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3.8.4 Aspects to consider by selection of field survey and laboratory equipment

Radiation survey parameters that might be needed for site release purposes include surface activities, exposure rates, and radionuclide concentrations in soil. To determine these parameters, field measurements and laboratory analyses may be necessary. For certain radio-nuclides or radio-nuclide mixtures, both alpha and beta radiations may have to be measured. In addition to assessing average radiological conditions, the survey objectives should address identifying small areas of elevated activity and determining the extent and level of residual radioactivity.

Figure 3.10 Flow diagram for selection of field survey instrumentation for direct measurements and analysis of samples
Figure 3.10 Flow diagram for selection of field survey instrumentation for direct measurements and analysis of samples.

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Additionally, the potential uses of radiation instruments can vary significantly depending on the specific design and operating criteria of a given detector type. For example, a NaI(Tl) scintillator can be designed to be very thin with a low atomic number entrance window (e.g., beryllium) such that the effective detection capability for low energy photons is optimized. Conversely, the same scintillant material can be fabricated as a thick cylinder in order to optimize the detection probability for higher energy photons. On the recording end of a detection system, the output could be a ratemeter, scaler, or multi-channel analyzer as described in Section 3.8.2. Operator variables such as training and level of experience with specific instruments should also be considered.

With so many variables, it is highly unlikely that any single instrument (detector and readout combination) will be capable of adequately measuring all of the radiological parameters necessary to demonstrate that criteria for release have been satisfied. It is usually necessary to select multiple instruments to perform the variety of measurements required.

Selection of instruments will require an evaluation of a number of situations and conditions. Instruments must be stable and reliable under the environmental and physical conditions where they will be used, and their physical characteristics (size and weight) should be compatible with the intended application. The instrument must be able to detect the type of radiation of interest, and the measurement system should be capable of measuring levels that are less than the DCGL (see Section 3.3.7).

For gamma radiation scanning, a scintillation detector/ratemeter combination is a common instrument of choice. A large-area proportional detector with a ratemeter is recommended for scanning for alpha and beta radiations where surface conditions and locations permit; otherwise, an alpha scintillation or thin-window GM detector (for beta surveys) may be used.

For direct gamma measurements, a pressurized ionization chamber or in-situ gamma spectroscopy system is recommended. As an option, e.g., a NaI(Tl) scintillation detector may be used if cross-calibrated to a pressurized ion chamber or calibrated for the specific energy of interest. The same alpha and beta detectors identified above for scanning surveys are also recommended for use in direct measurements. In Figure 3.10 a flow diagram for the selection of field survey instrumentation for direct measurements and analysis of samples is given.

There are certain radio-nuclides that, because of the types, energies, and abundances of their radiations, will be essentially impossible to measure at the guideline levels, under field conditions, using state-of-the-art instrumentation and techniques. Examples of such radio-nuclides include very low energy pure beta emitters, such as 3H and 63Ni, and low energy photon emitters, such as 55Fe and 125I. Pure alpha emitters dispersed in soil or covered with some absorbing layer will not be detectable because the alpha radiation will not penetrate through the media or covering to reach the detector. A common example of such a condition would be 230Th surface contamination covered by paint, dust, oil, or moisture. In such circumstances, sampling and laboratory analysis would be required to measure the residual activity levels unless surrogate radio-nuclides are present as discussed in Section 3.3.6.2.

The number of possible design and operating schemes for each of the different types of detectors is too large to discuss in detail within the context of this document. For a general overview, lists of common radiation detectors along with their usual applications during surveys are provided in Table 3.47, Table 3.48 and Table 3.49. Appendix B contains specific information for various types of field survey and laboratory analysis equipment currently in use. Continual development of new technologies will result in changes to these listings.
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Detector type Detector description Application Remarks

Gas Proportional < 1 mg/cm2 window; probe area 50 to 1000 cm2 Surface scanning; surface contamination measurement Requires a supply of appropriate fill gas.
< 0.1 mg/cm2 window; probe area 10 to 20 cm2 Laboratory measurement of water, air, and smear samples
No window (internal proportional) Laboratory measurement of water, air, and smear samples

Air Proportional < 1 mg/cm2 window; probe area ~50 cm2 Useful in low humidity conditions

Scintillation ZnS(Ag) scintillator; probe area 50 to 100 cm2 Surface contamination measurements, smears
ZnS(Ag) scintillator; probe area 10 to 20 cm2 Laboratory measurement of water, air, and smear samples
Liquid scintillation cocktail containing sample Laboratory analysis, spectrometry capabilities

Solid State Silicon surface barrier detector Laboratory analysis by alpha spectrometry

Passive, integrating electret ion chamber < 0.8 mg/cm2 window, also window-less, window area 50-180 cm2, chamber volume 50-1,000 ml

Contamination on surfaces, in pipes and in soils Useable in high humidity and temperature

Table 3.47 Radiation detectors with applications to alpha surveys
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Detector type Detector description Application Remarks

Gas Proportional < 1 mg/cm2 window; probe area 50 to 1000 cm2 Surface scanning; surface contamination measurement Requires a supply of appropriate fill gas. Can be used for measuring very low-energy betas.
< 0.1 mg/cm2 window; probe area 10 to 20 cm2 Laboratory measurement of water, air, smear, and other samples
No window (internal proportional) Laboratory measurement of water, air, smear, and other samples

Ionisation (non-pressurised) < 1-7 mg/cm2 window Contamination measurements, skin dose rate estimates

Geiger-Mueller 2 mg/cm2 window, probe area 10 to 20 cm2 Surface scanning, contamination measurements, laboratory analyses
Various window thickness; few cm2 probe face

Special scanning applications
Scintillation Liquid scintillation cocktail containing sample Laboratory analysis; spectrometry capabilities
Plastic scintillator Contamination measurements

Passive, integrating electret ion chamber < 7 mg/cm2 window, also window-less, window area 50-180 cm2, chamber volume 50-1,000 ml

Low energy beta including 3H contamination on surfaces and in pipes

Useable in high humidity and temperature

Table 3.48 Radiation detectors with applications to beta surveys
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Detector type Detector description Application Remarks

Gas Ionisation Pressurized ionization chamber; non-pressurized ionization chamber

Exposure rate measurements
Geiger-Mueller Pancake (< 2 mg/cm2 window) or side window (~30 mg/cm2) Surface scanning; exposure rate correlation (side window in closed position)

Low relative sensitivity to gamma radiation
Scintillation NaI(Tl) scintillator; up to 5 cm by 5 cm Surface scanning; exposure rate correlation High sensitivity; cross calibrate with PIC (or equivalent) or for specific site gamma energy mixture for exposure rate measurements. Detection of low-energy radiation
NaI(Tl) scintillator; large volume and “well” configurations Laboratory gamma spectrometry
CsI or NaI(Tl) scintillator; thin crystal Scanning; low-energy gamma and X-rays
Organic tissue equivalent (plastics) Dose equivalent rate measurements

Solid State Germanium semi-conductor Laboratory and field gamma spectrometry and spectroscopy

Passive, integrating electret ion chamber 7 mg/cm2 window, also window-less, window area 50-180 cm2, chamber volume 50-1,000 ml Useable in high humidity and temperature

Table 3.49 Radiation detectors with applications to gamma surveys
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some absorbing layer will not be detectable because the alpha radiation will not penetrate through the media or covering to reach the detector. A common example of such a condition would be 230Th surface contamination covered by paint, dust, oil, or moisture. In such circumstances, sampling and laboratory analysis would be required to measure the residual activity levels unless surrogate radio-nuclides are present as discussed in Section 3.3.6.2.
The number of possible design and operating schemes for each of the different types of detectors is too large to discuss in detail within the context of this document. For a general overview, lists of common radiation detectors along with their usual applications during surveys are provided in Table 3.47, Table 3.48 and Table 3.49. Appendix C contains specific information for various types of field survey and laboratory analysis equipment currently in use. Continual development of new technologies will result in changes to these listings.