Fluence Density Research Articles

Study of mechanical strains in implanted silicon slices

In this paper the influence of Ar + and He + ion bombardment on mechanical stress formation in silicon and SiSiO 2 structures is studied. Ion bombardment was carried out at energies of 50–175 keV, ion flux densities of 0.1–10 μA cm −2 and fluences up to 10 17 cm −2. Double-beam laser interferometry was used for contactless registration with high accuracy of deformation of the samples during the process of ion implantation. We obtained that the stresses induced by ion implantation in silicon are a complex function of ion kind, fluence and flux density. The sign of strains for the oxidized slices is opposite to that for the unoxidized ones. Large deformations are obtained during He + implantation in the oxidized samples. The experimental results are directly dependent on ion damage level. The formation of mechanical stresses in SiO 2 is related to the amount of SiO and SiSi bonds which changes under ion bombardment.

A Mechanistic Study of the Interaction of Ultraviolet Laser Radiation with Low Density Polymers

AbstractUltraviolet laser radiation at 248 nm and 266 nm was used to ablate low density (0.04 to 0.14 g/cm3) microcellular polystyrene and TPX foams at laser fluences between 0.1 and 1000 J/cm2, The dependence of the etch rate on laser fluence and foam density is consistent with a linear absorption mechanism for the initial stages of polystyrene ablation. Studies of the morphology and chemical composition of the ablated foams as a function of laser fluence and foam density show that the degree of melting increases at the higher foam densities. Blackbody and radical specie contributions to optical emission from the ablated foam were readily resolved for correlation with laser fluence, foam density, absorption spectra, and etch depth. Temperatures in excess of 3000 °K are calculated from emission spectra observed during the ablation of polystyrene foams. From this data we conclude that chemical and thermal changes in the ablated foam are influenced by the absorption spectrum as well as the density of the foam.

Use of SSNTD kodak BCA 80-15 for thermal neutron flux control

Solid state nuclear track detector material Kodak BCA 80-15 can be used as well for neutron dosimetry as for quick measurement and control of neutron fluxes and neutron flux distributions. For quantitative measurements it is necessary to know the relation between neutron fluence and track density in form of a calibration curve. In the presented work the calibration curves for thermal neutrons of Cf-252 neutron irradiation facilities have been measured experimentally. To determine the accuracy and reproducibility the experiments have been repeated several times and the optical counting of track densities have been carried out by different persons. The results show a linear relation between track density and thermal neutron fluence in an interval from 8·108 to 3·1010 n/cm2. The accuracy depending on experimental procedures will be discussed.

Intrinsic and effective sensitivities of analysis by X-ray fluorescence induced by protons, electrons, and photons

The intrinsic analytic sensitivity to trace elements of X-ray fluorescence induced by protons, electrons and photons, defined in terms of an irreducible background and a “perfect” detector, is much lower than that observed with present source strengths. As ion flux densities increase and spatial resolutions improved, the sensitivity is likely to be limited by sample heating and by the Frenkel pair radiation damage. At a proton fluence density of ∼10 20/cm 2, the displacement damage is complete; the sample is amorphous. If the fluence density is limited to this value, then, at 1 μm resolution, the ultimate sensitivity of proton induced X-ray analysis will be about one atom of the trace element per million atoms of the host.

Hydrogen migration under avalanche injection of electrons in Si metal-oxide-semiconductor capacitors

Aluminum/silicon dioxide/silicon capacitors in which the oxide has been grown thermally under ultra-dry (≲1 ppm H2O) conditions and subsequently treated by low temperature water diffusion have been characterized electrically and chemically. Avalanche injection of electrons has been observed to produce the complex charging behavior previously observed in similar systems, which includes electron trapping and interface positive charge generation. Secondary ion mass spectrometry depth profiling of these structures has shown that electron injection also results in hydrogen transport. This is the first direct observation of hydrogen redistribution under the influence of an electron current. We demonstrate a linear relationship between injected charge fluence and areal density of hydrogen localized at the SiO2/Si interface. These results indicate that hydrogen release correlates with interface state generation, but not with bulk oxide trapping.

Generalized concepts in large-scale laser isotope separation, with application to deuterium

Optical laser isotope separation (LIS) and chemical process concepts are developed that arise in the large-scale application of LIS processes with continuous process streams. The discussion applies to one-photon, two-photon and multiphoton LIS and the separation of a multiple-component mixture. It is partly based on a novel reaction chamber which provides throughout its volume a constant energy fluence or power density. Concepts discussed are the isotopic depletion, isotopic and process selectivity, isotopic abundances, abundance quotients α* and β*, the photon utilization factor, and the material balance equations in the LIS plant. An expression is derived that gives the total process cost per unit separated product in terms of the isotopic selectivity, the isotopic depletion, and the natural abundance of the desired species. A total-cost minimization is carried out which gives the optimum isotopic depletion of the process as a function of isotopic selectivity, photon cost parameters, and parameters characterizing the photochemical reaction and the feed costs. The expressions are applicable to ’’closed cycle’’ as well as ’’once through’’ feed processing. The results are used to estimate the cost per kg of heavy water as a function of laser efficiency and isotopic selectivity, for a formaldehyde-based deuterium separation method.

Copper vapor laser unstable resonator oscillator and oscillator-amplifier characteristics

We have studied the properties of copper vapor lasers configured as oscillators with unstable resonator optics and as power amplifiers. With the unstable resonator, a 26-fold reduction in the oscillator beam divergence has been achieved over the value observed with stable optics without any power degradation. Time-dependent beam divergence effects in the unstable resonator have also been measured and explained. Small-signal gain, saturation fluence, and available stored energy density parameters have also been measured in oscillator amplifier experiments for both the 510.6 and 578.2 nm transitions as a function of the copper number density. These results quantify the maximum power per unit length available from these devices and furthermore, demonstrate efficient extraction of power from copper vapor lasers operated as amplifiers.

Multiphoton decomposition of hexafluoroacetone: Effects of pressure, fluence, wavelength, and temperature on the decomposition yield and C-13 and O-18 isotopic selectivity

The yield and isotopic selectivity of the infrared multiphoton induced decomposition of hexafluoroacetone have been studied in both parallel and focused beam geometries as a function of temperature, pressure, fluence, and photolysing wavelength. At low conversions, the chemical stoichiometry of the decomposition process is simple, CF3COCF3 nhν CO + 2CF3. Several potential interferences are discussed but it is concluded that the yield of CO directly reflects the yield of the primary dissociation event. Pressure effects are complex and not amenable to quantitative description, however, varying parameters such as fluence and photolysing wavelength lead to predictions which give a clear qualitative picture. In particular, it is possible to distinguish parametric variations due to spontaneous dissociation and dissociation dependent upon energy pooling. Isotopic selectivity provides a powerful insight in this area. Preliminary data on the effect of temperature have been obtained at constant incident fluence and number density. Varying temperature probes the contribution of initially excited species (hot-bands). Reducing the temperature has no effect on the blue-edge of the absorption band but leads to a narrowing on the red-edge which is more marked for parallel beam geometries. The form of the wavelength dependence is in agreement with theoretical predictions. The observed carbon-13 isotopic selectivities of the dissociation are in basic agreement with those predicted from spectroscopy in that αCO is much higher than αC2F6. Very high values for this selectivity have been observed approaching αCO=1000 under extreme conditions. The equivalent selectivities for oxygen-18 are small, α?2. The hexafluoroacetone molecule is demonstrated to be a good model for study of multiphoton events and the data presented offer a powerful basis set for theoretical treatment.

Depleted zone formation and recovery during neutron irradiation

Abstract Seeger zones or depleted zones are believed to be responsible for increased hardening in neutron irradiated materials. This investigation is an attempt to calculate the density of the Seeger zones as a function of temperature, neutron flux, fluence and dislocation density. The calculations assume an initial zone size at creation and then follow the annealing behavior of the zone. The zone anneals by absorbing vacancies and interstitials and by emitting vacancies. The annealing rates are controlled by the concentrstion of vacancies and interstitials. These concentrations are in turn dependent upon the Seeger zone density. The point defect concentrations are determined from chemical reaction rate equations.

The simulation of fission damage in U 3Si

The feasibility of simulating fission damage in U 3Si by means of ion bombardment is examined theoretically and experimentally. For ambient temperature irradiations, the changes in X-ray diffraction patterns produced by neutron irradiation can be reproduced by bombardment with 2 MeV argon. The values of ion fluence and fission density to give the same degree of damage are in the ratio 1: 1000 which is in good agreement with the calculated ratio assuming that the damage is proportional to the amount of energy deposited in atomic collisions. Radiation swelling in the bombarded layer has been measured by interferometry and has been found to agree with values obtained from neutron irradiated material. Microhardness measurements suggest that although U 3Si is hardened by irradiation, U 3Si 2 undergoes irradiation softening.