Optimizing the Flat Top on the 9660 is automatic and easy; manual adjustment is
not
necessary. Automatic adjustment of the Flat Top time is performed by initiating the
Auto BDC (ballistic Deficit Correction) function. A sophisticated algorithm measures
the detector pulses, determines the range of detector rise times, and sets the digital fil-
ter trapezoid flat top for full charge integration. After the 9660 has adjusted the flat
top, variations in detector rise time will not affect the output.
The instructions presented below assume the 9660 is set up as outlined in “Spectros-
copy System Setup” on page 34. The BDC does not change the Rise Time setting; use
the 5.6
µ
s default value or set as desired. For additional information on setting the Rise
Time please refer to Appendix B, “Performance Adjustments”.
To initiate the Auto BDC process, perform the following steps:
1.
Adjust the
60
Co radioactive source for an incoming count rate (ICR) between
1 and 20 kHz. The Auto BDC may fail to converge if the incoming count
rate is not within this count rate range. The ICR can be verified by viewing
the Status Page. Select
MCA | Adjust | Status
from the Acquisition and
Analysis window, then “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 BDC op-
erates properly with spectral peaks located within 25% to 100% of the system
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 system
dynamic range. Once the Auto BDC has completed, replace the calibration
source with the sample to be analyzed and adjust the system gain as required.
2.
Initiating Auto BDC:
Note: The Gain and Pole/Zero should be set prior to initiating the Auto BDC
function.
Select the Filter Device Adjust screen (under the MCA | Adjust menu) and
press the AUTO BDC “Start” button. The BDC BUSY LED on the 9660 will
illuminate for the duration of the process. During this time, normal operation
of the 9660 is suspended while detector rise time data is being acquired.
Note
Spectral data acquired during the Auto BDC process may
be corrupted and should be discarded or cleared when the
auto BDC Process has completed.
Upon completing a successful BDC and setting of the Trapezoid Flat Top,
the BDC BUSY LED will extinguish. Verify the new Flat Top setting. The
Filter Device screen must be updated to show the new value, momentarily
User’s Manual - ICN 9231014G
45
Detector Matching