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Gaseous Detectors vs Scintillation Detectors

Gaseous Ionization Detectors

Gaseous ionization detectors are widely used in nuclear power plants, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and Geiger-Mueller regions, with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors.

Advantages and Disadvantages depending on Detector Voltage

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.

  • Ionization Region. 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 depends 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. 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.
  • Proportional Region. In the proportional region, the charge collected increases with a further increase in the detector voltage, while the number of primary ion pairs remains unchanged. Increasing the voltage provides the primary electrons with sufficient acceleration and energy to ionize additional atoms of the medium. These secondary ions formed are also accelerated, causing an effect known as Townsend avalanches, which creates a single large electrical pulse. Even though there is a large number of secondary ions (about 103 – 105) for each primary event, the chamber is always operated such that the number of secondary ions is proportional to the number of primary events. It is important because the primary ionization depends on the type and energy of the particles or rays in the intercepted radiation field. The number of ion pairs collected divided by the number of ion pairs produced by the primary ionization provides the gas amplification factor (denoted by A). The gas amplification in this region can increase the total amount of ionization to a measurable value. The charge amplification process greatly improves the detector’s signal-to-noise ratio and reduces the subsequent electronic amplification required. 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. Proportional counter detection instruments are very sensitive to low levels of radiation. Moreover, proportional counters are capable of particle identification and energy measurement (spectroscopy). Different energies of radiation and different types of radiation can be distinguished by analyzing the pulse height since they significantly differ in the primary ionization.
  • Geiger-Mueller Region. In the Geiger-Mueller region, the voltage and thus the electric field are so strong that secondary avalanches can occur. These avalanches can be triggered and propagated by photons emitted by atoms excited in the original avalanche. Since these photons are not affected by the electric field, they may interact far (e.g., laterally to the axis) from the primary avalanche, and the entire Geiger tube participates in the process. A strong signal (the amplification factor can reach about 1010) is produced by these avalanches with shape and height independently of the primary ionization and the energy of the detected photon. Detectors operated in the Geiger-Mueller region can detect gamma rays and all types of charged particles that can enter the detector. These detectors are known as Geiger counters. The main advantage of these instruments is that they usually do not require any signal amplifiers. Since the positive ions do not move far from the avalanche region, a positively charged ion cloud disturbs the electric field and terminates the avalanche process. In practice, the avalanche’s termination is improved by using “quenching” techniques. In contrast to proportional counters, the energy of incident radiation particles can not be distinguished by Geiger counters since the output signal is independent of the amount and type of original ionization.

Scintillation Counters

A scintillation counter or scintillation detector is a radiation detector that uses the effect known as scintillation. Scintillation is a flash of light produced in a transparent material by passing a particle (an electron, an alpha particle, an ion, or a high-energy photon). Scintillation occurs in the scintillator, a key part of a scintillation detector. In general, a scintillation detector consists of:

  • Scintillator. A scintillator generates photons in response to incident radiation.
  • Photodetector. A sensitive photodetector (usually a photomultiplier tube (PMT), a charge-coupled device (CCD) camera, or a photodiode) converts the light to an electrical signal, and electronics process this signal.

The basic principle of operation involves the radiation reacting with a scintillator, which produces a series of flashes of varying intensity. The intensity of the flashes is proportional to the energy of the radiation, and this feature is very important. These counters are suited to measure the energy of gamma radiation (gamma spectroscopy) and, therefore, can be used to identify gamma-emitting isotopes.

Scintillation counters are widely used in radiation protection, an assay of radioactive materials, and physics research because they can be made inexpensively yet with good efficiency and can measure both the intensity and the energy of incident radiation. Hospitals worldwide have gamma cameras based on the scintillation effect; therefore, they are also called scintillation cameras.

The advantages and disadvantages of scintillation counters are determined by the scintillator. The following features are not general for all scintillators.

Advantages of Scintillation Counters

  • Efficiency. The advantages of a scintillation counter are its efficiency and possible high precision and counting rates. These latter attributes result from the extremely short duration of the light flashes, from about 10-9  (organic scintillators) to 10-6 (inorganic scintillators) seconds.
  • Spectroscopy. The intensity of the flashes and the amplitude of the output voltage pulse are proportional to the energy of the radiation. Therefore, scintillation counters can be used to determine the energy and the number of the exciting particles (or gamma photons). For gamma spectrometry, the most common detectors include sodium iodide (NaI) scintillation counters and high-purity germanium detectors. The NaI(Tl) scintillator has a higher energy resolution than a proportional counter, allowing more accurate energy determinations. On the other hand, if a perfect energy resolution is required, we have to use a germanium-based detector, such as the HPGe detector.

Disadvantages of Scintillation Counters

  • Hygroscopicity. A disadvantage of some inorganic crystals, e.g., NaI, is their hygroscopicity, a property that requires them to be housed in an airtight container to protect them from moisture.
  • NaI(Tl) has no beta or alpha response and poor low-energy gamma response.
  • Liquid scintillators are relatively cumbersome.
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, Instrumentation, 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