Previous experimental results of the present authors on the energy loss of 18-Mev protons in aluminum are corrected to give a value for the mean excitation potential $I=168\mathrm{}$ ev. It is pointed out that recent work on the range-energy relationship for protons in aluminum may indicate a variation of $I$ with proton energy which is considerably larger than that to be attributed to the nonparticipation of the $K$ electrons.

• Received 22 September 1952

DOI:https://doi.org/10.1103/PhysRev.89.1163