Due to the fact that pulse rise time is coincident with the fall time of the previous pulse in the Preloaded Filter pulse processor, it has a definite throughput advantage over all other pulse shaping methods. So, for the first time, it was possible to achieve throughput rates of60Co after pileup rejection in excess of 100 kc/s, at substantially better resolutions than those of other high rate shaping amplifiers.
Based on a functional description of the standard Ge(Li) spectrometer some of its shortcomings at high counting rates are
discussed and, as a possible solution to the problem, an outline is given of an experimental high rate gamma spectroscopy
with real time compensation of counting losses.
Reviewing the current status of real-time correction of counting losses in nuclear pulse spectroscopy, the pileup problem
is identified as the last question not resolved satisfactorily up to now. Correction of pileup losses in provided, at least
in principle, by the classical pulse generator method, however, severe limitations in test frequency prohibit its application
to real-time correction of counting losses. A solution is offered by the novel principle of the virtual pulse generator which
obviates the shortcomings of the classical method simply by not introducing pulses into the spectroscopy system. Instead,
the probability for pileup-free pulse processing is determined by suitable tests of the system status at arbitrarily high
test frequencies. After a discussion of the principles of the new method and its application to a real-time correction system
experimental evidence is provided for the complete correction of counting losses of more than 98% under conditions of stationary
as well as variable counting rates up to the limit of stable operation of the underlying spectroscopy system which is 800
000 c/s for an experimental high-rate gamma spectrometer.
An overload resistent input stage with PA step response equalizer, automatic self-alignment of previous pulse subtraction gain, and a novel automatic discriminator are valuable improvements for the Preloaded Filter (PLF) pulse processing system. Measurements with an RC-type preamplifier also make clear that the PLF gives superior results not only with transistor reset preamplifiers.
High rate, high resolution gamma-spectrometry with real-time correction of counting losses is made possible by combining the novel Preloaded Filter (PLF) pulse processor with the Virtual Pulse Generator (VPG) counting loss correction method. A spectrometry system for high-speed activation analysis based on the PLF processor, VPG correction and a high resolution LOAX detector is tested up to 850 kc/s.
Two commercially available digital filters with selectable, fixed time constants and triangular pulse response are discussed
to outline their potential advantages over traditional analog filters with semi-Gaussian pulse shape. A solution for the “resolution
or throughput rate” dilemma is offered by the preloaded digital filter fulfilling the postulate for the ideal adaptive filter
with optimum resolution at any counting rate. Throughput rates of >100 kc/s are demonstrated for the preloded digital filter
at resolutions superior to those of fixed shaping time filters.
By adapting noise filtering to individual pulse intervals, the Preloaded Filter (PLF) pulse processor (1) combines high resolution with optimum throughput efficiency. As a consequence, its output pulse interval distribution contains strong non-random components which render conventional ADC dead-time correction an impossibility. Quantitative correction of dead-time and pileup losses of the PLF processor may be achieved, however, with the Virtual Pulse Generator (2), together with a new, distribution-independent method of measuring ADC losses which is based on a pulse counting technique.
Quantitative gamma spectrometry at high counting rates will grow to become an accepted analytical tool as soon as pulse processing systems are available which combine maximum throughput, high resolution and best possible suppression of pulse pileup induced background. A high efficiency pileup rejector, analog FIFO memories and the novel Preloaded Filter pulse processing system will be presented as potential steps to the achievement of this goal. Net Throughput Rate/Resolution is proposed as a new measure of performance in high counting rate spectrometry, and is applied to the comparison of different systems to show that a threefold improvement upon the conventional Gated Integrator is possible.
Pileup losses in nuclear pulse spectrometry also depend on energy as lower energies produce narrower pulses which in turn
have better chances to avoid pulse pileup. Consequently, in our present system individual energy-dependent pileup correction
factors are calculated for all events, making it what very probably may be called the first perfect implementation of Loss-Free
Counting. Temporal response and quantitative performance of the new system are tested over the whole range of counting rates
(up to 106 c/s) and counting losses (up to 99%) by means of short-lived isomeric transitions and a fast rabbit system.