The rise time setting can be optimized separately to achieve the best count rate/resolu
-
tion compromise. However, the optimum flat top for a detector depends somewhat on
the rise time selection.
Therefore, the best correction for ballistic deficit will be achieved by checking the flat
top setting if the rise time is increased or decreased by a factor of two or more.
Triangular Shaping
Triangular shaping may give enhanced resolution performance for small detectors hav
-
ing little variability in charge collection time. To set the unit for triangular shaping,
adjust the rise time to the desired value and set the flat top to zero.
Trade-Offs
The processing time (Shaping) is set by the Rise Time and Flat Top selections and is
generally a compromise between optimizing throughput and resolution. Having the
ability to independently set the Rise Time and Flat Top allows greater flexibility when
optimizing the processing time or shaping for a wide variety of detector applications.
As in any signal processing application, a performance tradeoff exists between high
resolution and high throughput.
For example when using a small Ge detector, 5.6 µs rise time and 0.8 µs flat top set-
tings provide optimum resolution over a wide range of count rates. However, a 2.8 µs
rise time and 0.6 µs flat top will degrade low count rate resolution performance
slightly, but results in less resolution broadening and peak shift over a much wider
count rate range.
For ultra high counting and throughput rates, rise time and flat top settings of less than
1 µs may be used. For this case, optimum resolution is traded off for increased count
rate performance. For high resolution detectors, longer rise time settings offer a better
signal to noise (S/N) ratio and longer flat top settings reduce the effects of ballistic
deficit. However, as the system count rate increases, resolution may degrade more rap
-
idly due to increased processing time and the effects of pulse pile-up.
For most Ge detector applications, digital trapezoidal shaping provides Gaussian
equivalent resolution with half the processing time. Faster processing time means the
Lynx DSA provides significantly greater throughput than a traditional analog system
with its processing or shaping times set for equivalent resolution.
When using small Ge detectors which are optimized for high count rate performance,
throughputs of 100 kcps can be achieved. To achieve 100 kcps and higher throughput,
the highest spectral peak must not exceed 80% of full scale.
Appendix C - Shaping Adjustments
190 Lynx™ Digital Signal Analyzer
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