Patent GB-718537-A (GB)
Title: An improved radiation dosimeter
IPC: G01T1/04
Filed: 1952-03-19
Abstract: 718,537. Radiation dosimeters. NUCLEONIC & RADIOLOGICAL DEVELOPMENTS, Ltd., and NICHOLLS, A. M. Feb. 4, 1953 [March 19, 1952], No. 7085/52. Class 98(1) A chemical dosimeter for measuring the strength of X-ray or gamma-ray radiation contains, preferably in a closed transparent vessel, a solution of the formazan of a suitable tetrazolium salt in a suitable halongenated hydrocarbon, or in an inert organic solvent containing a suitable dissolved halogenated hydrocarbon. A formazan of a suitable tetrazolium salt means one which, on exposure of the solution to radiation, gives a perceptible change in optical density and yields a tetrazolium salt which is insoluble in the solvent employed; e.g formazans derived from 2, 3, 5 triphenyl-tetrazolium chloride (triphenyl formazan), neotetrazolium, salt i.e. 2, 2 (p-diphenylene) bis (3, 5-diphenyl-tetrazolium chloride) (neo-formazan), or from the dimethoxy derivative of neo-tetrazolium salt (blue di-formazan). A suitable halogenated hydrocarbon means one in which the tetrazolium salt produced by oxidation of the formazan is insoluble or substantially so, which does not react with the formazan, and which is not so coloured as to mask the colour change produced on exposure of the solution to radiation. Preferably a chlorinated hydrocarbon, e.g. carbon tetrachloride, is used. To provide an additional source of halogen ions, and so to increase the reactivity of the solution to radiation, a solid halogenated hydrocarbon may be dissolved in the solvent, e.g. when carbon tetrachloride is the solvent, 10 per cent by weight of carbon tetrabromide may be dissolved therein. In the case of triphenyl formazan, the reaction is assisted by adding to the solution a hydrogen acceptor, e.g. succinic acid, malonic acid, maleic acid, fumaric acid, oleic acid, succinic anhydride or succinic imide, but the effect is not permanent. The solution changes colour when exposed to daylight (but not to artificial light with no ultra-violet content) and should therefore be 'enclosed in a light-tight container. This effect can be temporarily-reduced by adding a solution of succinic acid in alcohol to the solution. To measure the change of optical density, a light beam of a wavelength selected for the formazan used is directed through a tube of solution to a light intensity measuring device, which may e.g. operate a circuit to terminate an X-ray exposure, Alternatively, the density may be estimated by eye, or by comparison with a standard. As shown in Fig. 4, glass tubes 10, 11, 12. sealed at both ends with a cellulose nitrate cap, lie within a block 14 of rubber enclosed by an opaque synthetic plastic cover 15 with removable end caps 16. The tubes may contain three solutions of different official density of the same formazan, so that they will decolorize at different dosages of radiation. Alternatively the centre tube 11 may contain the detector solution, tube 12 may contain the same formazan solution inhibited by addition of hydroquinone or quinhydrone so that no colour change takes place, and tube 10 may contain an aqueous alkaline solution of a tetrazolium salt which will indicate by a colour change inadvertent exposure of the tubes to daylight. In the dosimeter shown in Figs. 6, 7, the tube 11 is mounted within an opaque cylinder 17 by transparent discs 18. One disc 18 carries filters 19, 20, 21 of different optical density. By comparing the density of the solution with the filters by eye, the radiation dosage can be estimated. In another form of dosimeter, the tube containing formazan is surrounded by other tubes containing the same formazan solution but of varying optical densities and inhibited by the inclusion therein of hydroquinone, for comparison purposes. A further form of dosimeter has a tube with comparison filters which may be viewed by the public, and which decolarizes completely when the dosage is less than fatal. Two other tubes, which are visible only when the casing is broken open by an authorised person, decolorize at two different fatal dosages.