BS ISO 7503-1:2016 pdf download – Measurement of radioactivity — Measurement and evaluation of surface contamination Part 1 : General principles
Due to the inherent shortcomings of both the direct measurement and the indirect evaluation of surface contamination, in many cases, the use of both methods in tandem can help ensure results which best meet the aims of the evaluation.
5.2 Direct method The direct method is the best approach whenever possible. In the direct method, the monitor probe is moved over a surface, with the face of the probe at a minimal distance of approximately 3 mm from the surface. The probe shall be kept stationary for a minimum to obtain sufficient accuracy. This measurement can then be used to determine the radiation emitted from the surface. There are many circumstances where the above measurement might not be possible. A surface may be so convoluted that it is not possible to monitor it directly, or the background radiation may be so high that it is impossible to obtain meaningful results from the measurements; however, these results should be recorded because a calibration could be provided later. In these instances, an indirect measurement has to be made using a wipe test.
5.3 Indirect method (wipe tests) A test procedure is often carried out using a filter paper or other wipe, typically 20 mm to 60 mm in diameter, which can be placed in commercial holder for measurement. The filter paper should be wiped over the area, usually at least 100 cm 2 , or whatever area is locally defined for the surface that may be contaminated with radionuclides. The filter paper can either be placed in a lab counter drawer to assess the level and type of activity, or sent to a radiochemistry laboratory for a full assessment of nuclide type and activity. In both instances, all measurements should be traceable to national standards or governed by local requirements. Wipe tests can be either “dry wipe” or “wet wipe”. In general, it is a senior health physics professional who makes the decision on which to use. The indirect surface evaluation contamination method is described in detail in ISO 7503-2. 5.4 Wipe test uncertainties A brief discussion on uncertainties is given in 10.3.
6 Radionuclide identification and spectral analysis
Normally, the radionuclides are known. If not, they need to be identified. Radionuclide identification of contaminants using hand-held instruments is only practicable where the contaminants are gamma emitting nuclides with energies in the range of 50 keV to 1500 keV. If the contaminant does not emit photons in this range, it may not be possible to identify the radionuclide with hand-held instruments. In cases such as an accident or where only one radionuclide is in use, it may not be necessary for it to be determined as the contamination is known. Otherwise, more sophisticated techniques such as beta and alpha spectroscopy are required and these techniques are usually only available in a well-equipped laboratory where samples from the contaminated site can be prepared and analysed.
Small hand-held instruments exist that permit spectroscopic analysis of gamma radiation. In general, the instruments use a small, approximately 40 mm × 40 mm, NaI crystal as the principle detector.The sensitivity of a NaI crystal to gamma radiation makes these instruments particularly useful as “search and locate” devices particularly for finding and identifying lost or hidden gamma sources. However, it is not possible to make an accurate assessment of contamination levels using this type of instrument. A small NaI crystal connected to a multichannel analyser (MCA) permits spectral analysis of the ambient radiation. The MCA may also contain an electronic library of many common nuclides and their associated photo-peaks.
The instrument shall be properly calibrated before use in a calibration facility that can provide traceability to national standards. The calibration should confirm not only the dose rate accuracy but also that the Multi Channel Analyser (MCA) has been correctly set up. If the MCA is not properly set up, the instrument is not able to perform automatic nuclide identification. The user should understand that automatic nuclide identification is limited to those nuclides in the instrument library. If peaks occur in the gamma spectrum, that are not automatically identified, the photon peak energy should be assessed from the spectrum and the literature consulted to try and identify the parent radionuclide. Alternatively, the user should consult with an experienced health physics professional or radio chemist.