Amplifier pole-zero cancellation

Some high specification amplifiers have built-in facilities to help with pole-zero cancellation. For example, the ORTEC 672 Spectroscopy Amplifier provides automatic cancellation at the press of a button. Some amplifiers provide LED over/under indicators to assist with the adjustment. Nevertheless, because of the importance of this setting I would advocate that every gamma spectrometry laboratory should have ready access to an oscilloscope. With experience, a quick look at the pulses coming from the preamplifier or the amplifier can quickly reassure one that everything is normal, or lead one to a solution if it isn t. 

Points to note are the pole-zero cancellation circuit at the start of the chain, so that the amplifier is matched to the... 

There are, however, difficulties. Accurate correction for pulse loss depends upon the pulser pulses accurately mimicking the detector pulses. The rise time and the fall times of the pulser pulses should be identical to those of detector pulses. Leaving aside the fact that preamplifier output pulses have a variable rise time, none of the readily available pulsers allow detailed control of the fall time. Bear in mind that the fall time of the preamphfier pulses depends upon the time constant of the feedback circuit in the preamplifier, and that pole-zero cancellation within the amplifier matches the shaping circuits to the input pulse fall time. The consequence of this is that it may not be possible to pole-zero correct the pulser pulses and the detector pulses together. At anything more than a low count rate, many detector pulses may be incorrectly measured by the ADC if they occur close in time to a pulser pulse. 

As with the preamplifier, the scintillation amphfier need not be of such a demanding low noise specification as would be needed for semiconductor systems. In the manufacturers catalogues, a distinction is commonly made between amplifier , suitable for low-resolution spectrometry, and spectroscopy amplifier intended for high-resolution spectrometry using semiconductor detectors. Typical simple amplifier modules provide pole-zero cancellation and automatic base line restoration. The pulse shaping time options provided are often limited on such instruments and may need to be selected internally. Because of the faster rise time of scintillation pulses, the time constants provided are usually within the range 0.2 to 2 or 3 (its. 

An oscilloscope for looking at pulses. Unless your amplifier has automatic correction built in to it, this item is essential for pole-zero cancellation. The specification does not need to be high, but must have a 50 MHz band-width, the ability to measure rise times down to a few nanoseconds and have a voltage sensitivity of 5mV per division. An oscilloscope with a storage option is a bonus, allowing a more considered examination of pulses. 

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. 

Using a spectroscopy amplifier with carefully adjusted pole-zero cancellation and a good multichannel analyzer, collect a spectrum of Co-60. In the central channel of the 1.3 MeV peak, at least 10,000 counts should be collected. 

Note that the transistor reset preamplifier with its step pulse output has no need for PZ adjustment. If such a preamplifier is used, the pole-zero potentiometer should be adjusted to its fully anticlockwise position which corresponds to infinite time constant. Certain amplifiers provide a switch to disable the PZ cancellation. 

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