Dosimetric Characteristics of HZE Particles in Space.

Abstract

The name HZE Particles has become customary for the small fraction of galactic heavy primaries with Linear Energy Transfer (LET) values substantially larger than those of conventional nuclear radiations. HZE particles represent a component of ionizing radiation in space that is essentially unexplored as to its biological significance. Although cellular destruction by single hits has been demonstrated in pilot experiments, broader information on dose/effect relationships is missing. General agreement seems to exist merely on the negative proposition that conventional dosimetric concepts and units are rnot applicable. Eye flashes experienced by the astronauts on lunar missions, presumably triggered by HZE particles, have deepened concern lest microlesions from HZE particles might pose a subtle yet insidious hazard especially for non-proliferating cell systems in the human body, such as the CNS.The composition of the primary spectrum of HZE particles in space as well as the physical characteristics of their interaction with matter are well understood. However, the pertinent information reported by physicists usually is presented in terms not directly suitable for dosimetric interpretation. This report attempts to present the basic data on the physics of HZE particles with special consideration of the needs of the experimenter planning to study effects on biological systems. FINDINGSDiscrepancies between energy spectra of galactic heavy primaries reported by various investigators require establishing a compromise model before flux densities and hit frequencies in special targets are analyzed. For better descriptiveness, the model is converted to the range spectrum which allows direct reading of flu:: densities for varying shield thickness or target depth. The iron group (Z = 24 to 28) as the heaviest component of th -HZE particle spectrum is selected for detailed analysis. It is shown that the LET distribution shows a steep negative slope with hit frequencies dropping to less than one hit of maximum LET per cc tissue per 24 hours. As an alternate way of presentation, the LET distribution is also established in terms of integral track length which furnishes a more complete picture of the energy dissipation pattern i tissue. The problems posed by the peculiar LET distribution of HZE particle irradiation for the design of both radiobiological experimentation and dosimetric instrumentation are pointed out.ii