Abstract
The determination of zinc contained in the sample such as air-borne by non-destructive, neutron activation analysis by measuring a γ-ray spectrum is disturbed by scandium accompanied with zinc. It is difficult to distinguish the photo-peak with 1115 keV of 65Zn (half life; 245 days) from that with 1120 keV of 46Sc (half life; 83.9 days), even though a Ge(Li) detector is used. These radioactive nuclides are produced by the thermal neutron irradiation of target elements, 64Zn and 45Sc whose thermal neutron cross section and natural abundance are 0.64 barn and 48.9%, and 23 barn and 100%, respectively.
In this report, after separation of neutron irradiated zinc and scandium by thin-layer chromatography (TLC), zinc was identified by a NaI (Tl) scintilation detector which was not superior to Ge(Li) detector in the resolution of measuring of γ-ray spectrum, but excellent in the counting efficiency and the maintainance. The sample solutions were prepared as follow; after irradiation of scandium oxide or zinc metal by thermal neutron (flux: 1 × 1012 n/cm2.sec) in a reactor for 6 hours, samples were dissolved with an acid. The solutions were developed on cellulose as an adsorbent, with a mixture of n-propanol and 3N HCl in a volume ratio (10 : 1) as a developer which is the most excellent one in several kinds of mixed solutions. The developed spot of zinc was overlapped on the acid front with sharp shape and the Rf values were 0.74 for zinc and 0.02 for scandium. The sample solutions including of different ratios of scandium to zinc such as 50 μg : 0 μg, 50 μg : 1 μg, 50 μg : 5 μg, 50 μg : 50 μg, 50 μg : 500 μg were developed by using the TLC system. No trace of scandium could be detected in each of these zinc fractions on the thin-layer plates. Next, the determination limit of zinc was examined by measuring γ-ray photo-peak of 65Zn and total γ-ray of 65Zn. Zinc was separated by the TLC system above mentioned from the sample solution containing cobalt, scandium, iron and zinc of which Rf values were 0.02, 0.02, 0.45 and 0.74, respectively, in order to exclude the radio-activities of 60Co, 46Sc and 59Fe which were produced by the irradiation of thermal neutron to target elements. These radioactive nuclides disturb the identification of the photo-peak of 65Zn by a NaI(Tl) detector for their long half-lives and γ-ray energy more than 1 MeV. More than 5 μg of zinc can be determined by measuring the γ-ray spectrum and moreover the determination limit of zinc was found to be about 1 μg with an error of 20% by counting total γ-ray of 65Zn in the zinc spot developed on the thin-layer plate.
