Showing papers on "Fluence published in 1990"

The significance of a fluence threshold for ultraviolet laser ablation and etching of polymers

Abstract: The minimum fluence at which a laser pulse (FWHM 20 ns; 248 nm) causes a change in the surface of a film of polyethylene terephthalate (=PET) or polyimide (KaptonTM) in air has been measured by probing the surfaces with visible laser pulses of <1 ns at delay times of 10–10 000 ns. At fluences at which a single pulse left a permanent etch pit, the probe pulse showed an intense and rapid (<60 ns) darkening of the surface which may be attributed to the scattering of the beam by gas bubbles at the polymer surface. In 100 ns, a blast wave was visible which could be tracked for over 1000 ns. At the end (∞ ns), the polymer surface was not darkened but an etch pit was apparent. Progressively decreasing the fluence showed that even at fluences <0.025 J/cm2 for PET and<0.050 J/cm2 for Kapton, a single pulse transiently blackened the surface but did not leave an etch pit. The threshold for the ablative photodecomposition of these polymers appears to lie at a value of the fluence that is well below the threshold for measurable etching by a single laser pulse.

Excimer Laser-Induced Surface Activation of Aln for Electroless Metal Deposition

Abstract: A new method for selective metal deposition on aluminum nitride (AIN) is presented. Excimer laser radiation is used for local decomposition of AIN in air. The degree of the surface modification depends on the fluence and number of pulses. A distinct wavelength dependence of the threshold fluence decomposition of AIN was found. The irradiated surface regions act as catalysts in conventional electroless metal plating baths. Structured metal coatings of various elements (Cu, Ni, Au, Pd, etc.) on AIN can thus be obtained in a simple way.

Wear property and structure of nitrogen implanted glassy carbon

Abstract: A study has been made of the correlation between wear resistance and crystalline structure of ion implanted glassy carbon. Nitrogen ions were implanted in glassy carbon with fluences ranging from 5 × 1014 to 5 × 1016 ions/cm2 at an energy of 150 keV. The target temperature during ion implantation was maintained constantly at −70, 25, and 200 °C. Wear tests were carried out with the system of glassy carbon and polishing silk disk on which water including diamond slurry was poured, using a conventional polishing machine. The surface layer structure was investigated by means of laser Raman spectroscopy. Nitrogen implantation causes the wear resistance to improve, and the reduction of wear rate is dependent on the ion fluence and the target temperature during ion implantation. As the fluence increases and the target temperature is lower than room temperature, the wear rate decreases drastically. Raman spectra show that the structure of ion implanted layers becomes what is called amorphous-like as the fluence increases and the target temperature is lowered. In conclusion, the wear resistance of glassy carbon is improved owing to the change in structure followed by nitrogen implantation at a relatively high fluence and low target temperature.

Radiobiology of alpha particles. I. Exposure system and dosimetry.

Abstract: A 238Pu alpha-particle exposure apparatus was designed and constructed for use in radiobiological studies with cultured cell systems. The system provides a wide dynamic range of absorbed doses and a uniform radiation field. Average dose rate in air was measured with a small-volume ionization chamber. Estimates of dose rate at the cell surface were obtained from measurements taken with a silicon surface barrier detector. Particle fluence uniformity and fluence rate were measured using track etch procedures. The design and dosimetric characterization of the apparatus are discussed.

Reversible modification of CdTe surface composition by excimer laser irradiation

Abstract: KrF excimer laser irradiation of CdTe at fluences below the melt threshold (≤75 mJ/cm2) removes surface layers and produces reversible changes in the surface composition that depend upon the laser fluence and number of laser pulses delivered to the surface. At fluences above ∼40 mJ/cm2 a Te‐rich layer is obtained. A stoichiometric composition can be restored by irradiation at reduced laser fluence. The primary desorption products are Cd and Te2, and the velocities of these species are well described by a Maxwellian distribution. The fluence‐dependent changes in CdTe surface composition are consistent with a photothermal mechanism based on the competition between formation and desorption of Te2 and desorption of Cd atoms from the laser‐irradiated surface.

Dynamics of excimer laser ablation of polyimide determined by time-resolved reflectivity

Abstract: The time-dependent intensity profile of pulsed KrF excimer laser radiation reflected from polyimide is determined over a range of laser fluences, from well below to above the ablation threshold. The reflected laser beam is truncated once the incident laser radiation exceeds a threshold fluence, i.e., truncation depends on the energy per unit area and not on the intensity, analogous to results for the ablation threshold and the etch depth per pulse. The threshold fluence for pulse truncation corresponds to the onset of ablation. The results indicate that the truncation is not due to laser plasma interactions at these fluences. A general mechanism is discussed involving a time dependent index of refraction.

Model for burning kinetics and shape of fluence-saturated spectral holes

Abstract: We propose a simple model for the study of fluence-broadened holes. The model is based on a strong dispersion of the burning rates and the burning process takes place over several decades of fluence. In this so-called semi-saturated regime, the hole depth varies almost linearly with the logarithm of the fluence and the hole width increases as the power 0.25 of the fluence, i.e. exponentially with the hole depth. A careful study of semi-saturated holes could allow us in principle to determine the homogeneous width free from the contribution of spectral diffusion. The hole shape is also discussed and can often be fitted either by Lorentzian or by logarithm of Lorentzian profiles. Several examples of semi-saturated holes can be found in previous works in the literature. As a further instance, we present our results on free-base tetraazaporphin in a Langmuir-Blodgett film. The reasonable agreement with the model allows us to evaluate the rate dispersion in this system and to ascribe it to barrier height fluctuation induced by the disordered molecular surrounding.

A Common Fluence Threshold for First Positive and Second Positive Phototropism in Arabidopsis thaliana

Abstract: The relationship between the amount of light and the amount of response for any photobiological process can be based on the number of incident quanta per unit time (fluence rate-response) or on the number of incident quanta during a given period of irradiation (fluence-response). Fluence-response and fluence rate-response relationships have been measured for second positive phototropism by seedlings of Arabidopsis thaliana. The fluence-response relationships exhibit a single limiting threshold at about 0.01 micromole per square meter when measured at fluence rates from 2.4 x 10(-5) to 6.5 x 10(-3) micromoles per square meter per second. The threshold values in the fluence rate-response curves decrease with increasing time of irradiation, but show a common fluence threshold at about 0.01 micromole per square meter. These thresholds are the same as the threshold of about 0.01 micromole per square meter measured for first positive phototropism. Based on these data, it is suggested that second positive curvature has a threshold in time of about 10 minutes. Moreover, if the times of irradiation exceed the time threshold, there is a single limiting fluence threshold at about 0.01 micromole per square meter. Thus, the limiting fluence threshold for second positive phototropism is the same as the fluence threshold for first positive phototropism. Based on these data, we suggest that this common fluence threshold for first positive and second positive phototropism is set by a single photoreceptor pigment system.

Computer simulation of high fluence oxygen profiles in silicon

Abstract: A fast computer programme has been developed to calculate high fluence oxygen distributions after implantation into unmasked silicon or through masking oxide layers. Using a TRIM-profile to represent the incremental fluence distribution, the oxygen redistribution due to swelling, sputtering and diffusion is calculated. The results show that for substoichiometric fluences the peak of the high dose profile is moving downwards for unmasked and upwards for masked silicon, due to swelling being the dominant process in the first case and sputtering being the dominant process in the latter case. Once a buried SiO2layer is formed the upper interface is always moving upwards, the lower one always downwards, but interface velocities and shapes strongly depend on the existence of a masking layer.

Amino acid decomposition induced by keV ion irradiation

Abstract: Molecular emission of H2, NH3, CO2 and dimer hydrocarbons has been detected during keV ion bombardment of alanine. 200 keV helium and argon beams produce high yield emission up to 103 molecules/ion for all the species. The time (or fluence) dependence of molecular emission has been investigated in the ion current range 10 nA to 1 μA over a 3 × 3 mm2 spot, with a quadrupole mass analyzer. We have observed a prompt emission of H2, while for heavier masses an incubation ion fluence is required (∼ 1013 ions/cm2). The observed effects have been correlated with the EPR signals, detected in the irradiated amino acids, mainly in alanine. The good linearity between the energy deposition and EPR signals allows to use such material to build dosimeters in the high fluence regimes.

Defect formation in implantation of crystalline Si by MeV Si ions

Abstract: The distributions of vacancy‐type defects and displaced Si atoms in Si(100) produced by the room‐temperature implantation of 1014–1016 12‐MeV 28Si+ ions/cm2 are measured with low‐energy positron‐ and ion‐beam techniques. The observed damage regions are reproduced by computer simulations. The distribution of displaced Si atoms coincides with the deposited energy distribution in elastic collisions. At the fluence of 1×1016 Si+/cm2, no crystalline structure was found in the peak region of the deposited energy at the depth of z=6 μm. Saturation of the divacancy concentration was observed at the ion fluences 3×1015 Si+/cm2 close to the surface (z 1 μm). In the region z 1 μm. This is also found in the simulated spatial structure of collision cascades.

The influence of fast neutron irradiation on the intra- and intergrain properties of the polycrystalline BiPbSrCaCuO system

Abstract: The influence of irradiation by fast neutrons with fluences from 3.3 x 10 16 n / cm 2 up to 3 x 10 18 n / cm 2 on the physical properties of polycrystalline Bi 0.7 Pb 0.3 SrCaCu 1.8 O x was examined. Studies of DC magnetization, AC susceptibility, transport and thermoelectric power were performed. The irradiation caused a decrease of T c , determined from the onset of diamagnetism, by as much as 31 K for a fluence of 3 x 10 18 n / cm 2 . A strong influence of neutron irradiation on both intra- and intergranular properties was observed. The defects within the superconducting grains created by neutrons caused an increase of the pinning forces which enhanced the critical magnetization current. A gradual decoupling of Josephson weak links with increasing neutron fluence was observed in transport and low field magnetization measurements. From the AC susceptibility measurements the irreversibility lines between the flux-creep and flux-flow regions were determined. An increase of the absolute values of thermoelectric power with rising fluence was noticed.

Surface-plasmon-enhanced multiphoton photoelectric emission from thin silver films.

Abstract: We report the enhancement of multiphoton photoelectric emission, from a thin silver film, by surface plasmons excited at the single-metal–vacuum interface. The surface plasmons are resonantly pumped by femtosecond laser pulses in the Kretchmann attenuated-total-internal-reflection configuration. A quantum yield approaching 2 × 10−8 at a photon fluence of 10 MW/cm2 from a two-photon process was observed, which is 5 × 103 higher than that of the bulk emission measured for the same film at the same power fluence.

Enhancement of Transport Critical Current Density of Ba 2 YCu 3 O 7-δ Thin Film by 28 MeV Electron Irradiation

Abstract: Effect of 28 MeV electron irradiation was investigated on the critical current density (Jc) of high-Tc superconductor Ba2YCu3O7-δ thin films. For the film irradiated with a fluence of 4.7×1016 electrons/cm2, Jc determined from I-V characteristics at temperatures from 75 to 85 K was enhanced by about 8%. The enhance ratio of Jc due to irradiation increased with fluence over the range investigated, while the critical temperature (Tc) was unchanged. The temperature dependence of Jc was not affected by electron irradiation.

Investigation of the Spectral Fluence of Neutrons from Spontaneous Fission of 252Cf by Means of Time-of-Flight Spectrometry

Abstract: A precision multidetector neutron time-of-flight (TOF) spectrometer and low-mass paral-lel-plate ionization chambers with a 252Cf deposit on the inner electrode are used to measure the spectral flu...

Laser‐induced sputtering of Ga0 and Ga+ from the GaP (110) surface: Its relation to surface imperfection

Abstract: We have measured emission of Ga+ ions and neutral Ga0 atoms induced by irradiation of the GaP (110) surface with laser pulses of subband‐gap energies. It is found that the Ga+ yield of partially annealed Ar+ ion bombarded surfaces is reduced by thermal annealing and also by repeated irradiation with laser pulses, while the Ga0 yield is not influenced to an important extent by these treatments. The Ga+ emission, which occurs at a lower fluence than Ga0 emission, is ascribed to emission from defect sites at surfaces. We suggest that measurements of laser‐induced particle emission for subband‐gap energies will be useful for detection and elimination of low‐density defects on surfaces.

In situ defect studies on Si+ implanted InP

Abstract: Transmission electron microscopy (TEM) was used to investigate the structural evolution of InP during 50 keV Si ion implantation at room temperature (RT) and during further in situ annealing. The authors describe a new thinning method to obtain reproducible, easily handled and strong samples exhibiting very thin and large areas. In situ TEM studies were performed during implantation at 80 keV up to approximately 3*1013 Si+/cm2. The defect density and size as a function of ion fluence is reported. Amorphisation sets in above a threshold fluence of approximately 2*1013 Si+/cm2. Further studies on implanted samples at various total fluences show that the whole implanted layer ( approximately 70 nm) becomes amorphous at approximately 1015 Si+/cm2. Below fluences of approximately 2*1013 Si+/cm2 in situ thermal annealing leads to complete recovery of induced defect clusters at T>or=500 K. The defect clusters induced by Si ion implantation anneal out during TEM observation at electron energies >or=100 keV, an effect which they ascribe to the low displacement energy of P atoms ( approximately 8 eV).

Sputtering yield and residual vacuum influence during titanium implantation into iron

Abstract: Ti implantation into iron-based alloys is known to improve tribological properties (low friction and wear) by formation of an amorphous Fe-Ti-C surface layer due to the interaction of the carbonaceous molecules in the residual vacuum with the surface during implantation. To state precisely the conditions of this amorphous-layer formation, thin evaporated iron targets were implanted with 110 keV Ti ions at room temperature with fluences ranging from 6 × 1016 to 3 × 1017 Ti/cm2 and at different residual pressures. Samples were analysed using backscattering spectrometry with 5.7 MeV 4He ions to obtain titanium profiles and to follow the evolution of sputtering yield versus fluence and residual vacuum pressure, and also for determining the amount of carbon and oxygen incorporated on the Ti-implanted surface as a function of fluence and pressure. Theoretical calculations of sputtering and high-fluence Ti distributions were performed and compared to experimental data. The importance of the reactions that occur between the implanted surface and the residual gases in the vacuum during implantation (C and O competition) is discussed.

Optical nonlinearities in carbon black particles

Abstract: We have characterized the nonlinear optical properties of carbon black particles in liquids and layers deposited on glass. We find that the limiting is dependent on the energy density (fluence) and that the material changes from a linear absorber to a nonlinear scatterer for fluence levels 0.2 J/cm2 and 0.38 J/cm2 for 0.532 jim, 14 ns and 1.064 pm, 20 ns laser pulses respectively. In this paper, we will discuss the possible mechanisms that have been proposed to explain the nonlinear scattering. These mechanisms are plasma formation, micro-bubble formation and change in index of refraction of the liquid surrounding the particles. We will show through a series of experiments that plasma formation is consistent with all of the experimental results while bubble formation may influence the limiting behavior at fluence levels substantially above the limiting threshold. In this model, the microscopic carbon particles are heated by linear absorption to a temperature at which a plasma can be created by the optical field. These microplasmas rapidly expand, thus scattering the incident light and limiting the transmittance.

Helium implantation effects in hard hydrogenated carbon layers

Abstract: Hard amorphous films of a‐C:D were deposited by a rf glow discharge in CD4. The implantation of 40‐keV 3He+ ions in these films was studied as a function of the helium fluence at temperatures between 100 K and room temperature. The trapped amounts of helium and the deuterium losses in the layer were measured in situ using nuclear reaction analysis with 0.5‐MeV D+ and 1‐MeV 3He+ beams, respectively, and subsequent ex‐situ elastic‐recoil detection analysis. A transient helium retention appears in the carbonized layers, occurring only at temperatures below 200 K. Above a critical fluence which depends on temperature and ion flux, outdiffusion of He is observed which is ascribed to the formation of diffusion channels by radiation damage. The deuterium depletion induced by helium bombardment below 200 K sets on in correlation with the helium outdiffusion, but is otherwise independent of the helium trapping.

A channeling study of ion-produced disorder in silicon carbide

Abstract: The RBS/C technique is used to study the disorder buildup in silicon carbide (SiC) produced by Al+ and Ga+ ions with energies of 40 and 90 keV. The damage rate increases sharply in the fluence range near the critical value for amorphization, Φ ≈ 1 × 1014 cm−2 and = 8×1014 cm−2 for Ga+ and Al+ ions, respectively. The defect concentration rises slowly with fluence outside this range. At fluences Φ > Φc, the concept of elastic energy loss density is used in the analysis of the amorphous layer thickness. A strong decrease of the critical energy loss density, Wc, is observed when the amorphous layer expands due to fluence accumulation.

Effect Of Neutron Irradiation On The Thermoelectric Properties Of Sige Alloys

Abstract: Zone-leveled and hot-pressed n- and p-type Si80Ge20 alloys were irradiated with neutrons to a fluence of 4 x 1018 n/sq cm and to a fluence of 5.4 x 1019 n/sq cm at a temperature of approximately 200-300 C. The effect of neutron irradiation on the thermoelectric properties of these alloys was evaluated. The carrier concentration and mobility (and hence the resistivity) were measured at room temperature while the thermal diffusivity was measured at 177-192 C both before and after the irradiation and after each subsequent 2-h heat treatment at 350 C, 600, and 1000 C. The irradiation increased the resistivity significantly, but the thermal conductivity decreased only by about 10-15 percent. This tends to indicate that the radiation produced only small defects (single pairs and small vacancy chains). The samples all returned to almost exactly their preirradiation state after the 1000 C anneal. This indicates that SiGe alloys can be operated in this neutron fluence at high temperatures without a degradation of thermoelectric properties.