Amorphous Tracks Research Articles

THE EFFECT OF HEAVY COSMIC-RAY IONS ON SILICATE GRAINS IN THE INTERSTELLAR DUST

Electronmicroscopic samples of crystalline Mg{sub 2}SiO{sub 4} forsterite were irradiated by energetic Ar, Fe, Kr, and Xe ions at room temperature. Tracks with a mean radius R{sub e} = 1.36 nm were observed after irradiation by 56 MeV Fe ions, while no tracks were induced by a 48 MeV Ar beam. Amorphization of forsterite grains by cosmic-ray (CR) Fe ions are discussed, including the effects of low temperature, ion velocity, and ion-induced crystallization. CR Fe ions induce amorphous tracks in crystalline forsterite only in the range 40-140 MeV, and the period of time for complete amorphization is tau{sub cr} approx 13,400 Myr. Our estimate is tau{sub cr} approx 1300 Myr for enstatite. Thus, heavy CR particles do not reduce the crystallinity of silicate grains within a reasonable time, as supposed previously. However, energetic ions can induce crystallization in amorphous solids, and this may be partially or fully responsible for the estimated 0.2% crystallinity of silicates in the interstellar medium.

Ion tracks in apatite at high pressures: the effect of crystallographic track orientation on the elastic properties of fluorapatite under hydrostatic compression

Static elasticity measurements at high pressures were carried out on oriented fluorapatite single crystals, some of which contained oriented amorphous ion tracks (ITs) implanted with relativistic Au ions (2.2 GeV) from the UNILAC linear accelerator at GSI, Darmstadt. High-pressure experiments on irradiated and non-irradiated crystal sections were carried out in diamond-anvil high-pressure cells under hydrostatic conditions. In situ single-crystal diffraction was performed to determine the high-precision lattice parameters, simultaneously monitoring the widths of X-ray diffraction Bragg peaks. High-pressure Raman spectra were analyzed with respect to the frequency shift and widths of bands, which correspond to the Raman-active vibrational modes of the phosphate tetrahedra. Swift heavy ion irradiation was found to induce anisotropic lattice expansion and tensile strain within the host lattice dependent on the ion-track orientation. The relatively low Gruneisen parameter for the ν 1b(A g) mode, which has been assigned to originate from the volume fraction of the amorphous tracks, and the γ(ν 1a)/γ(ν 1b) ratio reveals compressive strain on the amorphous ITs. The comparative compressibilities for the host lattice reveal approximately equivalent bulk moduli, but significantly different pressure derivatives (K T = 88.4 ± 0.7 GPa, ∂K/∂P = 6.3 ± 0.3 for non-irradiated, K T = 90.0 ± 1.7 GPa, ∂K/∂P = 3.8 ± 0.5 for irradiated samples). The axial compressibility moduli β −1 reveal significant differences, which correlate with the ion-track orientation [\( \beta_{a}^{ - 1} \) = 240 ± 5 GPa, \( \beta_{c}^{ - 1} \) = 361 ± 14 GPa, ∂\( \left( {\beta_{a}^{ - 1} } \right) \)/∂P = 11.3 ± 1.2, ∂\( \left( {\beta_{c}^{ - 1} } \right) \)/∂P = 11.6 ± 3.4 for irradiation ⊥(100); 246 ± 9 GPa, 364 ± 57 GPa, 9.5 ± 2.9, 14.7 ± 14.1 for irradiation ⊥(001), 230.7 ± 3.6 GPa, 373.5 ± 5.1 GPa, 19.2 ± 1.4, 20.1 ± 1.8 for no irradiation]. Line widths of XRD Bragg peaks in irradiated apatites confirm the strain of the host lattice, which appears to decrease with increasing pressure. By contrast, the bandwidths of Raman modes increase with pressure, and this is attributed to increasing strain gradients on the length scale of the short-range order. The investigations reveal considerable deviatoric stress on the [100]-oriented tracks due to the anisotropic elasticity, while the compression is uniform for the directions perpendicular to the tracks, which are aligned parallel to the c-axis. This difference might be considered to control the diffusion properties related to the annealing kinetics and its observed anisotropy, and hence to cause potential pressure effects on track-fading rates.

200 MeV silver ion irradiation induced structural modification in YBa2Cu3O7−y thin films at 89 K: An in situ x-ray diffraction study

We report in situ x-ray diffraction (XRD) study of 200 MeV Ag ion irradiation induced structural modification in c-axis oriented YBa2Cu3O7−y (YBCO) thin films at 89 K. The films remained c-axis oriented up to a fluence of 2×1013 ionscm−2, where complete amorphization sets in. The amorphous ion tracks, the strained region around these tracks, and irradiation induced point defects are shown to control the evolution of the structure with ion fluence. Secondary electrons emanating from the ion paths are shown to create point defects in a cylindrical region of 97 nm radius, which corresponds to their maximum range in the YBCO medium. The point defects are created exclusively in the CuO basal planes of fully oxygenated YBCO, which has not been possible, by other techniques including low energy ion irradiation and thermal quenching. The point defects led to a faster decrease in the integral intensity of XRD peaks at very low fluences of irradiation (Φ≤3×1010 ionscm−2) than what can be expected from amorphous tracks. The radius of amorphous ion tracks, estimated from the fluence dependence of integral XRD peak intensity beyond this fluence, was found to be 1.9 nm. Both point defect and the strained region around amorphous ion tracks are shown to contribute to the increase in the c-parameter at 89 K. The full width at half maximum (FWHM) of XRD peaks arising mostly due to the strained region around the ion tracks showed an incubation effect up to 1012 ionscm−2, before increasing at higher fluences. Fluence dependence of FWHM gives the cross section of the strained region as 37.9 nm2, which is more than three times the cross section of the amorphous ion tracks.

Behavior of crystalline silicon under huge electronic excitations: A transient thermal spike description

Recent experimental works devoted to the phenomena of mixing observed at metallic multilayers Ni/Si irradiated by swift heavy ions irradiations make it necessary to revisit the insensibility of crystalline Si under huge electronic excitations. Knowing that Ni is an insensitive material, such observed mixing would exist only if Si is a sensitive material. In order to extend the study of swift heavy ion effects to semiconductor materials, the experimental results obtained in bulk silicon have been analyzed within the framework of the inelastic thermal spike model. Provided the quenching of a boiling (or vapor) phase is taken as the criterion of amorphization, the calculations with an electron–phonon coupling constant g(300 K) = 1.8 × 10 12 W/cm 3/K and an electronic diffusivity D e(300 K) = 80 cm 2/s nicely reproduce the size of observed amorphous tracks as well as the electronic energy loss threshold value for their creation, assuming that they result from the quenching of the appearance of a boiling phase along the ion path. Using these parameters for Si in the case of a Ni/Si multilayer, the mixing observed experimentally can be well simulated by the inelastic thermal spike model extended to multilayers, assuming that this occurs in the molten phase created at the Ni interface by energy transfer from Si.

Single-ion tracks inGd2Zr2−xTixO7pyrochlores irradiated with swift heavy ions

Swift xenon ions (1.43 GeV) were used to systematically investigate the radiation response of pyrochlores in the ${\text{Gd}}_{2}{\text{Zr}}_{2\ensuremath{-}x}{\text{Ti}}_{x}{\text{O}}_{7}$ binary in the electronic energy loss regime. Ion-induced structural modifications were characterized by synchrotron x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The structure of ion tracks depends on the pyrochlore composition, and the damage cross section increases with the Ti content. In general, single ion tracks consist of an amorphous track core, surrounded by a crystalline, but disordered, defect-fluorite-structured shell. That is in turn surrounded by a defect-rich pyrochlore region. The decrease in the size of these different track zones, with increasing Zr content, is a result of the enhanced radiation stability of Zr-rich pyrochlore within individual swift heavy ion tracks.

Impact of track structure calculations on biological treatment planning in ion radiotherapy

Treatment planning for ion therapy requires precise knowledge about the biological effectiveness of particle beams, which is strongly determined by the microscopic radial energy deposition around individual ion tracks. We analyse different amorphous track structure models based on simple analytical formulae as well as on radial dose distributions derived by means of Monte Carlo simulations. Moreover, these track structure representations are used as input for the local effect model (LEM) in order to determine their impact on the relative biological effectiveness (RBE) of cell inactivation. It demonstrates the relevance of the inner part of the ion track with a radius of the order of a few nanometres. We show that simple analytical formulae for the radial dose distributions give good results for the prediction of cell inactivation. However, they strongly depend on the assumptions about the local dose in the track core. Additionally, we discuss the interdependence of track structure calculations with other model constituents such as target size and the choice of the biological input data for conventional photon irradiation.

TU‐EE‐A2‐02: Impact of Track Structure Calculations On Biological Treatment Planning in Carbon Ion Radiotherapy

Purpose: To analyze predictions of the relative biological effectiveness (RBE) for carbon ion treatment planning based on different descriptions of the radial dose distributions around ion tracks and to discuss the implications on clinically relevant depth dose profiles. Method and Materials: We investigate the impact of track structure on calculations of RBE values using the Local Effect Model (LEM). It calculates the RBE for cell lines or tissues starting from the corresponding experimental/clinical photon data and an amorphous track structure model. Different track structure models that use energy‐dependent as well as energy‐independent core radii are investigated and compared to in vitro and in vivo data. Additionally, we apply them to calculate the biologically effective dose along a spread‐out Bragg peak for typical chordoma treatments. Results: We show that the LEM is sensitive to different descriptions of the radial dose distributions. However, by changing a single model parameter of the photon reference curve, reasonable RBE values can be achieved for all dose distributions. The general improvement of predictions using a modified version of the LEM with an energy‐dependent core radius is demonstrated by comparison to in vitro data of human cell lines and to experimental data of the radiation tolerance of the rat spinal cord. Additionally, we find a larger therapeutic ratio for the modified model version relative to the original LEM for a typical treatment scenario for chordoma patients. Conclusion: The Local Effect Model is sensitive to the inner part (few nanometers) of different track structure descriptions. Since these interactions of ions with liquid water are neither well understood experimentally nor theoretically, more studies should address the energy deposition around ion tracks. This will be in particular useful for further optimization of carbon ion therapy in general and with respect to comparison with other treatment modalities like protons or IMRT.

Raman spectroscopy of apatite irradiated with swift heavy ions with and without simultaneous exertion of high pressure

Durango apatite was irradiated with energetic U ions of 2.64 GeV and Kr ions of 2.1 GeV, with and without simultaneous exposure to a pressure of 10.5 GPa. Analysis by confocal Raman spectroscopy gives evidence of vibrational changes being marginal for fluences below 5×1011 ions/cm2 but becoming dominant when increasing the fluence to 8×1012 ions/cm2. Samples irradiated with U ions experience severe strain resulting in crystal cracking and finally breakage at high fluences. These radiation effects are directly linked to the formation of amorphous tracks and the fraction of amorphized material increasing with fluence. Raman spectroscopy of pressurized irradiated samples shows small shifts of the band positions with decreasing pressure but without a significant change of the Gruneisen parameter. Compared to irradiations at ambient conditions, the Raman spectra of apatite irradiated at 10.5 GPa exhibit fewer modifications, suggesting a higher radiation stability of the lattice by the pressure applied.

Biophysical calculation of cell survival probabilities using amorphous track structure models for heavy-ion irradiation

Both the microdosimetric kinetic model (MKM) and the local effect model (LEM) can be used to calculate the surviving fraction of cells irradiated by high-energy ion beams. In this study, amorphous track structure models instead of the stochastic energy deposition are used for the MKM calculation, and it is found that the MKM calculation is useful for predicting the survival curves of the mammalian cells in vitro for 3He-, 12C- and 20Ne-ion beams. The survival curves are also calculated by two different implementations of the LEM, which inherently used an amorphous track structure model. The results calculated in this manner show good agreement with the experimental results especially for the modified LEM. These results are compared to those calculated by the MKM. Comparison of the two models reveals that both models require three basic constituents: target geometry, photon survival curve and track structure, although the implementation of each model is significantly different. In the context of the amorphous track structure model, the difference between the MKM and LEM is primarily the result of different approaches calculating the biological effects of the extremely high local dose in the center of the ion track.

Tailoring of refractive index profiles in LiNbO3 optical waveguides by low-fluence swift-ion irradiation

Proton-exchange LiNbO3 planar optical waveguides have been irradiated with swift ions (Cl 30 MeV) at very low fluences in the range 5 × 1010−5 × 1012 cm−2. Large modifications in the refractive index profiles, and therefore in the optical performance, have been obtained due to the generation of amorphous nano-tracks by the individual ion impacts. Moreover, a fine tuning of the refractive index can be achieved by a suitable control of the fluence (δn/δϕ ∼ 10−14 cm2 or δn ≈ 10−5 for δϕ = 109 cm−2). An effective medium approach has been used to account for those changes and determine the amorphous fraction of material. The results have been compared with information extracted from complementary RBS channelling experiments. From the calculated amorphous fractions a radius of ∼2 nm for the amorphous tracks have been estimated.

Cluster Effects within the Local Effect Model

The local effect model predicts the relative biological effectiveness (RBE) for different ions and cell lines starting from the corresponding experimental photon data and an amorphous track structure model. Here we present an extension of the model that takes cluster effects of single-strand breaks (SSBs) at the nanometer scale into account. In line with the main idea of the local effect model, we take the yields of SSBs and double-strand breaks (DSBs) from experimental photon data and use a Monte Carlo method to distribute them onto the DNA. We score clusters of SSBs where individual SSBs are separated by less than 25 bp as additional DSBs. Assuming that the number of DSBs is a measure of cell lethality, we derive a modified cell survival curve for photons that takes these cluster effects into account. In combination with an improved radial dose distribution, we find that the extended local effect model including cluster effects reproduces most experimental data better than the original local effect model and thus enhances the accuracy of the local effect model.

Tracks in epitaxial Si1−xGex alloy layers: Effect of layer thickness

Strain-relaxed epitaxial Si0.5Ge0.5 alloy layers were irradiated with 2.7-GeV 238U ions in the electronic stopping regime. Using transmission electron microscopy, clear evidence is found that details of track formation such as morphology, defect structure, and number density strongly depend on the thickness of the sample. Amorphous tracks of diameter of ∼5nm are formed at the outer edge (15–20nm thick) of a wedge-shaped sample. In thicker sample regions (30–40nm and ∼70nm), the structure of the tracks is crystalline and the tracks contain clusters of point defects and dislocation loops. The track morphology exhibits a more or less discontinuous character. The results are ascribed to higher thermal-spike temperatures in thin layers due to restricted energy dissipation and increased surface scattering of excited electrons.

Effect of high electronic excitation in swift heavy ion irradiated semiconductors

The damage evolution due to high electronic energy deposition during 375MeV Xe and 593MeV Au ion irradiation was studied in the semiconductors InP, GaP, GaAs, AlAs and Ge using RBS and TEM. In InP the high electronic energy deposition leads to the formation of amorphous tracks. In the other materials no effect of the electronic energy deposition on damage formation was found. The TEM investigations show that in InP each impinging ion produces a track. With the use of the extended thermal spike model lattice temperatures and maximum radii of the molten zones were calculated for InP and Ge. Whereas in InP the lattice temperature exceeds the melting temperature confirming the observed formation of amorphous tracks, this is not the case for Ge. The calculated radii of the molten zones in InP are in good agreement with the track radii determined from the TEM images.

Molecular dynamics simulation of amorphization in forsterite by cosmic rays

We have examined cosmic ray interactions with silicate dust grains by simulating a thermal spike in a 1.25 million atom forsterite (Mg 2SiO 4) crystal with periodic boundaries. Spikes were generated by giving a kinetic energy of 1 or 2 eV to every atom within a cylinder of radius 1.73 nm along the [0 0 1] direction. An amorphous track of radius ∼3 nm was produced for the 2 eV/atom case, but practically no amorphization was produced for 1 eV/atom because of effective dynamic annealing. Chemical segregation was not observed in the track. These results agree with recent experimental studies of ion irradiation effects in silicates, and indicate that cosmic rays can cause the amorphization of interstellar dust.

Synergy between thermal spike and exciton decay mechanisms for ion damage and amorphization by electronic excitation

A theoretical model is proposed to account for the damage and amorphization induced in LiNbO3 by ion bombardment in the electronic energy-loss regime. It relies on the synergy between the thermal spike generated by electron-phonon interaction and the nonradiative decay of localized self-trapped excitons. Calculations have been carried out to describe the effect of single impact as well as multiple impact high fluence irradiations. In the first case, the defect concentration profile and the radius of the amorphous tracks have been theoretically predicted and they are in good accordance with those experimentally determined. For high fluence irradiations 10 13 cm �2 the model predicts the formation of homogeneous amorphous surface layers whose thickness increases with fluence. The propagation of the crystalline-amorphous boundary has been determined as a function of irradiation fluence. Theoretical predictions are also in good agreement with experimental data on Si-irradiated 7.5 and 5 MeV LiNbO3 outside the region of nuclear collision damage.

Strain fields around high-energy ion tracks in α-quartz

Transmission electron microscopy has been used to image the tracks of high-energy Au+26197 (374MeV) and I+18127 (241MeV) ions incident in a nonchanneling direction through a prethinned specimen of hexagonal α-quartz (SiO2). These ions have high electronic stopping powers in quartz, 24 and 19keV∕nm, respectively, which are sufficient to produce a disordered latent track. When the tracks are imaged with diffraction contrast using several different reciprocal lattice vectors, they exhibit a radial strain extending outward from their disordered centerline approximately 16nm into the crystalline surroundings. The images are consistent with a radial strain field with cylindrical symmetry around the amorphous track, like that found in models developed to account for the lateral expansion of amorphous SiO2 films produced by irradiation with high-energy ions. These findings provide an experimental basis for increased confidence in such modeling.

Optical determination of three-dimensional nanotrack profiles generated by single swift-heavy ion impacts in lithium niobate

Three-dimensional (3D) profiles of single nanotracks generated by a low impact density of Cl ions at 46MeV have been determined by optical methods, using an effective-medium approach. The buried location of the maximum stopping power induces a surface optical waveguiding layer even at ultralow fluences (1011–1013at.∕cm2) that allows to obtain the effective refractive index profiles (from dark-mode measurements). Combining the optical information with Rutherford backscattering spectroscopy/channeling experiments, the existence of a surrounding defective halo around the amorphous track core has been ascertained. The 3D profile of the halo has also been determined.

Amorphous tracks induced by energetic ions in strongly anisotropic solids with layered structures

Common features of ion-induced tracks in layered structures are analysed in Y‒Ba‒Cu‒O, Bi‒Sr‒Ca‒Cu‒O and in semiconducting GeS and MoS 2. In all crystals, the conduction is poor along the normal layer. The anisotropic electron properties lead to the formation of an ion-induced broad and a narrow thermal spike in these solids. The contribution of the two spikes to the formation of tracks can be separated in GeS. When the gap energy is low or zero (Y‒Ba‒Cu‒O, Bi‒Sr‒Ca‒Cu‒O, MoS 2), only the narrow spike controls the track formation, and the localization of the energy deposition is the same as in insulators. However, the fraction of the deposited energy transferred to the narrow spike is only about one-third of that in insulators. The anisotropic features of tracks show the effect of the energy bands. Good quantitative agreement is found between the experimental data and the predictions of the author’s thermal spike model.

Optical investigation of the propagation of the amorphous–crystalline boundary in ion-beam irradiated LiNbO3

The effects of high-energy silicon (5MeV, 7.5MeV and 30MeV) irradiations have been optically investigated by the dark-mode m-lines technique. In all cases, an optically isotropic homogeneous layer is created after a certain critical fluence that depends on ion and energy. The structure of the layer has been investigated by micro-Raman spectroscopy and RBS/channeling. The inner boundary of the layer separating the amorphous and crystalline regions moves into the crystal on increasing fluence. The results are discussed based on the occurrence of a sharp threshold in the electronic stopping power leading to the formation of overlapped latent (amorphous) tracks.

Generation of amorphous surface layers in LiNbO3 by ion-beam irradiation: thresholding and boundary propagation

The refractive-index profiles induced by high-energy (5 MeV, 7.5 MeV) silicon irradiation in LiNbO3 have been systematically determined as a function of ion fluence in the range 1013–1015 cm-2. At variance with irradiations at lower energies, an optically isotropic (‘amorphous’) homogeneous surface layer is generated whose thickness increases with fluence. These results have been associated with an electronic excitation mechanism. They are discussed in relation to the well-documented phenomenon of latent (amorphous) track generation under ion irradiation, requiring a threshold value Se,th for the electronic stopping power Se. Our optical data have yielded a value of ≈5 keV/nm for such a threshold, within the range reported by independent single-track measurements. The propagation of the amorphous boundary into the crystal during irradiation indicates that the threshold value decreases on increasing the fluence. Complementary Rutherford backscattering–channeling and micro-Raman (on samples irradiated at 30 MeV) experiments have been performed to monitor the induced structural changes.