Krypton Irradiation Research Articles

Controlled disorder for the Yb2Ti2-xO7-2x (x = 0 to 1) series and corresponding radiation tolerance

The use of ceramics as radionuclide containment matrices for the safe long-term storage and disposal of high-level nuclear waste is studied with emphasis on chemistry flexibility, structural stability, and radiation tolerance. Here we investigate the flexibility of a particular series of compounds Yb2Ti2-xO7-2x (x = 0, 0.2, 0.52, 0.8, and 1) to form a solid solution between the end members Yb2Ti2O7 of pyrochlore structure through to Yb2TiO5 of fluorite structure. Whilst it is shown that cation disorder and corresponding transition from the ordered pyrochlore to disordered fluorite is facilitated via increasing the ytterbium to titanium ratio, it is also shown that sintering conditions may be controlled in such a way as to encourage the growth of the pyrochlore structure even in Yb2TiO5. The radiation response of these materials was tested in situ via 1 MeV krypton irradiation coupled with transmission electron microscopy characterisation. The critical fluence of ions required to transition the crystalline phase to amorphous was found to increase with increasing disorder from Yb2Ti2O7 to Yb2TiO5. However, of the two forms (pyrochlore and fluorite) of Yb2TiO5 studied via ion irradiation that with the greater pyrochlore structure and so more ordered also had the lower critical temperature, 434 K, for maintaining crystallinity during irradiation.

Dislocation loop evolution in Kr‐irradiated ThO2

AbstractThe early stage of microstructural evolution of ThO2, under krypton irradiation at 600, 800, and 1000°C, was investigated using in situ transmission electron microscopy (TEM). Dislocation loops grew faster, whereas their number density decreased with increasing irradiation temperature. Loop density was found to decrease with ion dose. Interstitial dislocation loops, including Frank loops with Burgers vector of a/3〈111〉 and perfect loops with Burgers vector of a/2〈110〉, were determined by traditional TEM and atomic resolution–scanning TEM techniques. Atomistic and mesoscale level modeling are performed to interpret experimental observations. The migration energy barriers of defects in ThO2 were calculated using density‐functional theory. The energetics of different dislocation loop types were studied using molecular dynamics simulations. Loop density and diameter were analyzed using a kinetic rate theory model that considers stoichiometric loop evolution. This analysis reveals that loop growth is governed by the mobility of cation interstitials, whereas loop nucleation is determined by the mobility of anion defects. Lastly, a rate theory model was used to extract the diffusion coefficients of thorium interstitials, oxygen interstitials, and vacancies.

Novel complex ceramic oxides, Ln2TiO5 (Ln = La, Sm, Gd, Tb, Dy, Ho, Er, and Yb), for polyphase nuclear waste‐forms

AbstractAs part of a broader study of ceramic nuclear waste‐forms, four different lanthanide titanates were fabricated; La0.1Sm0.1Gd0.1Tb0.1Dy0.3Ho0.1Er0.2YbTiO5, Sm0.3Gd0.3Dy0.3Yb1.1TiO5, Sm0.1Gd0.4Dy0.4Yb1.1TiO5, and Sm0.2Gd0.2Dy0.2Yb1.4TiO5. The aim was to produce single‐phase novel materials with cubic symmetry, capable of incorporating a wide variety of cations and with acceptable radiation tolerance. The chemistry flexibility and radiation tolerance are some of the major desirable properties for nuclear waste‐form materials. By using multiple lanthanides the average lanthanide radius can be controlled and consequently the structure, along with properties such as radiation tolerance. The radiation tolerance was assessed using in situ 1 MeV krypton irradiation and transmission electron microscopy characterization. Those materials for which cubic symmetry was achieved displayed better radiation tolerance; a greater critical fluence of ions (Fc) was required for the crystalline to amorphous transition, and a lower temperature was required to maintain crystallinity (Tc) during irradiation.

Local ion irradiation-induced resistive threshold and memory switching in Nb2O5/NbO(x) films.

Resistive switching devices with a Nb2O5/NbOx bilayer stack combine threshold and memory switching. Here we present a new fabrication method to form such devices. Amorphous Nb2O5 layers were treated by a krypton irradiation. Two effects are found to turn the oxide partly into a metallic NbOx layer: preferential sputtering and interface mixing. Both effects take place at different locations in the material stack of the device; preferential sputtering affects the surface, while interface mixing appears at the bottom electrode. To separate both effects, devices were irradiated at different energies (4, 10, and 35 keV). Structural changes caused by ion irradiation are studied in detail. After successful electroforming, the devices exhibit the desired threshold switching. In addition, the choice of the current compliance defines whether a memory effect adds to the device. Findings from electrical characterization disclose a model of the layer modification during irradiation.

Krypton irradiation damage in Nd-doped zirconolite and perovskite

Understanding the effect of radiation damage and noble gas accommodation in potential ceramic hosts for plutonium disposition is necessary to evaluate their long-term behaviour during geological disposal. Polycrystalline samples of Nd-doped zirconolite and Nd-doped perovskite were irradiated ex situ with 2 MeV Kr + at a dose of 5 × 10 15 ions cm −2 to simulate recoil of Pu nuclei during alpha decay. The feasibility of thin section preparation of both pristine and irradiated samples by Focused Ion Beam sectioning was demonstrated. After irradiation, the Nd-doped zirconolite revealed a well defined amorphous region separated from the pristine material by a thin (40–60 nm) damaged interface. The zirconolite lattice was lost in the damaged interface, but the fluorite sublattice was retained. The Nd-doped perovskite contained a defined irradiated layer composed of an amorphous region surrounded by damaged but still crystalline layers. The structural evolution of the damaged regions is consistent with a change from orthorhombic to cubic symmetry. In addition in Nd-doped perovskite, the amorphisation dose depended on crystallographic orientation and possibly sample configuration (thin section or bulk). Electron Energy Loss Spectroscopy revealed Ti remained in the 4+ oxidation state but there was a change in Ti coordination in both Nd-doped perovskite and Nd-doped zirconolite associated with the crystalline to amorphous transition.

Titanium surface layers after irradiation with swift Kr and Xe ions

This paper presents the test results of the swift krypton (240 MeV) and xenon ion (130 MeV) irradiation influence on the titanium surface layer properties. These ions change the titanium surface topography in different ways. The irradiation contributes to the creation of a new phase in the surface layer. Radiation damage effects on the friction coefficient, wear, and microhardness of titanium after krypton and xenon ion irradiation were studied. Krypton irradiation does not influence the microhardness of the titanium. After xenon irradiation with a fluence of 1 × 10 18 ion/m 2 the microhardness increases by 13%; further increases of the fluence gradually reduce the microhardness of the surface layer. Swift Kr and Xe ion irradiation does not influence the friction coefficient of the titanium surface.

2 MeV ion irradiation effects on AlGaN/GaN HFET devices

AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with 2MeV protons, carbon, oxygen, iron and krypton ions with fluences ranging from 1×109cm−2 to 1×1013cm−2. DC, pulsed I–V characteristics, loadpull and S-parameters of the AlGaN HFET devices were measured before and after irradiation. In parallel, a thick GaN reference layer was also irradiated with the same ions and was characterized by X-ray diffraction, photoluminescence, Hall measurements before and after irradiation. Small changes in the device performance were observed after irradiation with carbon and oxygen at a fluence of 5×1010cm−2. Remarkable changes in device characteristics were seen at a fluence of 1×1012cm−2 for carbon, oxygen, iron and krypton irradiation. Similarly, remarkable changes were also observed in the GaN layer for irradiations with fluence of 1×1012cm−2. The results found on devices and on the GaN layer were compared and correlated.

Irradiation effects on AlGaN HFET devices and GaN layers

AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with protons as well as carbon, oxygen, iron and krypton ions of high (68 and 120 MeV) and low (2 MeV) energy with fluences in the range from 1 × 107 to 1 × 1013 cm−2. High energy irradiation with protons, carbon and oxygen produced no degradation in devices while krypton irradiation at the fluence of 1 × 1010 cm−2 resulted in a small reduction of 2% in the transconductance. Similarly, for GaN samples irradiated with protons, carbon and oxygen at high energy no changes were seen by XRD, PL and Hall effect, while changes in lattice constant and a reduction in PL intensity were observed after irradiation with high energy krypton. Low energy irradiation with carbon and oxygen at a fluence of 5 × 1010 cm−2 results in small change in the device performance while remarkable changes in device characteristics are seen at a fluence of 1 × 1012 cm−2 for carbon, oxygen, iron and krypton irradiation. Similarly changes are also observed by XRD, PL and Hall effect for the thick GaN layer irradiated at the fluence of 1 × 1012 cm−2. The device results and GaN layer properties are strongly correlated.

High energy irradiation effects on AlGaN/GaN HFET devices

The effect of proton, carbon, oxygen and krypton irradiation on AlGaN HFET devices has been studied. Irradiation was performed at 68 and 120 MeV with fluences in the range from 1 × 107 to 1 × 1013 cm−2. Before and after irradiation, dc and pulsed I–V characteristics, loadpull and S-parameters of the AlGaN HFET devices were measured. A thick GaN reference layer was characterized by x-ray diffraction, photoluminescence and Hall measurements before and after irradiation. Proton, carbon and oxygen irradiation show no degradation in devices while krypton irradiation shows a small change at a fluence of 1 × 1010 cm−2 in the device characteristics. The device results are correlated with the thick GaN results.

Irradiation effects in Cu/W multilayers: Ion beam mixing and structural evolution

Irradiation effects in the immiscible Cu/W system are investigated for helium 70 keV and krypton 340 keV irradiations at liquid-nitrogen temperature. Evolution of the [(3.1 nm Cu/4.2 nm W)×12] multilayer was characterized by x-ray diffraction measurements. Limits of solubility were observed in both matrixes for both irradiations. As expected, a strong thermal spike effect is evidenced in copper layers under krypton irradiation where almost no solubility is observed. For helium irradiations very similar limits of solubility are observed in both matrixes. These latter results can be analyzed by a model with two atomic fluxes: one due to ballistic displacement and the other due to chemically driven thermally activated transport. A structural study shows a transition to an amorphous state for heavy ion irradiation in the large cascades above a critical solubility threshold.

Precipitation of copper in iron under swift ion irradiations

After irradiation was carried out on the Fe1.34 at% Cu supersaturated solution with 2.5 MeV electrons and 188 MeV oxygen ions at 290°C, we report new results about copper precipitation under 5.07 GeV krypton irradiation. The precipitation observed on line by electrical resistivity and after irradiation by tomographic atom probe experiments and microhardness measurements, shows that the kinetics of copper precipitation (in term of dpa calculated using the modified Kinchin–Pease model) is hundred times larger with GeV krypton ions than with MeV electrons and is the same under oxygen and electron irradiations. We show that such a behaviour can be explained only by assuming that the energy deposited in electronic excitations along the path of swift ions is actually able to induce precipitation in metallic supersaturated solid solutions.

Radiation Damage of a Glass-Bonded Zeolite Waste Form Using Ion Irradiation

ABSTRACTGlass-bonded zeolite is being considered as a candidate ceramic waste form for storing radioactive isotopes separated from spent nuclear fuel in the electrorefming process. To determine the stability of glass-bonded zeolite under irradiation, transmission electron microscope samples were irradiated using high energy helium, lead, and krypton. The major crystalline phase of the waste form, which retains alkaline and alkaline earth fission products, loses its long range order under both helium and krypton irradiation. The dose at which the long range crystalline structure is lost is about 0.8 dpa for helium and 0.1 dpa for krypton. Because the damage from lead is localized in such a small region of the sample, damage could not be recognized even at a peak damage of 50 dpa. Because the crystalline phase loses its long range structure due to irradiation, the effect on retention capacity needs to be further evaluated.

Changes in magnetic properties of magnetite Fe3O4 ceramics induced by high energy heavy ion irradiation

Study of the change induced by heavy ion irradiation in the magnetic properties of magnetite ceramics Fe3O4 can be important for applications. In this work, we report on the effect of the high electronic excitation induced by lead ions on the magnetic properties of highly densified Fe3O4 ceramics. The physical changes induced by irradiation have been investigated by magnetization measurements and Mössbauer spectroscopy, whereas the damage morphology has been observed by TEM. Variations of the saturation magnetization have shown original behaviour versus fluence: the magnetization starts to decrease at low fluence before recovering its initial value at higher fluence. TEM observations have shown the presence of extended spherical defects in the ion wakes. This damage morphology is very similar to that observed for krypton irradiation of the garnet Y3Fe5O12 at low electronic stopping power, which is characterized by a very small amorphous volume. In agreement with this observation, Mössbauer spectroscopy, recorded below the Verwey transition, has shown the absence of a paramagnetic phase. Moreover, Mössbauer spectra exhibited an irradiation-induced anisotropy of the hyperfine magnetic field which revealed the presence of defect-induced anisotropy. Such a result led us to the hypothesis that magnetic domain walls could be pinned by the extended defects, at least for low fluences.

Does continuous trail of damage appear at the change in the electronic stopping power damage rate?

Y3Fe5O12 samples have been irradiated by Kr ions with incident energies between 9 and 37 MeV/A and Xe ions between 9 and 25 MeV/A. In this way a range of electronic stopping power between 7 and 28 MeV/μm has been explored. The irradiated samples have been etched chemically. A homogeneous track etching radius is observed after xenon irradiation but not after krypton irradiation. This result indicates that at an electronic stopping power value (dE/dx) of 17±3 MeV/μm a change in the damage morphology occurs: above this value the latent track becomes continuous cylinder. This value of stopping power is markedly higher than the dE/dx (8±1 MeV/μm) at which we have previously observed a change in the latent track damage rate and in the damage morphology from spherical to cylindrical shape of the defects.

In-situ HVEM study of migration of bcc/glass interfaces in TiCr 2, induced by 1 MeV krypton irradiation at 80–180 K

The Laves phase TiCr 2 is readily amorphized by 1 MeV krypton irradiation at temperatures below about 250 K. However, with continuing irradiation the glassy phase (a-TiCr 2) gradually transforms to a metastable bcc structure without measurable change of local composition. The results of direct observations of migrating a-TiCr 2/bcc interfaces at temperatures in the range 80–180 K yield an effective activation energy of 0.027 eV for the interface velocity. A model for this irradiation-induced phase transformation is discussed in terms of dynamic reconstruction within displacement cascades. The phenomenon has been termed “cascade-driven migration of structural interfaces”.

An in situ HVEM study of the migration of bcc/glass interfaces in TiCr2 induced by 1 MeV krypton irradiation at 80-180 K

The Laves phase TiCr2 is readily amorphized by 1 MeV Kr irradiation at temperatures below about 250 K. After an ion dose of order 1019 m-2, however, the amorphous phase (a-TiCr2) gradually transforms to the metastable bcc structure without measureable change of composition. This reaction proceeds by a nucleation and growth mechanism and by growth of any bcc material in the initial microstructure. An in situ study of the temperature dependence of the interface velocity in the temperature range 80-180 K has been conducted in the HVEM-Tandem User Facility at Argonne, which allows simultaneous ion irradiation and TEM observation. A single tilt liquid helium stage was employed with a sample of Cr+30 at. pct. Ti, prepared for TEM by mechanical dimpling and ion milling. The sample initially was composed of two phases, the Laves phase TiCr2 and the Cr-rich bcc phase β. During the experiment the temperature was measured and controlled by a platinum resistance thermometer attached to the stage cold block.