MeV 12C Research Articles

Dynamic Deformation in Nuclear Graphite and Underlying Mechanisms.

Time-dependent deformation in nuclear graphite is influenced by the creation and migration of radiation-induced defects in the reactor environment. This study investigates the role of pre-existing defects such as point defect clusters and Mrozowski cracks in nuclear graphite IG-110. Separate specimens were irradiated with a 2.8 MeV Au2+ beam with a fluence of 4.38 × 1014 cm-2 and an 8 MeV C2+ beam with a fluence of 1.24 × 1016 cm-2. Microscopic specimens were either mechanically loaded inside a transmission electron microscope (TEM) or subjected to ex situ indentation-based creep loading. In situ TEM tests showed significant plasticity in regions highly localized around the Mrozowski cracks, resembling slip or ripplocation bands. Slip bands were also seen near regions without pre-existing defects but at very high stresses. Ex situ self-ion irradiation embrittled the specimens and decreased the creep displacement and rate, while heavy ion irradiation resulted in the opposite behavior. We hypothesize that the large-sized gold ions (compared to the carbon atoms) induced interplanar swelling as well as cross-plane channels for increased defect mobility. These findings illustrate the role of pre-existing defects in the dynamic relaxation of stresses during irradiation and the need for more studies into the radiation environment's impact on the mechanical response of nuclear graphite.

The quantitative damage and impurity depth profiling of the MgO single crystal

Ion implantation is frequently used method for the simulation of material damage caused by its exposure to harsh environments. The induced material damage and impurities accumulation will exhibit strong depth dependency in the case of its exposure to charged particles due to particle energy losses mechanisms. In presented study, we have performed an ion implantation of 4 MeV C3+ ions with three different fluences (1.5, 5 and 10 × 1015 cm−2) in the MgO [100] crystal in order to simulate structural damage induced by energetic particles. The damage depth profiles have been obtained by Elastic Backscattering Spectrometry in channeling orientation (EBS/C) using 1.86 MeV protons. EBS/C spectra were analyzed with the in house developed phenomenological Channeling SIMulation (CSIM) code. In addition to the damage depth profiles, with the new upgraded version of CSIM, concentration depth profile of implanted atoms (impurities) have also been determined. EBS/C spectra obtained profiles have been compared with the results of depth-resolved (micro) Photoluminescence spectroscopy (μPL) of the implanted MgO crystal cross-section. EBS/C and μPL obtained profiles for all investigated samples show very good consistency. This opens up the possibility for usage of EBS/C method in material analysis of lighter than bulk impurity concentration profiling.

Beam optimization of a heavy ion microbeam for targeted irradiation of mitochondria in human cells

Mitochondria, organelles of the cytoplasm, are the power plants of the cell and thus their function is important for the survival of cells. Mitochondria are known to depolarize after targeted irradiation, but the effects on cells are still unclear. The aim of this work is to investigate the effects of mitochondrial depolarization on growth and survival of cells. We performed targeted irradiation with 55 MeV C5+-ions at the ion-microbeam SNAKE at the 14 MV tandem accelerator in Garching near Munich, with a beam spot size of ∼1 µm. Approx. 6% of the mitochondrial area was irradiated in 74 cells with 5,120 carbon ions homogenously distributed over a square area of 13.2 µm2. Cell growth was investigated by observing the cells for 3.5 days via live-cell phase-contrast microscopy and evaluating the number of vital cells. While the number of irradiated cells remained constant during the observation, the unirradiated control group showed exponential growth. An additional particle track detector test with polycarbonate revealed that 4% parasitic ions hit the cells up to 500 µm away from the target, forming a so-called halo and inducing a mean parasitic dose of (2 ± 2) Gy on the cells. This dose alone, when applied in cell nuclei, is large enough to reduce the survival and growth of cells significantly and overrides any effects caused by targeted irradiation of mitochondria. Subsequently, several methods to reduce the halo were investigated. A significant reduction in halo size and number of ions in the halo could be achieved by using C6+-ions instead of C5+-ions. The slit openings, correction of lens errors, and the beam spot size had minor influence on the halo size but could achieve a reduction in halo dose. Overall, a 97% reduction in halo area and a halving of halo dose were achieved.

Relative efficiency of radiophotoluminescent glass detectors in low energy ion beams

In heavy charged particle dosimetry, the luminescence efficiency strongly depends on linear energy transfer (LET) and characterization of the detectors' efficiency as a function of the particle LET is very important. Available literature data for the efficiency of radiophotoluminescent (RPL) glass detectors are scarce and especially for low energy range no data was found. In this study, detectors made of the most widely known RPL material (silver activated phosphate glass) were exposed to various low energy ion beams (1H, 7Li, and 12C). For every ion beam, the linear dose (fluence) response of the RPL glass detectors was confirmed for the investigated dose (fluence) range and the RPL efficiency relative to 60Co gamma rays was evaluated. The relative efficiency decreased as LET increased but not as a unique function of LET, where the relative efficiency (expressed for doses in RPL glass) decreased from 0.85 ± 0.06 for the 5 MeV 1H ion beam (LETw = 8.1 keV/μm) to 0.017 ± 0.001 for the 15 MeV 12C ion beam (LETw = 673.8 keV/μm). More experimental data are still needed but results obtained in this study may motivate the application and testing of microdosimetric models for RPL glass detectors.

Fast Heavy-Ion-Induced Anion-Molecule Reactions on the Methanol Droplet Surface.

To gain insight into complex ion-molecule reactions induced by MeV-energy heavy ion irradiation of condensed matter, we performed a mass spectrometric study of secondary ions emitted from methanol microdroplet surfaces by 2.0 MeV C2+. We observed positive and negative secondary ions, including fragments, clusters, and reaction products. We found that a wider variety of negative ions than positive ions (such as C2H-, C2HO-, C2H5O-, and C2H3O2-) were formed. We performed measurements for deuterated methanol (CH3OD) droplets to identify the hydrogen elimination site of the intermediates involved in the reactions and to reveal the mechanism that generates various negative reaction product ions. Comparing the results of CH3OD with CH3OH droplets, we propose that the primary formation mechanism is association reactions of anion and neutral fragments, such as CH3O- + CO → C2H3O2-. Quantum chemical calculations confirmed that the reactions can proceed with no barrier. This study provides new insights into the importance of rapid anion-molecule reactions among fragments as the mechanism that generates complex molecular species in fast heavy-ion-induced reactions.

Measured and GATE predicted PET-image-based activity range verification for carbon therapy: a phantom study

In-beam PET is one of the imaging-based methods for monitoring carbon therapy by reconstruction of positron-emitters produced by incident particles. The accurate Monte Carlo simulation is necessary for range verification and therapeutic dose monitoring by means of the in-beam PET. GATE is capable of performing in-beam PET imaging but very few studies assess the accuracy of activity range measured from PET imaging with GATE simulated results. In this work, we present both experimental and GATE simulation studies of in-beam PET imaging of a phantom, which is irradiated by carbon ion beam, for range verification. The experiment data is acquired at Wuwei Heavy Ion Cancer Treatment Center in Gansu, China, with a dual-head plate PET prototype. The PET prototype consists of 2 panels, each arranged in 2 × 2 matrix. Each detector block is composed of 20 × 20 LYSO crystal array coupled with the position-sensitive photomultiplier tube (HAMAMATSU H8500). A homogeneous plastic phantom is irradiated with monoenergetic 131.05 MeV and 190.19 MeV 12C ion pencil beams. The irradiation and full response of PET prototype are simulated with GATE macros. A novel mathematical model is established, which is capable of calculating and revealing the variation of the positron activity. Our focus is on the reconstructed activity ranges obtained by the experimental measurement and GATE MC prediction combined with the positron activity distribution calculation mathematical model. Results show that the reconstructed activity distribution of experimental data and GATE MC prediction are in good agreement. The measured and simulated 1D activity peak and falling edge positions are within 1.0 mm in all cases. The 20%, 50% and 80% PET peak positions predicted by GATE are close to measurements. These results indicated that it is feasible to assess the accuracy of activity range measured from the dual-head plate PET system with the modeled GATE hadron-PET simulation.

Characterisation of Channel Waveguides Fabricated in an Er3+-Doped Tellurite Glass Using Two Ion Beam Techniques

Two methods were proposed and implemented for the fabrication of channel waveguides in an Er-doped Tellurite glass. In the first method, channel waveguides were fabricated by implanting 1.5 MeV and 3.5 MeV energy N+ ions through a special silicon mask to the glass sample at various fluences. Those waveguides implanted at a fluence of 1.0 × 1016 ions/cm2 operated up to 980 nm, and showed green upconversion of the Erbium ions. In the second method, channel waveguides were directly written in the Er3+: TeO2W2O3 glass using an 11 MeV C4+ ion microbeam with fluences in the range of 1 · 1014–5 · 1016 ions/cm2. The waveguides worked in single mode regime up to the 1540 nm telecom wavelength. Propagation losses were reduced from the 14 dB/cm of the as-irradiated waveguides by stepwise thermal annealing to 1.5 dB/cm at λ = 1400 nm.

Polydimethylsiloxane as protecting layer to improve the quality of patterns on graphene oxide

Graphene oxide (GO) foils have been covered with poly(dimethylsiloxane) (PDMS) via spin coating and then heated in oven to strengthen the bond of the polymerized composite. Micro ion beam has been used to directly pattern the realized film and to promote the localized reduction in the ion irradiated GO areas in vacuum. The lessening of the disorder in the carbon crystal structure and the defects production are demanding in all the graphene-based materials applications. Micro beams of 5 MeV C3+ ions with a spot of 1.5 μm × 10 μm (focused beam) and unfocused beams with size of 3 × 3 mm2 have been employed to irradiate the produced composites on a small localized region and wide areas respectively. The poly(dimethylsiloxane) foil, used to cover GO, confers flexibility to the created patterns and allows the oxygen degassing from GO. The quality and the fidelity of the patterns have been investigated by Atomic Force Microscopy (AFM). The ion beam induced structural changes in the coated and uncoated graphene oxide foils have been studied by Raman spectroscopy confirming that PDMS can be used as protecting layer to improve the quality of patterns obtained on graphene oxide by the 5.0 MeV C3+ ions beam writing.

Point defect creation by proton and carbon irradiation of α-Ga2O3

Films of α-Ga2O3 grown by Halide Vapor Phase Epitaxy (HVPE) were irradiated with protons at energies of 330, 400, and 460 keV with fluences 6 × 1015 cm−2 and with 7 MeV C4+ ions with a fluence of 1.3 × 1013 cm−2 and characterized by a suite of measurements, including Photoinduced Transient Current Spectroscopy (PICTS), Thermally Stimulated Current (TSC), Microcathodoluminescence (MCL), Capacitance–frequency (C–f), photocapacitance and Admittance Spectroscopy (AS), as well as by Positron Annihilation Spectroscopy (PAS). Proton irradiation creates a conducting layer near the peak of the ion distribution and vacancy defects distribution and introduces deep traps at Ec-0.25, 0.8, and 1.4 eV associated with Ga interstitials, gallium–oxygen divacancies VGa–VO, and oxygen vacancies VO. Similar defects were observed in C implanted samples. The PAS results can also be interpreted by assuming that the observed changes are due to the introduction of VGa and VGa–VO.

Charge collection efficiency of scCVD diamond detectors at low temperatures

The charge collection efficiency (CCE) in electronic grade, single crystal chemical vapor deposition (scCVD) diamond detector was studied using different ions of the MeV energy range from room temperature down to 47 K. The measurements were performed by using the ion beam induced charge (IBIC) technique, with 0.8 MeV H+, 3 MeV He2+, 5.6 MeV Li2+, and 12.8 MeV C4+ ion beams. The total accumulated charge as a function of temperature was extracted from the measured pulse height spectra. A similar decreasing trend of the detector CCE with temperature was observed for all ion species. A plateau was observed from room temperature to 145 K followed by a sharp, step like, decrease in the temperature range of 65 K < T < 145 K. Below 65 K, the CCE reaches another plateau with small variations down to 47 K. The decrease in collected charge was observed in the same temperature window for each ion. However, the level of the CCE decrease varied. For He, Li and C, the CCE decreased to 24.7 ± 0.4 %, 23.6 ± 1.3 %, 25.9 ± 0.8 % respectively while 39.3 ± 0.8 % for H. The reduction of the CCE is in agreement with earlier reported results obtained by alpha particles which was associated with the changes in lifetime of excitons in diamond.

Effects of Energetic Carbon-Cluster Ion Irradiation on Lattice Structures of EuBa2Cu3O7−x Oxide Superconductor

C-axis-oriented EuBa2Cu3O7−x oxide films that were 100 nm thick were irradiated with 0.5 MeV C monoatomic ions, 2 MeV C4 cluster ions and 4 MeV C8 cluster ions at room temperature. Before and after the irradiation, X-ray diffraction (XRD) measurement was performed using Cu-Ka X-ray. The c-axis lattice constant increased almost linearly as a function of numbers of irradiating carbon ions, but it rarely depended on the cluster size. Cluster size effects were observed in the XRD peak intensity and the XRD peak width. With increasing the cluster size, the decrease in peak intensity becomes more remarkable and the peak width increases. The experimental result implies that the cluster ions with a larger size provide a more localized energy deposition in a sample, and cause larger and more inhomogeneous lattice disordering. As such, local and large lattice disordering acts as a pinning center for quantum vortex; energetic carbon-cluster ion irradiation will be effective for the increment in the critical current of EuBa2Cu3O7−x superconductors.

Impact of pre-existing crystal lattice defects on the accumulation of irradiation-induced damage in a C/C composite

A carbon-fibre reinforced carbon-matrix (C/C) composite was irradiated with 30 MeV C6+ ions to a peak damage of ∼25 dpa. Ion irradiation-induced microstructural changes were mainly studied using Raman spectroscopy. The irradiation-induced crystal lattice defect accumulation in the C/C composite was compared with a reference of PCIB graphite (nuclear-grade). It shows that a high concentration of pre-existing crystal lattice defects in the studied C/C composite have a significant impact on the unexpectedly high disordering of the crystal lattice observed along the entire ion range. In comparison, PCIB graphite with much less pre-existing crystal lattice defects behaves in a more predictable manner with the irradiation damage accumulated in a narrow high dpa region. We rationalised that a large number of pre-existing crystal lattice defects in the C/C composite lead to a stronger electron-phonon coupling and play an important role on the formation of stable crystal lattice defects due to electronic energy loss during ion irradiation. The present results have implications for the development of C/C composites for radiation-tolerant applications, in terms of the crystal lattice defect elimination in the as-manufactured microstructure. Additionally, this investigation identifies a fundamental knowledge gap in the electronic energy loss effect on the irradiation damage produced in carbon-based materials at intermediate ion energies.

Comparative study of the MeV ion channeling implantation induced damage in 6H-SiC by the iterative procedure and phenomenological CSIM computer code

Due to its unique material properties, such as extreme hardness and radiation resistance, silicon carbide has been used as an important construction material for environments with extreme conditions, like those present in nuclear reactors. As such, it is constantly exposed to energetic particles (e.g., neutrons) and consequently subjected to gradual crystal lattice degradation. In this article, the 6H-SiC crystal damage has been simulated by the implantation of 4 MeV C3+ ions in the (0001) axial direction of a single 6H-SiC crystal to the ion fluences of 1.359 1015 cm-2, 6.740 1015 cm-2, and 2.02 1016 cm-2. These implanted samples were subsequently analyzed by Rutherford and elastic backscattering spectrometry in the channeling orientation (RBS/C &amp; EBS/C) by the usage of 1 MeV protons. Obtained spectra were analyzed by channeling simulation phenomenological computer code (CSIM) to obtain quantitative crystal damage depth profiles. The difference between the positions of damage profile maxima obtained by CSIM code and one simulated with stopping and range of ions in matter (SRIM), a Monte Carlo based computer code focused on ion implantation simulation in random crystal direction only, is about 10%. Therefore, due to small profile depth shifts, the usage of the iterative procedure for calculating crystal damage depth profiles is proposed. It was shown that profiles obtained by iterative procedure show very good agreement with the ones obtained with CSIM code. Additionally, with the introduction of channeling to random energy loss ratio the energy to depth profile scale conversion, the agreement with CSIM profiles becomes excellent.

The quantitative 6H-SiC crystal damage depth profiling

The hexagonal silicon carbide (6H-SiC) is one of materials used in nuclear applications, and as such is exposed to crystal damage inducing by variety of energetic particles like neutrons. In this article the 6H-SiC crystal lattice damage was introduced by the 4 MeV C3+ and 4 MeV Si3+ channelling ion implantation at the room temperature. The implantation of C and Si ions (so called self-ions) to the set of different fluences, achieves a 6H-SiC crystal lattice damage more similar to what the exposure to neutrons would produce. The 6H-SiC crystal damage has been investigated by the Elastic Backscattering spectra taken in the channeling orientation (EBS/C). EBS/C spectra of the implanted 6H-SiC samples were taken with 1.725 MeV and 1.860 MeV protons. By fitting the EBS/C spectra, the quantitative 6H-SiC crystal damage depth profiles were obtained. Further, the cross section of crystal's implanted region has been scanned with the micro-Raman (μR) technique for a comparison. In this way, the qualitative analysis of a non-crystalline phase as a function of the crystal depth was independently determined. Additionally, a scanning electron microscopy (SEM) image was taken of the implanted crystal cross sections. The comparison of the crystal damage profiles obtained by fitting EBS/C spectra with the corresponding ones obtained with the μR and SEM techniques shows very good consistency between them.

BaF2 Ridge Waveguide Operating at Mid-Infrared Wavelength

We report on the fabrication of optical ridge waveguide in barium fluoride (BaF2) crystal by 15 MeV C5+ ions irradiation with femtosecond laser ablation. The near-field modal profile and propagation loss of the waveguide at mid-infrared wavelength 4 µm are investigated by using end-face coupling system. We implement a series of annealing treatment and it efficiently reduces the propagation loss of the waveguide. The confocal Raman spectra demonstrate that the lattice structure of BaF2 crystal does not change largely after C5+ ion irradiation.

Swift heavy ion beam modified MoS2- PVA nanocomposite free-standing electrodes for polymeric electrolyte based asymmetric supercapacitor

Swift heavy ion (SHI) beam irradiation technique is one of the sophisticated techniques used for modifying material properties to design efficient devices. In this report, the casted MoS2- PVA nanocomposite films have been irradiated with 80 MeV C6+, 100 MeV Si9+, and 120 A g9+ ion beams to achieve enhanced conductivity and have been exploited as an electrode material for asymmetric supercapacitor. PVDF-HFP: TPAI gel polymer electrolyte having 3 wt.% TPAI content showed the highest conductivity and was chosen for the iodide redox gel polymer electrolyte system. The pristine MoS2-PVA nanocomposite films having an electrical conductivity of 2.5E-5 S/cm exhibited specific capacitance of 65.27 F/g whereas the films irradiated with 120 MeV Ag9+ ion beams showed an electrical conductivity of 9.7E-3 S/cm and enhanced specific capacitance of 186.67 F/g. The enhanced capacitance is attributed to the high current density generated due to the increased conductivity of the electrodes.

Laser-driven collisionless shock acceleration of ions from near-critical plasmas

This paper overviews experimental and numerical results on acceleration of narrow energy spread ion beams by an electrostatic collisionless shockwave driven by 1 μm (Omega EP) and 10 μm (UCLA Neptune Laboratory) lasers in near critical density CH and He plasmas, respectively. Shock waves in CH targets produced high-energy ∼50 MeV protons (ΔE/E of ≤30%) and 314 MeV C6+ ions (ΔE/E of ≤10%). Observation of acceleration of both protons and carbon ions to similar velocities is consistent with reflection of particles off the moving potential of a shock front. For shocks driven by CO2 laser in a gas jet, ∼30 MeV peak in He ion spectrum was detected. Particle-in-cell simulations indicate that regardless of the target further control over its density profile is needed for optimization of accelerated ion beams in part of energy spread, yield and maximum kinetic energy.

Electron-loss and destruction processes in collision of MeV/atom carbon cluster ions with rare gases

Singly charged cluster-ion yield ϕ1 of MeV/atom carbon cluster ions, generated using He and Ne gases as a charge-changing gas in tandem accelerator, is analytically formulated by six cross sections on the basis of the rate equation. These cross sections for electron-loss process from a cluster were estimated with use of the time-dependent quantum theory, assuming binary collision treatment and independent electron model. The cross sections in ϕ1 estimated for MeV/atom Cn (n = 2–4) clusters were found to show the sub-linear dependence on the atom number n. The gas-pressure dependences of the ϕ1 values calculated for the 2.5 MeV C2+ and C4+ ions using Ne gas are in good agreement with the experimental data obtained at TIARA.

Comparison of the thermoluminescence properties of NaCaPO4:Dy3+ phosphors irradiated by 75 MeV C6+ ion and γ-rays

The thermoluminescence (TL) properties of sol-gel synthesized NaCaPO4:Dy3+ phosphors irradiated by different sources of γ and carbon-ion beam form the main focus of this article. The TL study of NaCaPO4:Dy3+ phosphor was discussed for the samples irradiated by different doses of Co60 γ-rays, Cs137 γ-rays and a 75 MeV C6+ ion-beam. The effects of irradiation on the structural and photoluminescence (PL) properties were explored on the basis of results obtained from X-ray diffraction (XRD), Fourier-transform infrared spectroscopy and PL emission spectra. Not much change was observed in the XRD patterns, except for the loss of crystallinity due to the radiation exposure. Different TL characteristics such as linearity, dose-response and TL signal fading were also studied and the trapping parameters were determined using the Glow Curve Deconvolution method. NaCaPO4:xDy3+ has shown potential to be used for TL dose measurements of γ-radiation and also for C6+-ion beam measurements.

Ti3AlC2, a candidate structural material for innovative nuclear energy system: The microstructure phase transformation and defect evolution induced by energetic heavy-ion irradiation

For the structure materials applied in the innovative nuclear energy system, the strongly environment radiation source is always a big concern which will severely degrade the materials performance especially at high temperature. To explore the mechanisms of the anti-irradiation properties in Ti3AlC2, a typical MAX phase material showing excellent irradiation damage tolerance and resistance to amorphization, we conducted a series of 1 MeV C4+ ions irradiation experiments on them at different temperatures (RT, 300°C, 500°C and 800°C). Through Grazing Incidence X-ray Diffraction (GIXRD), Raman spectra (Raman), slow positron annihilation Doppler Broadening Spectroscopy (DBS) and high resolution Transmission Electron Microscopy (HRTEM), the anti-irradiation properties were systematically investigated. For the first time, an entire microstructure phase transformation process of Ti3AlC2 from α to β to γ and to perfect fcc structure phase induced by irradiation at RT and it is inverse (recovery) process of phase transformation under high temperature (≥300 °C) irradiation conditions are found and confirmed. And lots of simple vacancies are induced by irradiation and the density of them gets saturated above 5 × 1015 ions/cm2 fluences. These processes of phase transformation and recovery and vacancy saturation phenomenon are the primary reasons for why Ti3AlC2 has excellent irradiation damage tolerance and resistance to amorphization. In addition, the micro strain and lattice parameters are also affected by microstructure transformation and have been discussed. In a word, Ti3AlC2 materials show good anti-irradiation properties especially at high temperature, and now it is a primary candidate as the coating of the cladding material and the spallation target beam windows material in Chinese ADS project. And studies on this kind of materials provide a promising new concept and way for designing the structural materials for innovative nuclear energy system.