PROPERTY OF AMETEK, INC.
–
ORTEC
®
ORTEC
®
EASY-NIM
™
928 Suite
932512A / 0114
30
2
2
2
2
1
1
1
var
)
1
L
H
(
*
4
n
/
n
/
B
n
/
n
/
B
B
var
B
G
100
*
)
spectrum
ZDT
from
(
Area
spectrum
error
from
error
%
A
CORR_ERR
spectrum is analyzed
6
as a regular spectrum most of the time, with a few exceptions as listed below.
●
To calculate the peak area uncertainty, the error spectrum is used. If the peak limits are from
L
and
H
channels, then the background variance is calculated as:
where:
B
1
= sum of background counts for the channels adjacent to the peak start (low-energy) channel
L
B
2
= sum of background counts for the channels adjacent to the peak end (high-energy) channel
H
n
1
= the number of low background points (
n
1
= 1, 3, or 5) used
n
2
= the number of high background points (
n
2
= 1, 3, or 5) used
The peak area uncertainty is calculated from:
where
G
is the sum of counts, from the error spectrum, from channels
L
to
H
.
●
In our ISOTOPIC software’s ISOWAN32 analysis engine, the peak fitting routine fits all the library peaks as
singlets to calculate the peak centroid, peak start and end channels, and peak background. A linear
background under the peak is assumed during the peak fitting process.
●
The error spectrum is always used to calculate the uncertainties of counts whenever needed. For example, if
peak deconvolution is needed, the error spectrum is used to find the best fit for the peak background.
3.5.4.
Choosing a ZDT Mode
When the counting rate is essentially constant during the time required to acquire the spectrum, the standard mode
—
ZDT Off
— is the preferred mode; only the uncorrected spectrum is collected and stored in the spectrum file. But, if
the counting rate varies significantly during the measurement time, the standard mode will not yield the proper dead-
time-corrected counting rate. This can be most easily understood by noting that the uncorrected mode compensates
for dead-time losses by extending the real counting time. Hence a sample containing both a short-lived high-activity
isotope and a long-lifetime lower-activity isotope will experience very high dead-time losses during the first few
seconds of the measurement, as the short-lifetime isotope decays rapidly. This high dead time will cause the counting
time to be extended after the short-lived isotope has decayed to zero activity, and the system will count the low-
activity isotope for the extra time. Consequently, the average activity of the short-lived isotope will be
underestimated.
If you anticipate significantly varying counting rates during the time taken to acquire the spectrum, the
CORR_ERR
ZDT
mode should be used. The
CORR_ERR
mode corrects for dead-time losses over minuscule time intervals by adding
counts to the ZDT spectrum in proportion to the instantaneous ratio of real time to live time. Thus, the dead-time
correction can correctly track rapidly changing counting rates. The
CORR_ERR
mode should be used whenever the
counting rate might change significantly during the measurement time. In addition to the rapidly-decaying isotope
6
Using our gamma-ray spectrum analysis software such as GammaVision
®
or ISOTOPIC.
