A Chemical Method for the Measurement of Quantity of Radiation

Abstract

The international unit of roentgen radiation has been defined in terms of the amount of ionization produced in a cubic centimeter of air under certain specified conditions. This ionization is to be measured by means of a standard chamber the specifications of which have not yet been agreed upon by the various committees interested in the problem. However, even when the standard instrument is decided upon, it will not be a practical device for general use. The primary standard for the United States will be at the Bureau of Standards, and there will be copies of it in a few laboratories, but for the most part measurements of radiation quantities will have to be made by means of some secondary standard. Ionization chambers of various sorts are already in use for this purpose, and these can be calibrated by direct comparison with a standard instrument, provided they meet certain minimum requirements. It seems, however, that there is a need for a simple, accurate, reliable method of measuring quantity of radiation which is not dependent on specialized apparatus. This would permit easy duplication of results in various parts of the world without the necessity for actual comparison of instruments. Wood (1) has proposed such a method with Drosophila eggs, but in this case various technical difficulties may arise if a trained biologist is not available. A very satisfactory method could be developed from a chemical change induced by radiation, provided it possessed certain characteristics. The chemical substances used must be fairly inexpensive, and easily obtainable in the required purity. The method of measuring the reaction produced should be simple enough to be mastered by the average trained technician or radiological interne, and it should not require expensive or elaborate apparatus. A readily measured change should be produced by amounts of radiation of the order of magnitude of those used in therapy, and there should be no spontaneous reaction going on for which corrections need be made. A great many chemical changes are produced by the action of radiation, but very few of these are suitable for the purpose here considered. The first type of reaction which may be thought of is the darkening of a photographic film. In this case a standard material is readily obtained and handled, and within a rather wide range the amount of change produced may be measured quite easily. However, the sensitivity is so great that complete blackening occurs with quantities of radiation of the order of 1 per cent of that required to produce a skin erythema. That is, with a regular therapy type of radiation, the time of exposure would have to be too short to measure conveniently, or the distance from the X-ray target to the film very great. The use of either very long distances or very short times introduces difficulties which need not be discussed here. Also standard films must be made and developed with each set of test films and even then slight variations in development technic and solutions may make accurate comparison difficult. The apparatus for determining the density is no simpler than many ionization instruments. This reaction, therefore, is not practical for use in such a standard procedure.