Ionization path (track): The trail of ion pairs produced by an ionizing radiation in its passage through matter. Ionizing radiation: Any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, in its passage through matter. Secondary radiation: Radition resulting from absorption or other radiation in matter. It may be either electromagnetic or particulate. Kerma: The quotient of dEtr by dm, where dEtr is the sum of the initial kinetic energies of all the charged particles liberated by indirectly ionizing particles in a volume element of the specified material, and dm is the mass of the matter in that volume element. K - dm Kerma has the same dimension as absorbed dose and both quantities have the same special unit, the rad. The unit in the International System of Units (SI) is the gray, symbol Gy; 1 Gy = 1 J kg~^ = 100 rad. Latent period: The period or state of apparent non-effectiveness between the time of exposure of tissue to an injurious agent and response to that exposure. Linear energy transfer (LET): The average amount of energy lost per unit of particle track length. Low-LET: Radiation energy transfer characteristic of electrons, x-rays, and Gamma rays. High-LET: Radiation energy transfer characteristic of heavy charged particles or fast neutrons. Linear no threshhold hypothesis: The assumption that a dose-effect curve derived from data in the high dose and dose-rate ranges may be extrapolated linearly through the low dose and low dose range to zero, implying that, theoretically, any amount of radiation may cause some damage. Low earth orbit (LEO): Orbit at an altitude of approximately 500 km.
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