Radiation Protection and Environment

: 2020  |  Volume : 43  |  Issue : 2  |  Page : 59--60

238U/226Ra or 232Th/228Ra: What is being measured in dietary items by gamma spectrometry?

DD Rao 
 Editor, RPE; Ex. Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Correspondence Address:
D D Rao
Editor, RPE; Ex. Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra

How to cite this article:
Rao D D. 238U/226Ra or 232Th/228Ra: What is being measured in dietary items by gamma spectrometry?.Radiat Prot Environ 2020;43:59-60

How to cite this URL:
Rao D D. 238U/226Ra or 232Th/228Ra: What is being measured in dietary items by gamma spectrometry?. Radiat Prot Environ [serial online] 2020 [cited 2020 Sep 19 ];43:59-60
Available from: http://www.rpe.org.in/text.asp?2020/43/2/59/293625

Full Text

The title of this editorial seems to be very simple, common, and nothing may be new to occupy editorial space, but the author has been observing contradictory publications regarding the measurement and assessment, for many years and is being continued. In this editorial, consciously, no references are provided, but easily one may “Google” and find the contradictory publications.

238 U and232 Th and their decay radionuclides are naturally occurring radioactive materials (NORMs), existing in the earth's crust since its formation. Researchers undertake an assessment of radiation exposure to members of the public by conducting measurements to evaluate238 U and232 Th in several matrices. It is a well-known fact that direct measurement of238 U and232 Th is rather difficult and complex in environmental matrices such as soil, sediment, vegetation, water, and other dietary components. Researchers make use of indirect technique, the widely used simple gamma spectrometry using either a NaI(Tl) or high-purity germanium detector for their determination based on their decay radionuclides. The technique is simple and at the same time is highly accurate if proper calibrations are performed. However, it is still an indirect measurement method as it is based on the measurement of decay radionuclides which emit gamma radiations and assume that238 U and232 Th are in secular equilibrium with their daughter products. The accuracy of measured concentrations in the matrices is solely dependent on the status of assumed secular equilibrium.

The daughter radionuclides used for the determination of238 U are214 Pb and214 Bi; similarly for232 Th, they are228 Ac,212 Pb,212 Bi, and208 Tl. In general, a standard protocol of measurement is followed by sealing the sample container and keeping it for a month. However, this assumed equilibrium is obtained only between226 Ra and its daughters and228 Ra and its daughters. The decay of238 U includes long half-life radionuclides of234 U (2.5 × 105 years) and230 Th (7.5 × 104 years) before226 Ra (1600 years), and similarly, the decay of232 Th daughter radionuclide228 Ra has half-life of 5.75 years. The assumption of secular equilibrium between238 U and its daughters and232 Th and its daughters is true for matrix-like soil/sediment, as it is in undisturbed condition for as many years as required. In other matrices such as water, vegetation, and other dietary items, the secular equilibrium cannot be assured/assumed. The transfer rates of238 U/226 Ra and daughters and232 Th/228 Ra and daughters are not the same and differ in orders. The average transfer factors for soil to vegetation for238 U,228 Ra,232 Th, and226 Ra are 4.8 × 10−2, 1.9 × 10−1, 3.4 × 10−5, and 1.9 × 10−2, respectively. Since the transfer rates being different, the Ra, U, and Th transfer to the vegetation is different. The daughters of226 Ra/228 Ra are all short lived and will have a secular equilibrium between soil and vegetation. Hence, the determined radionuclides represent only226 Ra and228 Ra in vegetation, dietary items, and water by gamma spectrometry.

The assessment like an evaluation of annual effective dose from the consumption of vegetation and other dietary items should also be based226 Ra and228 Ra and not their parent radionuclides of238 U and232 Th. The dose conversion factors (DCFs) vary widely and are different for these radionuclides (ICRP-119 for adults, ingestion, and members of the public). For example, the DCFs in Sv/Bq for238 U,226 Ra,232 Th and228 Ra are 4.5 × 10−8, 2.8 × 10−7, 2.3 × 10−7 and 6.9 × 10−7 respectively.

This author, as editor of this journal, has been viewing several papers with the above contradictions and has been advising the authors for technical modifications in the initial stages itself before marking them for peer review. Many scientific papers were also published in various journals where reviewers/editors underscore this technicality and clear through the papers. This author along with coauthors has published a detailed scientific paper (Radiation Protection and Environment, Vol. 40, 2, 2017) on this subject for making use of proper measurement techniques and there from the assessments.

Therefore, when the indirect gamma spectrometry technique is used for the determination of NORM in dietary items, the following precautions should be taken.

Mention that the measured radionuclides are226 Ra and228 Ra and not238 U and232 ThUse DCFs of228 Ra and226 Ra only for the assessment of committed effective doseSecular equilibrium may be assumed only for soil and the measurement may represent238 U and232 Th.