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Using Bragg ionization curves, measurements of the average stopping power of organic polymers relative to that of water have been made for 70 MeV protons losing about 30 MeV in the absorbers. By comparing calculated curves to experimental ones, I‐values for the polymers were obtained, relative to the I‐value for Al. For repeated measurements with A‐150 tissue equivalent plastic, the standard deviation of the ratio of the average stopping powersq was ±0.06%. It was found that q depends on the year of production of A‐150. For other absorbers, the uncertainty of q is estimated to be ±0.2%, with a corresponding uncertainty of the I‐values of ±1.5%. Most measured I‐values exceed values calculated with the additivity rule by about 10%. During the course of the measurements, fluctuations of the proton energy from the cyclotron of up to 0.8% were found. |
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