In many ways, proton irradiation overcomes the drawbacks of electron and neutron irradiation. The penetration depth of protons at a few MeV can exceed 40 mm and the damage profile is relatively flat such that the dose rate varies by less than a factor of 2 over several tens of micrometers. Further, the depth of penetration is sufficient to assess such prop­erties as irradiation hardening through microhardness measurements, and stress corrosion cracking through crack initiation tests such as the slow strain rate test.

Figure 34 shows schematics of 3.2 MeV proton and 5 MeV Ni2+ damage profiles in stainless steel. Super­imposed on the depth scale is a grain structure with a grain size of 10 pm. Note that with this grain size, there are numerous grain boundaries and a significant irradiated volume over which the proton damage rate is flat. The dose rate for proton irradiations is 2-3 orders ofmagnitude lower than that for electrons or ions, thus requiring only a modest temperature shift, but as it is still 102-103 times higher than neutron irradiation, modest doses can be achieved in reason­ably short irradiation time.

The disadvantages are that because of the small mass of the proton compared to heavy ions, the recoil energy is smaller and the resulting damage morphology is characterized by smaller, more widely spaced cascades than with ions or neutrons. Also, as only a few MeV are required to surmount the Cou­lomb barrier for light ions, there is also a minor amount of sample activation that increases with proton energy.