RADIOISOTOPES

Show abstractHide abstract

For assessment of radiation doses due to ¹⁴C released from nuclear facilities, at is necessary to know the background level of environmental ¹⁴C originated from the natural cosmic-ray production and the past atmospheric nuclear weapons tests The authors selected grapes as an indicator of ¹⁴C levels in the environment including atmosphere, and determined the specific activities of ¹⁴C in ethanol extracted from wine made from grapes cultivated in several prefectures or unknown places in Japan in each year from 1990 to 2000.
The specific activities of ¹⁴C were almost uniform all over Japan an each year, though they showed the weak tendency to slightly decrease with increasing population densities an the prefecture where the wine grapes were cultivated. It is considered that this small regional variation in the specific activities of ¹⁴C arose from Suess effect, because population densities can be regarded as an index of the amount of CO₂ emission from fossil fuel combustion.
The specific activates of ¹⁴C gradually decreased in the 1990s. That is, they were 260 mBq/gC in 1990, and became 244 mBq/gC in 2000. This ¹⁴C level in 2000 was still about 7% higher than the pre-bomb level. The decrease in ¹⁴C originated from the past atmospheric nuclear weapons tests could be described by a single exponential function with a half-time of 10.3 years in 1990 s. This half-time was shorter than that in the Europe, Canada and the South Pacific. This difference in the decreasing rate between Japan and other countries may be explained by a fact that the amount of CO₂ emission from fossil fuel combustion continued to increase in Japan in this period.
The specific activities of ¹⁴C in wine ethanol determined in this study showed good agreement with those in atmospheric CO₂ predicted by a mathematical model They were also in the same levels as the ¹⁴C specific activities in atmospheric CO₂, tree leaves, grass and plant food in Japan and other countries, which were determined by the benzene synthesis liquid scintillation counting or other methods These methods of ¹⁴C determination are not so simple as the method adopted in this study. It is therefore considered that the method of determination of the ¹⁴C activities in wine ethanol is simple and precise enough to estimate the ¹⁴C activities in atmospheric CO₂ and plants including crops, and thus contributes to assessment of doses due to environmental ¹⁴C.

View full abstract

Download PDF (2241K)

A spreadsheet software, Microsoft Excel, was used to calculate beta-ray spectra, which are necessary to obtain bremsstrahlung data for betaemitting nuclides. The results of the present calculation were compared with numerical data listed in ICRU Report 56, Appendix D.
The calculated spectra for 14 nuclides among 36 nuclides of importance listed in ICRU Report were in good agreement, 20 showed slight discrepancy, and two, ²⁰⁴Tl and ²¹⁰Bi, did not agree.
Among the above-mentioned 20 nuclides, those which emit single beta-ray could be made to agree with ICRU data by multiplying a newly modified correction factor. For the remaining nuclides correction was not applied because no detailed information e.g. maximum energy and intensity of beta rays is given in ICRU Report.
To calculate“|Γ (γ+ iy) |²” of the Fermi function in the formula of a beta-ray energy spectrum, two types of calculation were carried out: 1) direct calculation of infinite product and 2) calculation with approximation used in ICRU Report. The results of both calculations agreed well.

View full abstract

Register with J-STAGE for free!