Interpretation of microbeam experiments for manned space flight

Abstract

The track of a very heavy charged particle in tissue is composed of a very small central core of ionization caused by the particle itself, surrounded by a much wider area of ionization caused by the secondary particles ejected from the central core. Thus, most of the volume of the track is secondary radiation, and a living cell in the path of the track would probably be affected mostly by proton and electron radiation (1). Since no accelerators are yet available for irradiating mammalian tissues with heavy particles such as exist in small numbers in outer space, a microbeam of deuterons has been developed which will simulate the biological effect of these particles (2). The beam has a diameter as small as 0.025 mm and is arranged so that precise areas of a mammal can be irradiated. The accompanying paper (3) describes this apparatus and the results obtained with it. Briefly, it has been shown (1) that it takes enormous doses of radiation delivered through a microbeam 0.025 mm in diameter to cause destruction of the brain cells in the beam path; (2) that the epithelial cells of the lens of the eye are quite sensitive to this type of radiation, but the microbeam disrupts only one or two such cells and this is insufficient to cause a cataract; and (3) that the hair follicles are quite sensitive to this beam, and relatively small doses to the individual hair follicle will cause the hair from that follicle to turn gray. At the time these results were obtained, they seemed somewhat at variance with current thinking in this area. Since that time a number of other radiobiological observations have made these findings seem quite plausible. The present paper is a discussion of recent developments in radiobiology, the way in which the microbeam results correlate with these concepts, and from these an assessment of the possible hazards from the heavy cosmic-ray particles for manned space flight.