Displacement Cross Sections Research Articles

Spectroscopic studies on natural fluorapatites irradiated with 10 MeV electrons

Fluorapatites are one of the candidate matrices for hosting the nuclear waste from molten salt reactors. To assess their radiation stability and long-term performance, natural analogue study approach was undertaken. Naturally occurring fluorapatites containing with carbonate substitutions were irradiated with 10 MeV electron beam radiation for doses 20 MGy. X-ray diffraction, Raman and Fourier Transform infrared spectroscopy show no evidence of structural degradation or phosphate condensation. However, a change in the intensity of ν3 Phosphate Raman bands was observed indicating that the phosphate tetrahedra is affected by the electron beam consistent with the displacement cross-section calculations for each constituent element. Overall, the study confirmed that fluorapatite possess good structural stability against electron radiation upto 20 MGy.

Combined electronic excitation and knock-on damage in monolayer MoS2

Electron irradiation-induced damage is often the limiting factor in imaging materials prone to ionization or electronic excitations due to inelastic electron scattering. Quantifying the related processes at the atomic scale has only become possible with the advent of aberration-corrected (scanning) transmission electron microscopes and two-dimensional materials that allow imaging each lattice atom. While it has been shown for graphene that pure knock-on damage arising from elastic scattering is sufficient to describe the observed damage, the situation is more complicated with two-dimensional semiconducting materials such as ${\mathrm{MoS}}_{2}$. Here, we measure the displacement cross section for sulfur atoms in ${\mathrm{MoS}}_{2}$ with primary beam energies between 55 and 90 keV, and correlate the results with existing measurements and theoretical models. Our experimental data suggests that the displacement process can occur from the ground state, or with single or multiple excitations, all caused by the same impinging electron. The results bring light to reports in the recent literature, and add necessary experimental data for a comprehensive description of electron irradiation damage in a two-dimensional semiconducting material. Specifically, the results agree with a combined inelastic and elastic damage mechanism at intermediate energies, in addition to a pure elastic mechanism that dominates above 80 keV. When the inelastic contribution is assumed to arise through impact ionization, the associated excitation lifetime is on the order of picoseconds, on par with expected excitation lifetimes in ${\mathrm{MoS}}_{2}$, whereas it drops to some tens of femtoseconds when direct valence excitation is considered.

Structural characterization, defect evolution and luminescence properties of natural CaF2 under 10 MeV electron irradiation

Present study investigates the response of various coloured variants of natural fluorite (CaF2) to 10 MeV electron beam irradiation at accumulated dose of 5–10 MGy. The fluorite specimens did not show any post irradiation radioactivity. However, after irradiation the specimens developed purple colouration of different shades depending on the dose. In-depth characterization of the as-received and irradiated specimens using XPS and Raman spectroscopy confirm the relocation of fluorine (F) anions at interstitial sites and formation of metallic calcium (Ca). The calculated displacement cross sections for F and Ca indicate that it is easier to form fluorine anion interstitials within lattice structure. However, the estimated lower fraction of interstitial fluorine (∼14%) as compared to metallic Ca (>50%) is attributed to their surface diffusion and desorption. Electron irradiation was found to cause photoluminescence quenching in all the CaF2 specimens that is likely caused by a radiation-induced formation of competing recombination channels.

General displacement function for displacement damage cross-section calculation

The estimation of displacement damage rate requires the prior knowledge of displacement damage cross-section and displacement function is one part in the displacement damage cross-section calculation. The NRT model, which is used to calculate the displacement function for many years, always overestimates the number of displaced atoms in material. Although the arc-dpa model proposed recently can give correct results for the materials irradiated by neutron, there is still no suitable displacement function that can be used to accurately obtain both neutron and charged particles displacement cross-section comparing with experimental data. In the present work, a general displacement function is proposed, which is composed by power function model, arc-dpa model and BCA model. The model parameters are obtained for aluminum, iron, copper and tungsten. The displacement damage cross-sections for the materials irradiated by neutron and charged particles calculated using the general displacement function agree well with available experimental data.

Resonance self-shielded multigroup atomic displacement cross section for assessing neutron irradiation damage

Accurate calculation of neutron-induced displacement damage is a key step for the subsequent investigations on neutron irradiation damage. Due to the use of multigroup displacement damage cross sections, the resonance self-shielding effect of neutron-induced nuclear reaction must be taken into account. In addition to the method proposed by the authors in 2019, two additional methods are proposed in the present work to consider the self-shielding effect on the calculation of neutron irradiation-induced displacement damage. The two newly proposed methods allow directly using self-shielded multigroup nuclear reaction cross sections without infinite-dilution cross sections, even though they are not as consistent as the first one in the case of weak resonance self-shielding effect. The resulting resonance self-shielding corrections on the displacement damage of 56Fe in a typical sodium-cooled fast reactor are quite close for the three different methods. The slight differences (within ∼ 2 %) are much smaller than the self-shielding corrections, which are about −11.3 % for the dominant neutron elastic scattering-induced damage and −10.4 % for the total displacement damage.

Measurement of displacement cross-sections of Nb irradiated by protons with kinetic energy range between 0.4 and 3 GeV

In high-intensity proton accelerator facilities, it is crucial to evaluate the damage of beam-interception materials and accelerator components, such as a superconducting magnet coil and cavity. The displacement per atom (dpa) is used as a damage index derived by integrating the particle flux and the displacement cross section. Although the dpa is employed as the standard, the experimental data of displacement cross section are scarce for a proton in the energy region above 20 MeV. To obtain the data for superconducting materials for high-intensity accelerators and magnets, we measured the displacement cross section of Nb for proton irradiation with a kinetic energy range between 0.4 and 3 GeV at J-PARC. The present experimental results were compared with the calculation of PHITS code and Karlsruhe Institute of Technology (KIT) evaluation using both NorgerttRobinson-Torrens (NRT) and the athermal recombination corrected dpa (arc-dpa) models. The experiment showed that the widely utilized NRT model overestimates the cross section by 50%. It is also found that the arc-dpa model shows remarkably good agreement with the present data.

Improved methods for calculating electron, positron, and photon radiation-induced displacement damage cross sections

Radiation damage is a key challenge for materials. Accurate evaluation of radiation damage is a starting point for investigating the behaviors of materials used in a radiation environment. The present work proposes improved methods for calculating the primary radiation damage cross sections induced by light particles, including electrons, positrons, and photons. The main improvements compared with previous methods are: (1) accurate Mott cross section rather than the McKinley-Feshbach analytical approach is used for electron and positron scattering; (2) data and methods recommended by the International Commission on Radiation Unit and measurements (ICRU) are systematically applied for computing the stopping powers of electron and positron; (3) accurate differential cross section by combining different models instead of some simple approaches is used for pair production. The electron, positron, and photon irradiation-induced displacement cross sections for various important elements covering a large range of atomic numbers (from Li to U) are computed with the improved methods for energy up to 100 MeV. The calculated displacement cross sections are also tabulated so that they can be simply used for further applications.

Understanding natural radiation induced cationic and anionic defects in natural fluorite single crystals through spectroscopic investigations

Fluorite single crystals, CaF2 are multifunctional crystals that have potential applications in environmental, thermo-luminescent and accidental dosimetry. A comprehensive investigation of the co-located naturally occurring colourless and coloured (yellow, sea-green, purple, blue, black) fluorites (CaF2) doped with bivalent and multivalent cations was carried out for the cationic and anionic sub-lattice defects generated due to natural irradiation from associated actinide containing pyrochlore grains. Detailed spectroscopic analyses have been used to map the various cationic and anionic defect structures such as O3− and O2--Vacancy type defect complexes in yellow fluorites, F-centres and irradiation induced metallic Ca in purple, blue and black fluorites. The confinement of the radiation damage only to the anionic sub-lattice was explained in terms of higher displacement cross-section for F (59 b) as compared to Ca (33 b) as well as lower energy requirement to create F displacements and vacancies. It is observed that the fluorite retained its phase stability under natural radiation exposure and elemental substitutions.

Silicon Solar Cells for Post-Detonation Monitoring and Gamma-Radiation Effects

The feasibility of using solar cells for post-detonation monitoring, and more broadly, gamma-ray monitoring, is evaluated using Monte Carlo simulations and experiments in this work. We measured the short-circuit current Isc response of commercial silicon (Si) solar cells to 137Cs and 60Co gamma rays. A clear response of both mono- and polycrystalline Si solar cells to 137Cs and 60Co gamma rays was obtained in good agreement with the simulations. Radiation effects in solar cells due to accumulated gamma-ray dose were noticed as the drop in Isc and open-circuit voltage Voc. The atomic displacement cross section of the produced secondary fast electrons and nonionizing energy loss (NIEL) concepts were revisited to understand the principal gamma-radiation damage mechanism in solar cell devices. Analytical computations of and NIEL of electrons convoluted with simulated Compton electron distributions in Si enabled a fundamental understanding of the gamma-radiation effects and recovery mechanism in solar cells, further supporting the experimental results. Different from the ionization effects in the polymer and glass layers of a solar cell/panel, displacement damage in the Si p-n layer from gamma rays or fast electrons is much less than that from massive particles, which directly affects the charge collection performance fundamental to solar cell operation.

Recommendation for computing neutron irradiation damage from evaluated nuclear data

Accurate evaluation of displacement damage is important for studying the nuclear materials after irradiation. The present work investigates neutron-induced displacement damage evaluation via damage cross sections. There are potential discrepancies between different evaluations on the damage cross sections induced by the (n,γ) reaction and potential discontinuity at 20 MeV incident neutron energy. These two issues are however not important for evaluating neutron irradiation damage in fission and fusion reactors. Due to some issues associated with the recoil distributions in evaluated nuclear data files, the corresponding damage cross sections are questionable, such as those of 52Cr in JEFF-3.3, 55Mn in JEFF-3.3 and ENDF/B-VIII.0, and 180,182,183,184,186W in ENDF/B-VIII.0. Therefore, the present work recommends using two-body kinematic considerations and the differential cross sections of light particles for evaluating neutron irradiation damage (via either displacement cross sections or PKA spectra), unless the recoil distributions have been pre-validated/verified. The recommended method reduces the 70% discrepancy between different nuclear data evaluations to 13% for evaluated displacement damage of tungsten in ITER tokamak.

Some approaches to the calculation of the normalized Mott cross section, displacement cross section, and the Mott correction to the Bethe formula

A new exact representation for the normalized Mott cross section is obtained. Some earlier exact and approximate methods for its calculation are compared with the method proposed in this work. It is demonstrated that applying the suggested rigorous method, along with the approximate method of Lijian et al., is most preferable for the above calculations. Some modifications of Lijian’s method are proposed. Based on these modifications and the Lijian method itself, new analytical expressions are obtained for the Mott corrections to the Bethe stopping formula and the primary displacement cross section by fast electrons. It is shown that the highest fitting accuracy of the displacement cross sections is provided by the twice modified Lijian method. • A new exact representation for the normalized Mott cross section is proposed. • The modified and twice modified Lijian−Qing−Zhengming methods are suggested. • Based on these methods, a new analytical expression for Mott’s correction is obtained. • We also get in this way a new expression for the atomic displacement cross section.

Catalyzing Bond-Dissociation in Graphene via Alkali-Iodide Molecules.

Atomic design of a 2D-material such as graphene can be substantially influenced by etching, deliberately induced in a transmission electron microscope. It is achieved primarily by overcoming the threshold energy for defect formation by controlling the kinetic energy and current density of the fast electrons. Recent studies have demonstrated that the presence of certain species of atoms can catalyze atomic bond dissociation processes under the electron beam by reducing their threshold energy. Most of the reported catalytic atom species are single atoms, which have strong interaction with single-layer graphene (SLG). Yet, no such behavior has been reported for molecular species. This work shows by experimentally comparing the interaction of alkali and halide species separately and conjointly with SLG, that in the presence of electron irradiation, etching of SLG is drastically enhanced by the simultaneous presence of alkali and iodine atoms. Density functional theory and first principles molecular dynamics calculations reveal that due to charge-transfer phenomena the CC bonds weaken close to the alkali-iodide species, which increases the carbon displacement cross-section. This study ascribes pronounced etching activity observed in SLG to the catalytic behavior of the alkali-iodide species in the presence of electron irradiation.

Temperature-dependent displacement cross section of graphene and its impurities: measuring the carbon adatom migration barrier

Temperature-dependent displacement cross section of graphene and its impurities: measuring the carbon adatom migration barrier

Evaluation of displacement damage in solids induced by fast positrons: Modeling and effect on vacancy measurement

• The latest theoretical models of radiation damage in solid and numerical results of Mott differential cross section were used; • Displacement cross section by positrons for elements in most commonly used solids was calculated and the difference between positron and electron radiation was emphasized; • Displacement thresholds of kinetic energy for positrons in 30 elemental solids were presented; • Defects spatial distribution was obtained using Monte Carlo modeling. Positron Annihilation Technique is very sensitive to vacancy defects and small vacancy clusters and has been used as supplementary for TEM in radiation damage research. In this paper, we evaluated the displacement damage in solids induced by positrons and its effect on defects measurement, which has not been considered carefully in most PAT application. Displacement damage could only be produced when kinetic energy of positron exceeds a threshold determined by average displacement energy and the mass of atoms. Based on the latest models of displacement damage and numerical Mott elastic differential cross section of positrons interacting with atoms, displacement cross section for fast positron in solids was calculated and compared with that for electrons. In the energy range from 1 MeV to 100 MeV, electrons will produce more displacement defects than positrons do, especially in high-Z materials. Number of total defects depends on the primary displacement cross section and the size of damage cascades. To get distributions of defects and terminating position of positrons in materials, a Monte Carlo tool with full damage model was developed and validated. From the results of Monte Carlo modeling, the defects -generated by mono-energy positron beam have shallower distribution than the stopping position of positrons, indicating separation of defects and positrons and this fact benefits defects measurement because displacement damage produced by fast probing positron beam have little affection on the measurement. But for the positron beams from radioactive sources (such as 22 Na and 64 Cu) with continuous energy spectrum (from 0 to E max ), the depth profile of defects and terminated position of positrons overlap with each other and this fact indicated that defects measurement in materials will be affected by extra defects produced by positrons. The density of defects produced by positron beam is quite low with a typical value of about 0.01–0.1 cm −3 per positron in low-Z materials, which could be comparable with the defect density limitation if enough accumulated positrons are implanted into materials during measurement. For commonly used structural metals (such as Fe, Mn, Cu and metals with Z number higher than 40), using positron from 22 N and 64 Cu directly to measure defects density is quite safe because the energy is smaller than the displacement threshold.

Measurements of Displacement Cross Section of Tungsten under 389-MeV Proton Irradiation and Thermal Damage Recovery

For validating the number of displacements per atom (dpa) for tungsten under high-energy proton irradiation, we measured displacement cross sections related to defect-induced electrical resistivity changes in a tungsten wire sample under irradiation with 389-MeV protons under 10 K. The Gifford–McMahon cryocooler was used to cool the sample using a conductive coolant via thermal conduction plates of oxygen-free high-conductivity copper and electrical insulation sheets of aluminum nitride ceramic. In this experiment, the displacement cross section was 1612 ± 371 b for tungsten at 389 MeV. A comparison of the experimental displacement cross sections of tungsten with the calculated results obtained using Norgett–Robinson–Torrens (NRT) dpa and athermal recombination-corrected (arc) dpa cross sections indicates that arc-dpa was in better agreement with the experimental data than NRT-dpa; this is similar to the displacement cross sections of copper. From the measurements of damage recovery of the accumulated defects in tungsten through isochronal annealing, which is related to the defect concentration of the sample, approximately 20% of the damage was recovered at 60 K. This trend was similar to those observed in other experimental results for reactor neutrons.

Measurement of displacement cross-sections of copper and iron for proton with kinetic energies in the range 0.4 – 3 GeV

ABSTRACT To estimate the structural damages of materials in accelerator facilities, displacement per atom (dpa) is widely employed as a damage index, calculated based on the displacement cross-section obtained using a calculation model. Although dpa is applied as standard, the experimental data of the displacement cross-section for a proton in the energy region above 20 MeV are scarce. Among the calculation models, difference of about factor 8 exist, so that the experimental data of the cross-section are crucial to validate the model. To obtain the displacement cross-section, we conducted experiments at the Japan Proton Accelerator Research Complex (J-PARC). The displacement cross-section of copper and iron was successfully obtained for a proton projectile with the kinetic energies, 0.4, 0.8, 1.3, 2.2, and 3 GeV. The results were compared with those obtained using the widely utilized Norgertt–Robinson–Torrens (NRT) model and the athermal–recombination–corrected dpa (arc-dpa) model based on molecular dynamics (MD). It was found that the NRT model overestimates the present displacement cross-section by 3.5 times. The calculation results obtained using with the arc-dpa model based on the Nordlund parameter show remarkable agreement with the experimental data. It can be concluded that the arc-dpa model must be employed for the dpa calculation for the damage estimation of copper and iron.

Cathodoluminescence of cerium dioxide: Combined effects of the electron beam energy and sample temperature

In-situ cathodoluminescence (CL) spectroscopy is used to study point-defect formation in cerium dioxide (CeO2) by high-energy electrons (400 keV-1,250 keV) at ∼100 K, 200 K, and 300 K in a high-voltage electron microscope (HVEM). Complementary CL spectra are also obtained for 20-keV electron excitation at ∼300 K in a scanning electron microscope (SEM). Experiments were carried out on a single crystal and polycrystalline sintered sample. The more prominent and broad emission band centered at a photon energy of ∼4.2 eV is ascribed to F+ centers (oxygen vacancies) produced by the high-energy electron irradiation. Two other weaker CL bands centered at ∼2.3–2.4 eV and 2.8–2.9 eV at 300 K are related to trivalent cerium ions, corresponding to 5d → 4f radiative transitions, regardless of electron energy. Similar spectra are recorded at 100 K and 200 K, yet with shifts and broadening of the latter emission bands. A maximum of all CL bands is found at ∼600 keV electron energy for the polycrystalline sample regardless of temperature, and for the single crystal at 300 K. In contrast, a continuous increase versus electron energy is observed for the 4.2-eV band of the single crystal at 100 K and 200 K above a threshold corresponding to the oxygen displacement energy. The dependence of CL intensities on the primary electron energy is analyzed on the basis of the interplay of the luminescence cross section and oxygen displacement cross section. The electron energy dependence of ionization cross sections is addressed by relying on secondary electron spectra computed with the PHITS code which is benchmarked against other computer codes such as PENELOPE, as implemented in Geant4. The F+ center band intensity is strongly reduced in the polycrystalline sample with respect to the single crystal in the same irradiation conditions regardless of temperature due to grain boundaries.

Measurement of displacement cross section of structural materials utilized in the proton accelerator facilities with the kinematic energy above 400 MeV

For damage estimation of structural material in the accelerator facility, displacement per atom (DPA) is widely employed as an index of the damage calculated based on the displacement cross section obtained with the calculation model. Although the DPA is employed as the standard, the experimental data of displacement cross section are scarce for a proton in the energy region above 20 MeV. Among the calculation models, the difference exists about 8 times so that experimental data of the displacement cross section is crucial to validate the model. To obtain the displacement cross section, we conducted the experiment in J-PARC. As a preliminary result, the displacement cross section of copper was successfully obtained for 3-GeV proton. The present results showed that the widely utilized the Norgertt-Robinson-Torrens (NRT) model overestimates the cross section as suggested by the previous experiment for protons with lower energy.

Influence of temperature on the displacement threshold energy in graphene

The atomic structure of nanomaterials is often studied using transmission electron microscopy. In addition to image formation, the energetic electrons impinging on the sample may also cause damage. In a good conductor such as graphene, the damage is limited to the knock-on process caused by elastic electron-nucleus scattering. This process is determined by the kinetic energy an atom needs to be sputtered, i.e. its displacement threshold energy Ed. This is typically assumed to have a fixed value for all electron impacts on equivalent atoms within a crystal. Here we show using density functional tight-binding simulations that the displacement threshold energy is affected by thermal perturbations of atoms from their equilibrium positions. This effect can be accounted for in the estimation of the displacement cross section by replacing the constant threshold energy value with a distribution. Our refined model better describes previous precision measurements of graphene knock-on damage, and should be considered also for other low-dimensional materials.

Evaluation of displacement cross-section for neutron-irradiated 15-15Ti steel and its swelling behavior in CiADS radiation environment

We report on the proposed method of evaluating the displacement cross-sections in NRT and BCA-arc-dpa models. The method is based on the NJOY code in conjunction with the IOTA code and is applicable to neutron energy in the range of 0.1 neV–20 MeV, defined by using NJOY and nuclear library files with data up to 20 MeV. The displacement cross-section has been calculated for natural iron, 15-15Ti, 316L, Eurofer, ODS, and T91 steels. The displacement cross-sections for 15-15Ti steel were used to estimate radiation damage and swelling level under irradiation conditions of the China Initiative Accelerator Driven System (CiADS) subcritical core. The swelling analysis has shown, that in CiADS radiation environment, the 15-15Ti steel has a good margin to cladding failure due to swelling and, therefore, might be a cladding material for CiADS.