RT Book, Whole DB /z-wcorg/ DS http://worldcat.org ID 796815662 LA English T1 The r-unit at 320 and 160 Mev A1 McMillan, Edwin M., Blocker, Wade., Kenney, Robert W., U.S. Atomic Energy Commission., Lawrence Radiation Laboratory., PB U.S. Atomic Energy Commission, Technical Information Service PP Oak Ridge, Tenn. YR 1950 AB The r-unit is used today in at least two different ways: as a measure of radiation dosage, and as a measure of x-ray intensity. The meaning of the former is fairly well defined in terms of energy absorbed per gram of tissue, and the standard thimble chambers are calibrated to read in these terms. The meaning of the latter is much less clear. As originally defined, the r-unit referred to the ionization in air due to secondary electrons in equilibrium with the primary beam of x-rays. With low-energy x-rays equilibrium is easily attained since the range of the secondaries is small compared with the absorption distance of the primaries in air, but with high-energy-x-rays this is no longer true. Since a clearly defined equilibrium no longer exists, the old definition must be abandoned and the question remains: what should the r-unit mean in connection with the intensity of a beam of high-energy x-rays?