Alpha particle track production in polycarbonate foils amplified by electrochemical etching

We have examined two of the parameters concerned with alpha particle track production in 250 μm thick polycarbonate foils as part of our work to find micro-distribution of the dose delivered to the endosteal surface of bone out to a distance of 10 μm by 239Pu alphas ('Adapting the Polycarbonate Detector and Electrochemical Etching to the Microdosimetry of 239Pu in Bone', G. B. Stillwagon, S. J. Su, and K. Z. Morgan, Trans. Health Phys. Soc., Atlanta, Georgia, 1977). These parameters are the efficiency of track production by alpha particles as a function of time. The efficiency was determined by varying the air pressure in our vacuum-sealed alpha-calibrator over a wide range and observing the energy degradation and number of alphas incident from a 2.0 μC 239Pu source. The alphas were detected in a surface barrier detector which provided input into a Canberra Model 8100 multichannel analyzer. Then polycarbonate foils were irradiated at the same air densities (temperature and pressure correction included) using the same counting geometry as before. The foils were then etched in 28% KOH at 24 deg C for 4 hours using a voltage of 800 V and frequency of 2 KHz. When efficiency of track production vs absorber thickness was plotted a curve showing the 'Bragg-peak' was obtained. The peak was approximately a factor of 2 above the flat portion of the curve. For the track diameter vs etching time determination, foils were irradiated by 3 commercially available alpha sources - two 0.1 μC 241Am sources and one 0.2 μC mixed nuclide source containing 239Pu, 241Am and 244Cm. The foils were then etched in 45% KOH at 24 deg C and 800 V at 2 KHz for times ranging from 15 min to 4 hours. Observable tracks ranged from 3.8 μm at 30 minutes etching time to 84 μm at 4 hours. Ongoing work includes the examination of the reduction in efficiency noted as the etching time is decreased and the actual dose determination of the 239Pu alphas in rat, dog, and human bone. Current results in this area will be presented also. (author)