Recombination Region

The relationship between a detector’s applied voltage and pulse height is very complex. Pulse height and the number of ion pairs collected are directly related. As was written, voltages can vary widely depending upon the detector geometry, 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 for detecting ionizing radiation. These regions are shown below. The alpha curve is higher than the beta and gamma curve from the recombination region to part of the 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 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. More current is produced in the proportional counting region by alpha particles 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.

The electric field is not large enough at low voltage to accelerate electrons and ions. The electrons and ions can recombine soon after they are produced, and only a small fraction of the produced electrons and ions reach their respective electrodes. As the detector voltage increases, however, an increasingly large fraction of the ions produced will reach the electrodes. This increase continues until the “saturation” voltage is attained. The operating voltage range where this occurs is referred to as the recombination region. Detectors are not operated in this region because neither the number of recombinations nor the number of ion pairs initially produced can be determined accurately.

References:

Radiation Protection:

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

Nuclear and Reactor Physics:

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

See above:

Gaseous Detectors