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3.4.4 Photon emitting radionuclides

Energy specific measurements of gamma radiation are of use if identification of the isotopes present at a site is a requirement. It may help if measurements of a specific isotope are required against a background, possibly varying with time or position, of another radionuclide (such as naturally occurring radium or 40K).

Although simple instruments may be valuable in locating or delineating areas of high activity, at levels near to the natural background they must be used with care if statistical counting effects or local variations in background are not to be misinterpreted as variation in contaminant. It may be the case that integration times of simple instruments have to be set sufficiently long that spectrometric methods could give a more accurate result in less time. It must be emphasized that background radiation levels can vary rapidly, not only spatially but also with time due to changes in solar radiation or due to radon releases from the ground changing with atmospheric pressure.

As an example, if there is a known area of contamination on a site, staff may be instructed not to survey in detail the area of most concentrated contamination, but to explore the outer limits of contamination so as to be able to delineate the extent of the contaminated area.

Figure 3.6 shows a typical in-situ gamma spectrometry measurement with the detector placed at 1 meter above the soil surface. At this height, 85-90% of the gamma radiation detected is originating from a circle with radius of 10 meter from the detector. The in-situ gamma spectrometer will depending on its design, at a height of 1 meter, effectively detect radionuclides to a depth of up to 15-50 cm. The effective area observed by this detector (>300 m2) may, in fact, give a more representative picture of contamination than conventional sampling and analysis.

Figure 3.6 Area observed by an in-situ gamma ray spectrometer at 1 m above the ground. Depicted is the percent contribution to total 662 keV primary flux for a typical [^137^]Cs source distribution from past atmospheric weapons testing fall-out.
Figure 3.6 Area observed by an in-situ gamma ray spectrometer at 1 m above the ground. Depicted is the percent contribution to total 662 keV primary flux for a typical Cs-137 source distribution from past atmospheric weapons testing fall-out.

It should be mentioned first that there are limitations in using in-situ spectrometry. Due to the nature of radionuclide transport through matter (soil and air) and to the attenuation of ionising radiation, in-situ spectrometry is, for the most part, limited to the measurement of gamma rays and some X-ray emitters.

The ideal site for collecting a gamma spectrum is a large (20 m diameter or more) fiat, open area with little or no natural or man-made obstruction. For standard measurements, the height of the detector above the ground is an important parameter. One meter is often chosen for reasons of convenience; the higher the detector the greater the area which contributes to the measurement (and the faster an area may be surveyed, although this is at the expense of lateral resolution).

For undisturbed soils, the actual depth profile of the radionuclide of interest is highly dependent on whether it is present as a naturally occurring gamma ray emitter or it was released into the environment from anthropogenic sources and, if so, the time of the release, the mobility of the radionuclide in that specific environment, and the position of release (deposited on the surface, released from a buried pipe, etc.). Usually, naturally occurring emitters (e.g., 40K, 238U, 232Th) are distributed approximately uniformly throughout the soil. Those that are present as the result of nuclear weapons testing fall-out (e.g., 137Cs) tend to be distributed with the activity decreasing exponentially with depth. In the case of a very recent accidental airborne release, the 137Cs probably would be distributed only on the soil surface. In such a case 134Cs will also be measured.

In some cases, in-situ spectrometry has been used to determine soil depth profiles directly using differential attenuation for those nuclides which emit two (or more) gamma rays, analysis of the scattered component of the radiation, or (with a lead shield) measurements of the angular incidence of the radiation. Demonstration of this technique was conducted at a former United States weapons production facility using a p-type germanium closed-end coaxial detector to determine the surface soil concentration of uranium. The depth profile of 238U was obtained to a depth of 5-10 cm by observing the attenuation of 63 keV line with respect to the 93 keV line.

Multi-line isotopes such as 134Cs can be used to determine an approximate depth profile by virtue of the differential attenuation of gamma rays of different energies. This approach is limited by the small number of radioisotopes having the suitable spectra and which also are commonly found on contaminated sites.

There is no special sample preparation required for counting samples using a germanium detector or a sodium iodide detector beyond placing the sample in a known geometry for which the detector has been calibrated. The samples can be measured as they arrive at the laboratory, or the sample can be dried, ground to a uniform particle size, and mixed to provide a more homogeneous sample if required by the standard operation procedures.