PCM-2 Manual.pdf - Voss Associates

1400 to 1900 volts) using long count times (at least 300 seconds per voltage).
Once the test has been started, no operator intervention is required; it is therefore
advised that this plateau be nm overnight.
Once the background measurements have been completed, a plateau screen is
available for each detector. Page through these displays and verify that all
detectors have similar voltage curves. Note the voltage range over which alpha
background rates begin to increase; use the left and right arrow keys to move the
vertical cursor line and note that the exact count rates displayed at the bottom of
the screen are displayed for each voltage. Best results will be obtained at a
voltage slightly below the point at which alpha background rates begin to increase.
The SOURCE PLATEAU program determines the response of anyone detector to
alpha and beta sources. Select starting and ending voltages which bracket the
operating point determined from the background plateau results and count times
appropriate to the sources to be used. If large sources are available, it should not
be necessary to correct for background rates. If a background plateau has already
been run using the same voltage increments and covering a range which matches
the source plateau high voltage range, background corrections may be applied.
Both alpha and beta source plateaus should be run to obtain a full set of results.
Once an operating voltage has been determined, enter it through the OVERRIDE
PARAMETERS screen to set all detectors to this value. Individual detector
voltages may be adjusted through the DETECTOR PARAMETERS screen or the
VIEW PLATEAU display for that channel. Return to the BACKGROUND
AVERAGES screen and verify that all detectors give reasonable count rates at the
selected voltage. It is recommended that source plateaus be run on at least two
detectors to insure that the operating voltage selected will be appropriate for the
entire system. Verify similar results for all detectors that are plateaued.
Detector Efficiency
Calibration
Using the EFFICIENCIES conunand under the CALIBRATION heading, each
detector must be calibrated with alpha and beta sources of known activities. 100
square em "plate" sources are preferred for this purpose as opposed to smaller
"coin" sources. High-activity sources should be used if possible, since they will
provide accurate results with short count times.
Enter the activities of the sources to be used (the instrument will correct for
source decay if given an initial calibration date and the isotope half-life). Follow
the instructions displayed to select a detector; position the source on that detector
and begin the count. Once a satisfactory efficiency measurement has been
completed, store that value and move the source to the next detector. After all
detectors have been calibrated with one source (alpha or beta), repeat the
procedure with the other after selecting the correct channel type.
PCM2.MANI REV AI April 1995 2-7