ARTÍCULO
Swift heavy ion irradiation of water ice from MeV to GeV energies
Fecha
2013Registro en:
0004-6361, e1432-0746
10.1051/0004-6361/201321636
Autor
Mejia Guaman, Christian Fernando
Domaracka, Alicja
Dartois, Enmanuel
Godard, Marie
Frota Da silveira, Enio
Ferreira De barros, Ana Lucía
Boduch, Philippe
Rothard, Hermann
Brunetto, Rosario
Ding, Jing Jie
Pino, Thomas
Chabot, Marin
Thomas, Jean Charles
Institución
Resumen
Context. Cosmic ray ion irradiation affects the chemical composition of and triggers physical changes in interstellar ice mantles in
space. One of the primary structural changes induced is the loss of porosity, and the mantles evolve toward a more compact amorphous
state. Previously, ice compaction was monitored at low to moderate ion energies. The existence of a compaction threshold in stopping
power has been suggested.
Aims. In this article we experimentally study the effect of heavy ion irradiation at energies closer to true cosmic rays. This minimises
extrapolation and allows a regime where electronic interaction always dominates to be explored, providing the ice compaction cross
section over a wide range of electronic stopping power.
Methods. High-energy ion irradiations provided by the GANIL accelerator, from the MeV up to the GeV range, are combined with
in-situ infrared spectroscopy monitoring of ice mantles. We follow the IR spectral evolution of the ice as a function of increasing
fluence (induced compaction of the initial microporous amorphous ice into a more compact amorphous phase). We use the number
of OH dangling bonds of the water molecule, i.e. pending OH bonds not engaged in a hydrogen bond in the initially porous ice
structure as a probe of the phase transition. These high-energy experiments are combined with lower energy experiments using light
ions (H, He) from other facilities in Catania, Italy, and Washington, USA.
Results. We evaluated the cross section for the disappearance of OH dangling bonds as a function of electronic stopping power. A
cross-section law in a large energy range that includes data from different ice deposition setups is established. The relevant phase
structuring time scale for the ice network is compared to interstellar chemical time scales using an astrophysical model.
Conclusions. The presence of a threshold in compaction at low stopping power suggested in some previous works seems not to
be confirmed for the high-energy cosmic rays encountered in interstellar space. Ice mantle porosity or pending bonds monitored by
the OH dangling bonds is removed efficiently by cosmic rays. As a consequence, this considerably reduces the specific surface area
available for surface chemical reactions.