Showing papers on "Fluence published in 2017"

Effect of wavelength and beam width on penetration in light-tissue interaction using computational methods

Abstract: Penetration depth of ultraviolet, visible light and infrared radiation in biological tissue has not previously been adequately measured. Risk assessment of typical intense pulsed light and laser intensities, spectral characteristics and the subsequent chemical, physiological and psychological effects of such outputs on vital organs as consequence of inappropriate output use are examined. This technical note focuses on wavelength, illumination geometry and skin tone and their effect on the energy density (fluence) distribution within tissue. Monte Carlo modelling is one of the most widely used stochastic methods for the modelling of light transport in turbid biological media such as human skin. Using custom Monte Carlo simulation software of a multi-layered skin model, fluence distributions are produced for various non-ionising radiation combinations. Fluence distributions were analysed using Matlab mathematical software. Penetration depth increases with increasing wavelength with a maximum penetration depth of 5378 μm calculated. The calculations show that a 10-mm beam width produces a fluence level at target depths of 1–3 mm equal to 73–88% (depending on depth) of the fluence level at the same depths produced by an infinitely wide beam of equal incident fluence. Meaning little additional penetration is achieved with larger spot sizes. Fluence distribution within tissue and thus the treatment efficacy depends upon the illumination geometry and wavelength. To optimise therapeutic techniques, light-tissue interactions must be thoroughly understood and can be greatly supported by the use of mathematical modelling techniques.

1.5 MeV electron irradiation damage in β-Ga2O3 vertical rectifiers

Abstract: Vertical rectifiers fabricated on epi Ga2O3 on bulk β-Ga2O3 were subject to 1.5 MeV electron irradiation at fluences from 1.79 × 1015 to 1.43 × 1016 cm−2 at a fixed beam current of 10−3 A. The electron irradiation caused a reduction in carrier concentration in the epi Ga2O3, with a carrier removal rate of 4.9 cm−1. The 2 kT region of the forward current–voltage characteristics increased due to electron-induced damage, with an increase in diode ideality factor of ∼8% at the highest fluence and a more than 2 order of magnitude increase in on-state resistance. There was a significant reduction in reverse bias current, which scaled with electron fluence. The on/off ratio at −10 V reverse bias voltage was severely degraded by electron irradiation, decreasing from ∼107 in the reference diodes to ∼2 × 104 for the 1.43 × 1016 cm−2 fluence. The reverse recovery characteristics showed little change even at the highest fluence, with values in the range of 21–25 ns for all rectifiers.

Texturing metal surface with MHz ultra-short laser pulses

Abstract: We show, for the first time to our knowledge, the role the heat accumulation plays on the evolution of ultra-short pulse laser-induced surface structures morphology when varying fluence, the number of scans and the repetition rate from 100 kHz up to 2 MHz. We demonstrate how to tailor the size of micro-spikes from nearly ten microns to several tens of microns by a systematic variation of both fluence and overlap. We believe our results will contribute to an in deep understanding of the mechanisms underlying laser surface structuration at high repetition rates.

Preparation and characterization of aluminum oxide nanoparticles by laser ablation in liquid as passivating and anti-reflection coating for silicon photodiodes

Abstract: In this study, we have prepared aluminum oxide (Al2O3 nanoparticles) NPs with size ranging from 50 to 90 nm by laser ablation of aluminum target in ethanol. The effect of laser fluence on the structural, morphological and optical properties of Al2O3 was demonstrated and discussed. X-ray diffraction XRD results confirm that the synthesized Al2O3 NPs are crystalline in nature. The sample prepared at 3.5 J/cm2/pulse exhibits single phase of γ-Al2O3, while the XRD patterns of the nanoparticles synthesized at 5.3 and 7.5 J/cm2/pulse show the co-existence of the α-Al2O3 and γ-Al2O3 phases. Nanostructured Al2O3 films have been used as anti-reflecting coating and surface passivation layer to improve the photoresponse characteristics of silicon photodiode. The experimental data showed that the optical energy gap decreases from 5.3 to 5 eV as the laser fluence increases from 3.5 to 7.3 J/cm2. The lowest optical reflectivity was found for silicon photodiode deposited with a single layer of Al2O3 prepared at 3.5 J/cm2/pulse. The effect of laser fluence on the refractive index and extinction coefficient of the nanostructured Al2O3 film was studied. The photosensitivity of the silicon photodiode increased from 0.4 to 1.4 AW−1 at 800 nm after depositing Al2O3 prepared at 3.5 J/cm2/pulse, followed by rapid thermal annealing at 400 °C for 60 s.

Realization of Multilevel States in Phase‐Change Thin Films by Fast Laser Pulse Irradiation

Abstract: Multilevel storage techniques are promising for increasing storage density and for reducing energy consumption in the application of phase-change materials based memory devices. However, accurately controlling the phase transitions as well as understanding the underlying switching mechanisms are still under investigation. In this study, nonvolatile optical multilevel switching in single-layered GeTe phase-change films prepared by laser ablation is demonstrated to be feasible and accurately controllable at a time scale of nanoseconds. For this purpose, an ns UV laser pump-probe setup is adapted for the investigations. It is found that each laser pulse excitation (duration: 20 ns, wavelength: 248 nm) with a fluence of 26 mJ cm−2 induced a partial crystallization whereas complete crystallization is achieved by superposition of single pulses. In the reverse process, a single pulse excitation at a fluence of 112 mJ cm−2 leads to reamorphization of GeTe thin films. Regarding the switching times, complete crystallization and reamorphization are determined to be accomplished after ≈300 ns and ≈3 ns, respectively. Furthermore, it is shown that the local microstructure phase transformation processes are correlated to the observed reversible switching of the optical reflectivity, which are important for understanding the underlying mechanisms of the phase-change-materials-based multilevel phase-change memory.

Work Function Modulation of Molybdenum Disulfide Nanosheets by Introducing Systematic Lattice Strain

Abstract: Tuning the surface electronic properties of 2D transition metal dichalcogenides such as Molebdenum disulfide (MoS2) nanosheets is worth exploring for their potential applications in strain sensitive flexible electronic devices. Here in, the correlation between tensile strain developed in MoS2 nanosheets during swift heavy ion irradiation and corresponding modifications in their surface electronic properties is investigated. With prior structural characterization by transmission electron microscopy, chemically exfoliated MoS2 nanosheets were exposed to 100 MeV Ag ion irradiation at varying fluence for creation of controlled defects. The presence of defect induced systematic tensile strain was verified by Raman spectroscopy and X-ray Diffraction analysis. The effect of ion irradiation on in-plane mode is observed to be significantly higher than that on out-of-plane mode. The contribution of irradiation induced in-plane strain on modification of the surface electronic properties of nanosheets was analyzed by work function measurement using scanning Kelvin probe microscopy. The work function value is observed to be linearly proportional to tensile strain along the basal plane indicating a systematic shifting of Fermi surface with fluence towards the valence band.

Effects of laser fluence and liquid media on preparation of small Ag nanoparticles by laser ablation in liquid

Abstract: This study aims to assess a method for preparation of small and highly stable Ag nanoparticles by nanosecond laser ablation in liquid. Effect of liquid medium and laser fluence on the size, morphology and structure of produced nanoparticles has been studied experimentally. Pulses of a Nd:YAG laser of 1064 nm wavelength at 35 ns pulse width at different fluences were employed to irradiate the silver target in different environments (water, ethanol and acetone). The UV-Visible absorption spectra of nanoparticles exhibit surface plasmon resonance absorption peak in the UV region. STEM and TEM micrographs were used to evaluate the size and shape of nanoparticles. The stability of silver colloids in terms of oxidation at different liquid media was analyzed by SAED patterns. The results showed that characteristics of Ag nanoparticles and their production rate were strongly influenced by varying laser fluence and liquid medium. Particles from 2 to 80 nm of diameter were produced using different conditions and no oxidation was found in ethanol and acetone media. This work puts in evidence a promising approach to produce small nanoparticles by using high laser fluence energy.

Fluence Threshold Behaviour on Ablation and Bubble Formation in Pulsed Laser Ablation in Liquids.

Abstract: The analysis of the ablation yield and bubble formation process during nanosecond pulsed-laser ablation of silver in water has been performed by stroboscopic videography, time-resolved X-ray radiography and in-situ UV-Vis spectroscopy. This process was studied as function of lens-target distance and as function of laser fluence. We show that both the ablation yield and the cavitation bubble process follow a threshold behaviour as function of fluence, which is linked to the efficiency of coupling of energy at the water-target interface. Although ablation happens below this threshold, quantitative material emission is linked to the bubble formation. Above threshold both bubble size and ablation follow a linear behaviour.

Spectral distribution of particle fluence in small field detectors and its implication on small field dosimetry

Abstract: PURPOSE Correction factors for the relative dosimetry of narrow megavoltage photon beams have recently been determined in several publications. These corrections are required because of the several small-field effects generally thought to be caused by the lack of lateral charged particle equilibrium (LCPE) in narrow beams. Correction factors for relative dosimetry are ultimately necessary to account for the fluence perturbation caused by the detector. For most small field detectors the perturbation depends on field size, resulting in large correction factors when the field size is decreased. In this work, electron and photon fluence differential in energy will be calculated within the radiation sensitive volume of a number of small field detectors for 6 MV linear accelerator beams. The calculated electron spectra will be used to determine electron fluence perturbation as a function of field size and its implication on small field dosimetry analyzed. METHODS Fluence spectra were calculated with the user code PenEasy, based on the PENELOPE Monte Carlo system. The detectors simulated were one liquid ionization chamber, two air ionization chambers, one diamond detector, and six silicon diodes, all manufactured either by PTW or IBA. The spectra were calculated for broad (10 cm × 10 cm) and narrow (0.5 cm × 0.5 cm) photon beams in order to investigate the field size influence on the fluence spectra and its resulting perturbation. The photon fluence spectra were used to analyze the impact of absorption and generation of photons. These will have a direct influence on the electrons generated in the detector radiation sensitive volume. The electron fluence spectra were used to quantify the perturbation effects and their relation to output correction factors. RESULTS The photon fluence spectra obtained for all detectors were similar to the spectrum in water except for the shielded silicon diodes. The photon fluence in the latter group was strongly influenced, mostly in the low-energy region, by photoabsorption in the high-Z shielding material. For the ionization chambers and the diamond detector, the electron fluence spectra were found to be similar to that in water, for both field sizes. In contrast, electron spectra in the silicon diodes were much higher than that in water for both field sizes. The estimated perturbations of the fluence spectra for the silicon diodes were 11-21% for the large fields and 14-27% for the small fields. These perturbations are related to the atomic number, density and mean excitation energy (I-value) of silicon, as well as to the influence of the "extracameral"' components surrounding the detector sensitive volume. For most detectors the fluence perturbation was also found to increase when the field size was decreased, in consistency with the increased small-field effects observed for the smallest field sizes. CONCLUSIONS The present work improves the understanding of small-field effects by relating output correction factors to spectral fluence perturbations in small field detectors. It is shown that the main reasons for the well-known small-field effects in silicon diodes are the high-Z and density of the "extracameral" detector components and the high I-value of silicon relative to that of water and diamond. Compared to these parameters, the density and atomic number of the radiation sensitive volume material play a less significant role.

Structural, optical and magnetic properties of N ion implanted CeO2 thin films

Abstract: The present study reports the structural, morphological, optical and magnetic properties of N ion implanted CeO2 thin films deposited by RF magnetron sputtering technique. These CeO2 thin films were implanted with N ions having an energy of 80 keV with varying fluencies of 1 × 1015, 1 × 1016 and 6 × 1016 ions per cm2, respectively. X-ray diffraction measurements show that as deposited films had predominantly (111) orientations. There is a significant change in the crystalline nature of these films after implantation compared to pristine film. RBS measurements confirm the presence of N ions in CeO2 thin film with the highest fluence. The closely packed circular shaped nanoparticles were observed through AFM images both in pristine and N ion implanted CeO2 films and these were agglomerated on the surface at a fluence of 6 × 1016 ions per cm2. The crystalline structure and defect related information were evident through Raman spectroscopy. Raman results show that the crystalline structure is maintained even after implantation while the defect related peak is highest for the fluence value of 1 × 1015 ions per cm2 and decreases thereafter. The magnetic measurements show enhanced ferromagnetic ordering in N ion implanted CeO2 films compared to pristine film. The saturation magnetization is highest for the lowest fluence of N ions (1 × 1015 ions per cm2) which decreases with ion fluences. This diverse defect nature of oxygen vacancies (VO) in the N ion implanted CeO2 thin films mediates the ferromagnetic ordering.

Neutron irradiation test of depleted CMOS pixel detector prototypes

Abstract: Charge collection properties of depleted CMOS pixel detector prototypes produced on p-type substrate of 2 kΩ cm initial resistivity (by LFoundry 150 nm process) were studied using Edge-TCT method before and after neutron irradiation. The test structures were produced for investigation of CMOS technology in tracking detectors for experiments at HL-LHC upgrade. Measurements were made with passive detector structures in which current pulses induced on charge collecting electrodes could be directly observed. Thickness of depleted layer was estimated and studied as function of neutron irradiation fluence. An increase of depletion thickness was observed after first two irradiation steps to 1 1013 n/cm2 and 5 1013 n/cm2 and attributed to initial acceptor removal. At higher fluences the depletion thickness at given voltage decreases with increasing fluence because of radiation induced defects contributing to the effective space charge concentration. The behaviour is consistent with that of high resistivity silicon used for standard particle detectors. The measured thickness of the depleted layer after irradiation with 1 1015 n/cm2 is more than 50 μm at 100 V bias. This is sufficient to guarantee satisfactory signal/noise performance on outer layers of pixel trackers in HL-LHC experiments.

Laser-induced Translative Hydrodynamic Mass Snapshots: mapping at nanoscale

Abstract: Nanoscale thermally assisted hydrodynamic melt perturbations induced by ultrafast laser energy deposition in noble-metal films produce irreversible nanoscale translative mass redistributions and results in formation of radially-symmetric frozen surface structures. We demonstrate that the final three-dimensional (3D) shape of the surface structures formed after resolidification of the molten part of the film is shown to be governed by incident laser fluence and, more importantly, predicted theoretically via molecular dynamics modeling. Considering the underlying physical processes associated with laser-induced energy absorption, electron-ion energy exchange, acoustic relaxation and hydrodynamic flows, the theoretical approach separating slow and fast physical processes and combining hybrid analytical two-temperature calculations, scalable molecular-dynamics simulations, and a semi-analytical thin-shell model was shown to provide accurate prediction of the final nanoscale solidified morphologies, fully consistent with direct electron-microscopy visualization of nanoscale focused ion-beam cuts of the surface structures produced at different incident laser fluences. Finally, these results are in reasonable quantitative agreement with mass distribution profiles across the surface nanostructures, provided by their noninvasive and non-destructive nanoscale characterization based on energy-dispersive x-ray fluorescence spectroscopy, operating at variable electron-beam energies.

Thermal dynamics of pulsed-laser excited gold nanorods in suspension.

Abstract: Photothermal reactions of metallic nanostructures, such as gold nanorods show appealing structural relaxations, such as bubble formation or particle modification. We have employed a pump-probe method to record the structural relaxations of a suspension of gold nanorods upon femtosecond laser excitation by pulsed X-ray scattering both with wide-angle and small-angle sensitivity. Single-pulse reactions include transient bubble formation at 20 J m-2 and irreversible nanorod reshaping at 30 J m-2. Thus the window for reversible excitation is very narrow. Additionally we could map the time-domain and fluence behaviour in a wide range to characterize the relaxations comprehensively. The polarized laser pulse first selectively excites nanorods aligned with the laser electric field, but at higher fluence non-aligned rods are also transformed. At low fluence this transformation happens in the solid state, while at higher fluence the rods melt.

A study of the 160 MeV Ni7+swift heavy ion irradiation effect of defect creation and shifting of the phonon modes on MnxZn1–xO thin films

Abstract: MnxZn1–xO thin films were successfully synthesized by the dip coating technique. The thin films were irradiated by Ni7+ swift heavy ions (SHI) with 1 × 1013 ions per cm2 fluence, and their structural, electrical, Raman spectral and surface morphological properties were investigated. X-ray diffraction patterns confirmed the P63mc space groups, and the crystallite size increased after SHI irradiation, due to electron rearrangements. I–V studies revealed enhanced conductivity after Ni7+ SHI irradiation and showed the ohmic nature of the sample. The irradiation sensor efficiency and responsibility were calculated by using I–V data, which revealed impressive results. Photoluminescence (PL) measurements were performed to determine the evolution of defects and defect-annealing during ion irradiation; enhancement in the luminosity of pure and 5% Mn substituted ZnO thin films was observed. The presence of the Raman active strongest optical phonon mode of ZnO at 436.19 cm−1 revealed that ZnO with hexagonal wurtzite structure supported the XRD results. Atomic force microscopy (AFM) images revealed the formation of nano-bunches on the surface and enhanced the surface roughness and skewness of the irradiated samples, due to coulombic interactions between electrons and ions.

Defects responsible for lifetime degradation in electron irradiated n-GaN grown by hydride vapor phase epitaxy

Abstract: The effects of room temperature 6 MeV electron irradiation on the donor concentration, deep trap spectra, and diffusion lengths of nonequilibrium charge carriers were studied for undoped n-GaN grown by hydride vapor phase epitaxy. Changes in these parameters begin at a threshold electron fluence of 5 × 1015 cm−2. The diffusion lengths after this fluence decrease by a factor of 3, accompanied by a drastic increase in the density of deep electron traps with the level near Ec – 1 eV. There is a strong correlation between the changes in the density of these traps and the diffusion length of irradiated n-GaN, indicating that these centers control the lifetime in radiation damaged n-GaN. This is in sharp contrast to the starting material, where the lifetimes are controlled by other deep electron traps at Ec – 0.56 eV. The concentration of the latter is not strongly affected by high energy electron irradiation.

Properties of HPK UFSD after neutron irradiation up to 6e15 n/cm2

Abstract: In this paper we report results from a neutron irradiation campaign of Ultra-Fast Silicon Detectors (UFSD) with fluences of 1e14, 3e14, 6e14, 1e15, 3e15, 6e15 n/cm2. The UFSD used in this study are circular 50 micro-meter thick Low-Gain Avalanche Detectors (LGAD), with a 1.0 mm diameter active area. They have been produced by Hamamatsu Photonics (HPK), Japan, with pre-radiation internal gain in the range 10-100 depending on the bias voltage. The sensors were tested pre-irradiation and post-irradiation with minimum ionizing particle (MIPs) from a 90Sr based \b{eta}-source. The leakage current, internal gain and the timing resolution were measured as a function of bias voltage at -20C and -30C. The timing resolution was extracted from the time difference with a second calibrated UFSD in coincidence, using the constant fraction method for both. The dependence of the gain upon the irradiation fluence is consistent with the concept of acceptor removal and the gain decreases from about 80 pre-irradiation to 7 after a fluence of 6e15 n/cm2. Consequently, the timing resolution was found to deteriorate from 20 ps to 50 ps. The results indicate that the most accurate time resolution is obtained at a value of the constant fraction discriminator (CFD) threshold used to determine the time of arrival varying with fluence, from 10% pre-radiation to 60% at the highest fluence. Key changes to the pulse shape induced by irradiation, i.e. (i) a reduce sensitivity of the pulse shape on the initial non-uniform charge deposition, (ii) the shortening of the rise time and (iii) the reduced pulse height, were compared with the WF2 simulation program and found to be in agreement.