PCM-2 Manual.pdf - Voss Associates

Geometry Factor
Alpha & Beta
Channels Active
Alpha Threshold
Beta Threshold
Alpha & Beta
Efficiencies
Alpha & Beta RDAs
Alpha & Beta
High Fail
Alpha & Beta
Low Fail
Alpha & Beta
Weight Factors
Background count rates are multiplied by this factor before being used to test for
detector sensitivity. This converts the detector's average background count rates
into count rate per unit area in order to compensate for difference in detector size.
It is used only for channel sensitivity test calculations. All detector sizes are
referenced to one-third of a full size triple detector. As an example, the default
geometry factor for channel 0 is always 1.0 .
Detector(s) marked as inactive are not used in any computations. This is done to
keep the instrument operating if one detector fails.
Particles which cause the detector to produce a pulse exceeding this relative
amplitude are counted as alpha particles. Lower amplitude pulses register as
betas. The default alpha threshold of 45 % is seldom changed.
Particles which produce pulses below this relative amplitude are counted as either
alpha or beta. The default beta threshold of 5 % is seldom changed.
The fraction of particles (expressed as a percentage) emitted by the source of
interest in all directions (411") which are counted by the detector,
Contamination of these activity levels is detected with a probability equal to the
system RDA confidence level.
Channels which show background levels above these limits will be considered too
noisy or contaminated to use, and will remove the instrument from service.
Channels with backgrounds below these levels are assumed to have failed. The
instrument will not operate if this occurs.
These parameters control the speed with which the computed average background
rates follow changes in actual background count rates. The weight factors smooth
statistical fluctuations in background measurements by synthesizing an exponential
moving-average. Increasing the weighting factor will enhance the smooting
function at the expense of extending the time constant associated with recognizing
step changes in the real average background count rate. The formula for
computing the weighted average background count rate is:
iliold x WF) + R new
WF + 1
5-4
PCM2MAN/ R'", A/April 1995