Radiation Dosimetry and Protection

below this limit, exposure in roentgens is related to the energy absorption in most biological tissues by a sensibly constant factor of proportionality. 2 The reasons for the abandonment of the roentgen as a universal unit of dose have been treated at length in the literature (4, 5, 6) and they will not be discussed here. It will suffice to comment that, by virtue of the new unit adopted, the concept of dose remains unaltered whatever the type and energy of the ionizing radiation, and that, in principle at least, the measure­ ment of radiation dose need not necessar ily involve a measu rement of ioniza­ tion. In most instances, however, measurements of gas ionization in cavity chambers are likely to remain the method of choice for some time because of their relative ease and because of the basic property of the chamber which is represented by the familiar expression of Gray (7). 1. In this equation , Dm is the dose in the waIls of the cavity in ergs/gm., Jm is the number of ion pairs per unit mass of gas in the cavity, W the energy in ergs necessary to form a pair of ions in the gas, and Pm the ratio of the mass stopping power of the waH material to that of the gas for the ionizing particles associated with the incident radiation. Some readers will recall that accept