Patent GB-997338-A (GB)
Title: A method and apparatus for producing x-rays of particular wave lengths and applications therefor
Inventor: WATT JOHN STANLEY
IPC: H01J35/00
Filed: 1963-10-30
Abstract: 997, 338. Radiation sources for measurement and analysis. AUSTRALIAN ATOMIC ENERGY COMMISSION. Oct. 30, 1963, No. 42938/63. Heading G1A. [Also in Division H5] X-rays of aparticular wave length are emitted by a secondary material when the material is excited by the low energy gamma radiation (less than 500 kev.) from a gamma ray emitting radioisotope, The radioisotope, the secondary material, and a gamma ray shield are arranged so that the radiation from the secondary material emerges substantially free of radiation from the radioisotope. The radioisotope, and the secondary material are chosen so that the energy of at least some of the gamma rays is greater than that of the K or L absorption edge of the secondary material, and the K or L X-rays produced have the desired wave lengths. The X- rays produced by the secondary material may be arranged to excite a third material to cause the third material to emit its characteristic X-radiation further shielding being provided so that the radiation from the third source emerges substantially free of radiation from either the second or primary sources. The X-radiation may be used in the measurement of coating thickness or in the analysis of the constituents of materials. Fig. 1 shows one form of generator in which the gamma rays from a radioisotope source 1 impinge on the inner surface of a hollow truncated cone of material 4 which generates X-radiation 7 of the required wavelength, the direct radiation from the primary source 1 being prevented by the block of shielding material 5. The secondary material may be backed by shielding material and one suggested form is shown in Fig. 3 in which a lead box 9 is provided behind the secondary material, the box containing two hollow cones 10 and 11 of shielding material tipped at their ends 12 with secondary material. Fig. 2 shows an alternative form of generator in which gamma rays from source 1 in container 2 impinge on the sheets of secondary material 4 backed by shielding material 8 direct radiation from the source being absorbed by the shielding material. An arrangement for measuring the thickness of surface coating or for analysing a material is shown in Fig. 4 in which the generator of X-rays is housed, together with a scintillation detector 15, in a lead block 13. Radiation 7 from the X-ray generator excites the fluorescent radiation of the coating material, or a constituent of the material of the sheet 18, and the fluorescent radiation is detected and measured by the scintillation counter 15 (without energy analysis). In one of the examples given in the specification a primary source of Gadolinium 153 is used with a caesium chloride secondary material to find the thickness of tinning on steel plate, the scintillation crystal being of sodium iodide covered by a 0. 002 inch silver foil to absorb back scattered X-rays from the base plate thus enhancing the sensitivity. In another example for measuring the silver weight on photographic films and paper the secondary material of the first example is replaced by tellurium and the scintillation crystal covered with 0.003 inch palladium foil. A similar set up to the first described above. except that the scintillation crystal was covered with 0.0025 inch cadmium foil, is described for measuring the antimony content of lead plates while the second set up described above can be used to determine the cadmium content of zinc plates. A method-of determining the proportions of cadmium and silver as nitrates in a solution of water is also described using a Thulium-170 primary source, as thulium oxide dispersed in a beryllium oxide matrix, and alternative secondary materials of tellurium and antimony, the radiation from the secondary material passing through a 0.002 inch tin foil to eliminate the K X-rays, the scintillation counter measuring the radiation after it had passed through 1 cm of the solution. Using the alternative secondary materials a set of figures is obtained from which it is shown the constituents of the solution may be calculated. It is also suggested that the generator may be used to calibrate X-ray counters of both proportional and scintillation types.