PCM-2 Manual.pdf - Voss Associates

Efficiencies
Shield Factors
Detector efficiencies are determined by measuring calibrated sources and then
dividing the measured count rates by the known activities of the sources. When
the efficiency calculation function is selected, the user is asked to enter activities
for one alpha and one beta source, then select a detector and channel to calibrate.
The selected detector will acquire counts in the specified charmel for the count
time selected. When the count cycle is completed a new detector efficiency will
be calculated and displayed on the "Measured Efficiency" line. The operator must
press the F2 key to store the new efficiency, it is not automatically saved. The
count cycle may be repeated by pressing ENTER. Efficiency values may also be
directly edited via the detector parameter display.
If the hand probe option is installed and the hand probe board is communicating
properly, an efficiency test may be run for the hand probe detector as well. As
for the plateaus, pressing F5 selects the hand probe detector and starts the test. If
the hand probe option is installed but not communicating, a message will appear
in the "Select Detectors" screen informing the user that the hand probe is not
communicating. To view the test results, press F5 to select the hand probe
detector. Efficiency data may be viewed even if the hand probe board has a
communication failure.
Statistical Variance Test
Two count cycles are initiated, the first with the unit unoccupied and the second
with a user in counting position. Shield factors are calculated for all detectors
simultaneously, and may be saved either individually or as a set. Since the
background count rates used to calculate shield factors are low, it is recommended
that count times of at least 100 seconds (preferably more) be used for accuracy.
This test acquires counts from all active channels for N count cycles where N is
selected by the operator. Each count cycle is T seconds long where T is the
selected or calculated count time currently used for measurements. After
completing N cycles the instrument will calculate the mean, the variance and the
variance divided by the mean for every active channel. These results will then be
presented in tabular form and may be printed if desired. If the variance divided
by the mean for any channel falls outside the range 0.67 to 1.50, the channel is
probably noisy.
PCM2.MANI REV AI April 1995 7-7