Thermo Scientific SAM12, Manual

Page: 109 / 176

Issue: 1.0 Technical Description - Physics
Thermo Fisher Scientific SAM12 User Manual 6-5
A
Attenuation Factor
is an allowance for attenuation
of a directional background field by articles being
measured. Values of A can be determined for each
detector by following the test procedure in
Attenuation (page 5-28)
.
DL
Detection limit
or Minimum Detectable Count Rate
(cps)
M
Act
Minimum Detectable Activity (variable unit)
is
the minimum Activity the system can detect within
the calculated monitoring time. It is very dependent
on the user programmed False Alarm rate and the
prevailing background conditions and thus will
continuously vary. The current Minimum
Detectable Activity (MDA) may be viewed in
Information (page 5-16) in the units specified in on
the Options sub-menu (see Options (page 5-6) ).
The MDA calculation is detailed in Minimum
Detectable Activity (MDA) and High Background
Criterion (page 6-7)
.
R
Act
Activity of Contamination (variable unit)
While the SAM12 is not occupied, background is monitored in
10 second periods, each measurement contributing to a “rolling
average”, produced over a fixed 100 second period: i.e. 10
measurements. Measurements subsequent to the 10
th
are added
to the results store with the oldest being discarded. In this way
a mean background is stored and constantly updated for each of
the four detectors. These four mean values are summed to give
an overall mean Background value for the whole system.
In relatively stable background conditions, 100 second “rolling
average” accumulates an accurate background count. It is,
however, slow to respond to sudden background disturbance
and drift, making it is necessary to screen each count before
incorporating it into the rolling average. Consequently to guard
against inaccurate measurements and false alarms, the SAM
reads and checks the counts from each detector every second.
A changing background conditions exists if either of the
following conditions are true:-
Any one detector count shows a M
σ
change from the stored
average for that channel:
[ ]
av a av
B C B
M
≥ −
where
B
av
is the rolling average for any one detector.
C
a
is the counts registered in the last one second background
update.
Background Update
Changing Background