Background Rate Reset - Mirion Technologies GEM-5 User Manual

Appendix A Algorithms
Dividing through by
Thus the closer the Variance Ratio is to zero the closer the average is to a Poisson
distribution. Values larger than one imply that a noise source such as low gas flow,
electronic noise, or a fluctuating background, is affecting the background rate.
The background rate, variance and variance ratio can be viewed on the F3 –
Background History screen for each detection zone.

Background Rate Reset

At every background update the new instantaneous background rate
to the current upper and lower background rate reset limits. If
limit (over two background count intervals) then the running average for that detector
is discarded (reset) and a new average is computed using only the latest value. For a
smaller background level change that is within the upper and lower reset limits, the
background average will change gradually to the new level within the Background
Average Period. On Alpha/Beta units, the background reset for the Alpha and Beta
channels function independently.
The Background Reset Upper and Lower Reset Limits are defined in Equation 22 and
Equation 23.
Equation 22 Background Reset Upper Limit
UpperLimit
(22)
Equation 23 Background Reset Lower Limit
LowerLimit
(23)
...where
Background Collect Time (default = 5 s). Note that a longer Background Collect
Time ( T ) results in tighter limits, i.e. the unit is more responsive to real changes in
background.
Special Cases:
•
•
278
yields:
t
R
= + ×
R K
∆
B
T
R
= − ×
R K
∆
B
T
R is the current (Average) Background Count Rate and
B
b
R ≥
If , then the T term is removed from the UpperLimit in Equation 22
R
b
l B
above.
< <
If , then the
0 R R
l B
R changes to R from the LowerLimit in Equation 23.
B
l
σ
= ×
2
, and hence the variance ratio is zero.
R t
B B
B
b
B
b
T term is removed and the second occurrence of
b
R is compared
l
R is outside either
l
T is the
B