S. Bouffard

Swift heavy ions in insulating and conducting oxides: tracks and physical properties

Damage induced in several oxide materials by swift heavy ions is presented. The discussion is based on results obtained on the following materials [Y3Fe5O12, AFe12O19 (A = Ba, Sr), BFe2O4 (B = Ni, Mg, Zn), ZrSi2O4, SiO2 quartz, Al2O3, high Tc superconductors (YBa2Cu3O7 − δ and Bi2Sr2CaCu2O8)] which have been irradiated by ions with atomic number ranging between 6 (12C) and 92 (238U) and energies between 0.05 GeV and 6 GeV. The damage cross section A has been deduced using several physical characterisations like Mossbauer spectrometry, saturation magnetisation measurements, channeling Rutherford backscattering, infrared absorption and electrical resistance measurements. Depending on the material and on the value of the electronic stopping power (dE/dx) the damage cross section varies between 10−17 and 10−12 cm2. Using medium and high resolution transmission electron microscopy and chemical etching of the latent track, an electronic stopping power evolution of the damage morphology has been observed leading to the definition of an effective radius Re of the latent track which can be linked to the damage (amorphous) cross section A by the relation Re = √A/π. Moreover there is a direct correlation between these values and the damage morphology: for Re > 3 nm the latent tracks are long and cylindrical, conversely for Re < 3 nm the damage is inhomogeneous along the latent track. The effect of the irradiation temperature, of the crystallographic orientation, of the initial electrical resistivity and of the oxygen stoichiometry will be presented. In opposition to what has been usually believed it will be shown that alumina (Al2O3) is indeed sensitive to the electronic stopping power. Moreover the velocity of the incident ion has a direct influence on the damage production: the lower the velocity, the higher the damage.

Swift heavy ion modification of polymers

Abstract We have studied the effects of dense electronic excitations on vinyl linear polymers, by performing swift heavy ion, SHI, irradiations. We used different ion beams, from C to Pb, provided by the GANIL accelerator at energies of a few MeV/u. We mainly studied the physico-chemical modifications induced in polyethylene (PE), but the role of chemical substitutions has also been investigated by studying the swift heavy ion induced damage in polyvinylidene fluoride (PVDF) and polystyrene (PS). The irradiated samples were analysed ex-situ by means of Fourier transform infrared spectroscopy in the transmission mode. Any contact with air was avoided: oxidation does not occur. Swift heavy ion irradiations are characterised by a significant increase of the yield of chain scission associated with unsaturated end groups. Moreover, we have evidences for specific modifications, i.e. modifications which are never observed when using low ionising particles as electrons or gamma-rays; namely the creation of alkyne and allene end groups. These specific modifications are little sensitive to chemical substitutions. For instance, alkyne formation is observed in PE, PVDF and PS. A simultaneous ionisation of the different atoms of a constituting monomer is likely needed for triggering the formation of the specific swift heavy ion induced modifications. On the other hand, if we pay attention to the physico-chemical modifications which are easily induced by low ionising particles, it is observed that increasing the electronic stopping power does not significantly modify the creation yield of, for instance in PE, trans-vinylene or trans-trans diene groups.

Simulation of the primary stage of the interaction of swift heavy ions with condensed matter

Abstract In this paper the swift heavy ion interactions with condensed matter are studied from the point of view of the modifications induced in the electronic subsystem of the target. A Monte Carlo method is used to describe event by event the interactions of the projectile with the target electrons as well as the evolution of the electronic subsystem. The validity of the method and the results are discussed. This detailed picture of the excited target could be used for further explanations and calculations of the damage creation by electronic excitation. We have focused our attention on two materials whose electronic properties are different: graphite and quartz. For both materials a quantitative analysis of the energy deposition mechanism is given.

Physico-chemical modifications induced in polymers by swift heavy ions☆

Abstract We have studied the effects of dense electronic excitations on the physico-chemical modifications of polymers by performing heavy ion irradiations in the energy range of some MeV amu −1 . The effects induced by swift heavy ion irradiations were compared to those induced by MeV electron irradiations performed in similar experimental conditions in polyethylene and poly(vinylidene difluoride) films. The irradiated samples were analysed ex situ by means of Fourier transform infrared spectroscopy in the transmission mode. High electronic stopping power, (d E /d x ) e , irradiations are characterised by a significant increase of the yield of chain scission. Vinyl terminal unsaturations are recorded with radiochemical yields increasing markedly as (d E /d x ) e increases. Moreover, characteristic bands of alkyne terminal groups were observed only above a threshold of (d E /d x ) e . The overall set of results is analysed taking into account the radial dose deposition predicted by numerical calculations and the defect creation measured after electron irradiations. This allows us to elucidate the kind of chemical modifications influenced by the local dose rate; i.e. for which a specific effect due to a dense electronic excitation appears.

Etching threshold for ion tracks in polyimide

Abstract Tracks of various heavy ions with energies up to 13 MeV/u were studied using the technique of selective chemical etching. It was found that for homogeneous track etching the energy loss of the ions has to surpass a threshold of about 450 eV/A. In a transition regime between 180 and 450 eV/A etching was possible but the mean diameter of the resulting pores showed a wider distribution than pores at higher energy losses. In order to describe this observation, the radial dose distribution was calculated using a Monte Carlo simulation code. Inhomogeneous etching is interpreted as due to the spatial fluctuations of the deposited energy along the ion path.

Basic phenomena induced by swift heavy ions in polymers

Abstract The modifications induced by the high energy ions in materials may be understood only if the particularities of the energy transfer from the ion to the target are considered. This is true in particular for polymers. Indeed the ion irradiations are characterised by their large value of the linear energy transfer and certainly by their spatial distribution of the primary events and of the damages. The primary events are strongly correlated to each other: they are mainly located in a small cylinder centred on the ion path. The density of ionisation and the radius of this cylinder (the core region) depend on the projectile and on its velocity. Roughly, the higher the ion energy, the larger the radius is and the lower the ionisation density is. So the dose (the amount of energy absorbed by unit mass of the target) is not sufficient to characterise a high energy ion irradiation. In this paper we describe the physical aspects of the ion irradiation and we shall demonstrate that three parameters are needed to have a non-ambiguous description of these irradiations. They should be, for example, the atomic number, the velocity and the fluence.

Swift heavy ion irradiation of polystyrene

Abstract We have studied by Fourier transform infrared spectroscopy the chemical modifications induced by swift heavy ions in polystyrene. The overall destruction yield becomes very significant at high electronic stopping power ( d E d x ) e and the radiation sensitivity of this aromatic polymer merges typical values of equivalent aliphatic polymers as polyethylene. So, the radiation resistance conferred, at low ( d E d x ) e , to polystyrene by the phenyl side group is lost at high ( d E d x ) e . The creation of different unsaturated groups is also observed. They correspond to the vibration frequencies of alkyne, allene, vinylidene, vinyl and trans-vinylene groups. Moreover, the emergence of a wide band without any structure around 1600 cm−1 reveals the presence of double bonds with different degrees of conjugation. By comparison with the modifications induced in polyethylene, we discuss the role played by the phenyl side group and we consider the multiple ionisation of the aromatic ring as one possible way to induce the observed modifications.

Phase transformation induced in pure zirconia by high energy heavy ion irradiation

Abstract Samples of monoclinic zirconia were irradiated with heavy ions having incident energies in the range of a few hundred MeV giving then rise to a slowing down essentially caused by high electronic excitations. The characterizations of the samples by X-ray diffraction and complementary Raman spectroscopy analyses revealed two main features. First, in the electronic stopping power regime, it is only when the electronic energy loss is above a threshold near 13 keV nm−1 that monoclinic zirconia undergoes a transformation to the tetragonal phase. Second, the evolution of the amount of the tetragonal phase with the ion fluence exhibits a sigmoidal shape suggesting a mechanism for phase transformation which very likely needs two ion impacts.

Phase transformation of polycrystalline zirconia induced by swift heavy ion irradiation

Abstract Polycrystalline samples of monoclinic zirconia (α-ZrO 2 ) have been irradiated at room temperature with 190 MeV 36 Ar and 170 MeV 84 Kr ions in the electronic slowing down regime. Room-temperature X-ray diffraction (XRD) and micro-Raman spectroscopy measurements show consistently that a phase transition to the tetragonal form (β-ZrO 2 ) occurs for 170 MeV 84 Kr ion irradiation above an electronic stopping power value around 15 MeV μm −1 . The kinetics of the transition were monitored by on-line XRD measurements on the same sample. No such phase transformation is seen with 190 MeV 36 Ar ion irradiation for an electronic stopping power value around 6 MeV μm −1 . The plot of the tetragonal phase fraction deduced from XRD measurements vs fluence is analysed with single-impact and double-impact kinetic models. The data seem to be in favour of a double ion impact process.

Damage induced by high electronic stopping power in SiO2 quartz

Abstract Heavy ion irradiations of α-quartz have been performed using 1 MeV 4 He, 30 MeV 16 O, 110 and 168 MeV 58 Ni, 188 MeV 127 I and 3.5 GeV 238 U. The induced damage has been determined using channeling Rutherford backscattering with a 4 He beam of 3 MeV. In the electronic stopping power range investigated (1.5–104 MeV cm 2 mg −1 ), the electronic stopping power damage efficiency increases by at least four orders of magnitude. Moreover, irradiations have also been performed in channeled and random conditions. The influence on the track creation mechanism of inner-shell excitations was checked by comparison between channeling and random irradiations. It is not necessary to invoke inner-shell excitations followed by an Auger decay in order to account for the damage efficiency.