Showing papers on "Fluence published in 1980"

Deuterium separation at high pressure by nanosecond CO2 laser multiple‐photon dissociation

Abstract: Photochemical deuterium separation is evaluated at pressures up to 1 atm using 2 ns duration CO2 laser pulses to achieve multiple‐photon dissociation (MPD) as the isotopic separation step. Photochemical performance is compared for Freon 123 (2,2‐dichloro‐1,1,1‐trifluoroethane), difluoromethane, and trifluoromethane based on deuterium optical selectivity in absorption, photoproduct yield, and single‐step deuterium enrichment factor. The absorption coefficient versus energy fluence is measured from 0.01 to 3 J/cm2 fluence for CF3CDCl2, CDF3, and CHDF2; added buffer gas results in an order‐of‐magnitude increase in the CDF3 absorption coefficient. The deuterium optical selectivity in absorption at 0.5 J/cm2 fluence with added buffer is 80 for CF3CDCl2 at 10.65 μ, 800 for CHDF2 at 10.48 μ, and 2500 for CDF3 at 10.21 μ. The absorption coefficients and hence optical isotopic selectivities are dependent on fluence, and the optical selectivity attains a maximum value of 8000 for CDF3 below 0.01 J/cm2 fluence. The ...

Observations of a fcc helium gas-bubble superlattice in copper, nickel, and stainless steel

Abstract: Transmission electron microscopy is used to investigate the spatial arrangement of the small gas bubbles produced in several fcc metals by 30 keV helium ion irradiation to high dose at 300 K. In what is a new result for this important class of metals it is found that the helium gas bubbles lie on a superlattice having an fcc structure with principal axes aligned with those of the metal matrix. The bubble lattice constant al , is measured for a helium fluence just below the critical dose for radiation blistering of the metal surface (∼4 ± 1017 He/cm2). Implantation rates are typically ∼ 1014 He ions cm−2 sec−1. The values of al obtained for copper, nickel and stainless steel are (7.6 ± 0.3) nm, (6.6 ± 0.5) nm and (6.4 ± 0.5) nm respectively. Above the critical dose the bubble lattice is seen to survive in some blister caps as well as in the region between blisters. Bubble alignment is also observed in the case of hydrogen bubbles produced in copper by low energy proton irradiation to high fluence ...

Flux, fluence and implantation temperature dependence of disorder produced by 40 keV N+ ion irradiation of GaAs

Abstract: Measurements of the lattice disorder and the depth distribution of disorder produced by 40 keV N+ ion irradiation of GaAs have been carried out before and after thermal annealing. The results show a significant dependence of lattice disorder on ion flux, ion fluence and implant temperature. A bimodal depth distribution of disorder was also observed under certain implantation conditions. The results suggest that point defect production processes in III-IV binary semiconductors (GaAs) are similar to those of Si, whilst the annealing behaviour of GaAs is rather complex and is strongly dependent on implant temperature.

The influence of light- and heavy-ion irradiation on the structure, resistivity, and superconducting transition temperature of V 3 Si. A comparative study

Abstract: Channeling and x-ray diffraction measurements on Kr- and He-irradiated V3Si single crystals and films reveal different damage levels for fluences in cases where the superconducting transition temperature T chas been reduced by the same amount. This indicates that only special defect structures are responsible for the T c-reduction mechanism. In the fluence region where T cis decreasing, T ccorrelates with residual resistivity ϱo, independent of the kind of irradiation. However, at particle fluences where T csaturation occurs, different saturation values of ϱo are observed. The exponential decrease and the saturation of T cwith fluence are explained by a similar behavior of g9o versus fluence in the damage production and saturation processes. The increase of the lattice parameter is not uniquely dependent on the decrease of T c, but also on the amount of damage present.

The frequency, fluence, and pressure dependence of the absorption of pulsed CO2-laser radiation by SF6 at 140 K☆

Abstract: We report measurements of absorption of pulsed CO 2 laser radiation by SF 6 at 140 K. These measurements cover a broad range of fluence (10 −6 to 0.8 J/cm 2 ), SF 6 gas density (2 × 10 15 to 6 × 10 16 cm −3 ), and frequency (six CO 2 -laser frequencies within the SF 6 v 3 band). We employ two methods of data reduction including one that gives a simple phenomenological function of the three principal independent variables. We conclude that at low fluence a small fraction of the SF 6 molecules absorb the laser radiation and that collisions and higher fluence both increase that fraction. At higher fluence absorption by vibrationally excited molecules becomes increasingly important.

Multiphoton absorption of HF laser photons by molecules containing a hydroxyl group

Abstract: Multiphoton absorption (MPA) of HF laser radiation has been studied, as a function of pressure (15 Pa to 1.3 kPa) and fluence (2 mJ/cm2 to 75 J/cm2) for the series: water, methanol, methan‐d 3‐ol, ethanol, and 2,2,2‐trifluoroethanol. As the group attached to the –OH is made more complex, the quasicontinuum occurs after fewer excitation steps, and under ’’collisionless’’ conditions, the same degree of multiphoton excitation is found to require a lower fluence. For water, at pressures between 73 Pa and 1.3 kPa, the cross sections are considerably lower than those for the other molecules, and MPA requires fluences in excess of ∼75 J/cm2. The remaining molecules divide into two groups, the ’’small’’ molecules (CH3OH and CD3OH) and the ’’large’’ molecules (C2H5OH and CF3CH2OH). For the small molecules at low pressures, the cross sections decrease with increasing fluence, an effect which is thought to be due to anharmonic bottlenecking. As pressure increases, the fluence dependence of the cross sections disappear. For the large molecules, anharmonic bottlenecking appears to be reduced, due to the greater density of states, and cross sections increase with increasing fluence according to the empirical form: σ(E, P)=K′E b′ P a (where P is pressure, E is fluence and b′, a, and K′ are constants). The facility of HF laser‐induced collisionless multiphotondissociation of the –OH containing molecules is discussed in light of these results.

Multiphoton dissociation of ethyl chloride at 3.3 μm: Excitation mechanism and rate equations analysis

Abstract: Tunable 3.3 μm laser pulses were used to excite the CH stretching modes of ethyl chloride. Energy deposition vs fluence was measured optoacoustically. For 3.3 μm excitation, absorbed energy increases almost linearly with fluence, while for 10 μm excitation there is substantial saturation. Dissociation yields were measured as a function of fluence and of wavelength by gas chromatographic determination of C2H4 from C2H5Cl+nhν→C2H4+HCl. Much higher yields were observed for 3.3 μm excitation than for CO2 laser 10 μm excitation. Sharp resonances in the 3.3 μm dissociation yield spectrum match peaks in the fundamental and overtone absorption spectra. Overtone spectra show that for many rotational states the vCH=2 level (6000 cm−1) is in the quasicontinuum and that vCH=3 is always in the quasicontinuum. The resonant nature of the excitation allows the rate equations description for transitions in the quasicontinuum and continuum to be extended to the discrete levels. Absorption cross sections are estimated from ...

Thermomechanical stress degradation of metal mirror surfaces under pulsed laser irradiation

Abstract: Progressive degradation of the optical surface of metal mirrors in pulsed laser applications can result from cumulative plastic deformation under thermo-mechanical stress caused by relatively small temperature excursions during irradiation. An analysis of the transient stress-strain behavior of the near-surface region shows that elastic response is maintained below a limiting surface temperature rise (ΔT y ), which depends on the yield stress, modulus of elasticity, Poisson's ratio, and coefficient of thermal expansion of the metal. This temperature rise defines the threshold for first-pulse plastic yield at the surface. An elastic-plastic model of metal behavior shows that the increment of plastic strain accumulated during each successive laser pulse will be proportional to the temperature rise above 2 AT. This plastic strain will appear at the surface as slip bands and intergranular slip that progressively degrades the optical quality of the surface. The surface temperature rise of metal mirrors under pulsed laser irradiation depends on both the metal optical and thermal properties as well as the laser pulse parameters. The peak temperature excursion produced by a constant-flux (rectangular) pulse is expressed in terms of the optical absorptance, pulse fluence, pulse duration, thermal conductivity, density, and specific heat. Using this relationship, the pulse fluence threshold (F y ) for first-pulse plastic yield is derived. The threshold for multipulse accumulation of plastic deformation is 2 F y . Numerical values of these thresholds are calculated for copper under CO 2 laser irradiation. The appearance of well-defined slip bands on the surface of copper mirrors after irradiation by a mode-locked CO 2 laser pulse burst has been experimentally demonstrated. The theoretical quantification of thermomechanical stress degradation presented here is used to calculate the expected behavior of copper under the reported experimental conditions, and these results are correlated with the available experimental data.

Multiple effects of uv radiation (265–330 nm) on fungal spore emergence

Abstract: — We have investigated the influence of narrow-band UV radiation, 265–330 nm. on germination of spores of the fungus Cladosporium cucumerinum Ellis and Arth., using a Xe arc lamp and filters. Reciprocity of time and dose rate was demonstrated when fungal spores were subjected to UV radiation at 325 nm but failed to hold at 265 nm. Based on these findings, data on fluence response, and partial action spectra, we propose that there are two biologically active sites in this organism that are affected by radiation between 265 and 330 nm and that might be influenced by changes in the stratospheric ozone layer: a short-wave-sensitive site (265–295 nm) and a long-wave-sensitive site (300–330 nm). Data obtained with narrow-band interference filters confirmed previous reports of damage to nucleic acid from UV at 265–295 nm and in addition demonstrated significant inhibition by UV at 300–320 nm. Further studies of the 300 330 nm portion of the spectrum, using combinations of plastic and glass filters, showed that the influence of UV radiation in this region was primarily to produce a non-photoreactivable delay in germ-tube emergence.

Spectra and modeling of laser‐induced emission from multiple‐photon (λ=248.4 nm) irradiation of UF6

Abstract: The laser‐induced emission from KrF‐laser irradiation of gaseous, room temperature UF6 has been studied as a function of time, wavelength, pressure, and laser fluence. Spectra in the 200–900 nm region are presented at several sampling times. One group of 17 emission peaks in the 650–900 nm range has been assigned to the 3Γ6→1Γ7 transition in UF5. A possible kinetic mechanism is presented that is qualitatively consistent with the time‐resolved emission data over the experimental pressures and laser fluences investigated.

Thermomechanical stress degradation of metal mirror surfaces under pulsed laser irradiation

Abstract: Progressive degradation of the optical surface of metal mirrors in pulsed laser applications can result from cumulative plastic deformation under thermo-mechanical stress caused by relatively small temperature excursions during irradiation. An analysis of the transient stress-strain behavior of the near-surface region shows that elastic response is maintained below a limiting surface temperature rise (ΔT y ), which depends on the yield stress, modulus of elasticity, Poisson's ratio, and coefficient of thermal expansion of the metal. This temperature rise defines the threshold for first-pulse plastic yield at the surface. An elastic-plastic model of metal behavior shows that the increment of plastic strain accumulated during each successive laser pulse will be proportional to the temperature rise above 2 AT. This plastic strain will appear at the surface as slip bands and intergranular slip that progressively degrades the optical quality of the surface. The surface temperature rise of metal mirrors under pulsed laser irradiation depends on both the metal optical and thermal properties as well as the laser pulse parameters. The peak temperature excursion produced by a constant-flux (rectangular) pulse is expressed in terms of the optical absorptance, pulse fluence, pulse duration, thermal conductivity, density, and specific heat. Using this relationship, the pulse fluence threshold (F y ) for first-pulse plastic yield is derived. The threshold for multipulse accumulation of plastic deformation is 2 F y . Numerical values of these thresholds are calculated for copper under CO 2 laser irradiation. The appearance of well-defined slip bands on the surface of copper mirrors after irradiation by a mode-locked CO 2 laser pulse burst has been experimentally demonstrated. The theoretical quantification of thermomechanical stress degradation presented here is used to calculate the expected behavior of copper under the reported experimental conditions, and these results are correlated with the available experimental data.

Pulsed CO2 laser interaction with a metal surface at oblique incidence

Abstract: Thermal fluence deposition and surface pressure generation produced by a CO2 laser pulse (λ=10.6 μm, during 10 μs, maximum intensity 3 MW/cm2) have been measured as a function of angle of incidence ϑ on sheet aluminum in air. We find that air plasma ignition depends on the laser beam intensity I0 only, not on the surface‐normal flux I0 cosϑ. Conversely, the fluence deposition and surface pressure depend only on the product I0 cosϑ, and obey the square‐root and two‐thirds‐power dependences observed with simple I0 variation at normal incidence.

Fluence dependence of disorder depth profiles in Pb implanted Si

Abstract: The total, depth integrated disorder, induced by Pb implantation into Si at room temperature, initially increases rapidly with implantation fluence and then reaches a quasi saturation level where the increase with fluence is slow. Measurements of the depth distributions of the disorder, using high resolution low angle exit Rutherford Backscattering/Channelling analysis, suggest that the quasi saturation results from overlapping of disordered zones generated deep in the tail of the disorder-depth profiles. The depth of the disordered solid-crystal boundary, XD increases with ion fuence φ, according to the relation XD = x- + f[φ).[sgrave]., where x- is the most probable projected depth and [sgrave] the projected standard deviation of disorder generation. It is shown that this relationship is consistent with an approximately Gaussian depth distribution of disorder production.

Neutron transmutation doping of silicon. I. Electrical parameters versus fluence

Abstract: Thermal‐neutron transmutation doping of silicon produces phosphorus donors by the nuclear reaction 30Si(n,γ)31Si →31P+β−. Nearly exact compensation of residual impurities in float‐zone silicon is possible with careful monitoring of the neutron fluence. To achieve this goal, an understanding of the dependence on neutron fluence of the various room‐temperature semiconductor electrical parameters is required. An analysis of the fluence dependence of resistivity, Hall coefficient, mobility, and carrier concentration is presented.

Multiple‐pulse thermal coupling at 3.8‐μm wavelength

Abstract: Pulsed DF laser irradiation of 2024‐T3 aluminum in normal air produced increases in air‐plasma‐enhanced energy absorption after repeated exposures of the same target. For a sequence of pulses, the absorbed fluence per pulse increased on each shot until it reached a value more than twice that seen on the first pulse. The effect is believed to be due to surface roughening caused by melting and vaporization.

Determination of the exposure rate constant for 125I using a scintillation detector.

Abstract: A scintillation spectrometer was employed to determine the exposure rate constant for an 125I seed of the type used in therapeutic radiology. The method consisted of converting counting rate to photon fluence incident upon the detector, and then calculating the exposure rate from the photon fluence. Correlations are required for absorption of the 125I photons in the air path, in the aluminum window of the NaI crystal, and in the aluminum-oxide reflector. Corrections are also required for the escape of iodine K-characteristic radiation, and the intrinsic peak efficiency of the crystal. The experimentally determined exposure rate constant in a direction perpendicular to the long axis of the seed is 1.309 Rcm2mCi-1 h-1 +/- 5.8%, and this compares favorably with a calculated value of 1.284 R cm2mCi-1 h-1. The photon fluence as a function of angle of rotation about an axis perpendicular to the seed's long axis was also measured. These data were used to estimate an average exposure rate constant for 125I seeds of 1.089 Rcm2mC-1 h-1.

Evaluation of quantum and photoproduct yields in multiple‐photon dissociation for isotope separation

Abstract: A model is constructed which evaluates the minority isotope photoproduct yield and the number of photons required per unit product for isotope separation via infrared multiple‐photon dissociation by a Gaussian beam. Under most experimental conditions, these parameters are optimized if the peak central fluence is about twice the saturation fluence. The quantum yield and the photoproduct yield are then both about half that attainable if the same laser energy were instead delivered by a beam with a flat transverse profile. Specific applications to deuterium separation are also discussed.

SUPERCONDUCTIVITY AND ATOMIC ORDERING IN NEUTRON-IRRADIATED Nb3Ge.1

Abstract: The effects of high energy (E > 1 MeV) neutron irradiation on the properties of Nb3Ge prepared by chemical vapor deposition have been studied. Irradiation to a fluence of 3.4 × 1019 n/cm2 produces a decrease in the superconducting transition temperature Tc from 20.9 to 4.4 K and in the long range order parameter S from 0.86 to 0.46, and an increase in the lattice parameter ao from 5.142 A to 5.174 A. For a fluence of 6.5 × 1019 n/cm2, most of the Nb3Ge is transformed into a noncrystalline phase. The A15 phase is recovered after annealing

Optimization of two wavelength laser annealing of implanted semiconductors

Abstract: The output of a dual wavelength laser consisting of 106 μ m and 053 μ m wavelength pulses was used to irradiate an As ion implanted Si wafer The fluence values (joules/cm 2 ) required to reach threshold of melting were determined for various ratios of the two fluences The results show that a linear combination of the two fluences is approximately constant This indicates that the optimum heating efficiency, under the conditions of operation, occurs with the use of just one wavelength A numerical simulation of the heating process indicates that improved heating efficiency can be achieved by introducing delay into the time development of the IR output

Microstructural interpretation of the fluence and temperature dependence of the mechanical properties of irradiated AISI 316

Abstract: The effects of neutron irradiation on the mechanical properties of annealed and 20% cold-worked AISI 316 irradiated in EBR-II were determined for the temperature regime of 370 to 760/sup 0/C for fluences up to 8.4 x 10/sup 22/ n/cm/sup 2/ (E > 0.1 MeV). At irradiation temperatures below about 500/sup 0/C, both annealed and cold-worked material exhibit a substantial increase in the flow stress with increasing fluence. Furthermore, both materials eventually exhibit the same flow stress, which is independent of fluence. At temperatures in the range of 538 to 650/sup 0/C, the cold-worked material exhibits a softening with increasing fluence. Annealed AISI 316 in this temperature regime exhibits hardening and at a fluence of 2 to 3 x 10/sup 22/ n/cm/sup 2/ (E > 0.1 MeV) reaches the same value of flow stress as the cold-worked material.