The calibration of the 'Fingerhut' ionization chamber

The use of the "thimble" ionization chamber (Fingerhutkammer) has in the past been the only method for comparing the primary X-ray standards of different laboratories, hence the precise calibration of two thimble chambers is discussed in this paper. Five principal factors determine the accuracy of such a calibration, and it is shown that the errors are all, in general, in the same direction. It is shown that the X-ray beam must have a uniform section sufficient to cover the thimble chamber and yet not be so diaphragmed that off-focus radiation may impair the readings of the standard chamber. The chamber must be carefully aligned in the beam experimentally and not merely centered on the basis of its geometrical shape, since the distribution of measured ionization is found to vary considerably over the length of the chamber. In the investigation of the standard ionization chamber x the special conditions affecting the applicability of the inverse square law were described. The same general conditions were found to hold true for the thimble chamber, indicating the necessity of making a careful analysis of the diaphragm system before proceeding with the calibration. The "wall effect" of two thimble chambers was studied, and the very strong dependence upon quality of radiation indicated that any intercomparison by means of such a chamber will be valid only under identical conditions of voltage, filtration, and wave form of generator.