Cascade Energy Density Research Articles

Intermetallic Re phases formed in ion irradiated WRe alloy

Intermetallic Re precipitation at concentrations below the solubility limit is a puzzling phenomenon in neutron irradiated W. Ion irradiation has been unable to reproduce this, denying the community the ability to accurately simulate neutron damage microstructures and probe precipitate formation. We have recently been successful in inducing σ (WRe) and χ (WRe3) phase formations in W26Re irradiated with 350 keV Ne ions at 500 and 800 °C. The precipitation of these phases is related to the effects of cascade energy density and ballistic mixing during previous high energy self-ion irradiations and is concluded to have caused redissolution of precipitates and prevented their observation.

Fullerene ion (C 60+) damage in Si at 25°C

A series of low-dose (< 5 × 10 12 ions cm −2) fullerene ion implantations in silicon has been carried out at 25°C over the energy range 100–530 keV. The resulting damage was measured quantitatively by Rutherford backscattering (RBS), using 2.0 MeV helium ions. This is an extension of an earlier study [J.B. Mitchell, J.A. Davies, L.M. Howe, R.S. Walker, K.B. Winterbon, G. Foti and J.A. Moore, Proc. 4th Intl. Conf. on Ion Implantation in Semiconductors, (Plenum Press, New York, 1975), p. 493.] of cluster-ion damage in Si where we had observed a 15-fold increase in damage-creation efficiency — i.e., the number of displaced Si atoms per keV of deposited energy — in going from (8.8 keV) monatomic carbon to (53 keV) C 6 +. Using the same 8.8 keV per carbon, we find that a 530 keV fullerene (C 60 +) ion displaces 100 times more Si atoms than a 53 keV C 6 + ion; thus C 60 + exhibits an additional 10-fold increase in damage creation efficiency compared to C 6 +. At 100 keV, the deposited (nuclear) energy density θ within the central core of each C 60 + cascade (∼ 1.5 ev per Si atom) is considerably larger than the Si heat of melting. Not surprisingly, the observed number of displaced Si atoms also exceeds the theoretical cascade volume, thus providing strong evidence for some sort of spike effect. Despite the high damage levels involved, scanning tunneling microscopy revealed no evidence of any anomalous surface structures or craters. Comparison of the present C 60 + data with earlier Si implantation studies, where a variety of ions (C, Ga, As, Sb, Te, Bi) and energies (10–60 keV) had been used, confirms the previous suggestion that room temperature damage in Si is governed mainly by the cascade energy density θ (eV/atom).

The role of electron-phonon coupling in the formation of clustered vacancy defects in elemental metals from heavy-ion irradiation

Abstract The role of electron-phonon coupling in reducing the lifetime of the thermal spike of a collision cascade has been examined by calculating the power absorbed by the electronic system of the lattice and comparing the data with published results for the defect yield in a wide variety of elemental metals. A modified version of TRIM has been used to calculate the cascade energy density for all the irradiation conditions and materials considered. A strong dependence of the efficiency of dislocation loop production (defect yield) on the cascade energy density was determined, and only at constant cascade energy density were variations in the defect yield for different materials compared. Good correlation was found between the strength of electron-phonon coupling and the defect yield data for those materials that exhibit strong electron-phonon coupling. As the strength of this coupling decreased, other factors which influence the molten zone lifetime, such as the melting temperature, were found to become...

Radiation effects in FCC metals and intermetallic compounds: A molecular dynamics computer simulation study

Abstract We review recent results on the mechanisms of atomic mixing, radiation-induced disordering, and defect production and clustering induced by displacement cascades in Cu, Ag, Cu3Au and Ni3Al. We employ molecular dynamics computer simulation (MD) methods with isotropic many body potentials and recoil energies near the subcascade formation regime (up to 30 keV). Atomic mixing is shown to be dominated by diffusion in the locally molten cascade core in both the pure metals and the intermetallic compounds. Disordering of the intermetallics was found to take place in the core of the cascades. However, because of the short lifetime of the displacement cascade, chemical short range order is preserved in the molten zone. Our results reveal the appearance of very large vacancy and interstitial type defect clusters at high recoil energy and cascade energy density. Vacancies agglomerate and collapse into Frank dislocation loops as the result of the quenching of the cascade molten core. Large interstitial clusters are directly produced in cascades and form prismatic dislocation loops. The fraction of defects in clusters for low temperature cascades increases with recoil energy and approaches ∼70% and 60% for interstitials and vacancies, respectively, at recoil energies near the threshold for subcascade formation. In the case of intermetallic (A3B) compounds, the large energy required to produce and transport a superdislocation appears to inhibit the interstitial prismatic loop punching process and interstitials appear as isolated (100) dumbbells formed by pairs of majority-type atoms.

Simulation of interface broadening of sputter profiled isotopic silicon layers

Abstract The experimental SIMS profiles of Si30/Si28 multilayers depth profiled using 2–10 KeV Ne+, Ar+ and Xe+ (K. Wittmaack and D. B. Poker, Nucl. Instr. Meth. B47 (1990) 224) have been simulated using a diffusion approximation to ion mixing. Both the leading and trailing edges of peaks in the depth profiles could be fitted for all ion energies by mixing efficiencies of 25, 40 and 65 Å5 eV−1 respectively. These values are larger than the estimates of 5, 11 and 15 Å5 eV−1 from ballistic mixing. The additional contributions to the mixing, 20, 29 and 50 Å eV−1, scale with cascade energy density, suggesting that stimulated motion in thermal spikes is important. A simple spike model in which cascade defects are trapped at fixed sinks agrees with the experimental mixing efficiencies.

The characterisation of displacement-cascade collapse in Ni-Cr-Fe alloys

Displacement cascades produced by energetic lattice recoils are the primary damage state in neutron-irradiated metals, among which austenitic stainless steels are an important class of technological alloys. The vacancy constituent of this damage that survives in the form of collapsed vacancy loops is a major component of this damage state, and has been investigated in the present work as a function of alloy composition and the temperature. This has been done by irradiating single crystal foils of a range of high-purity model ternary Fe- xNi-15% Cr alloys ( x = 15–70%) with heavy ions, and then analysing the resulting damage by transmission electron microscopy. A full analysis has been achieved by measuring the areal density of loops and their size distribution for each irradiation condition, and hence obtaining the important parameters defect yield and collapse efficiency. By appropriate choice of ion energy, ion mass, ion dose and specimen temperature, we have been able to vary the factors such as cascade size, cascade energy density, cascade defect density and cascade overlap, and thus study their influence on cascade collapse. The results have been assessed in terms of current models of cascade processes in pure metals and alloys.

Lattice damage in ion implanted silicon-germanium alloys

The damage produced in Si 1− x Ge x alloys (0 ≤ x ≤ 1) by implantation of 70–100 keV 30Si + has been measured as a function of temperature and fluence by ion channeling. For all compositions, the damage efficiency decreased sharply as the implant temperature was increased between room temperature and 150°C. Furthermore, the damage efficiency in alloys of intermediate compositions (0.35 ≤ x ≤ 0.5) exceeds that in Ge, especially at elevated temperatures, despite the larger cascade energy density in Ge. It is shown that this behavior can be described based on a model in which the point defect mobility is the dominant factor controlling damage retention, rather than the cascade energy density. This approach provides a framework for understanding other temperature-dependent phenomena related to damage growth in SiGe alloys including dose rate effects and damage saturation in MeV implantation.

Fundamental aspects of ion beam mixing

Irradiation of solids with energetic particles results in the reorganization of constituent target atoms, i.e., ion beam mixing (IM). At low temperatures, IM is characterized by prompt (10 −10 s) diffusion processes which are localized in the vicinity of the displacement cascade. Mixing at low temperatures can cause the system to depart far from the equilibrium state. At elevated temperatures, the diffusion of radiation-induced defects extends the mixing to longer times and greater distances. These delayed IM processes tend to return the system toward equilibrium. Recent experimental progress has led to a qualitative understanding of the fundamental aspects of IM in both temperature regimes. This has been achieved through systematic measurements of the influence of temperature, dose, dose-rate, cascade energy density and chemical interactions on IM. The results of these experiments will be reviewed and compared to IM models based on collisional, thermal spike and radiation-enhanced diffusion processes. The relation of IM to other fundamental radiation damage effects will also be discussed.

Ion bombardment damage in α-quartz at 50–295 K

Abstract Backscattering/channeling measurements have been used to analyse the damage produced in single crystals of α-quartz bombarded with 15–120 keV Ne+, Ar+, Kr+, Sb+ and Bi+ ions at 50 and 295 K. Damage production rates of up to 20 times those predicted by linear cascade theory have been observed. The damage versus dose (φ) behaviour is determined and related to the average cascade energy density, θ ν , and average fractional damage, F D , occurring within the individual cascade. It appears that, when F D ≲ 2% , the damage buildup with dose exhibits three distinct regions of different behaviour: (1) at low values of φ the damage increases linearly with φ; (2) at intermediate values of φ, it appears that the damage is ∝ φ2; (3) at the highest values of φ, the damage level attains a saturation value. For F D ≳ 7% , the intermediate region (∝ φ2) does not appear. We have also measured the damage production rate due to the 2 MeV He+ ions used in the backscattering/channeling measurements. The damage production cross-sections are found to be approximately two orders of magnitude greater than the values calculated by assuming elastic scattering to be the only mechanism responsible for such damage production. Preliminary indications suggest that the MeV He+ ion damage results primarily from the inelastic energy deposition whereas the keV heavy ion damage results from elastic collision events.

Effects of cascade energy density on radiation induced segregation and ion beam mixing

The effects of cascade energy density on two ion-beam induced phenomena, radiation-induced-segregation (RIS) in near-surface regions and ion-beam mixing of thin surface layers, were investigated. RIS of Si to external surfaces of Ni-12.7 at.% Si alloys was measured for 2.57 MeV Kr and 1.5 MeV He bombardments. The Kr irradiation produces defects mostly in energetically dense displacement cascades, whereas the He irradiation produces most of the defects in low energy recoils. Thin Pt and Ni layers deposited on 〈111〉 oriented Si surfaces were ion-beam mixed using 80 keV Ne and 280 keV Kr irradiations. Dense cascades were found to inhibit RIS, but to enhance ion-beam mixing.

Transmission Electron Microscope Investigations Of Defects Produced By Individual Displacement Cascades In Si And Ge

SUMMARYMonocrystalline {111} Si and Ge specimens have been irradiated with atomic and molecular heavy ions in order to study the influence of the cascade energy density and the interaction between displacement cascades on the defect production. Transmission electron microscope (TEM) studies were performed to investigate the defect parameters. After atomic irradiation of Si and Ge the defects analysed (typical size 3–5 nm) are of interstitial type. Both defects with three‐dimensional strain centres and defects with a strongly asymmetric strain field were observed. An analysis of the yield and the defect size in Si as a function of ion dose suggests that the defects are formed within individual cascades rather than by a process involving overlapping cascades. Changes of the energy density locally deposited in the lattice do not affect yield and size significantly. Under otherwise similar experimental conditions, the average defect depth is significantly larger for Bi2+ irradiations than for Bi+ irradiations.

Correlations between ion and neutron irradiations: Defect production and stage I recovery

A comparison is made between results of recent neutron- and ion-irradiation experiments on metals. Specifically, lowtemperature defect production and stage-I annealing are discussed. Defect production efficiencies observed for neutron irradiations are shown to be consistent with results for ion irradiations. A phenomenological model describing both defect production and stage-I annealing in terms of thermal-spike induced recombinations is introduced. The stage-I recovery fraction for ion and neutron irradiations is found to correlate with cascade-energy density as represented by an appropriate nuclear stopping power.

Effect of cascade energy density on isochronal recovery in silver and copper

Effect of cascade energy density on isochronal recovery in silver and copper

Surface craters induced by displacement cascades in heavy metals

Past investigations of the erosion of surfaces under bombardment by energetic particles have mainly focused on the measurement of integral sputtering yields and the examination of surface structural changes by scanning electron microscopy. Both types of observations generally require the use of rather high particle doses (≳1015cm-2) and provide only indirect information on the primary sputtering mechanisms. For more direct information, transmission electron microscopy (TEM) was applied to study the interaction of individual displacement cascades with surfaces in thin Au films. First results from low dose (1010 - 1012cm-2) Bismuth ion irradiation revealed the existence of small craters where cascades intersect a surface. In the present paper it will be shown that this surface crater formation strongly depends on cascade energy and cascade energy density. Also, details of the observed crater contrast will be discussed in view of its implications in regard to crater formation mechanisms.Thin Au films, prepared by vacuum evaporation, were bombarded with Bismuth ions.

Effect of cascade energy density on the defect cluster type in heavy-ion-irradiated gold

For the quantification of defect production by energetic displacement cascades in metals, a knowledge of the types and numbers of defects created is required. In Au a considerable fraction of the defects generated in such cascades is present in large enough clusters to be observed by transmission electron microscopy (TEM) and analysed as to their structure, sizes, and types (interstitial or vacancy clusters). Heavy ion bombardments have provided an efficient means to study the defect production in cascades as a function of cascade energy. An important problem concerns the question whether visible interstitial clusters are formed in individual displacement cascades in addition to the vacancy clusters that are formed by collapse of the depleted zones. Two conditions may enhance the formation of interstitial clusters formed in cascades:1) An increase in displacement density as produced by overlapping two low energy cascades is expected to lead to a higher interstitial concentration in close vicinity of the depleted zones.