Erythema Doses in Absolute Units

OF all the various methods of determining the quantity and quality of the roentgen rays, the consensus of opinion to date would suggest that ionization in air. The first to propose such a unit, the so-called electrostatic unit, was the French physician Villard (1), as early as 1908. He defined as a quantimetric unit, that quantity of radiation that produces one electrostatic unit per cubic centimeter of air under normal conditions of pressure and temperature. From a scientific point of view the best definition of this unit was given by Behnken (2), in 1924, basing it on the valuable work of Friedrich (3), Duane (4) and Holthusen (5), as well as on his own investigations. Friedrich had already improved upon Villard's definition in 1918, Duane publishing along similar lines at about the same time. Behnken defines the unit as follows: The absolute unit of the roentgen-ray dose is obtained from that roentgen-ray energy, which, by fully utilizing the secondary electrons and by avoiding secondary radiation from the wall of the chamber, produces in one cubic centimeter of atmospheric air under normal conditions such a degree of conductivity that the quantity of electricity measured by saturation current equals one electrostatic unit. This definition corresponds practically to that of the older e-unit but avoids its original inaccuracies. Behnken, however, retains the name of “Roentgen,” as originally suggested by Solomon (6), to designate this unit. Solomon bases his roentgen unit on the ionization produced in a small graphite ionization chamber by a given amount of radium under specified conditions. In 1922 he defined his Roentgen-unit as follows: The absolute unit of the roentgen-ray dose is that intensity of the radiation which produces the same amount of ionization per second as one gram of radium element at a distance of 2 cm. from the ionization chamber (both in the same axis) and filtered with ½ mm. of platinum. The quantity of radiation is given by the radiation intensity expressed in R per second, multiplied by the time of application. Since experimentally the definition of the Solomon R-unit is not free from objection and since it depends at any rate in its applicability to a large degree upon the special type of indicating instruments used, the e-unit must be given the preference for general use. But even if Solomon's method takes second place as far as the definition of an absolute dosage unit is concerned, it has distinct practical advantages in the fact that radium is employed to control the constancy of a dosimeter and check its calibration. This advantage might well be combined with the e-unit method to help the practitioner in the constant control of his instrument.