Advantages and Disadvantages of Ionization Chambers

Detector of Ionizing Radiation - basic scheme
Detectors of ionizing radiation consist of two parts that are usually connected. The first part consists of a sensitive material, consisting of a compound that experiences changes when exposed to radiation. The other component is a device that converts these changes into measurable signals.

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 Neutron 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.(n,alpha) reactions of 10B

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.

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 upon the detector geometry and the 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 to detect ionizing radiation. Alpha particles are more ionising than beta particles and than gamma rays, so more current is produced in the ion chamber region by alpha than beta and gamma, but the particles cannot be differentiated. More current is produced in the proportional counting region by alpha particles than beta, but 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 ionisation 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 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.

 

 

 

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:

Ionization Chamber