Ludlum Measurements 2241-3, Manual

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Model 2241-3 & 3i Technical Manual
Section 8
Ludlum Measurements, Inc. Page 8-8 January 2016
instrument checkout procedure and the variables are saved under detector
setup number “1” when shipped from Ludlum Measurements, Inc.
The following subsection deals with exposure rate calibration. The detector
Calibration Constant (CC) and Dead Time Correction (DTC) are the two
primary parameters used in the exposure rate calibrations (R/hr and Sv/h).
These two constants are alternately varied to achieve linearity at the detector
non-linear operating regions. An example of the Ludlum Model 44-9 GM
detector calibration is given at the end of this section to illustrate the
algorithm used in determining the CC and DTC variables.
G
ENERAL
D
ETECTOR
S
ETUP
I
NFORMATION
The operating point for the instrument and probes is established by setting
the probe voltage and instrument sensitivity (
HV
and
DISC
). 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
radiation) are above the discrimination level and the unwanted pulses from
noise are below the discrimination level and are therefore not counted.
The total system gain can be controlled by adjusting either the instrument
sensitivity or the high voltage.
HV
controls the gain of the detector; and
DISC
(Discriminator) controls the instrument counting threshold (sensitivity).
In the special case of GM detectors, a minimum voltage must be applied to
establish the Geiger-Mueller characteristic. Further changes in
HV
will have
little effect on this type of detector.
GM Detectors
: The output pulse height of the GM detector is not
proportional to the energy of the detected radiation. Adjusting
DISC
will
have minimal effect on observed count rate unless the
DISC
setting is so low
that the instrument will double-pulse.