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Ionization Chamber – Ion Chamber

The ionization chamber, also known as the ion chamber, is an electrical device that detects various types of ionizing radiation. The voltage of the detector is adjusted so that the conditions correspond to the ionization region, and the voltage is insufficient to produce gas amplification (secondary ionization). Detectors in the ionization region operate at a low electric field strength, so no gas multiplication occurs. The charge collected (output signal) is independent of the applied voltage. Single minimum-ionizing particles tend to be quite small and usually require special low-noise amplifiers for efficient operating performance. Ionization chambers are preferred for high radiation dose rates because they have no “dead time,” a phenomenon that affects the accuracy of the Geiger-Mueller tube at high dose rates. This is because there is no inherent amplification of signal in the operating medium; therefore, these 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, pulse mode almost always uses proportional counters or Geiger counters. Detectors of ionizing radiation can be used both for activity measurements as well as for dose measurements. The dose can be obtained with knowledge about the energy needed to form a pair of ions.

Ionization Region
Gaseous Ionization Detectors - Regions
This diagram shows the number of ion pairs generated in the gas-filled detector, which varies according to the applied voltage for constant incident radiation. The voltages can vary widely depending on the detector geometry, gas type, and pressure. This 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 for detecting ionizing radiation. Alpha particles are more ionizing than beta particles, and gamma rays, so more current is produced in the ion chamber region by alpha than beta and gamma, but the particles cannot be differentiated. Alpha particles produce more current in the proportional counting region than beta. Still, by the nature of proportional counting, it is possible to differentiate alpha, beta, and gamma pulses. In the Geiger region, there is no differentiation of alpha and beta as any single ionization event in the gas results in the same current output.

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 equals the number of ion pairs produced by the incident radiation. It 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, so no gas multiplication occurs, and their current is independent of the applied voltage. They are preferred for high radiation dose rates because they have no “dead time,” a phenomenon that affects the accuracy of the Geiger-Mueller tube at high dose rates.

See also: Advantages and Disadvantages of Ion Chambers

Basic Principle of Ionization Chambers

ionization chamber - basic principleThe chamber has a cathode and an anode that are held at some voltage (perhaps 100 – 200 V), and the device is characterized by a capacitance determined by the electrodes’ geometry. Flat plates or concentric cylinders may be utilized to construct an ionization chamber. 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. As ionizing radiation enters the gas between the electrodes, a finite number of ion pairs are formed. The behavior of the resultant ion pairs is affected by the potential gradient of the electric field within the gas and the type and pressure of the fill gas. Under the influence of the electric field, the positive ions will move toward the negatively charged electrode (outer cylinder or plate), and the negative ions (electrons) will migrate toward the positive electrode (central wire or plate). The electric field in this region keeps the ions from recombining with the electrons. Collecting these ions will produce a charge on the electrodes and an electrical pulse across the detection circuit. The average energy needed to produce an ion in the air is about 34 eV. Therefore a 1 MeV radiation completely absorbed in the detector produces about 3 x 104 pairs of ions. However, it is a small signal, and this signal can be considerably amplified using standard electronics. A current of 1 micro-ampere consists of about 1012 electrons per second.

Ionization chamber construction differs from the proportional counter. The flat plate design is preferred for ionization chambers, or concentric cylinders may be utilized in the construction to allow for integrating pulses produced by the incident radiation. Proportional counters and Geiger counters usually utilize cylinder and central electrodes. The proportional counter would require such exact control of the electric field between the electrodes that it would not be practical.

Detection of Alpha Radiation using Ionization Chamber

For alpha and beta particles to be detected by ionization chambers, they must be given 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. But it does not mean alpha radiation cannot be detected by an ionization chamber.

For example, in some kinds of smoke detectors, you can meet artificial radionuclides such as americium-241, a source of alpha particles. The smoke detector has two ionization chambers, one open to the air and a reference chamber that does not allow the entry of particles. The radioactive source emits alpha particles into both chambers, which ionizes some air molecules. The free-air chamber allows the entry of smoke particles into the sensitive volume and changes the attenuation of alpha particles. If any smoke particles enter the free-air chamber, some ions will attach to the particles and not be available to carry the current in that chamber. An electronic circuit detects that a current difference has developed between the open and sealed chambers and sounds the alarm.

Detection of Beta Radiation using Ionization Chamber

For alpha and beta particles to be detected by ionization chambers, they must be given 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.

An ionization chamber may be, for example, used for the measurement of tritium in the air. These devices are known as tritium-in-air monitors. Tritium is a radioactive isotope, but it emits a very weak form of radiation, a low-energy beta particle similar to an electron. It is a pure beta emitter (i.e., beta emitter without accompanying gamma radiation). The electron’s kinetic energy varies, with an average of 5.7 keV, while the remaining energy is carried off by the nearly undetectable electron antineutrino. Such a very low energy of electron causes that the electron cannot penetrate the skin or even does not travel very far in the air. Beta particles from tritium can penetrate only about 6.0 mm of air. Designing a detector whose walls these beta particles can penetrate is practically impossible. Instead, the tritium-in-air monitor pumps the tritium-contaminated air right through an ionization chamber so that all of the energy of the beta particles can be usefully converted to producing ion pairs inside the chamber.

Detection of Gamma Radiation using Ionization Chamber

Gamma rays have very little trouble penetrating the metal walls of the chamber. Therefore, ionization chambers may be used to detect gamma radiation and X-rays, collectively known as photons, and for this, the windowless tube is used. Ionization chambers have a uniform response to radiation over a wide range of energies and are the preferred means of measuring high levels of gamma radiation. Some problems are caused by alpha particles being more ionizing than beta particles, and gamma rays, so more current is produced in the ionization chamber region by alpha than beta and gamma. Gamma rays deposit a significantly lower amount of energy into the detector than other particles.

Using a high-pressure gas can further increase the chamber’s efficiency. Typically a pressure of 8-10 atmospheres can be used, and various noble gases are employed. For example, high-pressure xenon (HPXe) ionization chambers are ideal for use in uncontrolled environments, as a detector’s response has been shown to be uniform over large temperature ranges (20–170°C). The higher pressure results in a greater gas density and thereby a greater chance of collision with the fill gas and ion-pair creation by incident gamma radiation. Because of the increased wall thickness required to withstand this high pressure, only gamma radiation can be detected. These detectors are used in survey meters and for environmental monitoring.

Detection of Neutrons using Ionization Chamber

Since the neutrons are electrically neutral particles, they are mainly subject to strong nuclear forces but not electric ones. Therefore, neutrons are not directly ionizing and usually have to be converted into charged particles before they can be detected. Generally, every type of neutron detector must be equipped with a converter (to convert neutron radiation to common detectable radiation) and one of the conventional radiation detectors (scintillation detector, gaseous detector, semiconductor detector, etc.).

Ionization chambers are often used as the charged particle detection device. For example, if the inner surface of the ionization chamber is coated with a thin coat of boron, the (n, alpha) reaction can occur. Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission.

(n,alpha) reactions of 10B

Moreover, isotope boron-10 has a 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 reacting with the boron atoms in the detector gas. Either method may be used to detect neutrons in a nuclear reactor. It must be noted that BF3 counters are usually operated in the proportional region.

References:

Radiation Protection:

  1. Knoll, Glenn F., Radiation Detection and Measurement 4th Edition, Wiley, 8/2010. ISBN-13: 978-0470131480.
  2. Stabin, Michael G., Radiation Protection and Dosimetry: An Introduction to Health Physics, Springer, 10/2010. ISBN-13: 978-1441923912.
  3. Martin, James E., Physics for Radiation Protection 3rd Edition, Wiley-VCH, 4/2013. ISBN-13: 978-3527411764.
  4. U.S.NRC, NUCLEAR REACTOR CONCEPTS
  5. U.S. Department of Energy, Instrumantation and Control. DOE Fundamentals Handbook, Volume 2 of 2. June 1992.

Nuclear and Reactor Physics:

  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
  9. Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.

See above:

Gaseous Detectors