(1 − 0)S(0) line brightness as a function of the cloud H₂ column density for different models of the interstellar CR proton spectrum impinging upon the cloud (see Fig. 3). Contours: CR spectral slope α (left) and the (attenuated) CR ionization rate inside the cloud (right). The five markers are our 3σ upper limits on the (1 − 0)S(0) line, which translate into upper limits on ζ and lower limits on α.