The RBE of Gamma-rays and Fission Neutrons for Acute and Sub-acute Effects in Irradiated Rabbits

Summary

Rabbits were given acute whole-body irradiation with either neutrons (mean energy 0·7 mev, dose range 400–1500 rads) or gamma-rays (mean energy 2·5 mev, dose range 1000–3000 R). The last acute death was at 32 days and the LD50/32 was 572 rads for neutrons and 1389 R for gamma-rays, giving an RBE of 2·4.

Acute deaths were primarily due to haemopoietic failure, but damage to the alimentary tract probably also played a part. Coprophagia (re-ingestion), which is normal in rabbits, may have increased the liability to post-irradiation infection.

Sub-acute lesions, between 2 and 7 months after irradiation, occurred only after exposure to neutrons and not after gamma-rays. Just over half were local lesions of limbs, related to the entry dose of neutrons, and the remainder were intrathoracic effusions. When these sub-acute effects were taken into account the RBE rose to 3·0.