Presently Hp(10) is surrogate to Effective dose, But this holds for lower energy photons (x-rays, photons from Cs or Co etc.) say having E upto 3 MeV and is not conservative estimate of Effective dose for photons having higher energy of 10-20 MeV or higher. Should we define Hp(30) or Hp(50) etc?
The significant difference between lower-energy and high-energy photons regarding the dose-distribution seems the depth dose curve. Indeed the maximum dose is achieved at greater depth for high-energy photons. Check Figure 1 of this proceedings publication: http://www.slac.stanford.edu/econf/C0709107/pdf/302.pdf.
Being exposed to photons of energies exceeding 3 MeV is typically very unlikely, except working in the vicinity of particle accelerators (and they should be switched off before entering). In general the majority of radiation exposed persons/workers are exposed to gamma radiation originating from isotopes with energies well below 1.5MeV. Therefore, it is not necessary to introduce a new dose equivalent value for this special case.
Looking at the relation of effective dose vs. equivalent dose, E/H*(10) is below 1 at least up to 10 MeV. So it is a good conservative estimate for photons < 10 MeV. For radiation > 10 MeV (as R. Schnell pointed out, such exposures are not very likely) this will more or less hold true because more and more of the radiation will go through the tissue without interaction. The half thickness for 10 MeV photons is already around 30cm.
@Schnell and strub@ thanks. What beyond 10 MeV photon energy especially for bremsstrahlung radiation emitted by electron synchrotron machines operated in GeV range.
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