Let’s assume gaseous ionization detectors. A basic gaseous ionization detector consists of a chamber filled with a suitable medium (air or a special fill gas) that can be easily ionized. As a general rule, the center wire is the positive electrode (anode), and the outer cylinder is the negative electrode (cathode), so that (negative) electrons are attracted to the center wire. Positive ions are attracted to the outer cylinder. The anode is at a positive voltage for the detector wall. As ionizing radiation enters the gas between the electrodes, a finite number of ion pairs are formed. Under the influence of the electric field, the positive ions will move toward the negatively charged electrode (outer cylinder), and the negative ions (electrons) will migrate toward the positive electrode (central wire). Collecting these ions will produce a charge on the electrodes and an electrical pulse across the detection circuit. However, it is a small signal. This signal can be amplified and then recorded using standard electronics.
Signal-to-noise ratio, SNR, is a measure used in science and engineering that compares the electrical output signal to the electrical noise generated in the cable run or the instrumentation.
Signal-to-Noise Ratio – Germanium Detectors
Total absorption of a 1 MeV photon produces around 3 x 105 electron-hole pairs, which is minor compared to the total number of free carriers in a 1 cm3 intrinsic semiconductor.
Particle passing through the detector ionizes the atoms of the semiconductor, producing the electron-hole pairs. But in germanium-based detectors at room temperature, thermal excitation is dominant. It is caused by impurities, irregularity in structure lattice, or by dopant. It strongly depends on the Egap (a distance between valence and conduction band), which is very low for germanium (Egap= 0.67 eV). Since thermal excitation results in the detector noise, active cooling is required for some types of semiconductors (e.g., germanium).
Note that a 1 cm3 sample of pure germanium at 20 °C contains about 4.2×1022 atoms, about 2.5 x 1013 free electrons, and 2.5 x 1013 holes. As can be seen, the signal-to-noise ratio (S/N) would be minimal. Adding 0.001% of arsenic (an impurity) donates an extra 1017 free electrons in the same volume, and the electrical conductivity is increased by 10,000. The signal-to-noise ratio (S/N) would be even smaller in doped material. Because germanium has a relatively low band gap, these detectors must be cooled to reduce the thermal generation of charge carriers (thus reverse leakage current) to an acceptable level. Otherwise, leakage current-induced noise destroys the energy resolution of the detector.