The use of deuteron microbeam for simulating the biological effects of heavy cosmic-ray particles.

atomic nuclei of elements heavier than carbon and as heavy as iron. They are stripped of their electrons and thus are highly charged. As these particles are slowed down in tissue, they interact with the elements of the tissue and cause very energetic 6 rays to be emitted. The result is a track in tissue having very dense ionization near the end of its path, and this is known as a "thindown." This thindown region may be as much as 0.025 mm in diameter and 1.5 mm long. Schaefer (1) has computed that the radiation dose in this ionization core may be as much as 10,000 rads at its center, tapering off to low values toward the edges. Only the particles having energies less than about one billion electron volts per nucleon will cause this type of damage, since particles of higher energies will form stars and dissipate their energies widely. It would be virtually impossible to shield against these latter particles in a space craft because even a very heavy shield would only slow down the very energetic particles to the point where they would become highly ionizing and thus dangerous. In assessing the biological effect of such particles it would obviously not be correct to apply the usual rules for tolerance doses-that is, to compute the total dose from the energy deposited in the body of an average man. For heavy particles the energy is all deposited in very small volumes, or "hot spots," leaving the rest unaffected. This then constitutes a special radiobiological problem. These particles cannot presently be generated in the laboratory with sufficient energy to be used for mammalian experiments. Consequently an indirect experimental approach had to be developed (2). This consisted in confining the 22-MeV deuterons from the Brookhaven 60-inch cyclotron in a beam 0.025 mm in diameter. This microbeam was arranged in such a way that it could be directed at any pre1 Research carried out at Brookhaven National Laboratory under the auspices of the U. S.