  Identification of the cosmic-ray (CR) `PeVatrons', which are sources capable
of accelerating particles to $\sim10^{15}$ eV energies and higher, may lead to
resolving the long-standing question of the origin of the spectral feature in
the all-particle CR spectrum known as the `knee'. Because CRs with these
energies are deflected by interstellar magnetic fields identification of
individual sources and determination of their spectral characteristics is more
likely via very high energy $\gamma$-ray emissions, which provide the necessary
directional information. However, pair production on the interstellar radiation
field (ISRF) and cosmic microwave background leads to steepening of the
high-energy tails of $\gamma$-ray spectra, and should be corrected for to
enable true properties of the spectrum at source to be recovered. Employing
recently developed three-dimensional ISRF models this paper quantifies the
pair-absorption effect on spectra for sources in the Galactic centre direction
at 8.5 kpc and 23.5 kpc distance, with the latter corresponding to the far side
of the Galactic stellar disc where it is expected that discrimination of
spectral features $>10$ TeV will be possible by the forthcoming Cherenkov
Telescope Array. The estimates made suggest spectral cutoffs could be
underestimated by factors of a few in the energy range so far sampled by TeV
$\gamma$-ray telescopes. As an example to illustrate this, the recent HESS
measurements of diffuse $\gamma$-ray emissions possibly associated with
injection of CRs nearby Sgr A$^*$ are ISRF-corrected, and estimates of the
spectral cutoff are re-evaluated. It is found that it could be higher by up to
a factor $\sim 2$, indicating that these emissions may be consistent with a CR
accelerator with a spectral cutoff of at least 1 PeV at the 95% confidence
level.
