Acute effects of high-energy protons and alpha particles in mice.

Ionizing radiation is one of the many potential hazards man must face as he endeavors to conquer cosmic space. The greatest source of this hazard in our solar system is the sun, which periodically bombards us with highly energetic protons and a particles. In most instances the radiation dose from solar flare particles is quite small. However, during periods of maximum solar activity the dose can become very intense and dangerous to man and animal. Freier and Webber (1) have recently reported that the dose from a particles in solar flares may equal that for protons of energy above a certain value, and that the flux of heavier nuclear particles is about 1 % of the a-particle flux in most solar flares. These authors have estimated that during a 10-day period in July, 1959, the integrated dose from the isotropic flux of a particles and protons would have reached 490 rads in air inside a spherical shell of water-shielding 2 grams per square centimeter thick, and that under the same conditions it would have reached 488 rads during a 6-day period in November 1960. Tissue dose from solar cosmic-ray irradiation will depend on both flux geometry and the rigidity of the particle spectrum. The maximum energy of the solar flare particles for the July 1959 events decreased from 200 to 100 MeV during the 24-hour period after the onset of the flare. If we take 5 cm as the nominal depth of the blood-forming tissues in man to calculate solar flare dose hazards (2), the esti-