ABSTRACT

Radiochemical NAA method has been studied for natural yttrium along with co-existing rare earth elements (REE). After neutron irradiation to the samples in the JRR4 reactor for a few hours with 6-8×10¹³ n/cm²sec, yttrium and REE mixture was separated as a group. Yttrium-90 was determined without further separation by a beta counting method using two appropriate absorbers. The interference due to other activities was reduced further by an anti-coincidence method in a well type NaI(Tl) scintillator as a guard counter. The pure hard beta ray, E_max = 2.3 MeV, emitted from ⁹⁰Y has a long range and was useful to discriminate other softer beta and gamma activities of REE nuclides. For terrestrial rocks and chondritic materials, the interference from REE did not exceed 10% of the overall signal recorded by an ordinary GM counter. The usefulness of this method was illustrated by applying milligram quantities of standard rock samples and a bulk chondritic sample of Maralinga, CK4. An extreme example was the 0.5-1 milligram sized Group II CAI inclusion samples hand-picked from the Allende chondrite. They have higher La/Y ratios, 30 to 250 times higher than the solar abundances. The sizes were sub-nanogram of Y and Ho. The lower abundance ratios of Y/Ho could be determined in these samples.