Behavior of40Kand137Cs on the radial direction of the annual ring of sugi (Cryptomeria japonica D. DON) was investigated by means of γ-ray spectrometry. It was confirmed that40Kand137Csmove from one annual ring to another. The concentration of the activity of these nuclides are almost constant in the heartwood, but changes markedly near the sap and heartwood boundary. Distribution profiles of these radioactive nuclides along the radial direction of the annual ring are similar to those of their stable isotopes and of another alkali metal elements such asNaandRb.The results suggest that alkali metal elements have intimate relation with the heartwood formation of sugi.
