Stopping Powers for Alpha Particles in Tissue Equivalent Materials and Their Application to the Estimation of the Radiation Dose to the Lungs due to Inhaled Radon Daughter Products.

Absorbed doses and dose equivalents to the bronchial epithelium resulting from exposure to airborne’ radon daughters have been calculated. The work was performed in two distinct parts; the establishment of the equilibrium activities developing in the lung due to exposure and_ the calculation of the absorbed doses and dose equivalents for unit equilibrium activity. The doses for unit activity were calculated using a computer program based on a cylindrical airway and a spherical target. The energy deposited in the target was calculated using alpha particle range-energy data for the lumenal air tissue and mucous, established as the experimental component of this work, using air, tissue equivalent gas and water as substitutes and alpha particle stopping powers in tissue derived from the experimental tissue equivalent gas range-energy data. Both the range-energy and derived stopping power data were represented by polynomial fitting functions for the purposes of the calculation. The method permitted the use of energy dependent quality factors for the calculation of the dose equivalents, an examination of the effect of straggling using alpha residual energy spectra peak widths obtained experimentally during the establishment of the range-energy data, and removed the need of normalization from one absorber to another. In addition, the effects of uncertainties, including the possible erroneous use of data for a tissue equivalent gas rather than a solid or liquid, were examined. For this, the sizes of the uncertainties involved were obtained experimentally by measuring range-energy data in a tissue equivalent solid and liquid as well as in a_ tissue equivalent gas. The doses were then recomputed using suitably amended polynomials. Although differences in the range-energy data for the various tissue equivalent absorbers were observed, no conclusions could be drawn as to the effect of phase in causing these differences. The calculation of the equilibrium activities was commenced with the specification of the radioactive atmospheres to which those at risk are exposed. These specifications, which were based on published data, were then used in a computer program based on the Weibel model A lung in order to calculate the rates of deposition of activity throughout the tracheobronchial tree. In the same program, these rates of deposition were combined with data, Page 2 again culled from the literature, relating to the translocation of activity by ciliary action and to clearance directly to the blood, in order to calculate the required equilibrium activities. Several situations were treated in order to examine the effect of various tidal volumes, mucous velocities, aerosol characterisics etc. Also, some of the radioactive atmospheres used previously were treated thereby testing the effect of the new deposition and clearance models. Calculated absorbed doses and dose equivalents for a number of airway diameters and target depths have been presented and compared with the results of other workers. Some observations on the most appropriate value of the quality factor have been made.