Thermo Scientific SAM12 Manual page 139

Issue: 1.0
Determining the operating
voltage for mid-high energy
nuclides
NBR method
Figure of Merit (FOM) method
Thermo Fisher Scientific
The scan can be aborted at any time by pressing
NOTE:
the 'Abort' button. The scan is performed on all detectors
simultaneously.
The first step is to perform a background scan. This is
followed by a scan with a
centre of the vault as possible. On completion of the scan the
users is given the option of saving the data by pressing the
'Save Scan' button. This data, along with its description, can
be retrieved via the 'Data' tab (see
36)). Once the 'Save' button has been pressed, the display
changes to show graphs of the scans.
There are two methods for determining the optimum operating
voltage depending on whether the NBR and CCM features will
be used, or whether the voltages will be set the "classic" way
by reviewing the Figure of Merit at each voltage setting. The
recommended method is that which allows the use of NBR and
CCM, which typically will also provide an operating voltage
which satisfies the Figure of Merit criteria.
When the graphs are displayed, select the 'NBR' option, and
then press the respective graph for a detector which will expand
it to fill the screen. The optimum operating voltage is that
which has a T1/T2 ratio of between 30 and 35. In order to find
this press the 'Table' button and the data will be displayed.
From the table, identify the operating voltage that has the
correct T1/T2. If you have to interpolate between two voltages,
assume a linear change. Enter this value into the 'HV Setting'
box and then press the 'Back' button.
The value below the graph will now have a Gold background
to it. This background colour highlights that the value has been
changed, but not yet saved. When all operating voltages have
been set, press the 'Apply Settings' button to save these values.
The 'Back' button must be pressed in order to
NOTE:
access the other functions.
NOTE : The following procedure tends to be iterative in nature
and included as a guide only. Different operational
requirements and background conditions may necessitate a
different choice of operating point. The final choice should be
at the discretion of the senior Health Physicist.
The Objectives are to:
137
Cs source, placed as near the
HV Scan Report (page 5-
SAM12 User Manual
Setting Up Procedure
9-5