Signal pulses from a gas-based detector (e.g. proportional detector, wire drift chambers, BF3, He3) 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 gas-based detector during the pulse event.
Connecting a proportional detector to the CSP:
The usual connection scheme of a proportional detector to the CSP is shown below, and is known as an 'AC coupled' circuit:
The charge sensitive preamplifier (CSP) shown in the figure above is shown as an 8 pin module. Cremat offers four different CSP modules (CR-110, CR-111, CR-112, and CR-113) which differ primarily by their gain. Proportional detectors will likely be best served by the CR-110, which has the lowest noise and greatest gain of the available models.
Adding protection to the preamplifier input
In the circuit shown above, the series resistor Rprotection has been added to help protect the preamplifier from the sparking that often occurs with gas-based detectors. This resistor (in the range from 100Ω to 220Ω) limits the current flowing to the preamplifier during this rapid discharge. Increasing this resistance increases the protection. This resistor, however, adds a small amount of electronic noise and also may limit the pulse rise time from the preamplifier.
A proportional detector based setup
An overview of the instrumentation for a gas-based (e.g. proportional detector) setup is shown below. To preserve low noise operation, we highly recommend keeping the cable between the detector and CR-Z-110 CSP as short as possible.
Given this overview, there are a number of choices and refinements still to be made. These are described below.
Do you need bias voltages in excess of 500V?
Cremat's CR-Z-110 and CR-Z-111 CSP instruments use BNC connectors at the 'bias in' and 'input' connections. BNC connectors are rated at 500V maximum. If your gas-based detector requires a bias greater than 500V you should use the '-HV' option which uses SHV connectors at these positions. The CR-Z-11X-HV instrument is rated at 2000V.
Choice of shaping time in the CR-S 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 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 short enough to accommodate the expected counting rate of the detection system.