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amplifier. The zero stabilizer controls the offset bias level so the peak will be maintained in its original position.
For both functions, the input pulse-height-to-channel-number relationship is: where:
Intercept = The channel number of the zero-height input pulse
Gain
= The relation between pulse height and channel number (slope of the curve)
Changes in either the intercept or gain can affect the positions of all the peaks in the spectrum.
When used with the zero stabilizer, both the zero intercept and the gain (slope) will be monitored to keep all the peaks in the spectrum stabilized. The zero stabilization and gain stabilization are separate functions but both will affect the position of the peaks in the spectrum.
The stabilization operates by keeping a peak centered in an ROI you have defined. The ROI should be made symmetrically about the center of a peak with reasonably good count rate in the higher channels of the spectrum. The ROI should be about twice the FWHM of the peak. If the region is too large, counts not in the peak will have an effect on the stabilization.
Before setting either stabilization peak, the coarse and fine gains should be set to the desired values, and optimization or pole-zero performed.
5.2.11.4. ZDT (Zero Dead Time) 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:
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783620H / 1013 5. MENU COMMANDS
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 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
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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.
! Off — Uncorrected Spectrum Only
In this mode, only the uncorrected spectrum (live time and real time with dead-time losses)
— also called the live-time-corrected or LTC spectrum — is collected and stored in the .SPC
file. The LTC spectrum can be used to determine exactly how many pulses at any energy were processed by the spectrometer. The corrected spectrum gives the best estimate of the total counts that would have been in the peak if the system were free of dead-time effects.
The uncertainty spectrum can be used to calculate the counting uncertainty, channel by channel, in the corrected spectrum.
NOTE
When the spectrometer is placed in ZDT mode, the throughput of the instrument is reduced somewhat as extra processing must be done on the spectrum; therefore, if the gamma-ray flux is not changing as a function of time, but absolute highest throughput is desirable, you may wish to store only the LTC spectrum in the MCB memory.
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U.S. Patent 6,327,549.
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! NORM_CORR — ZDT and Uncorrected Spectra Stored
When the ZDT mode is set to NORM_CORR, the two spectra stored are the LTC spectrum and the ZDT spectrum (corrected for the dead-time losses; real time only). Unfortunately, in the analysis of the ZDT spectrum, the uncertainty of the measurement cannot be determined using either spectrum.
NOTE
This mode is not useful for quantitative analysis if the counting rate varies significantly during the measurement time, particularly if the user desires an accurate counting rate and standard deviation calculation. When you select the NORM_
CORR mode, GammaVision ignores the ZDT spectrum and analyzes the LTC spectrum as it would for the Off ZDT mode.
! CORR_ERR — ZDT and Error Spectra Stored
In the CORR_ERR mode, the estimation of the statistical uncertainty is stored in place of the LTC spectrum, and is referred to as the error spectrum (ERR). In this mode, the ZDT spectrum is used to measure the counts in a peak, and the error spectrum is used to determine the uncertainty of the measurement made in the corrected spectrum.
For example, if the area of a peak is measured in the corrected spectrum by summing channels 1000 to 1100, the variance of the measurement can be determined by summing the counts in channels 1000 to 1100 in the error spectrum. Or, shown another way, the counts in channel i can be expressed as ± with a 1-sigma confidence limit, where N is the corrected spectral data and V is the variance (error) spectral data.
The live time is set to the real time within the analysis engine during the analysis of ZDT spectra.
Table 1 shows which spectra are collected in the three possible ZDT modes.
Mode
Off (ZDT Disabled)
NORM_CORR (ZDT–LTC Mode)
CORR_ERR (ZDT–ERR Mode)
Table 1. ZDT Modes.
Uncorrected
Spectrum
Yes
Yes
No
ZDT Corrected
Spectrum
No
Yes
Yes
ZDT Error
Spectrum
No
No
Yes
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