PCM-2 Manual.pdf - Voss Associates

To determine which detector(s) are the limiting factors, select the DETECTOR
PERFORMANCE screen from the DATA menu. This display sorts detectors by
performance, with the worst at the head of the list. If the problem detector(s) are
significantly worse than the rest, it may be worth (for example) increasing RDAs
in exchange for shorter count times.
Attempting to balance all of the detectors to equal backgrounds and/or efficiencies
is not required or recommended; computations are performed independently for
each detector to obtain maximum sensitivity in the shortest possible count time.
Excessive changes to individual detector parameters usually results in Ulmecessary
complication with little or no real change in performance. Detectors which are
not causing significant problems should be left alone.
Detector Parameters
Individual detectors may be adjusted through the DETECTOR PARAMETER
screen under the EDIT menu. Remember that any subsequent entries made
through the SYSTEM PARAMETER or OVERRIDE PARAMETER screens will
overwrite the changes made to individual detectors.
The need to modify detector parameters is most frequently motivated by a detector
which demands excessive count times due to high background or low efficiency.
The most direct approach to this problem is to raise the RDA level(s) for that
particular detector, accepting lower sensitivity in exchange for more practical
count times. This assumes that administrative policy does not preclude such
changes.
If both alpha and beta backgrounds for one detector are elevated, another useful
tactic is to reduce that detector's operating voltage by 10-20 volts. If beta counts
are normal but alphas are too high, the alpha threshold level may be increased by
up to 10%, which will move some of the excess alpha counts into the beta
channel. Increasing the beta threshold will similarly cause some very low energy
particles and extraneous noise to be completely ignored. Large changes to these
parameters will also impact detector efficiency and may therefore be somewhat
self-defeating. Changes to threshold settings should be accompanied by a replateau
of the affected detector.
In rare instances, a detector with acceptable counting performance may trigger
sensitivity failures due to high or low backgrounds. This is normally caused by
the presence of radiation or shielding which affects one detector more than the
others. If moving or re-orienting the unit is not acceptable, the detector's
geometry factor may be changed. This parameter is used only by the channel
sensitivity test, and will not change any test results. Background rates are simply
multiplied by the geometry factor before being used by the sensitivity test.
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