ORTEC DSPEC Pro, Руководство пользователя по оборудованию

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794380D / 0914 1. INTRODUCTION
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peak amplitude, the rising and falling edges are separated by a flattop to form a trapezoid. The
width of the flattop is adjusted to be long enough to match the longest charge collection time for
the detector employed. This allows time for the longer-charge-collection pulses to reach the
same maximum pulse amplitude as the faster-charge-collection pulses from gamma rays of the
same energy. Consequently, the effect of ballistic deficit is reduced, and the energy resolution is
dramatically improved at these short pulse widths.
Note that the selection of shorter pulse widths inevitably degrades the signal-to-noise ratio com-
pared to the optimum achieved at longer pulse widths. Thus, operating at shorter pulse widths
will compromise the energy resolution of low-energy gamma rays, for which the preamplifier
noise makes a significant contribution to the energy resolution. The signal-to-noise degradation
is independent of the ballistic deficit effect.
The flattop and other shaping controls are on the Amplifier 2 tab under Acquire/MCB Proper-
ties...; see Section 2.3.2 for a more detailed discussion.
1.6. 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,
is now corrected for dead-time losses. The standard
deviation in that counting rate is .
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