Amplifier high count rate

Proportional counter This radiation detector that produces a measurable amplified voltage pulse of height proportional to the energy of photons hitting it it gives a linear response at high counting rates. 

Figure 4.31 Pileup distortion at high counting rates. A pure iron specimen has been fluoresced to yield a counting rate of 44,000 counts/s at the detector, (a) A multiple-trace picture of the slow amplifier output on an oscilloscope first- and second-order pileup is visible, (b) The energy spectrum recorded without a pileup rejector, (c) The spectrum recorded using a pileup rejector. (Reprinted by courtesy of EG G ORTEC.)...

In this context, a high count rate would be when the duty cycle of the amplifier is more than 20 % and a low count rate when below 5 %. Duty cycle is the proportion of the time when the amplifier is busy and, assuming that the amplifier dead time is 6 times the shaping time constant, it can be estimated (as a percentage) as ... 

The reader should remember that throughput will also be constrained by the amplifier and that, at very high count rates, a resistive feedback preamplifier may lock up. Throughput of complete systems is discussed in Chapter 14. 

This is also caUed 1/f noise. It is associated with variations of direct current in all active devices, such as carbon resistors. The magnitude of this source of uncertainty is a function of current through the detector and on the effective frequency of the signal. Fortunately, it is independent of amplifier shaping time and is small compared to series and paraUel noise, but does increase at high count rates. 

In Chapter 4, I discussed random summing in connection with the pile-up rejection circuitry in amplifiers. We came to the conclusion that even with pile-up rejection there must be some residual random coincidences. There is then, whether or not pile-up rejection is available, a need to be able to correct for random summing in high count rate spectra. In some circles, there seems to be an assumption that pile-up rejection is 100% effective... 

Usually factory preset. First set main-amplifier PZ, then set prcamplilicr PZ to give stable baseline. Do these tests at high count rate. 

Figure 14.3 Throughput curves for two complete spectrometry systems using a transistor reset preamplifier (continuous line) and a resistive feedback preamplifier (dashed line), with each feeding the same high count rate amplifier and ADC (reproduced by permission of Canberra Nuclear)...

The essential functions of an amphfier were discussed in Chapter 4, Section 4.4. I suggested there that pulse processor would be a more appropriate name for this item than the historic amplifier . This is particnlarly true when considering high count rate systems. The data in Table 14.1 showed us that the pulse processor is the critical restraint on pulse throughput, mainly dne to pulse pUe-up (random summing) within it. The high cost of... 

The amplifier should be chosen carefully. This is likely to be the bottleneck at very high count rates. 

Besides the three major parts, modern Mossbauer spectrometers, to satisfy the requirement of high count rates, also utilize a fast pre-amplifier, a multichannel analyzer (which is now generally replaced by a computer or microprocessor), a calibration system (which is normally an interferometer) and a stable cryostat (such as simple He baths, He-flow cryostats, and He/" He refrigerators) during plotting of the Mossbauer spectrum. 

Coincidence and Dead-time Losses in y-Spectrometry. The influence of electronic effects at high-count rates on the performance of Ge(Li) detectors is considerable. The resolution of a detector can be degraded by effects within the amplification system, but these can be minimized by (i) the use of pole-zero cancellation, to prevent the pulse-height error caused by the tail of a preceding pulse and (n) baseline (or D.C.) restoration facilities to prevent similar errors caused by shifts in the apparent pulse baselines. The latter are a result of capacitative effects between the various stages of the overall amplifier, biased amplifier, and multi-channel analyser system. These effects can degrade the resolution of the detector but should not change the y-ray peak area. 

However, at high-count rate the probability of the loss of counts from a y-ray peak owing to the coincidence between two y-ray pulses detected at the same (or nearly the same) instant becomes important and has a significant effect on the accuracy of the measurement. Such losses cannot be accounted for by dead-time correction. It is important to realise that the coincidence loss depends not on the count rate at the multi-channel analyser, which might be low if a biased amplifier is used to select a region of interest, but on the total y-ray count rate at the detector. Since coincidence losses are rate dependent, samples and standards should be of comparable intensity or errors will result. 

Figure 5.6. Diagram of a low-energy, high-angle electron-impact spectrometer. (A) Electron gun (B) monochromator (180° spherical electrostatic energy selector) (C) electron optics (D) scattering chamber (E) analyzer (180° spherical electrostatic energy selector) (F) electron multiplier (G) amplifier and pulse discriminator (H) count-rate meter (I) multichannel scaler (J) X Y recorder (K) digital recorder. (After Kupperman et a/.<42))...

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