Abstract
The study on the effects of high LET radiation and chronic exposure on animals and plants is important in environmental radiation protection, because radiation effects in environment could be attributed to the chronic exposure to alpha emitters as well as gamma and beta emitters. Therefore, we studied the effects of heavy ions at NIRS-HIMAC with various high LET and of chronic exposure to gamma radiation on the growth of Enchytraeus japonensis (E. japonensis), a species of terrestrial earthworm, kindly donated by Dr. M. Myohara, (National Institute of Agrobiological Sciences, Tsukuba, Japan). The earthworm is known to grow very rapidly and increase more than 10 fold in population every half a month. The earthworm was exposed to gamma radiation or C, Ne, Si, Ar, Fe ion with the energy of 290, 400, 490, 500 and 500 MeV/u, respectively. The earthworm was then reared on plain agar medium in Petri dishes by occasional supply with powdered rolled oats and autoclaved distilled water, and the number of worms was enumerated 30 days after irradiation. Chronic exposure to gamma radiation was performed during 30 days rearing period. Heavy ions clearly showed stronger effects than gamma ray with respect to the growth inhibition of the earthworm. As LET was increased, heavy ions appeared to inhibit more effectively the growth, however the effects of Si, Ar and Fe ion were not significantly different. The dose or dose rate required to suppress 50% of growth was approximately 5Gy/day, total dose of 150Gy in chronic exposure compared with 22Gy in acute exposure.
