Ion beam induced defects in solids studied by optical techniques

Optical methods can provide important insights into the mechanisms and consequences of ion beam interactions with solids. This is illustrated by four distinctly different systems.X- and Y-cut LiNbO₃ crystals implanted with 8 MeV Au³⁺ ions with a fluence of 1 × 10¹⁷ ions/cm² result in gold nanoparticle formation during high temperature annealing. Optical extinction curves simulated by the Mie theory provide the average nanoparticle sizes. TEM studies are in reasonable agreement and confirm a near-spherical nanoparticle shape but with surface facets. Large temperature differences in the nanoparticle creation in the X- and Y-cut crystals are explained by recrystallisation of the initially amorphised regions so as to recreate the prior crystal structure and to result in anisotropic diffusion of the implanted gold.Defect formation in alkali halides using ion beam irradiation has provided new information. Radiation-hard CsI crystals bombarded with 1 MeV protons at 300 K successfully produce F-type centres and V-centres having the structure as identified by optical absorption and Raman studies. The results are discussed in relation to the formation of interstitial iodine aggregates of various types in alkali iodides. Depth profiling of and aggregates created in RbI bombarded with 13.6 MeV/A argon ions at 300 K is discussed.The recrystallisation of an amorphous silicon layer created in crystalline silicon bombarded with 100 keV carbon ions with a fluence of 5 × 10¹⁷ ions/cm² during subsequent high temperature annealing is studied by Raman and Brillouin light scattering.Irradiation of tin-doped indium oxide (ITO) films with 1 MeV protons with fluences from 1 × 10¹⁵ to 250 × 10¹⁵ ions/cm⁻² induces visible darkening over a broad spectral region that shows three stages of development. This is attributed to the formation of defect clusters by a model of defect growth and also high fluence optical absorption studies. X-ray diffraction studies show evidence of a strained lattice after the proton bombardment and recovery after long period storage. The effects are attributed to the annealing of the defects produced.     

Secondary ion emission from Cu(1 0 0) surfaces with atomic adsorbates (N, O, Cl, S and Br)

Several targets that consist of atomic species X (X = N, O, Cl, S, Br) adsorbed at hollow sites on the Cu(1 0 0) surface have been examined with low-fluence secondary ion mass spectrometry (SIMS). The positive and negative secondary ion (SI) abundance distributions, which show a range of characteristics, have been discussed with the aid of thermochemical data derived from ab initio calculations. In positive SIMS, CuX⁺ is never observed, while the only heteronuclear (mixed-atom) SI that is observed for all five systems is Cu₂X⁺. In negative SIMS, the dominant heteronuclear species for all systems is , except for N/Cu(1 0 0), which produces no , ions. Cu⁻ emission is observed only for O/Cu(1 0 0). By analogy with results from laser ablation studies of O/Cu targets, it is conjectured that Cu⁻ is a daughter product of the gas-phase dissociation of polyatomic Cu-O anion clusters.     

Elemental analysis of light constituents in thin plastic films

Experiments with 1-3 MeV ⁴He beams and 2.2-3.2 MeV proton beams have been performed in order to develop a precise and highly sensitive method for the detection and analysis of light elements in threat or drug materials using simultaneously the RBS and ERDA techniques. Commercially available plastic films have been used as samples. The 1 MeV ⁴He beam has been found to be inadequate owing to the rapid destruction of the targets. Best results have been obtained using proton beams with energies higher than 3 MeV. For 3.2 MeV proton beams, ERDA has been used for the analysis of the hydrogen constituent.     

Spectroscopic characterisation of a cross-flow plasma jet

The aim of this work is to study the thermal characteristics and electron density based on atomic and molecular emission of a new plasma jet at atmospheric pressure. The novelty of our jet is its generation with a single electrode, the plasma gas flowing perpendicularly to the RF powered electrode (13.56 MHz, 10³ V). Optical emission of the plasma was collected in two ways: the normal viewing mode and the axial viewing mode. The plasma characteristic parameters as function of helium flow-rate, plasma power and position of the investigated zone were studied. The excitation, vibrational and rotational temperatures are in the range of 1500-2350 K, 3500-4400 K and 450-1100 K, respectively. The electron number densities are in the range of 10¹³-10¹⁴ cm⁻³. For qualitative observations regarding the atomic and molecular processes in the plasma we used the relative intensities of the most representative lines of He, N₂, O, H and .     

Autoionization of N (q = 1-3) Rydberg states produced in high-energy collisions with He

Energy spectra of electrons ejected through autoionization decay of high-Rydberg states in high-energy collisions of N^q+ (q = 1-3) with He have been measured with high-resolution by using zero-degree electron spectroscopy. Several series of autoionizing lines were observed, corresponding to decays from N³⁺ 1s²2p(²P)nl Rydberg states produced in N³⁺ + He collisions, from N²⁺ 1s²2s2p(³P)nl Rydberg states produced in N²⁺ + He and from N⁺ 1s²2s2p²(⁴P)nl Rydberg states produced in N⁺ + He, respectively. Angular momentum distributions for the first or second peak of three series of Coster-Kronig electron transitions for N^q+ (q = 1-3) projectiles are also discussed, where the highly excited states are formed by electron excitation.     

380 keV proton irradiation effects on photoluminescence of Eu-doped GaN

The effect of 380 keV proton irradiation on the photoluminescence (PL) properties has been investigated for undoped and Eu-doped GaN. As the proton irradiation exceeds , a drastic decrease of PL intensity of the near band-edge emission of undoped GaN was observed. On the other hand, for Eu-doped GaN, the PL emission corresponding to the ⁵D₀→⁷F₂ transition in Eu³⁺ kept the initial PL intensity after the proton irradiation up to . Present results, together with our previous report on electron irradiation results, suggest that Eu-doped GaN is a strong candidate for light emitting devices in high irradiation environment.     

Density effects on the guided transmission of 3 keV Ne ions through PET nanocapillaries

Experiments for guided transmission of 3 keV Ne⁷⁺ ions through nanocapillaries in insulating PET polymers are reported. The ion guiding was studied for a two types of PET samples which consist of 200 nm capillaries with densities of and . The width of the emission profile and the fraction of transmitted ions were measured as a function of the capillary tilt angle. For the high capillary density the profile width of the transmitted ions is independent of the tilt angle in agreement previous studies. However, for the low-density sample the profile width was found to increase by a factor of 2 as the tilt angle increases from 0° to 8°. The results for the fraction of transmitted ions are used to evaluate the guiding angle, which specifies the guiding power of the material. The guiding powers were found to be equal for the two samples. The present results are discussed in terms of scaling laws, which have recently been established.     

A high charge state all-permanent magnet ECR ion source for the IMP 320 kV HV platform

A 320 kV high voltage (HV) platform has been constructed at Institute of Modern Physics (IMP) to satisfy the increasing requirements of experimental studies in some heavy ion associated directions. A high charge state all-permanent magnet ECRIS-LAPECR2 has been designed and fabricated to provide intense multiple charge state ion beams (such as 1000 eμA O⁶⁺, 16.7 eμA Ar¹⁴⁺, 24 eμA Xe²⁷⁺, etc.) for the HV platform. LAPECR2 has a dimension of ∅ 650 mm × 560 mm. The powerful 3D magnetic confinement to the ECR plasma and the optimum designed magnetic field for the operation at 14.5 GHz makes it possible to obtain very good performances from this source. After a brief introduction of the ECRIS and accelerator development at IMP, the conceptual design of LAPECR2 source is presented. The first test results of this all-permanent magnet ECRIS are given in this paper.     

Neutron interrogation of high-enriched uranium by a 4 MeV linac

For revealing unauthorized transport (illicit trafficking) of nuclear materials, a non-destructive method reported earlier, utilizing a 4 MeV linear accelerator for photoneutron interrogation, was further developed. The linac served as a pulsed neutron source for assay of highly enriched uranium. Produced in beryllium or heavy water by bremsstrahlung, neutrons subsequently induced fission in the samples. Delayed neutrons were detected by a newly designed neutron collar built up of 14 ³He counters embedded in a polyethylene moderator. A PC controlled multiscaler served as a time analyzer, triggering the detector startup by the beam pulse. Significant progress was achieved in enhancing the detector response, hence the sensitivity for revealing illicit material. A lower sensitivity limit of the order of 10 mg ²³⁵U was determined in a 20 s measurement time with a reasonable amount of beryllium (170 g) or of heavy water (100 g) and a mean electron current of 10 μA. Sensitivity can be further enhanced by increasing the measurement time.     

Surface exfoliation and defect structures in Si induced by 160 keV He and 110 keV H ion implantation

Cz n-type Si(100) wafers were implanted at room temperature with 160 keV He ions at a fluence of 5 × 10¹⁶/cm² and 110 keV H ions at a fluence of 1 × 10¹⁶/cm², singly or in combination. Surface phenomena and defect microstructures have been studied by various techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM). Surface exfoliation and flaking phenomena were only observed on silicon by successive implantation of He and H ions after subsequent annealing at temperatures above 400 °C. The surface phenomena show strong dependence on the thermal budget. At annealing temperatures ranging from 500 to 700 °C, craters with size of about 10 μm were produced throughout the silicon surface. As increasing temperature to 800 °C, most of the implanted layer was sheared, leaving structures like islands on the surface. AFM observations have demonstrated that the implanted layer is mainly transfered at the depth around 960 nm, which is quite consistent with the range of the ions. XTEM observations have revealed that the additional low fluence H ion implantation could significantly influence thermal growth of He-cavities, which gives rise to a monolayer of cavities surrounded by a large amount of dislocations and strain. The surface exfoliation effects have been tentatively interpreted in combination of AFM and XTEM results.     

Fabrication and nuclear analysis of isotopic N and N ion-implanted silicon standards

The fabrication of reliable isotopic nitrogen standards is achieved in Si through ¹⁴N and ¹⁵N ion implantation. 60 keV and ions were implanted at 400 °C up to ∼60% peak atomic concentration, yielding nitrogen-saturated silicon layers as measured using resonant nuclear reaction analysis. No isotopic effect has been observed. The nitrogen standards are validated by measurements of stability under ion irradiation. No significant desorption of nitrogen is observed either under a ⁴He⁺ ion fluence of 3.36 × 10¹⁶ cm⁻² or under a ¹H⁺ ion fluence of 8.60 × 10¹⁷ cm⁻², giving strong evidence that isotopic nitrogen standards can be achieved.     

Effects of 0.28-2.80 MeV proton irradiation on GaInP/GaAs/Ge triple-junction solar cells for space use

GaInP/GaAs/Ge triple-junction solar cells were irradiated with 0.28, 0.62 and 2.80 MeV protons with fluences ranging from 1 × 10¹⁰ cm⁻² to 1 × 10¹³ cm⁻². Their performance degradation is analyzed using current-voltage characteristics and spectral response measurements. The degradation rates of the short circuit current, open circuit voltage, and maximum power output increase with fluence, but decrease with increasing proton energy. It was also observed that the spectral response of the GaAs middle cell degrades more significantly than that of the GaInP top cell.     

The (He, tf) as a surrogate reaction to determine (n, f) cross sections in the 10-20 MeV energy range

The surrogate reaction ²³⁸U(³He, tf) is used to determine the ²³⁷Np(n, f) cross section indirectly over an equivalent neutron energy range from 10 to 20 MeV. A self-supporting ∼761 μg/cm² metallic ²³⁸U foil was bombarded with a 42 MeV ³He²⁺ beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL). Outgoing charged particles and fission fragments were identified using the Silicon Telescope Array for Reaction Studies (STARS) consisted of two 140 μm and one 1000 μm Micron S2 type silicon detectors. The ²³⁷Np(n, f) cross sections, determined indirectly, were compared with the ²³⁷Np(n, f) cross section data from direct measurements, the Evaluated Nuclear Data File (ENDF/B-VII.0), and the Japanese Evaluated Nuclear Data Library (JENDL 3.3) and found to closely follow those datasets. Use of the (³He, tf) reaction as a surrogate to extract (n, f) cross sections in the 10-20 MeV equivalent neutron energy range is found to be suitable.     

Monitoring the embrittlement of reactor pressure vessel steels by using the Seebeck coefficient

The degree of embrittlement of the reactor pressure vessel (RPV) limits the lifetime of nuclear power plants. Therefore, neutron irradiation-induced embrittlement of RPV steels demands accurate monitoring. Current federal legislation requires a surveillance program in which specimens are placed inside the RPV for several years before their fracture toughness is determined by destructive Charpy impact testing. Measuring the changes in the thermoelectric properties of the material due to irradiation, is an alternative and non-destructive method for the diagnostics of material embrittlement. In this paper, the measurement of the Seebeck coefficient () of several Charpy specimens, made from two different grades of 22 NiMoCr 37 low-alloy steels, irradiated by neutrons with energies greater than 1 MeV, and fluencies ranging from 0 up to 4.5 × 10¹⁹ neutrons per cm², are presented. Within this range, it was observed that increased by ≈500 nV/°C and a linear dependency was noted between and the temperature shift ΔT_41 J of the Charpy energy vs. temperature curve, which is a measure for the embrittlement. We conclude that the change of the Seebeck coefficient has the potential for non-destructive monitoring of the neutron embrittlement of RPV steels if very precise measurements of the Seebeck coefficient are possible.     

Photoneutron interrogation of low-enriched uranium induced by bremsstrahlung from a 4 MeV linac

Revealing smuggled nuclear material by passive γ-detection is hindered, because the weak radiation can easily be shielded. Neutrons, as penetrate shielding, represent a detection potential, by inducing fission in the nuclear material. A 4 MeV linear accelerator was used as a pulsed neutron source for active interrogation of U-bearing material. Produced in heavy water by bremsstrahlung, neutrons subsequently induced fissions in UO₂ samples. Delayed fission neutrons were detected in a neutron collar built up by ³He counters in a polyamide container. The counters were gated to be detached from high voltage during the electron pulse. Irradiation-measurement cycles were carried out with a 25 Hz pulse repetition rate as optimum setting. The time analyser start-up was externally triggered and synchronised by the electron beam pulse. The response of the system was studied as a function of the intensity of the electron current, the amount of heavy water, U enrichment, and total U content. Sensitivity limit was achieved as 0.5 g ²³⁵U and/or 30 g ²³⁸U in a 20 s measurement time (500 cycles) with the amount of heavy water of 100 g and a mean electron current of 2 μA. Because of the long decay time of the prompt (interrogating and fission) neutron pulse, about a half of the time interval (40 ms) between pulses is only available for counting delayed neutrons.     

Anisotropy of disorder accumulation and recovery in 6H-SiC irradiated with Au ions at 140 K

Single crystal 〈0 0 0 1〉-oriented 6H-SiC was irradiated with Au²⁺ ions to fluences of 0.032, 0.058 and 0.105 ions/nm² at 140 K and was subsequently annealed at various temperatures up to 500 K. The relative disorder on both the Si and C sublattices has been determined simultaneously using in situ D⁺ ion channeling along the 〈0 0 0 1〉 and 〈〉 axes. A higher level of disorder on both the Si and C sublattices is observed along the 〈〉. There is a preferential C disordering and more C interstitials are aligned with 〈0 0 0 1〉. Room-temperature recovery along 〈〉 occurs, which is associated with the 〈0 0 0 1〉-aligned interstitials that annihilate due to close-pair recombination. Disorder recovery between 400 and 500 K is primarily attributed to annihilation of interstitials that are misaligned with 〈0 0 0 1〉 and to epitaxial crystallization. Effects of stacking order in SiC on disorder accumulation are insignificant; however, noticeable differences of low-temperature recovery in Au²⁺-irradiated 6H-SiC and 4H-SiC are observed.     

Deuterium retention in plasma spray tungsten coatings exposed to low-energy, high flux D plasma

Two types of porous plasma spray tungsten coatings deposited onto stainless steel and graphite substrates were exposed to low-energy (76 eV ), high-flux (10²² D/m² s) D plasma to ion fluences of (3-4) × 10²⁶ D/m² at various temperatures. Deuterium retention in the W coatings was examined by thermal desorption spectroscopy and the D(³He,p)⁴He nuclear reaction, allowing determination of the D concentration at depths up to 7 μm. The relatively high D concentration (above 0.1 at.%) at depths of several micrometers observed after D plasma exposure at 340-560 K can be related to accumulation of D₂ molecules in pores, while at temperatures above 600 K deuterium is accumulated mainly in the form of D atoms chemisorbed on the inner pore surfaces. At exposure temperatures above 500 K, the D retention in the plasma spray W coating on graphite substrate increases significantly due to trapping of diffusing D atoms at carbon dangling bonds located at the edge of a graphite crystallite.     

Surface modifications and deuterium depth profiles in molybdenum irradiated with low-energy D ions

Depth profiles of deuterium trapped in single crystal Mo, polycrystalline Mo, and molybdenum trioxide film on polycrystalline Mo irradiated with 200 eV D ions have been measured up to a depth of 8 μm using the D(³He,p)⁴He nuclear reaction at a ³He energy varied from 0.69 to 4.0 MeV. For the D ion irradiation at 323 K to the highest ion fluence of 5 × 10²⁴ D/m², the D concentration decreases from several at.% in the near-surface layer to bulk values below 10⁻⁴ at.% for single crystal Mo and about 10⁻² at.% for polycrystalline Mo. The maximum D concentration in molybdenum trioxide film differs little in value from that for polycrystalline Mo. Blister formation at high fluences is observed for polycrystalline Mo and molybdenum trioxide film, but not for single crystal Mo. As the irradiation temperature increases from 323 to 493 K, the D retention in the polycrystalline Mo decreases from about 3 × 10²¹ down to about 2 × 10¹⁸ D/m².     

Structure and tribological performance of helium-implanted layer on Ti6Al4V alloy by plasma-based ion implantation

The present paper concentrates on tribological performance of Ti6Al4V alloy treated by helium plasma-based ion implantation with a voltage of −30 kV and a dose range of 1, 3, 6 and 9 × 10¹⁷ He/cm². X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM) and Atomic force microscopy (AFM) were used to characterize composition, structure and surface morphology, respectively. The variation of hardness with indenting depth was measured and tribological performance was evaluated. The uniform cavities with a diameter of several nanometers are formed in the helium-implanted layer on Ti6Al4V alloy. Helium implantation enhances the ingress of O, C and N and produces TiO₂, Al₂O₃, TiC, TiN in the near surface layer on their removal from the vacuum and exposure to normal atmospheric condition. In the near surface layer, the hardness of implanted samples increases remarkably comparing with the untreated sample, and the maximum peak increasing factor is up to 2.9 for the sample implanted with 3 × 10¹⁷ He/cm². A decrease in surface roughness, resulting from the leveling effect of sputtering and re-deposition during implantation, has also been observed. Comparing with the untreated sample, implanted samples have a good wear resistance property. And the maximum increase in wear resistance reaches over seven times that of the untreated one for the sample implanted with 3 × 10¹⁷ He/cm². The wear mechanism of implanted samples is abrasive-dominated.