  The radiolysis of a nitrogen-acetone mixture, condensed at 11 K, by 40 MeV
$^{58}$Ni$^{11+}$ ions is studied. These results are representative of studies
concerning solar system objects exposed to cosmic rays. In the Kuiper Belt,
region of Trans-Neptunian Objects (TNOs), acetone, molecular nitrogen and other
small molecules were detected and may be present on icy surfaces. Bombardment
by cosmic rays triggers chemical reactions leading to synthesis of larger
molecules. In this work, destruction cross sections of acetone and nitrogen in
solid phase are determined from a sequence of infrared spectra obtained at
increasing ion beam fluence. The results are analyzed and compared with those
of previous experiments performed with pure acetone. It is observed that the
molecular nitrogen column density decrease very fast, suggesting that nitrogen
quickly leaves a porous sample under irradiation. The behavior of acetone in
the mixture confirms that the ice formed by deposition of vapor mixture is more
porous than that of pure acetone ice. The most abundant molecular species
formed from the mixture during irradiation are: C$_3$H$_6$, C$_2$H$_6$, N$_3$,
CO, CH$_4$ and CO$_2$. Some N-bearing species are also formed, but with low
production yield. Comparing with pure acetone results, it is seen that
dissolving acetone in nitrogen affects the formation cross sections of the new
species: CH$_4$, CO$_2$ and H$_2$CO, for example, have formation cross section
smaller than the respective values for irradiated pure acetone, while those for
CO and C$_2$H$_6$ species are higher. This fact can explain the presence of
C$_2$H$_6$ even in regions on Pluto where CH$_4$ is not pure, but diluted in a
N$_2$ matrix together with less abundant species. These results also show the
formation of more complex molecules, such as HNCO, acetic acid and,
possibility, glycine.
