A computer-aided method of analysis of the attenuation by known materials of the exposure rate of a narrow beam of X-radiation generated by a constant medium potential at a thick smooth plane target is described. On the assumption that the average loss of energy of initially monoenergetic electrons is given by the Thomson-Whiddington relation, expressions are derived for the differential intensity of radiation leaving the surface of a solid plane target for a number of analytical approximations to the intensity distribution of the bremsstrahlung at its point of origin within the target. Particular forms of these approximations have been determined which yield attenuation curves closely matching those experimentally determined by Thoraeus for constant exciting potentials of 50, 60, 80 and 100 kV. They are found to be generally consistent with theoretical expectation and the few direct measurements available.

The attenuation of the intensity and the exposure-rate of brrmnstrahlung generated at constant potentials have heen calculated assuming that the spectral distrihution of the radiation at the target is given by the Kulenkampff-Kramera relation, and that attenuation coefficients may be represented with sufficient accuracy by expressions of the form CA3+ U. Values of C and U have heen computed from published data for a large number of photon energy intervals. Results for the exposure.rate are compared with published experimental data for narrow beams generated a t tungsten targets by potentials below G9.5 kv. Good agreement is obtained for t,hose cases where the initial filtration is sufficient t o remove target L-radiation and the beam is uncontaminated by fluorescence radiation from the filter. This close agreement indicates that the results presented for the attenuation of the radiation intensity, which have been computed in an analogous manner, are also correct under thr same conditions.

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Results of a recently described method of calculating the attenuation of the exposure-rate of X-radiation generated a t a constant potential are compared with published experimental data for radiation generated at tungsten targets by potentials above 69.5 kV, the V C ' K-absorption edge. It is shown that the K-radiation does not provide a satisfactory explanation for the slightly but consistently greater penetration observed experimentally, and that there is evidence that this is due to an initial loss of the less energetic bremsstrahlung by absorption within the target.

A recently described, computer aided method of attenuation analysis has been applied to free-air chamber measurements of the attenuation by up to 30mm A1 of the exposure rate of a narrow beam of X-radiation excited at a thick W target in a Be window tube by constant potentials of 60, 80, 100, 120 and 140 kV. It has been shown that the general characteristics of the intensity distributions of the bremsstahlung at its point of origin within the target found by an earlier analysis are fully confirmed but that, for radiation excited at potentials appreciably above the K-absorption edge of the tungsten target, the attenuation measurements must first be corrected for the pressure of target characteristic K-fluorescence radiation. A method for doing this from published estimates of the relative amounts of W K-fluorescence at various filtrations in radiation excited by the same potentials is described, and it is shown that these amounts come into reasonable agreement when translated to the target surface. Curves of the intensity distribution of the bremsstrahlung at its point of origin within the target are presented which are consistent with attenuation data corrected according to these estimates. They show a good general agreement with theoretical expectation.