Gaseous Ionization Detectors
Gaseous ionization detectors are widely used in nuclear power plants, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and Geiger-Mueller regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors.
Advantages and Disadvantages depending on Detector Voltage
The relationship between the applied voltage and pulse height in a detector is very complex. Pulse height and the number of ion pairs collected are directly related. As was written, the voltages can vary widely depending upon the detector geometry and the gas type and pressure. The figure schematically indicates the different voltage regions for alpha, beta and gamma rays. There are six main practical operating regions, where three (ionization, proportional and Geiger-Mueller region) are useful to detect ionizing radiation. These reqions are shown below. The alpha curve is higher than the beta and gamma curve from recombination region to part of limited proportionality region due to the larger number of ion pairs produced by the initial reaction of the incident radiation.
- Ionization Region. In the ionization region, an increase in voltage does not cause a substantial increase in the number of ion-pairs collected. The number of ion-pairs collected by the electrodes is equal to the number of ion-pairs produced by the incident radiation, and is dependent on the type and energy of the particles or rays in the incident radiation. Therefore, in this region the curve is flat. The voltage must be higher than the point where dissociated ion-pairs can recombine. On the other hand, the voltage is not high enough to produce gas amplification (secondary ionization). Detectors in the ionization region operate at a low electric field strength, selected such that no gas multiplication takes place. Their current is independent of the applied voltage, and they are preferred for high radiation dose rates because they have no “dead time”, a phenomenon which affects the accuracy of the Geiger-Mueller tube at high dose rates.
- Proportional Region. In the proportional region, the charge collected increases with a further increase in the detector voltage, while the number of primary ion-pairs remains unchanged. Increasing the voltage, provides the primary electrons with sufficient acceleration and energy so that they can ionize additional atoms of the medium. These secondary ions formed are also accelerated causing an effect known as Townsend avalanches, which creates a single large electrical pulse. Even though there is a large number of secondary ions (about 103 – 105) for each primary event, the chamber is always operated such that the number of secondary ions is proportional to the number of primary events. It is very important, because the primary ionization is dependent on the type and energy of the particles or rays in the intercepted radiation field. The number of ion pairs collected divided by the number of ion pairs produced by the primary ionization provides the gas amplification factor (denoted by A). The gas amplification that occurs in this region can increase the total amount of ionization to a measurable value. The process of charge amplification greatly improves the signal-to-noise ratio of the detector and reduces the subsequent electronic amplification required. When instruments are operated in the proportional region, the voltage must be kept constant. If a voltage remains constant the gas amplification factor also does not change. Proportional counter detection instruments are very sensitive to low levels of radiation. Moreover, proportional counters are capable of particle identification and energy measurement (spectroscopy). Different energies of radiation and different types of radiation can be distinguished by analyzing the pulse height, since they significantly differ in the primary ionization.
- Geiger-Mueller Region. In the Geiger-Mueller region, the voltage and thus the electric field is so strong that secondary avalanches can occur. These avalanches can be triggered and propagated by photons emitted by atoms excited in the original avalanche. Since these photons are not affected by the electric field, they may interact far (e.g. laterally to the axis) from the primary avalanche, the entire Geiger tube is participating in the process. A strong signal (the amplification factor can reach about 1010) is produced by these avalanches with shape and height independently of the primary ionization and the energy of the detected photon. Detectors, which are operated in the Geiger-Mueller region, are capable of detection of gamma rays, and also of all types of charged particles, that can enter the detector. These detectors are known as Geiger counters. Main advantage of these instruments is that they usually do not require any signal amplifiers. Since the positive ions do not move far from the avalanche region, a positively charged ion cloud disturbs the electric field and terminates the avalanche process. In practice the termination of the avalanche is improved by the use of “quenching” techniques. In contrast to proportional counters the energy or even incident radiation particle cannot not be distinguished by Geiger counters, since the output signal is independent of the amount and type of original ionization.
A semiconductor detector is a radiation detector which is based on a semiconductor, such as silicon or germanium to measure the effect of incident charged particles or photons. Semiconductor detectors are widely used in radiation protection, assay of radioactive materials and physics research because they have some unique features, can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation. These detectors are employed to measure the energy of the radiation and for identification of particles. Of the available semiconductor materials, silicon is 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. Semiconductor detectors, especially germanium based detectors, are most commonly used where a very good energy resolution is required. In order to achieve maximum efficiency the detectors must operate at the very low temperatures of liquid nitrogen (-196°C). Therefore, the drawback is that semiconductor detectors are much more expensive than other detectors and require sophisticated cooling to reduce leakage currents (noise).
Advantages of HPGe Detectors
- Higher atomic number. Germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction.
- Germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium.
- Very good energy resolution. The FWHM for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.
- Large Crystals. While silicon-based detectors cannot be thicker than a few millimeters, germanium can have a depleted, sensitive thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to few MeV.
Disadvantages of HPGe Detectors
- Cooling. The major drawback of HPGe detectors is that they must be cooled to liquid nitrogen temperatures. Because germanium has relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers to an acceptable level. Otherwise, leakage current induced noise destroys the energy resolution of the detector. Recall, the band gap (a distance between valence and conduction band) is very low for germanium (Egap= 0.67 eV). Cooling to liquid nitrogen temperature (-195.8°C; -320°F) reduces thermal excitations of valence electrons so that only a gamma ray interaction can give an electron the energy necessary to cross the band gap and reach the conduction band.
- Price. The disadvantage is that germanium detectors are much more expensive than ionization chambers or scintillation counters.
Advantages of Silicon Detectors
- 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.
- 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.
- Silicon-based detectors are very good for tracking charged particles, they constitute a substantial part of detection system at the LHC in CERN.
- Silicon detectors can be used in strong magnetic fields.
Disadvantages of Silicon Detectors
- Price. The disadvantage is that silicon detectors are much more expensive than cloud chambers or wire chambers.
- Degradation. They also suffer degradation over time from radiation, however this can be greatly reduced thanks to the Lazarus effect.
- High FWHM. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy.