The simplest geometry of a proportional counter is a gas-filled cylindrical conductive tube with a coaxial thin wire as shown in Fig. 2.1. The wire is connected to a positive high voltage and coupled via a capacitor to a charge sensitive preamplifier. For the detection of X-rays, the cathode tube has to have a window, transparent to the required energy band. X-rays entering the detector volume through the window interact with the detector gas. At X-ray energies up to 50 keV the predominant interaction process is the photo effect. The photo effect cross section scales as ZnE- 3, where E is the X-ray energy and Z is the atomic number of the detector gas and n « 4-5. The number N of electron-ion pairs generated by this event can be written as N = E/W, where E is the energy of the absorbed X-ray photon and W the average energy for the creation of one electron-ion pair in the detector gas (usually a noble gas with an additive of a molecular gas like CO2 or CH4). The average energy for the creation of an electron-ion pair depends on the detector gas and is about 2530 eV. A 1 keV X-ray photon creates 30-40 electron-ion pairs. Electrons and ions drift in the electrical field of the detector to the anode wire and the cathode, respectively. If the electrons gain enough energy over a mean free path length to ionize the detector gas, charge multiplication takes place. The actual charge reduplicates on average after each ionizing collision of the electrons. This happens in the vicinity
Fig. 2.1 Single wire proportional counter of the anode wire, where the electrical field is in the order of 105 Vcm-1. In this cylindrical geometry, the electrical field strength as a function of the radial distance r from the tube center is: dU/dr = Uo/[r(lnrc/ra)] with Uo = anode wire voltage, ra = anode wire radius, rc = cathode tube radius. The movement of electrons and ions extracts energy from the electrical field generating displacement currents on anode and cathode. The electrons move about three orders of magnitude faster than the ions and the majority of the charge is generated only several mean free path lengths away from the anode wire. Therefore, the waveform of the output signal of the detector has a small fraction with a short rise time, contributed by the electrons. The main portion of the signal, with a rise time of 100 |is or more, is generated by the movement of the ions. Not only charge multiplication takes place in the avalanche, but also the generation of UV photons both by excitation of gas atoms and by the neutralization of positive ions on arrival at the cathode. UV photons hitting the cathode induce the emission of electrons from cathode surface, when the work function of the cathode material is less than the photon energy. These electrons in turn can cause subsequent avalanches possibly leading to a permanent discharge of the counter. The addition of several % of a polyatomic gas (quench gas) to the detector gas prevents this problem. Quench gases absorb UV photons emitted by the noble gas and convert them via radiationless transitions finally into heat. Via charge exchange quench gases reduce also the number of noble gas ions arriving at the cathode. Quench gases can speed up the drift velocity of electrons quite dramatically reducing the influence of gas impurities .
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