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Pulse Electronics

The nuclear electronics industry has standardized the signal definitions, power supply voltages and physical dimensions of basic nuclear instrumentation modules (NIM). The standardization provides users with the ability to interchange modules, and the flexibility to reconfigure or expand nuclear counting systems, as their counting applications change or grow. Basic electronic principals, components and configurations which are fundamental in solving common nuclear applications are discussed below.

Preamplifiers and Amplifiers

Most detectors can be represented as a capacitor into which a charge is deposited, (as shown in Figure below ). By applying detector bias, an electric field is created which causes the charge carriers to migrate and be collected. During the charge collection a small current flows, and the voltage drop across the bias resistor is the pulse voltage.

Figure : Basic Detector and Amplification

The preamplifier is isolated from the high voltage by a capacitor. The rise time of the preamplifier’s output pulse is related to the collection time of the charge, while the decay time of the preamplifier’s output pulse is the RC time constant characteristic of the preamplifier itself. Rise times range from a few nanoseconds to a few microseconds, while decay times are usually set at about 50 microseconds.

Charge-sensitive preamplifiers are commonly used for most solid state detectors. In charge-sensitive preamplifiers, an output voltage pulse is produced that is proportional to the input charge. The output voltage is essentially independent of detector capacitance, which is especially important in silicon charged particle detection, since the detector capacitance depends strongly upon the bias voltage. However, noise is also affected by the capacitance.

To maximize performance, the preamplifier should be located at the detector to reduce capacitance of the leads, which can degrade the rise time as well as lower the effective signal size. Additionally, the preamplifier also serves to provide a match between the high impedance of the detector and the low impedance of coaxial cables to the amplifier, which may be located at great distances from the preamplifier.

The amplifier serves to shape the pulse as well as further amplify it. The long delay time of the preamplifier pulse may not be returned to zero voltage before another pulse occurs, so it is important to shorten it and only preserve the detector information in the pulse rise time. The RC clipping technique can be used in which the pulse is differentiated to remove the slowly varying decay time, and then integrated somewhat to reduce the noise. The unipolar pulse that results is much shorter. The actual circuitry used is an active filter for selected frequencies. A near-Gaussian pulse shape is produced, yielding optimum signal-to-noise characteristics and count rate performance.

A second differentiation produces a bipolar pulse. This bipolar pulse has the advantage of nearly equal amounts of positive and negative area, so that the net voltage is zero. When a bipolar pulse passes from one stage of a circuit to another through a capacitor, no charge is left on the capacitor between pulses. With a unipolar pulse, the charge must leak off through associated resistance, or be reset to zero with a baseline restorer.

Typical preamplifier pulses are shown in Figure .

Figure: Standard Pulse Waveforms

The dashed line in the unipolar pulse indicates undershoot which can occur when, at medium to high count rates, a substantial amount of the amplifier’s output pulses begin to ride on the undershoot of the previous pulse. If left uncorrected, the measured pulse amplitudes for these pulses would be too low, and when added to pulses whose amplitudes are correct, would lead to spectrum broadening of peaks in acquired spectra. To compensate for this effect, pole/zero cancellation quickly returns the pulse to the zero baseline voltage.

The bipolar pulse has the further advantage over unipolar in that the zero crossing point is nearly independent of time (relative to the start of the pulse) for a wide range of amplitudes. This is very useful in timing applications such as the ones discussed below. However, the unipolar pulse has lower noise, and constant fraction discriminators have been developed for timing with unipolar pulses.

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