High Fluence Range Research Articles

Fluence-dependent microstructure and nanomechanical property in Co-Ni-V medium entropy alloy thin films

Via tuning deposition power, a series of Co-Ni-V medium entropy alloy thin films, growing in a nanocolumn manner, with different microstructures and morphologies were synthesized. With increasing fluence, both the crystallinity and nanocolumn size experienced a process of first increasing and then decreasing, controlled by the adatom surface diffusion vs. deposition rate competition. Meanwhile, continuous strengthening with fluence was ascribed to improved bonding and reduced fraction of boundary at low fluence and amorphization reinforcement effect in high fluence range.

Microwave photoconductance decay measurements of n- and p-type silicon irradiated with neutrons and protons

N-type and p-type silicon were irradiated by neutrons and protons from low to high fluence range (108 − 1011 cm-2 for neutrons and 1010 − 1013 cm-2 for protons). The carrier recombination lifetime of each irradiated silicon was measured using a microwave photoconductance decay method, and the resistivity of the irradiated silicon was also measured. Both neutron and proton irradiation induced a similar lifetime at the same irradiation fluence. The defect formation rates were determined to vary with irradiation fluence from the inverse lifetime dependence on irradiation fluence. The sheet resistance was increased above a threshold fluence. The result shows that at least three different values of defect formation rate are present in the fluence range of 109 − 1015 cm-2, and it may have potential for practical use as low-dose dosimeters.

Surface layer morphology of the high fluence Fe implanted polyethylene - Correlation with the magnetic and optical behavior

Fe/Polyethylene nanocomposite was synthesized by ion beam implantation of 56Fe+ into bulk high-density polyethylene. Nanoscale surface morphology along with magnetic and optical behavior was investigated. The aim of the research was to investigate changes of polyethylene's surface layer morphology with the change of Fe implantation fluence in the high fluence range and to find correlations with the magnetic and optical behavior. Four implantation fluences were applied: 5 × 1016, 1 × 1017, 2 × 1017 and 5 × 1017 cm−2, while the implantation energy was 95 keV. Concentration profiles of implanted Fe were analyzed by Rutherford backscattering spectrometry, showing Fe concentration profile maxima closer to the surface with increasing implantation fluence. Cross-sectional transmission electron microscopy showed the formation of metallic nanoparticles with sizes in a range from below 1 nm up to few tens of nanometers, depending on the fluence, and for the highest implantation fluence, a continuous layer was formed. Magneto-optic Kerr effect magnetometry demonstrates weak ferromagnetic behavior for the 2 higher fluences, and superparamagnetic for the 2 lower fluences. The UV-VIS remission function spectra show the peak in the UV region, which we attribute to iron nanoparticles.

A Decade of Gamma-Ray Bursts Observed by Fermi-LAT: The Second GRB Catalog

Abstract The Large Area Telescope (LAT) aboard the Fermi spacecraft routinely observes high-energy emission from gamma-ray bursts (GRBs). Here we present the second catalog of LAT-detected GRBs, covering the first 10 yr of operations, from 2008 to 2018 August 4. A total of 186 GRBs are found; of these, 91 show emission in the range 30–100 MeV (17 of which are seen only in this band) and 169 are detected above 100 MeV. Most of these sources were discovered by other instruments (Fermi/GBM, Swift/BAT, AGILE, INTEGRAL) or reported by the Interplanetary Network (IPN); the LAT has independently triggered on four GRBs. This catalog presents the results for all 186 GRBs. We study onset, duration, and temporal properties of each GRB, as well as spectral characteristics in the 100 MeV–100 GeV energy range. Particular attention is given to the photons with the highest energy. Compared with the first LAT GRB catalog, our rate of detection is significantly improved. The results generally confirm the main findings of the first catalog: the LAT primarily detects the brightest GBM bursts, and the high-energy emission shows delayed onset as well as longer duration. However, in this work we find delays exceeding 1 ks and several GRBs with durations over 10 ks. Furthermore, the larger number of LAT detections shows that these GRBs not only cover the high-fluence range of GBM-detected GRBs but also sample lower fluences. In addition, the greater number of detected GRBs with redshift estimates allows us to study their properties in both the observer and rest frames. Comparison of the observational results with theoretical predictions reveals that no model is currently able to explain all results, highlighting the role of LAT observations in driving theoretical models.

Physics of picosecond pulse laser ablation

This study investigates the physical processes involved in picosecond pulse (20-28 ps FWHM) laser ablation of Al 6061, 316L stainless steel, and undoped crystalline Si (〈100〉) over a range of laser wavelength (355 nm and 1064 nm) and fluence (0.1-40 J/cm2). Experimental measurements of material ablation rate show enhanced removal at the 355 nm wavelength, primarily due to laser-plasma interaction (LPI) within the ablative plume that approaches an order of magnitude increase over the measured removal at 1064 nm. A transition in the ablation rate at 355 nm is identified around ∼10 J/cm2 above which the removal efficiency increases by a factor of two to three. Multi-physics radiation hydrodynamic simulations, considering LPI effects and utilizing a novel mixed-phase equation of state model, show that the transition in ablation efficiency is due to the onset of melt ejection through cavitation, where laser-driven shock heating sets the depth of melt penetration and the ensuing release wave from the ablation surface drives cavitation through the imposition of tensile strain within the melt. High-speed pump-probe imaging of the ejecta and ejecta collection studies, as well as scanning electron microscopy of the ablation craters, support the proposed cavitation mechanism in the higher fluence range. The ablation process is critically influenced by LPI effects and the thermophysical properties of the material.

Modification of magnetization ordering in Pt/Co/Pt trilayers depending on the scanning direction of a focused ion beam

Abstract Focused ion beam (FIB) irradiation was applied to an ultrathin Pt/Co/Pt film to create micrometer-sized, square-shaped regions with perpendicular orientation of magnetization embedded in an in-plane magnetized environment. The FIB parameters like ion fluence and scanning direction are used to control the magnetic properties of the irradiated areas. We have studied the magnetic and magnetooptical properties in the irradiated areas as a function of ion fluence by means of component selective magnetooptical Kerr microscopy that measures separately pure polar and pure longitudinal components of magnetization. Two fluence ranges that induce perpendicular magnetization components were observed. In the higher fluence range, four regions with independently inclined out-of-plane magnetization, depending on FIB fast scan direction, and an unexpected magnetic domain structure were distinguished inside the squares irradiated by FIB.

Contribution of irradiation-induced defects to hardening of a low-copper reactor pressure vessel steel

We investigated the fluence dependence of irradiation-induced solute cluster, dislocation loop, and very small defect to reveal the hardening mechanism in surveillance test specimens from a reactor pressure vessel steel with low-Cu content (0.04 wt%) using atom probe tomography (APT), weak-beam scanning transmission electron microscopy (WB-STEM), and positron annihilation spectroscopy. A high number density (>1023 m−3) of solute clusters mainly composed of Ni, Mn, and Si atoms were found in highly neutron irradiated specimens (∼1024 neutrons m−2 (E > 1 MeV)) by APT. These solute clusters were one of the main sources of hardening as reported previously. On the other hand, it was also revealed that dislocation loops were formed with a number density of ∼1022 m−3 in the high-fluence specimens by WB-STEM. The estimated hardening due to dislocation loops was more than half of the actual hardening, showing that dislocation loops are also main source of irradiation hardening at high neutron fluence with the solid experimental evidences. Regarding specimens subjected to a low neutron fluence (∼1023 neutrons m−2), very small defects, not detected by either WB-STEM or APT, were formed by positron annihilation spectroscopy. This result suggested that, at a low neutron fluence, the defects were the initial hardening source and they may grow the dislocation loops observed by WB-STEM at high fluence range.

Channeling STIM analysis of radiation damage in single crystal diamond membrane

The use of focused ion beam transmission channeling patterns to monitor the damage creation process in thin diamond single crystal membrane is described. A 0.8MeV proton beam from the Ruđer Bošković Institute nuclear microprobe was used to perform Channeling Scanning Transmission Ion Microscopy (CSTIM) measurements. CSTIM was used instead of RBS channeling because of (several orders of magnitude) lower damage done to the sample during the measurements. Damage was introduced in selected areas by 15MeV carbon beam in range of fluences 3·1015–2·1017ions/cm2. Contrary to Ion Beam Induced Charge (IBIC), CSTIM is shown to be sensitive to the large fluences of ion beam radiation. Complementary studies of both IBIC and CSTIM are presented to show that very high fluence range can be covered by these two microprobe techniques, providing much wider information about the diamond radiation hardness. In addition micro Raman measurements were performed and the height of the GR 1 peak was correlated to the ion beam fluence.

Study on the effect of ambient gas on nanostructure formation on metal surfaces during femtosecond laser ablation for fabrication of low-reflective surfaces

Nanostructure formation on bulk metals (silver, gold, copper and titanium) by femtosecond Ti-sapphire laser irradiation (775nm, 150fs) is studied aiming the production of low-reflectivity surfaces and the better understanding of the development process. The experiments were performed in nitrogen, air, oxygen and helium environments at atmospheric pressure. The samples were irradiated with fluences in the 0.1–2J/cm2 range and an average pulse number of 100 falling over a given area. The reflectivity of the treated surfaces was determined by a microspectrometer in the 450–800nm range and their morphology was studied by scanning electron microscopy. The gas ambience influenced the results via two effects: formation processes and the chemically-induced modifications of the nanostructures. In case of He the nanoparticle aggregates–otherwise generally present–are predominantly missing, which leads to a lower darkening efficiency. The presence of oxygen enhances the darkening effect for copper mostly at lower fluences, while causes a slow increase in reflectivity in the case of titanium (in case of pure oxygen) in the high fluence range. The surface morphology in case of nitrogen and air were quite similar probably due to their close molecular mass values.

Color-center production and recovery in electron-irradiated magnesium aluminate spinel and ceria

Single crystals of magnesium aluminate spinel (MgAl2O4) with (1 0 0) or (1 1 0) orientations and cerium dioxide or ceria (CeO2) were irradiated by 1.0 MeV and 2.5 MeV electrons in a high-fluence range. Point-defect production was studied by off-line UV–visible optical spectroscopy after irradiation. For spinel, regardless of both crystal orientation and electron energy, two characteristic broad bands centered at photon energies of 5.4 eV and 4.9 eV were assigned to F and F+ centers (neutral and singly ionized oxygen vacancies), respectively, on the basis of available literature data. No clear differences in color-center formation were observed for the two crystal orientations. Using calculations from displacement cross sections by elastic collisions, these results are consistent with a very large threshold displacement energy (200 eV) for oxygen atoms at room temperature. A third very broad band centered at 3.7 eV might be attributed either to an oxygen hole center (V-type center) or an F2 dimer center (oxygen di-vacancy). The onset of recovery of these color centers took place at 200 °C with almost full bleaching at 600 °C. Activation energies (~0.3–0.4 eV) for defect recovery were deduced from the isochronal annealing data by using a first-order kinetics analysis. For ceria, a sub-band-gap absorption feature, which peaked at ~3.1 eV, was recorded for 2.5 MeV electron irradiation only. Assuming a ballistic process, we suggest that the latter defect might result from cerium atom displacement on the basis of computed cross sections.

Monitoring radiation embrittlement during life extension periods

This paper presents guidelines to monitor the radiation embrittlement of reactor pressure vessels (RPV) during life extension periods (to 60 or 80 years) or for the long term operation of nuclear power plants (NPPs). The guidelines were developed in 2012–2013 by a Task Group of the international project LONGLIFE. The work performed responds to the need for guidance to treat long term irradiation effects within the ageing management of NPPs, since the standard RPV surveillance programmes were designed only to cover a time period of 40 years. The guidelines are intended to support specialists in the field and managers in the plant to choose among the most adequate techniques and methods available today to extend the use of their current RPV surveillance programme beyond design life, or implement a new programme when needed. The study performed identifies weaknesses in the ability of the standard surveillance programmes to provide data needed for long term operation, and proposes solutions and tools to solve and/or mitigate the lack or scarcity of surveillance material for their use in life extension. Guidance is also given on methods and strategies to generate reliable surveillance data in the high fluence range.

Solvent effect on the nonlinear absorption of 5,10-A2B2meso substituted porphyrins

The effect of the solvent on the nonlinear absorptive properties of two series of 5,10-A2B2 porphyrins was investigated with an open Z-scan technique in the ns time regime. The recorded responses, which varied between compounds and solvents, were fitted to a four-level model where the one-photon excited state absorption is followed by a two-photon process arising from the higher excited states. For most of the compounds the positive nonlinear absorption in toluene was stronger than that in DMF and chloroform. This was attributed to enhanced two-photon absorption in toluene. For DMF and chloroform the solvent effects were most likely to be compound specific. It was demonstrated that the high saturation intensity of two-photon absorption shifts the RSA/SA turnover into a higher fluence range, which is desirable for optical limiting applications. This saturation intensity of two-photon absorption varied between compounds and solvents. Additionally, nonlinear scattering contributed strongly to the open Z-scan responses for many compounds in chlorobenzene and chloroform-chlorobenzene solutions. This was associated with the photodegradation of chlorobenzene.

Surface modification of tungsten and tungsten–tantalum alloys exposed to high-flux deuterium plasma and its impact on deuterium retention

Samples of tungsten and tungsten–tantalum alloy (with 5 mass per cent of Ta) were exposed to high-flux deuterium plasma at different fluences. The surface modification was studied with scanning electron microscopy, and deuterium retention was measured by thermal desorption spectroscopy (TDS). In the high fluence range of ∼3.5 × 1026–1027 m-2, multiple large-size blisters are formed on the W surface, while blisters on the W–Ta surface are considerably smaller in size and number. Deuterium retention in this fluence range was found to be systematically higher in W than in W–Ta. Correlation between the evolution of the blistering patterns and the TDS spectra as a function of fluence suggests that trapping in the sub-surface cavities associated with blisters is the predominant trapping mechanism in tungsten in the case of high fluence exposures. We attribute the lower retention in W–Ta under the investigated conditions to the weaker blistering.

Drilling rate of five metals with picosecond laser pulses at 355, 532, and 1064 nm

Experimental results on picosecond laser processing of aluminum, nickel, stainless steel, molybdenum, and tungsten are described. Hole drilling is employed for comparative analysis of processing rates in an air environment. Drilling rates are measured over a wide range of laser fluences (0.05–20 J/cm2). Experiments with picosecond pulses at 355 nm are carried out for all five metals and in addition at 532 nm, and 1064 nm for nickel. A comparison of drilling rate with 6-ps and 6-ns pulses at 355 nm is performed. The dependence of drilling rate on laser fluence measured with picosecond pulses demonstrates two logarithmic regimes for all five metals. To determine the transition from one regime to another, a critical fluence is measured and correlated with the thermal properties of the metals. The logarithmic regime at high-fluence range with UV picosecond pulses is reported for the first time. The energy efficiency of material removal for the different regimes is evaluated. The results demonstrate that UV picosecond pulses can provide comparable quality and higher processing rate compared with literature data on ablation with near-IR femtosecond lasers. A significant contribution of two-photon absorption to the ablation process is suggested to explain high processing rate with powerful UV picosecond pulses.

FTIR analysis of polyethylene terephthalate irradiated by MeV He+

The degradation induced by MeV He+ ions onto polyethylene terephthalate (PET) is investigated as a function of fluence and electronic stopping power (dE/dx)e. Stack samples of four 3.6μm PET films are irradiated by 3.5MeV He+ under vacuum and room temperature at fluence ranging from 1013 to 1.5×1015He+cm−2. The entrance energies range from 3.5MeV for the front film to 0.573MeV for the rear one at the beginning of the irradiation and the corresponding mean (dE/dx)e lie for pristine PET between 158 and 264keV/μm. A Fourier transform infrared spectroscopy (FTIR) analysis is undertaken. The different stack films present similar damages with magnitude increase as the fluence goes up. Moreover, higher is the electronic stopping power (dE/dx)e more important is the damage. The evolution of the normalized integrated absorbance A(ϕ)/A0 of the internal reference 1410cm−1 band as a function of the fluence ϕ presents a complex behavior characterized by a two-exponential decay associated to low and high fluence range. The analysis reveals clearly that the A(ϕ)/A0 of the characteristic benzene ring 1577cm−1 band remains steady till fluence around 3×1014He+cm−2 and then decreases smoothly as fluence increases. The band appearing at 1610cm−1, assigned to mono substituted benzene, presents an integrated absorbance A(ϕ) which rises progressively and levels off beyond a critical fluence around 4×1014He+cm–2. The cross section of the creation of the monosubstituted benzene lies between 3×10−15 and 6×10−15cm2 for (dE/dx)e=158 and 264keV/μm, respectively.Drastic loss of crystallinity is induced by the MeV He+ ion irradiation in the polymer. The amorphization is monitored by the evolution of the specific bands of the trans and gauche ethylene glycol residue conformations. A sudden drop in the integrated absorbance of the 1340cm−1 band which characterizes the trans conformation is observed at fluence around 2×1014He+cm−2. On the other hand, a drastic increase is present in the integrated absorbance of the gauche conformation 1370cm−1 band that pairs up with 1340cm−1 band. This observation is a clear indication of the conversion of the trans ethylene glycol residue to gauche one under irradiation.

Microstructuring of fused silica by laser-induced backside wet etching using picosecond laser pulses

The laser-induced backside wet etching (LIBWE) is an advanced laser processing method used for structuring transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated for various materials, e.g. oxides (fused silica, sapphire) or fluorides (CaF 2, MgF 2), and applied for the fabrication of microstructures. In the present study, LIBWE of fused silica with mode-locked picosecond ( t p = 10 ps) lasers at UV wavelengths ( λ 1 = 355 nm and λ 2 = 266 nm) using a (pyrene) toluene solution was demonstrated for the first time. The influence of the experimental parameters, such as laser fluence, pulse number, and absorbing liquid, on the etch rate and the resulting surface morphology were investigated. The etch rate grew linearly with the laser fluence in the low and in the high fluence range with different slopes. Incubation at low pulse numbers as well as a nearly constant etch rate after a specific pulse number for example were observed. Additionally, the etch rate depended on the absorbing liquid used; whereas the higher absorption of the admixture of pyrene in the used toluene enhances the etch rate and decreases the threshold fluence. With a λ 1 = 266 nm laser set-up, an exceptionally smooth surface in the etch pits was achieved. For both wavelengths ( λ 1 = 266 nm and λ 2 = 355 nm), LIPSS (laser-induced periodic surface structures) formation was observed, especially at laser fluences near the thresholds of 170 and 120 mJ/cm 2, respectively.

Swift heavy ion irradiation-induced modifications in structural, magnetic and electrical transport properties of epitaxial magnetite thin films

The effect of swift heavy ion (SHI) irradiation (190MeVAg) on structural, electrical transport and magnetic properties of epitaxial magnetite (Fe3O4) thin films (thickness ∼70nm) grown on MgO⟨100⟩ oriented substrate have been investigated. The x-ray diffraction shows that at low fluence values up to 5×1011ions∕cm2, the strain in the films is relaxed, whereas, at higher fluence range 1×1012–1×1013ions∕cm2, the epitaxial relationship with the substrate is lost along with a phase transformation from magnetite to more oxidized magnetite phase (i.e., maghemite). The Verwey transition temperature measured by electrical transport is found to increase from 109 to 117K with the low fluence SHI irradiation, which is related to the irradiation induced strain relaxation and structural modifications. At higher fluences the system did not show Verwey transition and the resistance is also increased. The similar results were obtained by magnetization studies. The observed magnetization at 1T field is increased at low fluence suggesting the reduction of areas with frustrated exchange interactions associated with the cationic arrangement at the anti phase boundaries. At higher fluences it decreases monotonically, indicating the emergence of other phases. The observed modifications are explained on the basis of structural strain and disorder induced by swift heavy ions, which lead to modification of the interionic Coulomb potential at octahedral sublattices and bandwidth in this system.

Analysis of WWER-440 and PWR RPV welds surveillance data to compare irradiation damage evolution

It is known that for Russian-type and Western water reactor pressure vessel steels there is a similar degradation in mechanical properties during equivalent neutron irradiation. Available surveillance results from WWER and PWR vessels are used in this article to compare irradiation damage evolution for the different reactor pressure vessel welds. The analysis is done through the semi-mechanistic model for radiation embrittlement developed by JRC-IE. Consistency analysis with BWR vessel materials and model alloys has also been performed within this study. Globally the two families of studied materials follow similar trends regarding the evolution of irradiation damage. Moreover in the high fluence range typical of operation of WWER the radiation stability of these vessels is greater than the foreseen one for PWR.

Performance characteristics of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry at ∼265 and ∼200 nm

The analytical figures of merit of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry (UV-fs-LA-ICP-MS) using the 3rd and 4th harmonics of Ti:Sapphire (∼265 and ∼200 nm, respectively) were explored. For this purpose, elemental ratios of aerosols produced by LA of silicate glass (SRM NIST 610) were studied under varying fluence conditions ranging from moderate values of 2 J cm−2 up to 30 J cm−2, taking into account e.g. laser-induced (66Zn/65Cu) and particle size-related (238U/232Th) phenomena. It could, for instance, be shown that signal ratios were less dependent on the wavelength or laser repetition rate chosen. Furthermore, fractionation indices defined using the temporal drift of elemental ratios over two equal parts of the acquired signal were subject to systematic changes for threshold-close fluences. As a consequence, corresponding 42Ca-normalized values were found to deviate by more than 20% from unity. In contrast, LA at higher fluences resulted in less pronounced discrepancies, falling below 5% even for the most critical elements such as 66Zn, 111Cd, and 208Pb. The complete suppression of particle size-related fractionation quantified on the basis of the 238U/232Th-system turned out to be highly consistent with the absence of μ-sized particles which were measured by optical particle counting (OPC). The relative fraction of particles >0.5 μm was determined to be less than 5%, independent on the wavelength, fluence, or laser repetition rate chosen. Moreover, our results indicate the occurrence of ICP-induced elemental fractionation during analysis due to an increased mass loading of the plasma source if medium or high fluences are applied. Using the 66Zn/65Cu-ratio as a thermometric probe, the change in plasma ionization temperature among low and high mass loading conditions was estimated to be −900 K. Nevertheless, UV-fs-LA-ICP-MS analysis of different matrices (silicate glass SRM NIST 610 and brass (Zn ∼20%)) performed within the high fluence range was found to enhance the accuracy for non-matrix-matched calibration. Evidence is given that the enhancement observed mainly depends upon the suppression of laser- and/or transport-induced fractionation.

Effect of Irradiation on the Fracture Properties of Zr-2.5Nb Pressure Tubes at the End of Design Life

Abstract To determine the fracture properties of Zr-2.5Nb pressure tubes irradiated until the end of design life, cantilever beam, curved compact toughness, and transverse tensile samples were prepared from a typical pressure tube and irradiated in the high flux reactor OSIRIS at CEA, Saclay, France. Experiments were conducted on two batches of samples mounted in two irradiation inserts. Each insert held sixteen samples of each type of specimen. The first insert was irradiated to a fluence corresponding to approximately half of the design life in a CANDU3 reactor. The experimental results were reported in [1]. Samples in the second insert were irradiated for 10.5 years in OSIRIS and received a maximum neutron fluence of 2.61 × 1026 n/m2 (E > 1 MeV), being equivalent to 2.98 × 1026 n/m2 (E > 1 MeV) in a CANDU reactor, i.e., corresponding to ∼30 years operation in CANDU reactors at 80 % capacity factor. The present report describes the results of tensile, fracture toughness, and Delayed Hydride Cracking (DHC) tests and XRD microstructure analysis from the second batch of specimens. A continuous and gradual evolution in tensile, fracture, DHC properties, and dislocation densities is demonstrated without any evidence of a sudden change following the initial transitient at very low fluence. In the whole high fluence range, there is a very slow rate of increase in c-component dislocation density, strength, and DHC velocity and a slow reduction in elongation and Nb concentration in the β-phase. The a-type dislocation density and fracture toughness remain approximately constant. The results from the second insert of specimens confirm that, following the initial transient at very low fluence, there is little further change in the fracture properties of Zr-2.5Nb pressure tube material. Therefore, material properties behave in a stable and predicable manner to the end of a 30 years design life for CANDU reactor pressure tubes.