PM-7 Operating Manua.. - Voss Associates

Chapter 4 Algorithms
Counting Algorithms
The sum channel determines the minimum time required to measure the
specified RDA at the required confidence level while maintaining the
selected false alarm probability. That time is calculated as follows:
T = SF R B + z ( RDA × eff) + R
------------------------------------------------------------------------- B
RDA × eff
Where:
“T” Is the count time, in seconds.
2
Eq. 2.1
“SF” Is the SIGMA FACTOR which sets the false alarm rate. “SF” is the
multiplier of the standard deviation of the average background count
rate.
“R B ” Is the sum channel's average background count rate in counts per
second.
“z” Determines the confidence level, or probability of detection. z is the
multiplier of the standard deviation of the combined average
background and RDA count rate.
“RDA” Is the “Instrument RDA” or sum channel's Reliably Detectable
Activity expressed in units of dps.
“eff” Is the 4-pi efficiency of the sum channel computed as the ratio of
net count rate in counts per second over the decay-corrected source
activity in dps. The efficiency of the sum zone should be measured with
a calibration source of the isotope of interest placed in the center of the
portal.
If the calculated count time in the sum channel exceeds the selected
maximum count time, a high background alarm will occur.
Thermo Fisher Scientific Personnel Monitor -7 Techical Guide_ 4-1