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®
ORTEC
EASY-NIM™ 928 Suite
IMPORTANT
These MDA presets are not dynamically calculated. Each time you add an MDA preset to this table,
its Correction Factor value is calculated and stored in the MCB's memory. If you then load a
different library, change the efficiency calibration, or change the system geometry, the
spectroscopy application will not update the existing Correction Factors, and your MDA presets
may no longer be applicable.
3.5.
Zero Dead-Time (ZDT) Mode
An extended live-time clock increases the collection time (real time) of the acquisition to correct for input pulse train
losses incurred during acquisition due to system dead time. This corrected time value, known as the live time, is then
used to determine the net peak count rates necessary to determine nuclide activities.
As an example, consider the case where the spectrometry amplifier and ADC are 60% dead during the acquisition. the
elapsed real time will be:
If the N counts in the gamma-ray peak in the spectrum are divided by the elapsed live time, the resulting counting
rate, N / Live Time, is now corrected for dead-time losses. The standard deviation in that counting rate is
SQRT(N) / Live Time.
Unfortunately, extending the counting time to make up for losses due to system-busy results in an incorrect result if
the gamma-ray flux is changing as a function of time. If an isotope with a very short half-life is placed in front of the
detector, the spectrometer might start out with a very high dead time, but the isotope will decay during the count and
the dead time will be zero by the end of the count. If the spectrometer extends the counting time to make up for the
lost counts, it will no longer be counting the same source as when the losses occurred. As a result, the number of
counts in the peak will not be correct.
When a supported ORTEC MCB operates in ZDT mode, it adjusts for the dead-time losses by taking very short
acquisitions and applying a correction in real time — that is, as the data are coming in — to the number of counts in
the spectrum. This technique allows the gamma-ray flux to change while the acquisition is in progress, yet the total
counts recorded in each of the peaks are correct. The resulting spectrum has no dead time at all — in ZDT mode, the
data are corrected, not the acquisition time. Thus, the net counts in a peak are divided by the real time to determine
the count rate.
ZDT mode has a unique feature in that it can store both the corrected spectrum and the uncorrected spectrum, or the
corrected spectrum and the uncertainty spectrum. Therefore, supported MCBs allow you to choose between three
ZDT Mode settings on the ADC tab under MCB Properties...: Off, NORM_CORR, and CORR_ERR.
28
Live
Time
Real
Time
1
0.60
Live
Time
*
100
%
%
Dead
100
%
Time
932512A / 0114
PROPERTY OF AMETEK, INC. – ORTEC
®
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