Heavy ion irradiation of crystalline water ice

Cosmic ray amorphisation cross-section and sputtering yield^⋆

¹ CNRS-INSU, Institut d’Astrophysique Spatiale, UMR 8617, 91405 Orsay, France
e-mail: emmanuel.dartois@ias.u-psud.fr
² Université Paris Sud, Institut d’Astrophysique Spatiale, UMR 8617, bâtiment 121, 91405 Orsay, France
³ Centre de Recherche sur les Ions, les Matériaux et la Photonique (CEA/CNRS/ENSICAEN/Université de Caen-Basse Normandie), UMR 6252, CIMAP – CIRIL – Ganil, Boulevard Henri Becquerel, BP 5133, 14070 Caen Cedex 05, France
⁴ CNRS-IN2P3, Institut de Physique Nucléaire d’Orsay, UMR 8608, 91406 Orsay, France
⁵ Université Paris Sud, Institut de Physique Nucléaire d’Orsay, UMR 8608, IN2P3-CNRS, bâtiment 103, 91406 Orsay, France
⁶ Grand Accélérateur National d’Ions Lourds, CEA/DSM-CNRS/IN2P3, Boulevard Henri Becquerel, BP 55027, 14076 Caen Cedex 05, France
⁷ Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, 22451-900 Rio de Janeiro, RJ, Brazil

Received: 26 November 2014
Accepted: 21 February 2015

Abstract

Context. Under cosmic irradiation, the interstellar water ice mantles evolve towards a compact amorphous state. Crystalline ice amorphisation was previously monitored mainly in the keV to hundreds of keV ion energies.

Aims. We experimentally investigate heavy ion irradiation amorphisation of crystalline ice, at high energies closer to true cosmic rays, and explore the water-ice sputtering yield.

Methods. We irradiated thin crystalline ice films with MeV to GeV swift ion beams, produced at the GANIL accelerator. The ice infrared spectral evolution as a function of fluence is monitored with in-situ infrared spectroscopy (induced amorphisation of the initial crystalline state into a compact amorphous phase).

Results. The crystalline ice amorphisation cross-section is measured in the high electronic stopping-power range for different temperatures. At large fluence, the ice sputtering is measured on the infrared spectra, and the fitted sputtering-yield dependence, combined with previous measurements, is quadratic over three decades of electronic stopping power.

Conclusions. The final state of cosmic ray irradiation for porous amorphous and crystalline ice, as monitored by infrared spectroscopy, is the same, but with a large difference in cross-section, hence in time scale in an astrophysical context. The cosmic ray water-ice sputtering rates compete with the UV photodesorption yields reported in the literature. The prevalence of direct cosmic ray sputtering over cosmic-ray induced photons photodesorption may be particularly true for ices strongly bonded to the ice mantles surfaces, such as hydrogen-bonded ice structures or more generally the so-called polar ices.