PCM-2 Manual.pdf - Voss Associates

High & Low
Count Fail
Verify that the HI FAIL and LO FAIL limits provide adequate margin for normal
fluctuations in background count rate to remain between the two values. These
two parameters are found in the DETECTOR PARAMETERS and OVERRIDE
PARAMETERS screens under the EDIT menu selection.
Hardware induced high count failures exhibit the symptom of a noisy channel
evidenced by excessive count rates. The count rates can be observed in the VIEW
BACKGROUND AVERAGES, or VIEW GROSS COUNT RATES selections in
the test mode menu. If high count rates exist in only the alpha or beta channel,
but not both, the problem is almost certainly on the Modular Detector Board
(MDB) which should be replaced. Otherwise, the problem could also be caused
by a poor high voltage connection resulting from the MDB not being properly
seated onto the detector, or by an anode wire in the detector that is arcing to
ground. If the latter is the cause of the problem, the detector must be rebuilt.
Low count failures can be caused by insufficient flow of counting gas. If
adequate pressure and flow exist, a leak in the plumbing circuit or a rupture in the
mylar window of the detector could cause loss of counting gas. Other causes of
low count failure include broken anode wires in the detector or component failure
on the MDB. By swapping the MDB with one from a good detector zone, the
problem can be segregated to either the MDB if it follows the board to the other
zone or to the detector if it remains with that zone.
NOTE: Whenever an MDB is replaced, the DETECTOR DOWLOAD routine
must be invoked to restore the correct parameters to the affected detector.
Contaminated
Detector
A CONTAMINATED DETECTOR condition can only be flagged in the first
background update interval following a personnel contamination measurement.
Two conditions must exist before the condition will be flagged: 1.) The detector
must post an alarm during the contamination measurement, and 2.) That detector's
background count rate must be elevated above its previous average background by
a factor determined by the BACKGROUND SIGMA FACTOR. When a
contaminated detector condition is posted, the possibility of a statistical anomaly
causing the condition can be investigated by observing the background count rates
in the VIEW menu selection. If high count rates exist, the presence of
contamination can be verified with an independent counter. Short-half-life
isotopes may be allowed to decay in situ or contamination can be removed using
proper health physics procedures. The instrument can be restored to service by
clearing the condition in the DETECTOR STATUS screen of the STATUS menu
selection.
9-8
PCM2.MAN/ Rev A/April 1995