  Supernova remnants interacting with molecular clouds are ideal laboratories
to study the acceleration of particles at shock waves and their transport and
interactions in the surrounding interstellar medium. In this paper, we focus on
the supernova remnant W28, which over the years has been observed in all energy
domains from radio waves to very-high-energy gamma rays. The bright gamma-ray
emission detected from molecular clouds located in its vicinity revealed the
presence of accelerated GeV and TeV particles in the region. An enhanced
ionization rate has also been measured by means of millimetre observations, but
such observations alone cannot tell us whether the enhancement is due to low
energy (MeV) cosmic rays (either protons or electrons) or the X-ray photons
emitted by the shocked gas. The goal of this study is to determine the origin
of the enhanced ionization rate and to infer from multiwavelength observations
the spectrum of cosmic rays accelerated at the supernova remnant shock in the
unprecedented range spanning from MeV to multi-TeV particle energies. We
developed a model to describe the transport of X-ray photons into the molecular
cloud, and we fitted the radio, millimeter, and gamma-ray data to derive the
spectrum of the radiating particles. The contribution from X-ray photons to the
enhanced ionization rate is negligible, and therefore the ionization must be
due to cosmic rays. Even though we cannot exclude a contribution to the
ionization rate coming from cosmic ray electrons, we show that a scenario where
cosmic ray protons explain both the gamma-ray flux and the enhanced ionization
rate provides the most natural fit to multiwavelength data. This strongly
suggests that the intensity of CR protons is enhanced in the region for
particle energies in a very broad range covering almost 6 orders of magnitude:
from $\lesssim 100$ MeV up to several tens of TeV.
