Count-Rate Considerations

In gamma spectrometry you want to obtain the best data possible. In high count-rate applications, you have many counts in the spectrum, but other issues become relevant in making the choice of detector and the system electronics.

• Low — Below 100 cps
• High — Above 75,000 cps input rate
• Very High — Above 100,000 cps

A high-rate system must be operated at short shaping times to minimize the processing time per pulse. This will decrease the dead time and give maximum throughput. This shorter shaping time means the resolution is worse (peaks get wider) but not always significantly worse. Throughput is defined as the number of "useful" events stored in memory per second. Pulses which are too close together to be separated are called "pileup," and these can’t be used because you don’t know the separate energies of the pulses. The useful events (full energy peak area) is, of course, less than the total counts stored, but pileup events can’t be used and even degrade signal to noise ratio in the spectrum. So when you consider high count rates, you must count just the good counts not all the counts.

Standard ORTEC GEM/GMX preamplifiers have an energy rate limit of 145,000 MeV/sec while the LO-AX/GLP preamplifiers have a limit of 4000 MeV/sec. "Modified Resistor" GLP preamplifiers can be produced for special applications (e.g., safeguards) to a limit of 10,000 MeV/sec.

It is important to realize that pulsed reset preamplifiers do not saturate and are therefore an excellent choice if wide ranges of count-rate may occur (e.g., accident monitoring), but the reset process increases dead time. Thus, a reset preamplifier will produce fewer counts to memory than a resistor feedback preamplifier operating below its point of saturation.

The combination of the digital or analog shaping time chosen, the system processing dead time per pulse and the dead time due to the reset of the preamplifier (if not resistive), defines the system maximum throughput. Misleading claims are sometimes made in commercial literature about maximum achievable throughput of various electronic systems. However, the throughput limit is determined by the amplifier settings (or digital filter settings). These settings determine the dead time and resolution. So you select the settings based on the resolution you need, and this determines the throughput you can achieve.

By choosing the correct detector, you can improve the quality of the spectral data. You might think that choosing a small detector would give superior high count-rate performance. This might be true for certain low-energy applications where very good resolution at very short shaping times is important (Ref. 9), but this is not always the case. Recall Fig. 4. In this figure, you see that the large detector has "higher peaks and lower valleys." Thus, for throughput-limited work at intermediate to high energies, a collimated larger detector will produce better quality data than a smaller detector, even though both may have the same capability in terms of throughput to memory. The larger detector has a higher proportion of photopeak (good) events in its pulse stream than Compton background (bad) events in comparison to the smaller detector.

High Count-Rate "Rules of Thumb"

• What is the "worst" tolerable resolution? This defines amplifier shaping time, and thereby throughput limits.
• Low energy only (planar)? Planar detectors can operate at short shaping times; special resistor option can trade resolution for throughput with no reset losses as in TRP.
• High energy? Cconsider using a collimated large coax to improve the data quality.
  Fixed high rate situation? Adjust the count rate to operate at the point of maximum throughput.
• TRP (Plus) or resistive? TRP for wide dynamic count rate ranges (no saturation), but some loss of throughput.