The Automatic Pole/Zero (P/Z) feature will give good results for most detector appli-
cations and count rates. However, it may be necessary to optimize the P/Z compensa-
tion manually at extremely high count rates or for specific applications where the
digitally filtered trapezoid signal is prevented from returning monotonically to the
baseline. The feedback resistor on some RC preamps may exhibit non-ideal character-
istics which produce multiple time constants making the tail pulse fall time non-
monotonic. This behavior may become problematic at high count rates causing signifi-
cant baseline perturbations and resolution degradation. NaI detectors may have multi-
ple time constants due to AC coupled preamps and the scintillator interactions. In
these situations it might be possible to minimize performance degradation by fine tun-
ing the pole/zero manually. To adjust the Pole/Zero refer to “Manual Pole/Zero
Matching” on page 41.
To initiate the Auto Pole/Zero process, please follow the directions below:
1.
Adjust the
60
Co radioactive source for an incoming count rate (ICR) between
1 and 20 kHz. The Auto Pole/Zero may fail to converge if the incoming
count rate is not within this count rate range. The incoming count rate can be
verified by looking at ICR on the
MCA | Adjust | Status Page
of the
Acquisition and Analysis window. Select “Update” to update the ICR Status
whenever the radioactive source is adjusted.
Note
Although any radioactive source may be used, the most accurate adjustment is
obtained using simple sources such as
57
Co,
137
Cs or
60
Co. The Auto P/Z oper-
ates properly with spectral peaks are located within 25% to 100% of the sys-
tem dynamic range. However best performance is obtained with the system
gain adjusted to place the primary peaks within the top 75% to 95% of the sys-
tem dynamic range. These recommendations also apply when adjusting the
Pole/Zero manually on systems without Auto Pole/Zero. Once completed, re-
place the calibration source with the sample to be analyzed and adjust the sys-
tem gain as required.
2.
In the
MCA | Adjust
screen, click on the FILTER button, then click on the
Next button until the Adjust screen with the Auto P/Z button appears.
3.
Click the AUTO P/Z “Start” button, to start the Auto Pole/Zero. While the
pole/zero is converging, “Wait” will be posted in the Adjust screen's Status
box.
The pole/zero value can be viewed on the Filter Device Adjust screen under
“Pole/Zero” or on the Status Page. The pole/zero value must be updated each
time an Auto P/Z is performed or when the slider bar is changed. To update
the pole/zero value momentarily switch the Device Adjust screens by
clicking on the Stab button or the Gain button, then back to the Filter Device
Adjust screen.
User’s Manual - ICN 9231014G
39
Detector Matching
