J. Dural

A high-resistivity phase induced by swift heavy-ion irradiation of Bi: a probe for thermal spike damage?

Pure bismuth samples were irradiated at 20 K with swift heavy ions from 18O to 238U in the GeV range. The rate of the induced damage was deduced from in situ electrical resistance measurements. Above a threshold in the electronic stopping power Se equal to 24 keV nm-1, the damage is due to electronic slowing down. Above 30 keV nm-1, the electronic slowing down is efficient enough to induce latent tracks attributed to the appearance of a high-resistivity phase. The induced latent tracks radii can be up to 21.9 nm for Se=51 keV nm-1 which is the largest value reported so far for non-radiolytic materials. The evolution with Se of the latent tracks radii is calculated on the basis of the thermal spike model, assuming a realistic value for the electron-phonon coupling constant. A rather good agreement is obtained which supports the idea that the thermal spike could be operative in the observed radiation damage.

Velocity effect on the damage creation in metals in the electronic stopping power regime

Abstract The energy deposited on target electrons by a swift heavy ion has been studied for several years. The deposited energy density in the vicinity of the ion path is large as the velocity of the incident ion is low. The influence of the ion velocity on irradiation effect induced by electronic energy loss ( S e ) has been clearly demonstrated after irradiation experiments performed on the insulator Y 3 Fe 5 O 12 and suspected in metals. In that case the damage creation is enhanced at low ion velocities. In the present work, pure bismuth samples have been irradiated at 100 K by 42.9 MeV amu −1 Xe and 4 MeV amu −1 Kr ions with similar S e values (∼ 17 KeV nm −1 ). The resistivity increment Δϱ was measured in situ as a function of the ion fluence, Φt. The analyses of Δϱ(Φt) curves show that the Kr irradiation leads to a damage efficiency which is two to four times larger than that obtained after Xe irradiation. In addition, 2, 10 and 20 MeV amu −1 Pb-ion irradiations were performed at 20 K. The comparison of deduced latent track radii for the same S e values with previous results gives that lower velocity in irradiations produce larger tracks. These phenomena evidence that a strong ion velocity effect exists in bismuth. The thermal spike model with the ion velocity dependence of the initial energy deposition is used to determine the track radii as a function of S e values. We now find an agreement between calculated and experimental track radii in relation with the ion velocity. Moreover the electron number density n e of the quasi-free gas in bismuth is found to be 1.5 times the atomic density n a ( n e = 1.5n a ) and the S e threshold for continuous latent track formation, S et , is ranged from 24 to 31 keV nm −1 for ion energies ranging from 2 to 25 MeV amu −1 .

Impact parameter dependence of energy loss and target-electron-induced ionization for 27 MeV/u Xe35+ incident ions transmitted in [110] Si channels

Abstract We present original experiments providing information on the impact parameter dependence of stopping power in axial channeling and on electron impact ionization. A beam of 27 MeV/u Xe35+ ions (far from the equilibrium charge state in matter) has been transmitted through a Si crystal, parallel to the [110] axis. The very broad and out-of-equilibrium emerging charge state distribution (35 ⩽ Qout ⩽ 53) arises nearly only from electron impact ionization. The corresponding cross sections σ(Q → Q + 1) obtained, when fitting the experimental distribution by Monte Carlo simulations. The energy loss spectra measured for each Qout have been analyzed, using the same simulation program. The choice of Qout corresponds to a selection in the transverse energy distribution of the ions and on their accessible transverse space. Detailed information is thus obtained on “local stopping power”, i.e. in a given point of the transverse [110] space, and on its relation with the corresponding local electronic density. The respective influence of core and valence electrons is studied. For very well channeled ions, the energy loss, which is only induced by the valence electron gas, depends little on the local electronic density sampled by the particles. It is very close to the .energy loss corresponding to a homogeneous electron gas with density equal to the mean density of valence electrons.

Anomalous Enhancement in Defect Production in Gallium Irradiated by High-Energy Xenon Ions

Pure crystalline gallium samples have been irradiated at temperatures lower than 11 K with a high-energy Xe beam (27 MeV/a.m.u.) depositing 2 keV/A by electronic excitations. The results obtained with the aid of resistance variation measurements are compared with those obtained by an electron irradiation and with published results on pure metals. For the first time in a pure crystalline metal, it is found that high-energy ions are more efficient than electrons or low-energy ions for defect creation. The role of displacement threshold anisotropy is discussed, and it is suggested that the high electronic stopping power is responsible for the observed effect.

Observation of Transient Response of Nb Superconducting Thin Film to a Single-Heavy-Ion Impact

Transient response of a 2500 A thick superconducting Nb film to the impact of a single 129Xe50+ ion of total energy 3.2 GeV, transferring about 2.4 keV/A of electronic excitation energy, was observed by means of a superconductor-insulator-superconductor tunnel junction attached to the film some distance away from the impact point. The response had rise time of less than 150 ps (resolution limit of the electronic detection system) and decay time varying from 2 to 1.3 ns as function of temperature. Main experimental features, if not finer details, are in good agreement with Rothwarf and Taylor theory of nonequilibrium processes in superconductors.

Influence of intrashell excitation (n=2) on the population of mestastable states of H- and He-like krypton ions in channeling conditions

Abstract We have observed delayed K α photons emitted by 60 A MeV H-like and He-like krypton ions leaving a 37 μm silicon crystal, both for random and 〈110〉 axial alignment of the target. We could extract intrashell (2s→ 2p) excitation probabilities, which are compared with values deduced from PWBA calculations.

Electronic capture and excitation of highly charged channeled ions

Abstract Two aspects of heavy ion channeling are presented. The first aspect is related to the fact that channeled ions interact only with the most loosely bound target electrons. One can take benefit of this feature to study processes such as radiative electron capture (REC) and resonant transfer and excitation (RTE) in a dense quasi-free electron gas. The experimental work, performed at GANIL, devoted to these two processes is described. A possible extension to Nuclear RTE or NEEC (nuclear excitation by electron capture) studies is also described. The second aspect discussed is related to the periodicity of the potential experienced by channeled ions. We show that in a well chosen case this could lead to a significant and detectable coherent excitation of the projectile nucleus.

High energy heavy ion irradiation effects in MgOMgFe2O4 ceramics

Abstract Magnesium oxide single crystals containing magnesioferrite precipitates (MgFe2O4) have been obtained by thermal annealing at 1073 K of MgO single crystals implanted with 6 × 1016 100 keV 57Fe ions cm−2. The size of the aggregates ranges from 20 to 80 nm. These samples have been irradiated at GANIL with 3.4 GeV Xe ions at 1012 ions cm−2 fluence. The effects of these particles have been investigated by transmission electron microscopy (TEM) and conversion electron Mossbauer spectroscopy (CEMS). TEM observations reveal damaged regions inside the precipitates. The diameter of these zones ranges from 4 to 6 nm and their density roughly corresponds to the xenon fluence. The CEMS spectra show an increase of the paramagnetic component due to Fe3+ ions on the cost of the magnetically ordered components due to Fe3+ ions in octahedral and tetrahedral sites of the MgFe2O4 spinel structure. This is in agreement with the TEM observations and can be due to a local modification of the spinel structure in the damaged regions. Such irradiation defects are tracks induced by the very high electronic stopping power [ ( − d E d x ) e ≈ 1 keV A −1 ] of the Xe ions.

Channelling experiment with a cooled and decelerated H-like gold ion beam extracted from a storage ring

We have performed a channelling experiment in which a ion beam, extracted by radiative recombination from the storage ring ESR at GSI after deceleration and cooling, was directed onto a thin silicon crystal. The directional properties of channelling have been used to test the quality of the extracted beam. An upper limit of the beam emittance has been measured to be via x-ray measurements. Coincidence measurements between photons and ions transmitted after capture of one electron in the crystal have allowed us to study L-REC and the associated emission.

Electronic stopping effects in Fe60Co40 films irradiated with high energy ions

Abstract Thin films of the Fe60Co40 alloy produced by electron beam deposition under ultrahigh vacuum have been irradiated at liquid nitrogen temperature with high energy ions, 40–186 MeV N ions and 445 MeV Ag ions, on the French accelerator GANIL in Caen. The damage production has been continuously measured as a function of the ion fluence at 77 K using in situ electrical resistivity measurements. The experimental results are analyzed to obtain the defect production efficiency which is the ratio of the experimental to the theoretical displacement cross section. It is found for the lightest ions, 40–186 MeV N, that the defect production efficiency is lower than unity and can be understood on the basis of elastic collision effects only. On the contrary, an important defect production efficiency much higher than unity is observed in the case of the 445 MeV Ag irradiation where the slowing down of ions occurs mainly via inelastic collisions. These observations are new evidence of the spectacular damage creation which can be induced by very high energy deposition in electronic excitation in some metallic targets. A comparison and discussion are made regarding previous damage rate studies for other types of high electronic energy deposition in metallic targets.