Silicon-based semiconductor detectors are mainly used for charged particle detectors (especially for tracking charged particles) and soft X-ray detectors while germanium is widely used for gamma ray spectroscopy. A large, clean and almost perfect semiconductor is ideal as a counter for radioactivity. However, it is difficult to make large crystals with sufficient purity. The semiconductor detectors have, therefore, low efficiency, but they do give a very precise measure of the energy. Detectors based on silicon have sufficiently low noise even by room temperature. This is caused by the large band gap of silicon (Egap= 1.12 eV), which allows us to operate the detector at room temperature, but cooling is prefered to reduce noise. The drawback is that silicon detectors are much more expensive than cloud chambers or wire chambers and require sophisticated cooling to reduce leakage currents (noise). They also suffer degradation over time from radiation, however this can be greatly reduced thanks to the Lazarus effect.
Principle of Operation of Silicon Detectors
The operation of semiconductor detectors is summarized in the following points:
- Ionizing radiation enters the sensitive volume of the detector and interacts with the semiconductor material.
- Particle passing through the detector ionizes the atoms of semiconductor, producing the electron-hole pairs. The number of electron-hole pairs is proportional to the energy of the radiation to the semiconductor. As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band.
- Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit,
- This pulse carries information about the energy of the original incident radiation. The number of such pulses per unit time also gives information about the intensity of the radiation.
The energy required to produce electron-hole-pairs is very low compared to the energy required to produce paired ions in a gaseous ionization detector. In semiconductor detectors the statistical variation of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good. Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.
Properties of Silicon
There are many semiconductors in nature and others synthesized in laboratories; however, the best known are silicon (Si) and germanium (Ge). Silicon has the following properties:
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Purified silicon. Source: wikipedia.org License: Public Domain Silicon. Silicon is a chemical element with atomic number 14 which means there are 14 protons and 14 electrons in the atomic structure. The chemical symbol for Silicon is Si. Silicon is a hard and brittle crystalline solid with a blue-grey metallic lustre, it is a tetravalent metalloid and semiconductor.
- Silicon is mainly used for charged particle detectors (especially for tracking charged particles) and soft X-ray detectors. The large band-gap energy (Egap= 1.12 eV) allows us to operate the detector at room temperature, but cooling is prefered to reduce noise. Silicon based detectors are very important in high-energy physics. Since silicon-based detectors are very good for tracking charged particles, they constitute a substantial part of detection system at the LHC in CERN.
- The average energy for creation of electron-hole pair is 3.6 eV. This value is approximately 10 times lower compared to the ionization energy of gases used in proportional chambers, drift chambers and etc. And it leads to massive creation of charge carriers.
- Silicon has a high density of 2.329 g/cm3 and therefore the average energy loss per unit of length allows building thin detectors (e.g. 300 µm) that still produce measurable signals. For example, in case of minimum ionizing particle (MIP) the energy loss is 390 eV/µm.
The silicon detectors are mechanically rigid and therefore no special supporting structures are needed.- High mobility of the charge carriers (electrons – 1450 cm2V−1 s −1 , holes – 450 cm2V−1 s −1) means rapid charge carriers collection (≪10 ns) and short dead time. Width of the readout pulse at half maximum (full width at half maximum – FWHM) is about 20 ns.
- Silicon detectors can be used in strong magnetic fields.
In all cases, a particle of ionizing radiation deposits a portion of its energy along its path. Particle passing through the detector ionizes the atoms of semiconductor, producing the electron-hole pairs. For example, typical thickness of silicon detector is about 300 µm so the number of generated electron-hole pairs by minimum ionizing particle (MIP) passing perpendicular through the detector is about 3.2 x 104. This value is minor in comparison the total number of free carriers in intrinsic semiconductor of a surface of 1 cm2 and the same thickness. Note that, a sample of pure germanium at 20 °C contains about 1.26×1021 atoms, but also contains 7.5 x 1011 free electrons and 7.5 x 1011 holes constantly generated from thermal energy. As can be seen, the signal to noise ratio (S/N) would be minimal. The addition of 0.001% of arsenic (an impurity) donates an extra 1015 free electrons in the same volume and the electrical conductivity is increased by a factor of 10,000. In doped material the signal to noise ratio (S/N) would be even smaller. Cooling of the semiconductor is one way to lower this ratio.
See also: Doped Semiconductor
Improvement can be reached by use of a reverse-bias voltage to the P-N junction to deplete the detector of free carriers, which is the principle of the most silicon radiation detectors. In this case, negative voltage is applied to the p-side and positive to the second one. Holes in the p-region are attracted from the junction towards the p contact and similarly for electrons and the n contact.
Application of Silicon Detectors
Since silicon-based detectors are very good for tracking charged particles, they constitute a substantial part of detection system at the LHC in CERN. Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometers wide) strips of silicon to turn them into diodes, which are then reverse biased. As charged particles pass through these strips, they cause small ionization currents that can be detected and measured. Arranging thousands of these detectors around a collision point in a particle accelerator can yield an accurate picture of what paths particles take. For example, the Inner Tracking System (ITS) of a Large Ion Collider Experiment (ALICE) contains three layers of silicon-based detectors:
- Silicon Pixel Detector (SPD)
- Silicon Drift Detector (SDD)
- Silicon Strip Detector (SSD)
Silicon Strip Detectors
Silicon-based detectors are very good for tracking charged particles. A silicon strip detector is an arrangement of strip like shaped implants acting as charge collecting electrodes.
Silicon strip detectors 5 x 5 cm2 in area are quite common and are used in series (just like planes of MWPCs) to determine charged-particle trajectories to position-accuracies of the order of several μm in the transverse direction. Placed on a low doped fully depleted silicon wafer these implants form a one-dimensional array of diodes. By connecting each of the metalized strips to a charge sensitive amplifier a position sensitive detector is built. Two dimensional position measurements can be achieved by applying an additional strip like doping on the wafer backside by use of a double sided technology. Such devices can be used to measure small impact parameters and thereby determine whether some charged particle originated from a primary collision or was the decay product of a primary particle that traveled a small distance from the original interaction, and then decayed.
Silicon strip detectors constitute a substantial part of detection system at the LHC in CERN. Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometers wide) strips of silicon to turn them into diodes, which are then reverse biased. As charged particles pass through these strips, they cause small ionization currents that can be detected and measured. Arranging thousands of these detectors around a collision point in a particle accelerator can yield an accurate picture of what paths particles take.
For example, the Inner Tracking System (ITS) of a Large Ion Collider Experiment (ALICE) contains three layers of silicon-based detectors:
- Silicon Pixel Detector (SPD)
- Silicon Drift Detector (SDD)
- Silicon Strip Detector (SSD)
Delta E – E Detector – Telescope
In experimental physics, ΔE-E detectors, known as telescopes, are powerful devices for charged particles identification. In order to provide charged-particle identification, telescopes consisting of pairs of thin and thick surface-barrier detectors can be used. These detectors must be positioned in series. The velocity is deduced from the stopping power measured in the thin detectors (ΔE detectors). There is a strong correlation between the energy deposited in each detector. This correlation depends on the mass (A), the charge (Z) and the kinetic energy (E) of each particle. The mass is deduced from the range or from the total kinetic energy loss in the thicker detector (E detector).
Telescopes can be composed of several detectors (ionization chambers, silicon detectors and scintillators for instance) stacked in order to slow down incident charged particles, the first detector being the thinnest and the last one the thickest. CsI scintillation counters can be, for example used as final E counters. As an example of telescope, an assembly based on two front ΔE silicon detectors (10 or 30 µm) and an E silicon counter 1500 µm thick may be used for detection of high-energy charged particles.
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