In general, the ionization chamber and also the proportional counter are types of gaseous ionization detectors. These can be categorized according to the voltage applied to the detector:
As with other detectors, ionization chambers can be operated in current or pulse mode. In contrast, proportional counters or Geiger counters are almost always used in pulse mode. Detectors of ionizing radiation can be used both for activity measurements as well as for dose measurement. With knowledge about the energy needed to form an pair of ions – the dose can be obtained.
The ionization chamber, also known as the ion chamber, is electrical device that detects various types of ionizing radiation. The voltage of detector is adjusted so that the conditions correspond to the ionization region. The voltage is not high enough to produce gas amplification (secondary ionization).
Advantages of Ionization Chambers
- Current mode. Ionization chambers 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. This is due to the fact, there is no inherent amplification of signal in the operating medium and therefore these types of counters do not require much time to recover from large currents. In addition, because there is no amplification, they provide excellent energy resolution, which is limited primarily by electronic noise. Ionization chambers can be operated in current or pulse mode. In contrast, proportional counters or Geiger counters are almost always used in pulse mode. Detectors of ionizing radiation can be used both for activity measurements as well as for dose measurement. With knowledge about the energy needed to form an pair of ions – the dose can be obtained. The flat plate design is preferred because it has a well-defined active volume and ensures that ions will not collect on the insulators and cause a distortion of the electric field.
- Simplicity. Output current is independent of detector operating voltage. Observe the flat region of the curve in the ion chamber region. As a result, less regulated and thereby less expensive and more portable power supplies can be used with ion chamber instruments, and still offer a reasonably accurate response.
- Neutron Detection. In nuclear reactors, ionization chambers in current mode are often used to detect neutrons and belong to the Nuclear Instrumentation System (NIS). For example, if the inner surface of the ionization chamber is coated with a thin coat of boron, the (n,alpha) reaction can take place. Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission. Moreover, isotope boron-10 has high (n,alpha) reaction cross-section along the entire neutron energy spectrum. The alpha particle causes ionization within the chamber, and ejected electrons cause further secondary ionizations. Another method for detecting neutrons using an ionization chamber is to use the gas boron trifluoride (BF3) instead of air in the chamber. The incoming neutrons produce alpha particles when they react with the boron atoms in the detector gas. Either method may be used to detect neutrons in nuclear reactor.
Disadvantages of Ionization Chambers
- No Charge Amplification. Detectors in the ionization region operate at a low electric field strength, selected such that no gas multiplication takes place. The charge collected (output signal) is independent of the applied voltage and for single minimum-ionizing particles tends to be quite small and usually require special low-noise amplifiers for attaining efficient operating performance. In air, the average energy needed to produce an ion is about 34 eV, therefore a 1 MeV radiation completely absorbed in the detector produces about 3 x 104 pair of ions. However it is a small signal, this signal can be considerably amplified using standard electronics. A current of 1 micro-ampere consists of about 1012 electrons per second.
- Low Density. Gamma rays deposit significantly lower amount of energy to the detector than other particles. The efficiency of the chamber can be further increased by the use of a high pressure gas.
- For alpha and beta particles to be detected by ionization chambers, they must be provided with a thin window. This “end-window” must be thin enough for the alpha and beta particles to penetrate. However, a window of almost any thickness will prevent an alpha particle from entering the chamber. The window is usually made of mica with a density of about 1.5 – 2.0 mg/cm2.
A proportional counter, also known as the proportional detector, is an electrical device that detects various types of ionizing radiation. The voltage of detector is adjusted so that the conditions correspond to the proportional region. In this region, the voltage is high enough to provide the primary electrons with sufficient acceleration and energy so that they can ionize additional atoms of the medium. These secondary ions (gas amplification) formed are also accelerated causing an effect known as Townsend avalanches, which creates a single large electrical pulse.
Advantages of Proportional Counters
- Amplification. Gaseous proportional counters usually operate in high electric fields of the order of 10 kV/cm and achieve typical amplification factors of about 105. Since the amplification factor is strongly dependent on the applied voltage, the charge collected (output signal) is also dependent on the applied voltage and proportional counters require constant voltage. The high amplification factor of the proportional counter is the major advantage over the ionization chamber.
- Sensitivity. The process of charge amplification greatly improves the signal-to-noise ratio of the detector and reduces the subsequent electronic amplification required. Since the process of charge amplification greatly improves the signal-to-noise ratio of the detector, the subsequent electronic amplification is usually not required. Proportional counter detection instruments are very sensitive to low levels of radiation. Moreover, when measuring current output, a proportional detector is useful for dose rates
since the output signal is proportional to the energy deposited by ionization and therefore proportional to the dose rate.
- Spectroscopy. By proper functional arrangements, modifications, and biasing, the proportional counter can be used to detect alpha, beta, gamma, or neutron radiation in mixed radiation fields. Moreover, proportional counters are capable of particle identification and energy measurement (spectroscopy). The pulse height reflects the energy deposited by the incident radiation in the detector gas. As such, it is possible to distinguish the larger pulses produced by alpha particles from the smaller pulses produced by beta particles or gamma rays.
Disadvantages of Proportional Counters
- Constant Voltage. 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. The main drawback to using proportional counters in portable instruments is that they require a very stable power supply and amplifier to ensure constant operating conditions (in the middle of the proportional region). This is difficult to provide in a portable instrument, and that is why proportional counters tend to be used more in fixed or lab instruments.
- Quenching. For each electron collected in the chamber, there is a positively charged gas ion left over. These gas ions are heavy compared to an electron and move much more slowly. Free electrons are much lighter than the positive ions, thus, they are drawn toward the positive central electrode much faster than the positive ions are drawn to the chamber wall. The resulting cloud of positive ions near the electrode leads to distortions in gas multiplication. In practice the termination of the avalanche is improved by the use of “quenching” techniques.