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3.6.1 Non-intrusive radiological surveys

Contents
3.6.1.1 Design of non-intrusive radiological surveys
3.6.1.2 Scanning surveys
3.6.1.2.1 Walkover survey
3.6.1.2.2 Ground based vehicle survey
3.6.1.2.3 Airborne survey
3.6.1.2.4 Scanning for alpha emitting radionuclides
3.6.1.2.5 Scanning for beta emitting radio-nuclides
3.6.1.3 Direct measurements
3.6.1.3.1 Direct measurements for alpha emitting radionuclides
3.6.1.3.2 Direct measurements for beta emitting radionuclides
3.6.1.3.3 Direct measurements for photon emitting radio-nuclides

Ionising radiations (in particular, gamma radiation) can be detected in the field in real time, as example, with hand-held instruments. In contrast, most chemical contaminants can only be detected at some later time through laboratory measurement. As a consequence, non-intrusive radiation surveys (or ‘radiological surveys’) are a key component of any investigation on a potentially radioactively contaminated site. At present, there are no routinely used counterparts for detecting chemical contamination (with the possible exception of the use of gas monitoring equipment).

Radiological surveys, as with the other characterisation methods described in this section, should only be carried out by organisations experienced in undertaking such work. The guidance given below is not prescriptive or a method statement for carrying out a radiological survey, but a set of pointers to highlight important issues and good practice and to identify some common problems and mistakes.

The discussion summarised here is primarily from two references, which provide extensive information on the subject:

  • MARSSIM: The Multi-Agency Radiation Survey and Site Investigation Manual [USNRC-2002], [CIRIA-2007, CIRIA-2009] and www.marssim.com).
  • Environment Agency, Technical support material for the regulation of radioactively contaminated land [EA-1999].

Detailed information about existing non-intrusive radiological is provided later in this section.

Radiological surveys in the field can be broadly divided into two types: scanning surveys and direct (or point) measurements:

  • Scanning surveys. Radiation scanning surveys (sometimes called walkover radiation surveys, because they are typically undertaken on foot) are carried out using portable radiation detection equipment that responds rapidly to the presence of primarily gamma emitting radionuclide contamination on or close to the ground surface mostly nowadays combined with a global positioning system. The aim of these surveys is to identify rapidly the areal distribution of contamination at a site in order to focus further investigations. The results of the survey are generally presented in iso-plots or in tables and can give a good indication of the average value and of the relative levels of radioactivity across the site.
  • Direct (point) measurements. Direct measurements are carried out on the site to determine absolute values for certain parameters or to provide a better understanding of which radionuclides are present. Direct measurements tend to use instrumentation that is slower to respond or bulkier than that used for scanning surveys.

In general, a scanning radiological survey is carried out first, followed by point measurements (if necessary and sometimes called verification measurements) in areas of interest highlighted during the scanning survey.

The decision to use a measurement method as part of the survey design is determined by the survey objectives and the survey unit classification.

It should be noted that surveys in which data are recorded as equivalent dose (in, for example, µSv/hr) may be directly compared with other surveys. In contrast, surveys in which data are recorded as counts per second are not directly comparable with each other unless the same instrument has been used.

3.6.1.1 Design of non-intrusive radiological surveys

The first stage of designing the radiological survey is to identify the objectives of the work (see also Table 3.1, Examples of linkages between site characterisation design aspects and conceptual model). In most cases, this will consist of one or more of the following:

  • To determine if radiation levels (e.g., dose rate, contamination) on the site present are a hazard to site personnel or for the environment.
  • To determine the spatial distribution of radiation levels on the site.
  • To determine if radionuclides on the site present are a hazard to site personnel (if necessary):
    • To determine the spatial distribution of radionuclides on the site (if necessary).
    • To determine the degree of heterogeneity in the distribution of any contamination.
    • To determine the fingerprint of the radionuclides on the site.
  • To determine the size of the site to be surveyed.

Having identified the objectives, the questions in Table 3.40 can be of help to design the survey. The detailed survey design and equipment selection will depend on the site conditions and the radiation levels and/or radionuclides expected to be present. In general, three aspects will be considered: the type of radiation detector, its method of use and the scale of the survey grid.

Issue Remark
Radiation levels Which radiation levels are likely to be present? Based on environmental monitoring, primarily desk study (previous usage of radionuclides and amounts used).
Important because it is the primary drive in the selection of radiological monitoring equipment (see Section 3.8 and Appendix C).

What are the natural background radiation levels at the site? From previous monitoring from the area. If inadequate background information exists, it will be necessary to make measurements to assess this.

What is the detection limit required for the first action level? Based on the derived guideline levels for background radiation (see Section 2.2.2.4, Section 2.5 and Section 3.3.6).

Radionuclides Which radionuclides are likely to be present and at what activity levels? Based on environmental monitoring, primarily desk study (previous usage of radionuclides and amounts used).
Important because it is the primary drive in the selection of radiological monitoring equipment (see Section 3.8 and Appendix C).

What are the natural background levels of radioactivity at the site? From previous monitoring from the area. If inadequate background information exists, it will be necessary to make measurements to assess this.

What are the detection limits required for the radionuclides of interest? Based on the derived guideline levels for the radionuclides of interest (see Section 2.2.2.4, Section 2.5 and Section 3.3.6).
If the radionuclide fingerprint1 is known, it may be possible to infer the presence of a radionuclide by measuring the most easily detectible radionuclide in the fingerprint.

Size of the site What is the size of the site to be surveyed? Minimum the entire area of ground that has the potential to be contaminated and/or to assess the background.
Depending on the type of characterisation (see section 3.1.4) the design can be focused on known or suspected problems.
Important because this will drive selection of radiological monitoring equipment (see Section 3.8 and Appendix C), transportation and grid size.

Time and costs What are the time/cost limitations on the job? Financial and time constraints will often have a significant impact on the type of survey selected.

Table 3.40 Design Issues to be considered by non-intrusive radiation surveys.
.

3.6.1.2 Scanning surveys

Scanning is the process by which the operator uses portable radiation detection instruments to detect the presence of radio-nuclides on a specific surface (i.e., ground, wall, floor, equipment). The term scanning survey is used to describe the process of moving portable radiation detectors across a suspect surface with the intent of locating radionuclide contamination. Investigation levels for scanning surveys are determined during survey planning to identify areas of elevated activity. Scanning surveys are performed to locate radiation anomalies indicating residual gross activity that may require further investigation or action. These investigation levels may be based on the DCGLW, the DCGLEMC, or some other level as discussed in Section 3.3.2.7.

Small areas of elevated activity typically represent a small portion of the site or survey unit. Thus, random or systematic direct measurements or sampling on the commonly used grid spacing may have a low probability of identifying such small areas. Scanning surveys are often relatively quick and inexpensive to perform. For these reasons, scanning surveys are typically performed before direct measurements or sampling. This way time is not spent fully evaluating an area that may quickly prove to be contaminated above the investigation level during the scanning process. Scans are conducted which would be indicative of all radio-nuclides potentially present, based on the historical site assessment, surfaces to be surveyed, and survey design objectives. Surrogate measurements may be utilized where appropriate (see Section 3.3.6.2). Documenting scanning results and observations from the field is very important. For example, a scan that identified relatively sharp increases in instrument response or identified the boundary of an area of increased instrument response should be documented. This information is useful when interpreting survey results.
The following sections briefly describe techniques used to perform scanning surveys for different types of radiation. The instruments used to perform these measurements are described in more detail in Appendix C.

There are three main methods by which radiological monitoring equipment may be transported:

  • By hand.
  • In a ground-based vehicle (e.g., hand trolley, car).
  • By air.

The relative advantages and disadvantages of each approach are given below.

3.6.1.2.1 Walkover survey

This consists of a single person or two persons carrying up to approximately 15 kg of equipment. The walkover survey is suitable for areas up to a few hectares (both inside and outside buildings), and may be undertaken over relatively rough ground. As the equipment is carried by a single person, lightweight probes with little collimation are in general used. Due to technical improvement light weight computer controlled real-time spectrometry systems becomes available so that also multiple detectors can be employed.

3.6.1.2.2 Ground based vehicle survey

This consists of a ground-based vehicle, either hand-pushed or motorised, carrying up to approximately 500 kg of equipment. The vehicle survey is suitable for large (tens of hectares), flat open areas, for example, airfields or roadways. The vehicle survey has a number of advantages over the walkover survey, which are predominantly due to the increased mass that can be carried and the fact that the vehicle is weather-proof. Sophisticated electronics may be carried that allow real-time spectrometry, multiple detectors may be employed and large-area scintillation detectors can be used to achieve low detection limits. The main disadvantage of the vehicle survey compared to the walkover survey is that the site must be flat and open.

3.6.1.2.3 Airborne survey

In situations involving widespread contamination with sufficient gamma radiation emissions, aerial surveys can be a cost effective method for rapidly delineating and quantifying such areas. Helicopters are used for low-level work where maximum sensitivity is required, while an aeroplane or helicopter will be applied at higher attitudes. Positioning is generally accomplished with commercial navigation systems (e.g., GPS) which feed indicators to guide the pilot accurately along pre-selected routes. Gamma radiation, flight path, altitude and meteorological data are fed into an inboard data acquisition system for real time or post-flight analysis. Gamma radiation data including spectral data overlaid on aerial photographs indicate the location of the contamination very accurately.

The airborne survey is a rapid method, suitable for very large (thousands of hectares), rough or inaccessible areas. However, it has the disadvantage that individual measurements will be averaged over tens to hundreds of square metres.

In addition, overflying restrictions may apply on nuclear-licensed and defence sites, limiting the applicability of this technique. The IAEA TECDOC-1363 gives a state of the art overview of this technique [IAEA-2003a].

3.6.1.2.4 Scanning for alpha emitting radionuclides

Alpha scintillation survey meters and thin window gas-flow proportional counters are typically used for performing alpha surveys. Alpha radiation has a very limited range and, therefore, instrumentation must be kept close to the surface – usually less than 1 cm (0.4 in.). For this reason, alpha scans are generally performed on relatively smooth, impermeable surfaces (e.g., concrete, metal, drywall) and not on porous material (e.g., wood) or for volumetric contamination (e.g., soil, water). In most cases, porous and volumetric contamination cannot be detected by scanning for alpha activity and meet the objectives of the survey because of high detection sensitivities. Under these circumstances, samples of the material are usually collected and analyzed as discussed in Section 3.3.7. Determining scan rates when surveying for alpha emitters is discussed in Appendix B and Appendix C.

3.6.1.2.5 Scanning for beta emitting radio-nuclides

Thin window gas-flow proportional counters are normally used when surveying for beta emitters, although solid scintillators designed for this purpose are also available. Typically, the beta detector is held less than 2 cm from the surface and moved at a rate such that the desired investigation level can be detected. Low-energy (< 100 keV) beta emitters are subject to the same interferences and self-absorption problems found with alpha emitting radio-nuclides, and scans for these radio-nuclides are performed under similar circumstances. Determination of scan rates when surveying for beta emitters is discussed in Section 6.7.2.1.

3.6.1.2.6 Scanning for photon emitting radio-nuclides

Sodium iodide survey meters (NaI(Tl) detectors) are normally used for scanning areas for gamma emitters because they are sensitive to gamma radiation, easily portable and relatively inexpensive. The detector is held at a certain distance from the ground surface (~6 cm or 2.5 in up to 1 m or 40 in.) and moved in a meander or a serpentine (i.e., snake like, “S” shaped) pattern while walking at a speed that allows the investigator to detect the desired investigation level. A scan rate of approximately 0.5 m/s is typically used for distributed gamma emitting contaminants in soil; however, this rate must be adjusted depending on the expected detector response and the desired investigation level. Discussion of scanning rates versus detection sensitivity for gamma emitters is provided in Section 3.3.7.2.

Sodium iodide survey meters are also used for scanning to detect areas with elevated areas of low-energy gamma and X-ray emitting radio-nuclides such as 241Am and 239Pu. These sodium iodide detectors are specified in such a way that they are more sensitive for low-energy gammas and X-rays.

3.6.1.3 Direct measurements

To conduct direct measurements of alpha, beta, and photon surface activity, instruments and techniques providing the required detection sensitivity are selected. The type of instrument and method of performing the direct measurement are selected as dictated by the type of potential contamination present, the measurement sensitivity requirements, and the data quality objectives of the radiological survey. Direct measurements are taken by placing the instrument at the appropriate distance2 above the surface, taking a discrete measurement for a pre-determined time interval (e.g., 10 s, 60 s, etc.), and recording the reading. A one minute integrated count technique is a practical field survey procedure for most equipment and provides detection sensitivities that are below most derived concentration guideline levels (DCGLs). However, longer or shorter integrating times may be warranted (see Section 3.3.7.2 for information dealing with the calculation of direct measurement detection sensitivities).

Direct measurements may be collected at random locations in the survey unit. Alternatively, direct measurements may be collected at systematic locations and supplement scanning surveys for the identification of small areas of elevated activity (see Section 3.5.1). Direct measurements may also be collected at locations identified by scanning surveys as part of an investigation to determine the source of the elevated instrument response. Professional judgment may also be used to identify location for direct measurements to further define the areal extent of contamination and to determine maximum radiation levels within an area, although these types of direct measurements are usually associated with preliminary surveys (i.e., scoping, characterization, remedial action support). All direct measurement locations and results should be documented.

If the equipment and methodology used for scanning is capable of providing data of the same quality required for direct measurement (e.g., detection limit, location of measurements, ability to record and document results), then scanning may be used in place of direct measurements. Results should be documented for at least the number of locations required for the statistical tests. In addition, some direct measurement systems may be able to provide scanning data, provided they meet the objectives of the scanning survey.

The following sections briefly describe methods used to perform direct measurements in the field. The instruments used to perform these measurements are described in more detail in Section 3.8 and Appendix C.

3.6.1.3.1 Direct measurements for alpha emitting radionuclides

Direct measurements for alpha-emitting radio-nuclides are generally performed by placing the detector on or near the surface to be measured. The limited range of alpha particles (e.g., about 1 cm or 0.4 in. in air, less in denser material) means that these measurements are generally restricted to relatively smooth, impermeable surfaces such as concrete, metal, or drywall where the activity is present as surface contamination. In most cases, direct measurements of porous (e.g., wood) and volumetric (e.g., soil, water) material cannot meet the objectives of the survey. However, special instruments such as the long range alpha detector (see Appendix C) have been developed to measure the concentration of alpha emitting radio-nuclides in soil under certain conditions. Because the detector is used in close proximity to the potentially contaminated surface, contamination of the detector or damage to the detector caused by irregular surfaces need to be considered before performing direct measurements for alpha emitters.

3.6.1.3.2 Direct measurements for beta emitting radionuclides

Direct measurements for beta emitting radio-nuclides are generally performed by placing the detector on or near the surface to be measured, similar to measurements for alpha emitting radio-nuclides. These measurements are typically restricted to relatively smooth, impermeable surfaces where the activity is present as surface contamination. In most cases, direct measurements of porous (e.g., wood) and volumetric (e.g., soil, water) material cannot meet the objectives of the survey. However, special instruments such as large area gas-flow proportional counters (see Appendix C) and arrays of beta scintillators have been developed to measure the concentration of beta emitting radio-nuclides in soil under certain conditions. Similar to direct measurements for alpha emitting radio-nuclides, contamination of the detector and damage to the detector need to be considered before performing direct measurements for beta emitters.

3.6.1.3.3 Direct measurements for photon emitting radio-nuclides

There are a wide variety of instruments available for measuring photons in the field (see 0) but all of them are used in essentially the same way. The detector is set up at a specified distance from the surface being measured and data are collected for a specified period of time. The distance from the surface to the detector is generally determined by the calibration of the instrument because photons do not interact appreciably with air. When measuring X-rays or low-energy gamma rays, the detector is often placed closer to the surface to increase the counting efficiency. The time required to perform a direct measurement may vary from very short (e.g., 10 seconds) to very long (e.g., several days or weeks) depending on the type of detector and the required detection limit. In general, the lower the required detection limit the longer the time required to perform the measurement.

A collimator may be used in areas where activity from adjacent or nearby areas might interfere with the direct measurement. The collimator (usually lead, tungsten, or steel) shields the detector from extraneous photons but allows activity from a specified area of the surface to reach the detector.

Example 3.18: Direct measurement of gamma emitting radionuclide concentrations in the field

The portable germanium detector, or in-situ gamma spectrometer, can be used to estimate gamma emitting radionuclide concentrations in the field. As with the laboratory-based germanium detector with multi-channel analyzer, in-situ gamma spectrometry can discriminate among various radio-nuclides on the basis of characteristic gamma and X-ray energies to provide a nuclide-specific measurement. A calibrated detector measures the fluence rate of primary photons at specific energies that are characteristic of a particular radionuclide. This fluence rate can then be converted to units of concentration. Under certain conditions the fluence rate may be converted directly to dose or risk for a direct comparison to the release criterion rather than to the DCGLW. Although this conversion is generally made, the fluence rate should be considered the fundamental parameter for assessing the level of radiation at a specific location because it is a directly measurable physical quantity.

For outdoor measurements, where the contaminant is believed to be distributed within the surface soil, it may be appropriate to assume a uniform depth profile when converting the fluence rate to a concentration. At sites where the soil is plowed or overturned regularly, this assumption is quite realistic because of the effects of homogenization. At sites where the activity was initially deposited on the surface and has gradually penetrated deeper over time, the actual depth profile will have a higher activity at the surface and gradually diminish with depth. In this case, the assumption of a uniform depth profile will estimate a higher radionuclide concentration relative to the average concentration over that depth. In cases where there is an inverted depth profile (i.e., low concentration at the surface that increase with depth), the assumption of a uniform depth profile will underestimate the average radionuclide concentration over that depth. For this reason, EURSSEM recommends that soil cores be collected to determine the actual depth profile for the site. These soil cores may be collected during the characterization or remedial action support survey to establish a depth profile for planning a final status survey. The cores may also be collected during the final status survey to verify the assumptions used to develop the fluence-to-concentration correction.

For indoor measurements, un-collimated in-situ measurements can provide useful information on the low-level average activity across an entire room. The position of the measurement within the room is not critical if the radionuclide of interest is not present in the building materials. A measurement of peak count rate can be converted to fluence rate, which can in turn be related to the average surface activity. The absence of a discernible peak would mean that residual activity could not exceed a certain average level. However, this method will not easily locate small areas of elevated activity. For situations where the activity is not uniformly distributed on the surface, a series of collimated measurements using a systematic grid allows the operator to identify general areas of elevated contamination.

In-situ spectrometry is provided as one example of a useful tool for performing direct measurements for particular scenarios, but interpretation of the instrument output in terms of radionuclide distributions is dependent on the assumptions used to calibrate the method site-specifically. The depth of treatment of this technique in this example is not meant to imply that in-situ gamma spectrometry is preferred a priori over other appropriate measurement techniques described in this manual.

3.6.1.3.4 Direct radon measurements

Direct radon measurements are performed by gathering radon into a chamber and measuring the ionizations produced. A variety of methods have been developed, each making use of the same fundamental mechanics but employing different measurement processes. The first step is to get the radon into a chamber without collecting any radon progeny from the ambient air. A filter is normally used to capture charged aerosols while allowing the radon gas to pass through. Most passive monitors rely on diffusion of the ambient radon in the air into the chamber to establish equilibrium between the concentrations of radon in the air and in the chamber. Active monitors use some type of air pump system for the air exchange method.

Once inside the chamber, the radon decays by alpha emission to form 218Po which usually takes on a positive charge within thousandths of a second following formation. Some monitor types collect these ionic molecules and subsequently measure the alpha particles emitted by the radon progeny. Other monitor types, such as the electret ion chamber, measure the ionization produced by the decay of radon in the air within the chamber by directly collecting the ions produced inside the chamber. Simple systems measure the cumulative radon during the exposure period based on the total alpha decays that occur. More complicated systems actually measure the individual pulse height distributions of the alpha and/or beta radiation emissions and derive the radon plus progeny isotopic concentration in the air volume.

Care must be taken to accurately calibrate a system and to understand the effects of humidity, temperature, dust loading, and atmospheric pressure on the system. These conditions create a small adverse effect on some systems and a large influence on others.

1 A fingerprint of radionuclides is a method by which difficult to measure radionuclides are linked to a more easily detectable radionuclide (see Section 3.3.6.2).

2 Measurements at several distances may be needed. Near-surface or surface measurements provide the best indication of the size of the contaminated region and are useful for model implementation. Gamma measurements at 1 m provide a good estimate of potential direct external exposure.