  De-excitation gamma-ray lines, produced by nuclei colliding with protons,
provide information about astrophysical environments where particles have
kinetic energies of $10-100$ MeV per nucleon. In general, such environments can
be categorized into two types: the interaction between non-thermal MeV cosmic
rays and ambient gas, and the other is thermal plasma with a temperature above
a few MeV. In this paper, we focus on the latter type and investigate the
production of de-excitation gamma-ray lines in very hot thermal plasma,
especially the dependence of the line profile on the plasma temperature. We
have calculated the line profile of prompt gamma rays from $^{12}$C and
$^{16}$O and found that when nuclei have a higher temperature than protons,
gamma-ray line profiles can have a complex shape unique to each nucleus
species. This is caused by anisotropic gamma-ray emission in the nucleus rest
frame. We propose that the spectroscopy of nuclear de-excitation gamma-ray
lines may enable to probe energy distribution in very hot astrophysical
plasmas. This diagnostics can be a new and powerful technique to investigate
the physical state of a two-temperature accretion flows onto a black hole,
especially the energy distributions of the protons and nuclei, which are
difficult to access for any other diagnostics.
