Speaker
Description
Despite ²²⁶Ra and ²²⁸Ra deriving from different decay chains, the activity ratio in foods can reveal important aspects of radium transfer processes from soil to plants and further on along the food chain. Although ²²⁸Ra and ²²⁶Ra are frequently found in soils and waters at activity concentrations approximately 1:1, ²²⁸Ra has not yet been systematically measured on a comparable scale to ²²⁶Ra. There are no global compilations for typical ²²⁶Ra/²²⁸Ra activity ratios in soils available. Only several studies were focused on both Ra soil-to-plant transfer factors using the same plant species (Linsalata et al., 1989; Cooper et al., 1995; Sheppard et al., 2008 etc). In those papers there is evidence for a transfer factor of ²²⁸Ra being larger than that for ²²⁶Ra. The recent data analysis from these studies carried out as the initial phase of the present reseach showed that the difference is by a factor of about 1,7. However, most of publications have a lack of important supplementary data (pH value, temperature, soil moisture, chemical and granulometric composition of the soil, precipitations, etc.) that could potentially explain the observed differences in the transfer between ²²⁸Ra and ²²⁶Ra to plants.
One possible explanation for the differences in the transfer factors for ²²⁸Ra and ²²⁶Ra is that the parent isotopes ²³²Th and ²³⁰Th are present in different phases or with different sorption strengths. This can influence how much of the daughter radium is released and becomes biologically available. Currently, the transfer of both long-lived ²²⁶Ra and ²²⁸Ra isotopes are treated with the same transfer factor. Despite the relatively small contribution of radium isotopes to the total radiation dose, an underestimation of the dose caused by ²²⁸Ra is possible as it has one of the highest dose conversion factors for ingestion among the natural radionuclides.
This research initiated by the German Federal Office for Radiation Protection aims to examine whether the general assumption of isotope-nonspecific soil-plant transfer factors for ²²⁶Ra and ²²⁸Ra is justified. Testing this hypothesis, the research will either confirm the existing conservative assumption or incorporate the newly determined, nuclide-specific transfer factors for ²²⁶Ra and ²²⁸Ra into relevant areas. The study selected 5 locations in different regions of Germany with different concentrations of radium in the soil. Vegetables and fruits and the corresponding soil samples were collected mainly from farm fields. They include fruit and vegetable species typical for studied regions: potatoes, carrots, onion, spinach, cauliflower, apple, pear, strawberry, plum, and pasture grass. Corresponding to the plant locations, the soil in which the plants grow was sampled, so that the soil-plant transfer factors can be calculated. In addition, it is planned to conduct experiments with plants grown in containers in a phytochamber and outdoors.
The harvested crops are washed, dried at T = 105 C, ashed at T = 450 C for 24 h and measured with HP-Ge gamma spectrometer in the Petri dish geometry. The soil samples are dried at T = 105 C and measured also with HP-Ge gamma spectrometer in Marinelli beaker geometry. Further measurements of the subsequent crop and soil batches this year will provide an opportunity to identify more detailed trends in the accumulation of ²²⁸Ra and ²²⁶Ra isotopes by the studied crop species and thus to reveal the potential differences in their uptake by estimating the transfer factors, as well as access their contribution to the internal radiation dose.