PCM-2 Manual.pdf - Voss Associates

Theory of Operation
Operational Mode
Radioactive emissions cause ionization of the counting gas in the detector
chambers. The ions attach to the detector anode wire causing small votage pulses
on top of the static high voltage that is applied to the anode wire. Through
capacitive coupling, the pulses are stripped from the high voltage, discriminated
by pulse height, and amplified. The detector microprocessors count pulses,
convert the counts to count rates. Count rate information is communicated to the
system controller over an RS-485 bus. The system controller applies the
appropriate algorithms to update background count rates and measure for
contamination.
In its main task loop the computer program continually updates background count
rates for all detector channels, performs diagnostic checks, and monitors input
devices to determine if a person is to be measured. Numerous I/O devices are
used to prompt the user and verify correct positioning for a contamination
measurement. The results of the measurement are annunciated audibly and
visually. The measurement is principally a qualitative determination (an alarm
indicates that there is a high probability that the person is contaminated; no alarm
indicates a high probability of no contamination present). Notwithstanding, alarm
annunciation includes presentation of quantitative information, i.e., activity levels
are stated.
Three counting modes are supported; Preset All, Maximum Sensitivity (Fixed
Count Time) and Minimum Count Time. In each mode statistical control of the
counting exercise ensures that the performance of the monitor is optimized for
that mode's key parameters. The alarm set points (all three modes). RDA (mode
2) and count time (mode 3) are all computed for each new background
measurement. The parameters used include average background count rate, count
time, Sigma Factor (which controls false alarm probability), confidence level,
RDA (Reliably Detectable Activity) and detector efficency.
Preset All
Maximum Sensitivity
This mode maintains a fixed confidence level (probability of detection) for the
user-selected RDA and count time. False alarm proability is maintained at or
below a user-prescribed maximum. This mode is best used when a fixed release
limit is established and lower levels of activity are of negligible concern.
This mode is appropriate for ALARA type measurements. Using a fixed count
time that is selected by the user, the instrument applies the selected confidence
level to achieve the lowest possible RDA while maintaining a fixed false alarm
probability.
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