Ludlum Measurements 18, Manual

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Model 18 Analyzer Technical Manual
Section 6
Ludlum Measurements, Inc. Page 6-3 March 2012
S
CINTILLATION
D
ETECTORS
1. Set
THR
for 10 millivolts.
2. Set
WIN IN
/
OUT
to
OUT
.
3. Expose detector to a radiation source and observe count
rate with and without source as voltage is increased in 50
volt increments. Plot this data-vs-high-voltage.
4. Select an operating high-voltage point with minimum
change in count-rate-vs-high-voltage (i.e. on the plateau).
5. Record this high voltage for future operational check.
W
INDOW
C
ALIBRATION
1. To find the peak energy of an isotope, the operating
characteristics of the detector must be determined. A
plateau graph of 50-volt increments should be taken with
the
WIN IN
/
OUT
switch in the
OUT
position and plotted for
both background and a reference source, such as
137
Cs for
high energy or 241 Am for low energy.
Window calibration will be valid only with the specific detector to be
used with the Model 18. A check source will be required. A Model 500
Pulser will be required if the
WIN
calibration is shifted to another energy.
Using the pulser, set
THR
on at 10 millivolts and
WIN OFF
at 15 millivolts.
Example:
2. The peak energy of the source is located where the count
from the source "just starts" to appear above background.
Take note of the high voltage in this area. This will be the
region of the operating point for the reference source.
3. Place the
WIN IN
/
OUT
switch to
IN
, and set the
WIN
and
THR
controls. Reduce the high voltage below the point
where the source count is not seen above background.