A photographic personnel dosimeter for x-radiation in the range from 30 kev to beyond 1 MEV.

Abstract

Introduction Biological Considerations: When used for x-ray dosimetry, photographic film is at best a rough, secondary instrument. It has to be calibrated against primary standards, processed under controlled conditions, and, if possible, placed in a special holder designed to produce a suitable film response as a function of radiation energy. Ideally, the response of a radiation dosimeter should parallel closely the radiation damage to biological tissue. For the tactical radiation dosimeters used by the Military, the biological damage criterion usually selected is the lethal effect of the radiation on mammals. In the case of x-rays, the lethal exposure as measured in roentgens in free air was found to increase sharply below about 80 key (1). A sharp drop of dosimeter response below this energy is therefore sometimes thought to be desirable for tactical x-ray dosimetry. As most of the film dosimeters presently in use employ filters of high atomic number over at least a portion of the sensitive area (2, 3), they display this cut-off. In civilian applications, biological criteria other than lethality apply, and damage to superficially located biological structures must be considered. It is now believed that, for most types of biological action, the effect of radiation is proportional to the amount of energy imparted to tissue along unit track length of the secondary ionizing particles (linear energy transfer) (4); also, it has been shown (5) that the linear energy transfer of electrons rises sharply as their energy decreases from 100 key to 1 kev. If one assumes that for equal energy absorption per gram of tissue the relative biological effectiveness of x-rays below 100 kev does not decrease as the x-ray energy decreases, one may con conelude that a response cut-off of a radiation dosimeter below 80 kev is undesirable for most civilian applications. A ims and A ccomplishments of Present Study: Groups concerned with personnel monitoring have frequently expressed a demand for a film dosimeter whose response to x-radiation of energy below 100 kev parallels more closely the energy absorption in biological tissue, or at least that in air (i.e., the exposure in roentgens). The instrument described in this paper is, in part, an answer to this demand. Its response to x-radiation between 30 kev and 1 MEV lies within ±20 per cent of that of air for exposures ranging from roughly 250 mr or less to about 3 r, and within ±30 per cent for exposures ranging from about 3 r to 6 r. It is useful also for radiation energies higher than 1 MEV, although with slightly decreased accuracy, and its exposure range can be extended up to 600 r for high-energy emergency work. Principle of Film-Stack Dosimeter Basic Idea: The idea for the dosimeter here referred to as the "film-stack dosimeter" was first conceived by Allisy (6).