Thermo Scientific SAM12, Manual

Page: 111 / 176

Issue: 1.0 Technical Description - Physics
Thermo Fisher Scientific SAM12 User Manual 6-7
rounded up to the nearest whole second and compared to the
user programmed minimum and maximum monitoring times.
The actual monitoring time (T
mon
) used when monitoring is
then set according to the following criteria:
T
mon
= T
cal
when T
cal
> T
min
T
mon
= T
min
when T
cal
≤
T
min
If T
cal
> T
max
then a high background condition exists and
monitoring is not possible. See Notes regarding High
Background (page 5-62) .
Once a mean background count rate has been calculated, the
SAM will be ready to monitor articles, and the Minimum
Detectable Count Rate, also referred to as the detection limit
(DL) is found by solving the following formula for DL:
Therefore
E Z
DL
M
Act
Act
.
=
,
where Act is the activity unit
m Detectable Activity and Detection Limit may be viewed on
the Information sub-menu (see
Information (page 5-16) ) and
will be displayed in the units selected in the Options PAGE
(see
Options (page 5-6) ).
NOTE
:
MDA may differ from the Alarm level because:
Tmon is rounded up
Tmin may take effect
During the monitoring cycle, the monitoring time is subdivided
into timeslices. Each sum of the count rates on all detectors in
a slice is compared to the average count rate in the cycle up to
that point.
Changing Conditions exist where the following formula is
satisfied:
[ ]
av sl av
C C C
sl
N
≥ −
where
C
av
is the rolling average count rate within the monitoring cycle,
up to the point of the check.
C
sl
is the counts registered in the last time slice
Minimum Detectable
Activity (MDA) and High
Background Criterion
Changing Conditions
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+ + +
+
+ + =
+
mon B mon
eff
B
eff
mon
eff
B
eff
T t
P F
T
DL B
t
B
P
T
B
t
B
F DL
1 1
) (
4
1
2