Discharge Region – Ionization Detector

Detector of Ionizing Radiation - Geiger Tube
Detector of Ionizing Radiation – Geiger Tube

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.

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.

Discharge Region

Finally, at still higher voltages (above Geiger-Mueller region), the electric field generates a continuous discharge of the medium, with the chamber no longer being sensitive to any incident ionization. This region is not used for detection or measurement of ionizing radiation. If the Geiger tube voltage is increased above the end of the plateau the count rate begins to increase rapidly again, until the onset of continuous discharge where the tube cannot detect radiation, and may be damaged.

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