Model 42-49 Neutron Detector
Technical Manual
Section 5
Ludlum Measurements, Inc.
Page 5-1
December 2017
Calibration
Calibration
To calibrate the detector, the following items are needed:
Counting instrument: Ludlum Model 2200 Scaler/Ratemeter
241
AmBe neutron source or equivalent
137
Cs gamma source for gamma rejection check
Operating Voltage Plateau
Connect the Model 42-49 to a Model 2200.
Set Model 2200 input sensitivity to -2 mV.
Expose the detector to a 20 mrem/hr
241
AmBe neutron source.
“Run a plateau” as follows: Taking one-minute counts, record the
count rate for 25-volt incremental steps from 1000 volts through a
region in which the count rate steadily increases, becomes relatively
constant (“flattens out”), and then increases again. (A typical range
will be 1000-1250 volts.) The flattest portion of this region is the
desired plateau. The value in the middle of this region is the assumed
operating voltage of the detector, subject to the sensitivity and the
gamma rejection checks.
Calculate the sensitivity (cpm/mrem/hr) for the assumed operating
voltage as follows:
Rate
Equivalent
Dose
Rate
Count
y
Sensitivit
Section
5
