PCM-2 Manual.pdf - Voss Associates

Sensitivity failures can be caused by greater fluctuations in ambient background
than are allowed by the alpha and/or beta sensitivity factors. In such cases of
environmentally-induced failures the symptoms are not due to poor detector
performance. Greater tolerance for background fluctuations is attained by
reducing the sensitivity factors in the System Parameters screen under the Edit
menu.
Gas Flow
Adjustment
Living VVith Radon
If several detectors exhibit low background counts, the counting gas flow rate may
be inadequate. Increase the gas flow until acceptable backgrounds are obtained.
There may be considerable lag time between increasing the flow rate and
observing a resultant increase in count rate. The amount of lag time will depend
on the degree of counting gas starvation and the final flow rate setting.
Tvv'o options are provided to reduce the number of false alarms caused by radon
gas which attaches to clothing. Both may be enabled or disabled from the
n'-JSTRUMENT PARAMETERS screen located under the EDIT menu.
The first approach is to enable the RADON COMPENSATION feature. This uses
a proprietary computational algorithm which attempts to recognize the "signature"
of radon by comparing alpha and beta count rates from each detector. The second
(and more direct) approach is to simply disable alpha channel sum zone alarms,
since this is the most frequent alarm pattern caused by radon contamination. Sum
zone alarms are disabled by raising the sum zone RDA high enough so that singlechannel
alarms will always occur before a sum zone alarm will occur. This is
accomplished when sum zone RDA's are at least twice the single-channel RDA
(or maximum RDA if the instrument is operated in mode 2).
The radon compensation algorithm uses count information from both alpha and
beta detector channels, and operates on the assumption that non-natural
contaminants will be either pure alpha or pure beta emitters. For this reason,
radon compensation should NOT be enabled when isotopes such as uranium,
which emit BOTH alpha and beta particles, are to be measured.
Software
Maintenance
All changes made to instrument and detector parameters, sum zone groups, etc.
are immediately written both to the instrument's disk and to non-volatile memory
on the front panel computer board. Whenever the PCM-2 is powered up. it will
first attempt to reload its parameters from the disk; if the required files are
missing, the program will offer to re-create them using data from the front panel
board. The third option is to reinstate the factory default values and start over
from scratch. Before deleting any disk files, print a calibration report for the
PCM-2 to preserve the current parameters. If the wrong files are accidentally
deleted, this will avoid the need to completely re-calibrate the unit.
2-10
PCM2.MAN/ Rev A/April 1995