DSPEC 50
®
and DSPEC 502
®
Digital Gamma-Ray Spectrometer User’s Manual
932502G / 0618
1.2. Ballistic Deficit and Adjusting the Flattop Duration
In germanium detectors it takes a variable amount of time to collect all of the charge released in
the detector diode when a gamma ray deposits energy in the detector. The duration of this charge
collection time depends on the dimensions of the detector, the geometry of the electrodes, and
the points at which energy is deposited in the detector. A small, planar detector has charge col-
lection times that vary from 50 ns to 100 ns, whereas a large coaxial detector is characterized by
charge collection times that vary from 100 ns to 700 ns. The variability of the charge collection
time within a specific detector is the source of the resolution degradation described by the
ballistic deficit effect
.
The ballistic deficit becomes a significant source of resolution degradation when very short
shaping times are selected in order to reduce dead time and improve the high counting-rate limit.
For a digital filter having the shape of a symmetrical triangle in the time domain, the output
pulse begins to rise as soon as the gamma ray is detected. It continues to rise until it reaches a
peak amplitude at a time specified by the currently selected rise/fall time (this is the
Rise Time
parameter entered on the Amplifier 2 tab under
Acquire/MCB Properties...
; see Section 4.2).
Beyond this peak-amplitude time, the pulse falls back to the baseline to allow a subsequent
gamma ray to be processed. If all the charge has not been collected by the detector by the desig-
nated time for peak amplitude, there will be a deficit in the measured peak amplitude and a
broadening of the pulse width. The deficit in amplitude is called the
ballistic deficit
. Obviously,
events that result in a faster charge collection time will suffer less deficit and less increase in
pulse width than those yielding a slower charge collection time, even though the deposited
energy was identical. Thus, the ballistic deficit resulting from variable charge collection times
broadens the energy resolution for that gamma ray.
If the longest charge collection time is negligible compared to the rise time of the filter pulse, the
ballistic deficit will be imperceptible, and the energy resolution will not be degraded. Thus, at
the 10- to 20-ìs rise times typically selected for optimum signal-to-noise ratio (i.e., minimum
peak FWHM at low energies) the ballistic deficit problem can be ignored.
Long rise times create higher dead times and depress the maximum counting-rate limit. If it is
necessary to accept higher counting rates, the rise/fall times must be shortened accordingly. For
this case, the DSPEC 50 includes a flattop feature for the filter that can eliminate the resolution
broadening caused by ballistic deficit. Instead of a symmetrical triangle with a sharp point at the
peak amplitude, the rising and falling edges are separated by a flattop to form a trapezoid. The
width of the flattop is adjusted to be long enough to match the longest charge collection time for
the detector employed. This allows time for the longer-charge-collection pulses to reach the
same maximum pulse amplitude as the faster-charge-collection pulses from gamma rays of the
same energy. Consequently, the effect of ballistic deficit is reduced, and the energy resolution is
dramatically improved at these short pulse widths.
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