same
reading.