Stopping powers of Al and Mo have been measured for protons in the respecti
ve energy ranges of 0.3-3.0 and 0.4- 3.0 MeV with a surface barrier silicon
detector. The uncertainty of the present results is +/-0.35%. Above 0.8 Me
V the present results for Al agree well with the Aarhus data of Anderson et
al. [Phys. Rev. A 16 (1977) 1929] and the tabulated values of ICRU Report
49 (Stopping Powers and Ranges for Protons and Alpha Particles (Internation
al Commission on Radiation Units and Measurements, 1993)) (ICRU values). Be
low 0.8 MeV, however, ratios of the present data for Al to ICRU values show
a slight decrease with decreasing proton energy. The decrease amounts to s
imilar to 1% at 0.3 MeV. In the case of Mo the ratios also decrease with de
creasing proton energy below 2.0 MeV. The decrease reaches similar to 8% at
around the lowest energy, 0.4 MeV. Because the Bethe-Bloch formula is know
n to well reproduce stopping powers for protons above similar to 1 MeV, the
mean excitation energies, the I values, of Al and Mo have been extracted f
rom the results for 1-3 MeV protons. These I values are compared with publi
shed values. (C) 1999 Elsevier Science B.V. All rights reserved.