932502G / 0618 1. INTRODUCTION
! 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.
1.6.4. 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 under-
estimated.
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 propor-
tion 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 when-
ever the counting rate might change significantly during the measurement time. In addition to
the rapidly-decaying isotope example above, the CORR_ERR mode should be used when
monitoring cooling water flow from a nuclear reactor. The CORR_ERR mode accommodates
brief bursts of high-activity in the water flowing past the gamma-ray detector. Both the corrected
and error spectra are stored in the resulting spectrum file.
Note that the counts in the ZDT spectrum must be divided by the elapsed REAL time to compute
the dead-time corrected counting rate. It is important to note that the standard deviation in the
N
ZDT
counts in a gamma-ray peak in the ZDT spectrum is not . Instead the standard
deviation is obtained from the N
ERR
counts in the same peak ROI in the accompanying error
spectrum. The standard deviation in this case is . And the standard deviation in the
computed counting rate, , is .
13
To Next Page
Loading...
Need help?
General
