Signal pulses from a PMTs can usually best be read out using a charge sensitive preamplifier (CSP). This is due to the low noise characteristics of CSPs, as well as the integrating nature of the output signal which provides an output proportional to the total charge flowing from the PMT anode during the pulse event.
Just a couple examples of pulse-detection applications using PMTs are the detection of radiation events from scintillators, or the detection of laser pulses.
Connecting a PMT to the CSP:
The connection scheme of a photomultiplier tube to the CR-Z-PMT CSP is shown below:
This PMT biasing scheme directly connects the anode to the input of the preamplifier. In the case where the biasing of the PMT is made with the photocathode at ground and the anode at positive bias, then a HV blocking capacitor is necessary between the anode and the preamplifier input.
A PMT based detection setup
An overview of the instrumentation for a PMT detection setup is shown below. To preserve low noise operation, we highly recommend keeping the cable between the PMT and CR-Z-PMT CSP as short as possible.
Choice of shaping time in the CR-S-X shaping amplifier
In the diagram above, the CR-S-1us shaping amplifier is used. This shaping amplifier uses a 1 microsecond shaping time, and is a 'middle of the road' choice for many applications. There are, however, some guidelines to help you determine the best shaping time for your application:
1) Chose the shaping time that will give you a minimum of electronic noise. As was mentioned above, shaping amplifiers are electronic (band pass) filters - the shaping time is related to the band pass frequency. There are a few sources electronic noise in a typical detection system, and some of them differ in their power spectra. While it is outside the scope of this guide to quantify the total electronic noise or provide complicated formulae for calculating the noise minimum, there generally is a noise minimum within the range of available shaping times. Changing the shaping time of the CR-S-X instrument is a simple matter of swapping out the installed CR-200-X shaping amplifier module for that of another shaping time, so determining the noise minimum could be done experimentally.
2) Choose a shaping time that is long enough to collect all the charge from the PMT. While PMTs are fast detectors, the photon signal may have a duration lasting microseconds. For example, detection of a light signal from a CsI(Tl) scintillator occurs over a 2 microsecond duration. In this case and in other applications involving relatively slow signals, use a shaping time that is at least as long as the photon signal duration.
3) Choose a shaping time that is short enough to accommodate the expected counting rate of the detection system.