Thermo Scientific SAM12, Manual

Page: 140 / 176

Setting Up Procedure Issue: 1.0
9-6 SAM12 User Manual Thermo Fisher Scientific
•
Obtain consistent detector efficiencies, particularly
between opposite pairs, and
•
Obtain good overall system efficiency, and
•
Obtain low average background count and minimal
spread between detectors
When the graphs are displayed, select the ‘Gross Counting’
option, and then press the respective graph for a detector which
will expand it to fill the screen. Pressing ‘Table’ will show the
actual FOM and S
2
/B values – note that S
2
/B is proportional to
FOM and is included for those users who prefer this quantity.
For mid and high energies, an FOM ‘peak’ is usually evident
from the data. Determine the maximum value of FOM for each
detector -
as a starting point
. If no clear peak is visible or
several peaks exist, select a starting FOM value corresponding
to a background value (B) similar to that of the other detectors.
Select the detector operating point as follows:
1. The background count for each detector does not exceed
the following:
Typical Maximum
1 inch (25 mm)
10 cps/
μ
R/h
1000 cps/
μ
Sv/h
15 cps/
μ
R/h
1500 cps/
μ
Sv/h
2 inches (50 mm)
6 cps/
μ
R/h
600 cps/
μ
Sv/h
10 cps/
μ
R/h
1000 cps/
μ
Sv/h
2. Each background count does not differ by more than
30% from the mean value of all detectors.
3. Calculate the Mean Source Counts (Sm) for all
detectors. The value of the net source counts (S) for
each detector does not differ by more than 15% from
the mean value of (S) for all top, bottom, left and right
detectors and by no more than 25% for front and rear
detectors (if fitted).
If any detector exceeds any of these limits, alter the relevant
HV by 10 V and check the FOM is not significantly different
than the original. A total adjustment of 75 V is permissible.
This is a rough guide and individual circumstances may
demand wider variations.
If and when the background and source values are satisfactory,
calculate the detector efficiencies from the net source counts
(S) and source activity, at the chosen HV setting. Calculate the
overall system efficiency, which should be within the range
below: