PCM-2 Manual.pdf - Voss Associates

Background Plateau
Source Plateau
Due to the large number of detectors in the PCM-2, plateauing each with a single
source is a daunting proposition. An alternative is to use background radiation
over a long period (i.e., overnight) to verify that all detectors exhibit similar
responses. While this process takes the instrument out of service for several
hours, it may be run without any operator intervention.
Background plateau results have three main uses: First, this data is subtracted
from source plateaus to correct for background levels. Second, by paging through
the background plateau graphs it is possible to identify any detectors with unusual
response characteristics or arcing problems. Finally, the voltage range over
which source plateaus must be run may be narrowed to a small region around the
point at which alpha background counts begin to climb sharply.
The background plateau for each detector may be scanned using a vertical cursor
line. As the cursor is moved information below the graph is updated. The
information displayed includes the high voltage setting and the alpha and beta
counts at that setting. The operator may select the high voltage for the displayed
detector by pressing F2 while the cursor is positioned on the voltage desired. The
operator may also select high voltage via the override parameter or detector
parameter edit displays.
The background plateau data can be displayed as actual data or normalized data
(relative to the largest data point).
If the hand probe option is installed and the hand probe board is communicating
properly, a background plateau may be obtained for the hand probe detector as
well. Pressing F5 in the" Select Detectors" screen toggles the selection of the
hand probe detector. If the hand probe option is installed but the board is not
communicating, a message will appear in the "Select Detectors" screen informing
the user that the hand probe is not communicating. Pressing F5 when choosing to
view plateau data also selects the hand probe detector. Plateau data may be
viewed even if the hand probe board has a communication failure.
To determine the optimum detector voltage for a particular altitude, it is necessary
to plateau one or more detectors with alpha and beta sources. Satisfactory
perfoIDlance can usually be obtained by setting all detectors to the same voltage,
however the PCM-2 does allow for individual settings if desired. It is
recommended that source plateaus be run on at least two detectors to obtain
reliable data.
If calibration sources with high activities are used, it should not be necessary to
subtract our background rates since they will be much lower. For the best
7-2
PCM2.M/\NI Rev f\I/\pril 1995