ISO 13166:2020 pdf download Water quality — Uranium isotopes — Test method using alpha-spectrometry
4 Principle
The test sample is mixed with an aliquot of 232 U tracer, followed by equilibration of the sample prior to analysis. Chemical isolation of uranium is achieved by a concentration step (e.g. a precipitation) followed by a specific separation step (e.g. using ion exchange chromatography). The detection limit for measurement of a test portion of about 500 ml is approximately 5 mBq · l −1 with a counting time of about 200 000 s. Natural radionuclides such as 230 Th, 226 Ra and 228 Th can be present in water and can interfere with the counting of uranium isotopes if no chemical separation is carried out to remove these radionuclides from the test portion. Plutonium isotopes can also interfere, if present with detectable activities in the sample. The measured thin source is prepared by electrodeposition or coprecipitation and measured by alphaspectrometry using a grid chamber or a semiconductor-type apparatus.
Measurements rely on the interaction of alpha-particles with the detecting medium. This interaction creates a charge, which is amplified and output as a voltage pulse proportional to the deposited energy of the incoming alpha- particle. The electric pulse from the detector is analysed by the electronic systems. Data analysis software provides a spectrum, in which the number of pulses (counts) recorded in each energy interval is shown. The analysis of the count rates in the uranium isotopes alpha-energy windows allows the determination of the test sample activity concentration for 238 U, 235 U and 234 U, after correcting for the blank count rate, volume of the test sample and the total measurement yield (chemical yield and detection efficiency).
The chemical yield and detection efficiency are not necessarily determined separately, but are determined together by measuring the total measurement yield from the net count rate of 232 U, added as a chemical yield tracer.
In order to quantify any potential interference coming from the reagents, a blank sample is prepared in the same way as the test sample. This blank sample is prepared using a laboratory water.
For quality control, in order to quantify potential impurities in the tracer solution, another blank sample shall be prepared with addition of tracer.
The radioactive characteristics of the main uranium isotopes are given in Table 2 (References [8], [9]).
With a spectral resolution (FWHM full-width half-maximum height) of around 20 keV in best cases, alpha-spectrometry cannot easily resolve 233 U and 234 U, nor 235 U and 236 U, due to the similarity in their respective emission energies. However, 233 U and 236 U are normally not present in environmental samples or in quantities above their detection limits using alpha spectrometry (see Annex D).
5 Chemical reagents and equipment
5.1 General
The chemical reagents and equipment used for chemical treatment and preparation of the source are described in Annexes A to C, which give various alternatives. Where there are options, at least one of the options presented shall be used. Use only reagents of recognized analytical grade.
5.2 Chemical reagents
5.2.1 Laboratory water, used as a blank, as free as possible of chemical or radioactive impurities (e.g. uranium isotopes), conforming to ISO 3696, grade 3. Fresh rainwater is an example of water with a very low uranium activity concentration. The uranium activity concentration of this water can be evaluated at the same time as interferences from reagents or using another type of precise measurement, e.g. thermal ionization or inductively coupled plasma mass spectrometry.
5.2.2 232 U tracer solution, used to determine the total yield. It can also be used to calculate the chemical yield. The solution is prepared by the dilution of a suitable standard that provides traceability to national and international standards. The tracer solution shall be homogeneous and stable. The tracer solution concentration should be calculated to allow adding a small amount of this solution in order to be in the range of activity contained in the test portion. For example, the tracer solution concentration could be between 0,05 Bq · g −1 and 1 Bq · g −1 .ISO 13166:2020 pdf download