放射能汚染魚に関する研究-IV

放射性鉄の確認について

抄録

In the previous report, description was given to the analysis of radioactive elements separated from the muscle of a skipjack which had been caught by the R. V. “Shunkotsu-Maru” at a station near Bikini Atoll on June 19th, 1954. By ion exchanger technique, the presence of Zn⁶⁵ was proved in a fraction withdrawn by 5% ammonium citrate at pH 4.18 from a column of Dowex 50 in which the ashed material was absorbed. But in another fraction obtained by elution with 0.5% oxalic acid, the nature of radioisotopes involved could not been ascertained.
Because of the weakness of radioactivity remaining in the muscle, the present study was mainly directed to analyzing redioactivity in the liver of the fish by the techniques almost as same as discribed in the previous report. And this time the unknown element, which can be eluted with 0.5% oxalic acid, was also obtained same as found in the muscle sample (Fig. 1). The unknown element was identified as radioiron (Fe⁵⁵ and Fe⁵⁹) on the basis of the following evidences.
Elution curve of the element in question eluted from Dowex 50 with 0.5% oxalic acid well coincided with that of radioiron treated with the same procedure (Fig. 3). Another experiment for separation was made by flowing through a column of Dowex 1 with various concentrations. of HCl as influent; and both samples, the standard radioiron and the sample in question, gave quite similar elution curves in the portion of 0.6M HCl as shown in Fig. 4. Energy of two gamma rays emitted from the element were found by a scintillation gamma ray spectrometer to be 1.1 MeV and 1.3 MeV respectively. These gamma rays were considered to be emitted from Fe⁵⁹ (Fig. 5). However, as decay of the element was much slower than that of Fe⁵⁹ (Fig. 6), the coexistence of some other radioisotope with a longer half life than Fe⁵⁹ could be assumed. And the absorption coeficient of A1, Ag and Au for radiation of the element under study was calculated from three absorption curves shown in Fig. 7; values obtained are 115, 460 and 460cm2/g respectively. These values are almost the same with those of K X-rays exhibited by Fe⁵⁵ (Table 1).
It is more feasible to consider that the appearence of the elements of such lower mass number as radiozinc and radioiron would be resulted from the hit of neutrons at the time of the explosion, unless these elements would be contained originally in the weapon. Because these radioisotopes can not be grouped to the so-called fission products.
In view of a fact that these heavy metals have high affinity with, and are readily taken into living organisms, the pollution of water mass resulted from the explosion test should be considered more serious than usual contammation caused by fission products.