Radiation units and radiation instruments.

Abstract

The field of radiation dosimetry involves a close relationship between units and instrumentation that is similar to the proverbial chicken and egg relationship, in which it is difficult to state unequivocally which came first. This duality has had a profound influence on the historical development of the field. It also justifies the presentation of the background for this symposium in a way that intermingles units and instruments. It is to be understood that the principles of instrumentation, rather than the detailed design, construction, and operation of specific apparatus, will be considered. Basic Choice of Measuring Method A unit is the specific magnitude of a quantity, set apart by appropriate definition, which serves as a basis for comparison or measurement for other quantities of the same nature. Whenever a phenomenon of science is discovered, a measuring system, and eventually a unit, develops upon the basis of some conveniently observable effect of the new agent. It is not uncommon to find that familiar agents can be satisfactorily measured, for different applications, by more than one method. In the case of electric current, for example, the scientific unit is based on the magnetic effect of the current, whereas the practical unit is based on the unrelated electrochemical deposition effect. It should be neither surprising nor discouraging that in the younger field of radiology there should have been a multiplicity of units. In the early days of radiology, many of the newly discovered effects of ionizing radiation served as measuring methods, often with associated arbitrary units. The calorimetric method dates back to 1897. Chemical methods and fluorescence or scintillation were used in the first years of the present century. One of the earliest dose systems was founded on the coloration of the Sabouraud and Noiré pastilles. The photographic method, the primary means of detection, was made reasonably quantitative as early as 1905. In the following decade, radiation effect on electrical resistance was used. A wide variety of biological effects was scrutinized for dosimetry use. Ionization in gases gained an early lead. The first formal statement of an ionization unit was apparently that of Villard in 1908, defining a unit quantity of x-radiation as that which by ionization liberates one electrostatic unit of electricity per cubic centimeter of air under normal conditions of temperature and pressure. This is still the elementary interpretation of the present-day “roentgen.” The reasons for preference of the ionization method in air are of importance in the whole development of radiological units.