Model 12-4 Technical Manual Section 6
Ludlum Measurements, Inc. Page 6-4 May 2016
From your data, pick a high-voltage operating point between 2000 and
2800 cpm at 20 mrem/hr.
The operating point should show less than 10% deviation of count for
±25 volt change.
Record this count at your selected operating point. Record the sensitivity
in counts per minute/mrem/hr. Record the high-voltage operating
point.
Gamma Rejection
Remove the Model 42-31H detector from the moderator and place in a
10 R/hr
137
Cs gamma radiation field.
Take a one-minute count. If more than 10 counts are observed for the
one-minute period, decrease the operating voltage until the count rate
drops below 10 cpm; however, ensure that the HV remains in the
plateau region determined above.
Ensure that sensitivity for the selected operating voltage is
approximately 120 cpm/mrem/hr.
Return the Model 42-31H to the moderator.
Instrument Calibration
Connect the Model 12-4 counting instrument to a Model 500 Pulser.
Now adjust the Pulser to 4-6 mV.
Adjust the Model 12-4
HV control for a HV setting equal to the operating
point HV from the previous paragraph.
Multiply the sensitivity in counts/minute/mrem/hr times 80 for the
specific detector.
Adjust the Model 500 pulse rate to the calculated value. (Count rate
should be 9600 cpm ±10%.) Set the Model 12-4 range selector switch to
×10 and adjust the ×10 calibration potentiometer for a meter reading of
˝8˝.
Connect the Model 12-4 counter to the Model 12-4 detector assembly.
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