Upper limits and measurements on the product of proton energy content above 10 TeV and ambient density (top) and proton energy content above 10 TeV (bottom) vs. source distance. We assume the ambient density and distance estimates from the publications listed in Table A.2 and also show the corresponding values for VHE-detected shell-type SNRs. For the latter, we use the flux normalisations as reported in the publications listed in Table A.3. In several sources a gas clump correlation of γ-ray emission was observed. In these cases, filled points indicate the result if the low density values in the rarefied inter-clumpmedium are used in the calculation, while open points show the result if the high density values inside the clumps are assumed. We also show the canonical expectation, assuming an acceleration efficiency of Θ = 10%, a SN blast energy of E_SN = 10⁵¹ erg, and a power- law spectrum with spectral index of Γ = 2 (expected from standard first order Fermi acceleration) and Γ = 2.3 (corresponding to the average spectral index of HGPS sources) up to E_max = 1 PeV, assuming n = 1 cm⁻³ in the top panel. Furthermore, we show in the top panel the sensitivity of the HGPS (assuming a mean value of ~1.5% Crab) and the projected sensitivity of CTA (assuming a ten times higher sensitivity). Red points represent old sources (>10 kyr) that are not expected to accelerate protons to PeV energies, grey limits in the top panel indicate SNRs that are likely to interact with molecular clouds (see Table A.1). Values for Kepler’s SNR (source 2), G1.9+0.3 (1) and G330.2+1.0 (41 top, 19 bottom) are derived from the limits in Aharonian et al. (2008a) and Abramowski et al. (2014b).