STOPPING POWERS OF MYLAR FOR PROTONS FROM 4 TO 11.5 MEV

The stopping powers of Mylar for 4-11.5 MeV protons have been measured with an uncertainty of +/-0.35%. The present results have been compared with experimental values obtained by Rauhala and Raisanen [Nucl. Instr. and Meth. B 35 (1988) 130] and with calculated values of the ICRU Report 49 and Janni [At. Data Nucl. Data Tables 27 (1982) 147]. These values agree well with each other within stated uncertainties. Requiring that experimental stopping powers are best reproduced by the Bethe-Bloch formula, we have extracted the mean excitation energy, I-value, for Mylar from the present results. In this analysis, we used the theory of Ashley et al. [Phys. Rev. B 5 (1972) 2393; Phys. Rev. A 8 (1973) 2402] for the estimation of the Parkas correction and regarded the I-value as an adjustable parameter. The I-values obtained are 80.2 eV if we adopted Bichsel's shell correction [UCRL Report No. 17538 (1967)] and 80.8 eV when we used Bonderup's shell correction [K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 35 (1967) no. 17]. Considering the accuracy of theories this discrepancy is not physically significant. The present I-values agree well with the recommended value of 78.7 eV in the ICRU Reports 37 and 49 but disagree with the value of 75.3+/-1.1 eV which Hiraoka et al. [Med. Phys. 20 (1993) 135] have recently obtained.