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3.8.1 Radiation detection instrumentation

Contents
3.8.1.1 Radiation detectors
3.8.1.2 Gas-filled detectors
3.8.1.3 Scintillation detectors
3.8.1.4 Solid-State Detectors
3.8.1.5 Passive integrating detectors

A wide range of instruments is available for the detection of radioactivity. EURSSEM gives guidance and detailed description of instruments available and this is presented in Appendix C (not claiming to be 100% complete). A competent person, such as a specialist in radiological measurements or a radiological protection adviser, should select appropriate radiation detectors. Instruments should be used by suitably qualified and experienced staff (such as a health physics surveyor, or a radiation protection supervisor) that is capable of carrying out the survey whilst adhering to the appropriate quality control and health and safety rules.

The selected instrumentation should be appropriate to obtain the data required.

Different radiation detectors will be in general required to detect different types of radioactivity (alpha, beta and gamma). However, in most cases field radiological surveys focus on detection of gamma emitting radionuclides and, to some extent, of high-energy beta emitters. This is primarily because these are the most penetrating radiations and are easily detectable at distances of tens of centimetres to metres from the ground surface. Identification of alpha emitters or low-energy beta and gamma emitters is generally not possible during an on-site radiological survey of a contaminated site.

Appendix C contains:

  • Aspects to consider by selection of field survey and laboratory equipment;
  • Advised sensitivity of direct measurements and scanning survey techniques;
  • Short summaries of radiation detection principles applied in instruments;
  • Overview available α, β and γ instrumentation (not claiming to be 100% complete);
  • The following specifications will be dealt with:
    • System name;
    • Applicability for laboratory or field measurements;
    • Radiation detected;
    • Applicability to site surveys;
    • Operation;
    • Specificity/sensitivity;
    • Cost of equipment (assessment);
    • Cost per measurement (assessment).

Radiation instruments consist in principle out of two components:

  • A radiation detector;
  • An electronic equipment to provide power to the detector and to display or record detected radiation events.

This section identifies and very briefly describes the types of radiation detectors and associated display or recording equipment that are applicable to survey activities in support of environmental assessment or remedial action. Each survey usually requires performing some direct field measurements using portable instrumentation and collection of samples for laboratory analysis. The selection and proper use of appropriate instruments for both direct measurements and laboratory analyses are important factors in assuring that the survey accurately determines the radiological status of a site and meets the radiological survey data quality objectives. Section 3.4 provides specific information on sampling of different materials and Section 3.9 provides specific information on laboratory analysis of collected samples. 0 contains instrument specific information for various types of field survey and laboratory analysis equipment currently in use and commercially available.

3.8.1.1 Radiation detectors

The particular capabilities of a radiation detector will establish its potential applications in conducting a specific type of survey. Radiation detectors can be divided into four general classes based on the detector material or the application. These categories are:

  • Gas-filled detectors;
  • Scintillation detectors;
  • Solid-state detectors;
  • Passive integrating detectors.

These four categories will briefly be discussed in the next sections (see also Appendix C).

3.8.1.2 Gas-filled detectors

Radiation interacts with the fill gas, producing ion-pairs that are collected by charged electrodes. Commonly used gas-filled detectors are categorized as ionization, proportional, or Geiger-Mueller (GM), referring to the region of gas amplification in which they are operated. The fill gas varies, but the most common are:

  • Air;
  • Argon with a small amount of organic methane (usually 10% methane by mass, referred to as P-10 gas);
  • Argon or helium with a small amount of a halogen such as chlorine or bromine added as a quenching agent.

3.8.1.3 Scintillation detectors

Radiation interacts with a solid or liquid medium causing electronic transitions to excited states in a luminescent material. The excited states decay rapidly, emitting photons that in turn are captured by a photomultiplier tube. The ensuing electrical signal is proportional to the scintillator light output, which, under the right conditions, is proportional to the energy loss that produced the scintillation. The most common scintillant materials are NaI(Tl), ZnS(Ag), Cd(Te), CsI(Tl), LaCl(Ce) and LaBr which are used in radiation instruments. As examples NaI(Tl) detector used for gamma scanning surveys and direct measurements, and the ZnS(Ag) detector for alpha surveys, mostly direct measurements.

3.8.1.4 Solid-State Detectors

Radiation interacting with a semiconductor material creates electron-hole pairs that are collected by a charged electrode. The design and operating conditions of a specific solid-state detector determines the types of radiations (alpha, beta, and/or gamma) that can be measured, the detection level of the measurements, and the ability of the detector to resolve the energies of the interacting radiations. The semiconductor materials currently being used are germanium and silicon which are available in both n- and p-types in various configurations.

Spectrometric techniques using these detectors provide a marked increase in sensitivity in many situations. When a particular radionuclide contributes only a fraction of the total particle fluence or photon fluence, or both, from all sources (natural or man-made background), gross measurements are inadequate and nuclide-specific measurements are necessary. Spectrometry provides the means to discriminate among various radio-nuclides on the basis of characteristic energies. In-situ gamma spectrometry is particularly effective in field measurements since the penetrating nature of the radiation allows one to “see” beyond immediate surface contamination. The availability of large, high efficiency germanium detectors permits measurement of low abundance gamma emitters such as 238U as well as low energy emitters such as 241Am and 239Pu.

3.8.1.5 Passive integrating detectors

There is an additional class of instruments that consists of passive, integrating detectors and associated reading/analyzing instruments. The integrated ionization is read using a laboratory or hand-held reader. This class includes thermo-luminescence dosimeters (TLDs) and electret ion chambers (EICs). Because these detectors are passive and can be exposed for relatively long periods of time, they can provide better sensitivity for measuring low activity levels such as free release limits or for continuing surveillance. The ability to read and present data on-site is a useful feature and such systems are comparable to direct reading instruments.

The scintillation materials in Section 3.8.1.3 are selected for their prompt fluorescence characteristics. In another class of inorganic crystals, called TLDs, the crystal material and impurities are chosen so that the free electrons and holes created following the absorption of energy from the radiation are trapped by impurities in the crystalline lattice thus locking the excitation energy in the crystal. Such materials are used as passive, integrating detectors. After removal from the exposure area, the TLDs are heated in a reader which measures the total amount of light produced when the energy is released. The total amount of light is proportional to the number of trapped, excited electrons, which in turn is proportional to the amount of energy absorbed from the radiation. The intensity of the light emitted from the thermo-luminescent crystals is thus directly proportional to the radiation dose. TLDs come in a large number of materials, the most common of which are LiF, CaF2:Mn, CaF2:Dy, CaSO4:Mn, CaSO4:Dy, Al2O3:C.

The electret ion chamber consists of a very stable electret (a charged Teflon® disk) mounted inside a small chamber made of electrically charged plastic. The ions produced inside this air filled chamber are collected onto the electret, causing a reduction of its surface charge. The reduction in charge is a function of the total ionization during a specific monitoring period and the specific chamber volume. This change in voltage is measured with a surface potential voltmeter.