Canberra Lynx - C. Shaping Adjustments; Optimizing Processing Time; Shaping

C. Shaping Adjustments
Optimizing Processing Time
The digital filter employed in the Lynx DSA has a Triangular/Trapezoidal weighting
or shaping function.
Shaping
Shaping is adjusted by selecting the Rise Time and Flat Top, which determine the
Trapezoid pulse shape and optimizes performance for the specific detector, spectral
energy range and count rate. These adjustments, together, may also be referred to as
Ballistic Deficit Correction.
Note For more information on count rate performance, refer to Canberra’s Appli
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cation Note “Performance of Digital Signal Processors for Gamma Spectros
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copy”. Please contact your sales representative to request a copy.
Rise Time
The Rise Time sets the noise filtering characteristics of the Digital Filter.
Flat Top
The Flat Top allows for the charge collection time of the particular detector. Independ-
ent adjustment of the flat top allows the shaping function to be optimized for detectors
with long charge collection time, without a large increase in the overall processing
time.
For small detectors with minimal charge collection time variation or ballistic deficit,
the trapezoidal shape reduces to triangular shaping when the Flat Top is set to mini
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mum or zero. The triangular/trapezoidal shaping function is symmetrical. The fall time
cannot be set independently; it always equals the Rise Time selection.
Independent Adjustment
The Lynx DSA allows independent selection of rise time and flat top. A detector
with long charge collection times will require a flat top long enough to process all the
charge from the detector. If the flat top is too short, it may result in low side spectral
tailing and degraded resolution (However, if these symptoms occur at high rates only,
the pole zero setting may be misadjusted. In this case, first verify the correct P/Z set
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ting and readjust if necessary).
To set the flat top manually, start with a long value, then collect a spectrum and verify
good resolution and peak symmetry. Reduce the flat top and repeat the process. Con
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tinue until resolution and peak symmetry begin to degrade, then set the flat top to the
next higher value. The optimal (shortest) flat top will allow the best throughput.
User’s Manual 189