MeV Cluster Ions Research Articles

Study on the coloration response of a radiochromic film to MeV cluster ion beams

A radiochromic film, Gafchromic HD-V2, is applied to a possible method of measuring a two-dimensional (2D) spatial profile of MeV cluster ion beams. The coloration responses of the HD-V2 film to MeV carbon and gold cluster ion beams are experimentally investigated since some cluster effect may appear. The degree of the film coloration is quantified as a change in optical density (OD) by reading the films with an image scanner for high-resolution measurement of the 2D beam profile. The OD response of HD-V2 is characterized as a function of the ion and atom fluence for comparison. The dependences of the OD response on the cluster size, kinetic energy, and ion species are discussed. It is found that the sensitivity of the OD change is reduced when the cluster size is large. The beam profile of MeV cluster ion beams delivered from the tandem accelerator in TIARA is characterized from the measurement result using HD-V2 films. The present results show that the use of the Gafchromic HD-V2 film is suitable for the detail beam profile measurement of MeV cluster ions, especially C60 ions, whose available intensity is rather low in comparison with that of monatomic ion beams.

Atomic flows, coronas and cratering in Au, Si and [formula omitted

The traditional picture of ion irradiation effects in materials was that of a series of binary collisions. Since the 1980’s it has, however, become clear that collective many-body effects such as heat spikes and flow of liquid matter can be important in dense metals. Now very recent atomistic simulations indicate that during MeV cluster ion bombardment, the irradiation effects start to resemble macroscopic phenomena such as the splashes and coronas observed when heavy objects fall on water. In this article, we review recent work showing that the craters formed during cluster impacts start to behave like macroscopic cratering beyond a certain impactor size. We also present new results which show that for cluster sizes of only around 100 atoms, not only dense metals, but also brittle materials like Si and SiO2 start to exhibit hydrodynamic-like behaviour of collective atom flow and corona formation.

Nuclear tracks in solids: registration physics and the compound spike

Observations of GeV heavy ion and MeV cluster-ion tracks in crystalline solids give us new insight into registration physics. Thermal and ion explosion spikes no longer compete; a ‘compound spike’ accounts for both. Ion explosion dominates for surface tracks (electronic sputtering). And there can also be transient plasma stopping in the bulk. For clusters there are ‘vicinage effects’—both electronic and nuclear—which can influence track dimensions and structure. Displacement cascades in large energetic clusters may lead to projectile “fission” and coherent flow into sub-tracks. The absence of tracks in certain targets, and their size/structure in others, leads to a model of projectile assisted prompt anneal (PAPA) in ∼10 −11s, either partial or complete, often by swift epitaxy, on elemental lattices (e.g. silicon) or on compound sublattices (e.g. fluorite). Phase transformations are important, but simple target amorphization is rare—the exception, not the rule. For many targets the thermal spike (macroscopic) fails, since ‘point’ defects (atomistic) characteristic of the target, their motion, and the electronic band structure, determine latent track detail. Circumstances in which the Bragg Rule of Additivity fails completely are revealed, and the kinetic threshold for constructive phase transitions in tracks described. This same track physics applies generally also to geothermometry—the opposite time extremum (∼10 +11s)—where annealing is due to defect assisted delayed anneal (DADA). Differences between etching rates of induced and spontaneous fission tracks can be explained. The geothermobarometric “Wendt/Vidal effect” (2002)—combined pressure, temperature and stress (with time) influences on fission track annealing (in e.g. apatite)—is briefly discussed.

Materials Modifications with Cluster Beams: Bulk and Surface Modification

It is now well accepted that electronic excitation and ionization arising from the slowing down of swift heavy ions can lead to structural modifications in some metallic targets as it has been known for a long time in insulators. A rapid overview of some results obtained after GeV monoatomic heavy ion irradiations will be given. It will then be shown that new specific effects take place during irradiations with cluster ions. The projectiles used are energetic cluster beams: 10 to 40 MeV Au4 or C60 ions. The rates of linear energy deposition in electronic excitation are close for GeV monoatomic and for 10 MeV cluster ions, but the cluster ions have characteristic velocities which are one order of magnitude smaller than those of monoatomic ions. This leads to a strong spatial localization of the deposited energy during the slowing down process. The density of deposited energy can then reach values as high as a few 100 eV/atom. This very high density of energy deposited in the electronic system of the targets can lead to spectacular structural modifications: generation in the vicinity of the ion trajectories of isolated or agglomerated point defects, new crystalline phases, amorphized regions... After an overview of such damage induced in bulk metals, semiconductors, and insulators, we will discuss surface damage, consisting in the formation of bumps, craters, lava-flows on the target surface.

A comparison between tracks created by high energy mono-atomic and cluster ions in Y3Fe5O12

Samples of yttrium iron garnet (Y3Fe5O12 or YIG) were irradiated with Au4, Cn (2⩽n⩽20), C60 ions in the MeV energy range. Using transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS), the induced latent tracks are compared with those generated with GeV mono-atomic ions. The cluster ions create amorphous and continuous tracks as in the mono-atomic case. Irradiation at grazing incidence allows to visualise the dissociation of cluster ions inside a solid. Some of the tracks induced by high energy C60 ions contain a “bubble-like” structure inside the amorphous part of the tracks, which is in contrast to the tracks induced by smaller cluster and GeV mono-atomic ions for which no structure is seen. The diameters of the tracks induced by MeV cluster ions and by the low velocity mono-atomic ions are seen to follow the same curve as a function of the linear rate of energy deposition (dE/dx)e.

Getting off the track

The production of MeV/amu heavy-ion and MeV cluster-ion beams has allowed continuous damage tracks to be made in a wide variety of materials. Using simple phenomenological models of the track-formation process one can estimate in advance the morphology of the tracks that will result from a particular set of irradiation parameters, i.e., target material, ion type and energy. In this talk I shall discuss the use of these models and how they are applied in a specific example: the pinning of quantized magnetic-flux vortices in a high-temperature superconductor. For this application one must also employ models for the interaction of the vortex and the column of damage. On the basis of such simulations it is found that although damage tracks are extremely useful for increasing the flux pinning, and hence the critical current, it would be even better if one could control the track positions and radii over a wider range of values. A new development in nanotechnology will be discussed that may, indeed, allow this to be accomplished easily and inexpensively.

MeV cluster ion induced defect production in high Tc superconductors

Bi 2Sr 2Ca 1Cu 2O 8 + δ and YBa 2Cu 3O 7 − δ thin films were irradiated by 23 MeV C 60 and 24 MeV Au 5 cluster ions from a tandem accelerator. The defect production of the C 60 cluster ions in Bi 2Sr 2Ca 1Cu 2O 8 + δ exceeds that of 238U ions with an energy of 2.7 GeV. The analysis of the mechanism underlying the defect production leads to different conclusions: (i) for C 60 clusters we find independence of nuclear and electronic contributions to the defect production as in the case of atomic projectiles, (ii) for Au 5 cluster ions this independence has lost it's validity. A possible interpretation of the Au 5 results is that nuclear defects are annealed by the electronic energy loss. The size of the effective damage radii obtained for the Bi 2Sr 2Ca 1Cu 2O 8 + δ film may indicate that continuous tracks are formed in this film via the C 60 and Au 5 irradiation.

Vacancy related defect profiles in MeV cluster-ion irradiated silicon

Silicon samples have been irradiated with cluster ions, 16.2 MeV Au 4 and 23 MeV C 60, from the Orsay tandem accelerator in France. After the irradiation, Schottky diodes are formed and the samples are studied with deep level transient spectroscopy. Majority carrier spectra of bombarded n-type silicon are dominated by the single charge state of the divacancy ( E C − E T = 0.42 eV). Measured depth distributions of the divacancy suggest that the peak of elastic energy deposition appears at the same depth for Au 4 ions as for monoatomic gold ions with one fourth of the energy. For C 60 clusters the measured peak is closer to the surface than the peak of the vacancy distribution calculated with TRIM. Comparison with vacancy distributions also shows that the deep tail of the divacancy defect distribution is extended to greater depths. The interpretation is that divacancies are created not only by direct collisions, but also by diffusion and pairing of two single vacancies.

Acceleration of cluster ions to MeV energies

Gold and C 60 cluster ions have been accelerated to MeV energies with the EN tandem accelerator at Erlangen. Negatively charged cluster ions were produced in the sputter source and accelerated through both stages of the accelerator. The identification of the MeV cluster ions was performed by time-of-flight and energy measurements.

Vicinage effects in Rutherford backscattering intensities of MeV cluster ions penetrating thin layers of condensed matter

The intensities of Rutherford backscattering (RBS) from thin layers of 0.5 MeV H +, 1.0 MeV H + 2 and 1.5 MeV H + 3 ion beams have been measured with different targets of C, Si, Ge, TiO 2 and Au. It is found that the RBS intensity ratios within certain critical penetration depths on H + 2 and H + incidences and on H + 3 and H + incidences are less than unity. The cluster to atomic stopping power ratio and the nuclear separation between protons in cluster ions, within which the vicinage effect continues to occur, are determined from the reduction rate in the intensity ratio and the characteristic depth. It is shown from the experimental data that the equipartition rule holds for the stopping of 0.5 MeV protons in C and Si targets and that the characteristics nuclear separation is well explained in terms of v/ ω 0, when the contribution of core electrons, lost by ionization, is involved in the frequency ω 0 of the plasmon wakes trailing individual protons.