Rise time preamplifiers

The light decay time constant in Nal is about 0.25 (is. Typical charge sensitive preamplifiers translate this into an output pulse rise time of about 0.5 (is. Fast coincidence measurements cannot achieve the very short resolving times that are possible with plastic, especially at low gamma ray energies. 

The rise time T, of the pulse generated by a semiconductor detector can be measured at the output of a charge-sensitive preamplifier. If the preamplifier is sufficiency fast, T, is determined by the following factors ... 

The anode of a photomultiplier tube is connected by a resistor of R = kQ to ground. The stray capacitance is lOpf, the current amplification 10, and the anode rise time 1.5 ns. What is the peak amplitude and the halfwidth of the anode output pulse produced by a single photoelectron What is the dc output current produced by 10 W cw radiation at A = 500nm, if the quantum efficiency of the cathode is 77 = 0.2 Estimate the necessary voltage amplification of a preamplifier (a) to produce 1V pulses for single-photon counting and (b) to read 1 V on a dc meter of the cw radiation ... 

To give this a practical perspective, if we consider a preamplifier pulse with a rise time of, say, 200 ns, and were to transmit it along a 2 m cable the pulse would be slow (2m X 4/200ns is much less than 1). If the cable were 200 m long, the pulse would be fast and careful... 

Figure 4.7 Shape of the output pulse from a resistive feedback preamplifier (a) definition of rise time and fall time (b) actual rising edge shapes derived from a 45 % detector...

Ideally, we wish to collect the charge as quickly as possible and it would not be satisfactory if the preamplifier itself limited that process. The specification of a preamplifier will include a statement of the rise time of its output, again related to the input capacitance (e.g. < 20 ns at 30 pF input capacitance for the instrument referred to above). It is sufficient if this is small compared to the rise time of the detector pulses so that the effective rise time is determined by the detector, not by the preamplifier. 

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. 

We saw in Chapter 4, Figure 4.7 that this type of preamplifier provides an output pulse with a very fast rise-time... 

A pulse generator that can simulate detector pulses is perhaps not essential but is certainly desirable. It should provide pulses with variable rise time (perhaps 10 to 500 ns) and variable fall-time (perhaps 10 to 500 j,s). The output from the pulser is put into the TEST INPUT of the preamplifier. It allows a distinction to be made between detector problems and pulse processing problems. Some laboratories routinely use a pulser for dead time and random summing correction purposes. For systems with TRP preamplifiers, a simpler pulser providing square pulses would be adequate. 

In the measurement of short drift times, sometimes the condition that the rise time of cell and preamplifier be much smaller than t is not fulfilled. In such a case, the convolution of the excitation functions p(t) and the response function is calculated... 

The tail of the pulse passing through the differentiator forces the output to fall below and then rise towards the baseline with a time constant equal to that of the feedback circuit in the preamplifier. This is then transmitted through the integrator to the output pulse. A second pulse following close behind the first may find itself in the... 

---