  We study spontaneous radiation emission from matter, as predicted by the
Continuous Spontaneous Localization (CSL) collapse model. We show that, in an
appropriate range of energies of the emitted radiation, the largest
contribution comes from the atomic nuclei. Specifically, we show that in the
energy range $E\sim 10\,-\,10^{5}$ keV the contribution to the radiation
emission from the atomic nuclei grows quadratically with the atomic number of
the atom, overtaking the contribution from the electrons, which grows only
linearly. This theoretical prediction is then compared with the data from a
dedicated experiment performed at the extremely low background environment of
the Gran Sasso underground National Laboratory, where the radiation emitted
from a sample of Germanium was measured. As a result, we obtain the strongest
bounds on the CSL parameters for $r_C\leq 10^{-6}$ m, improving the previous
ones by more than an order of magnitude.
