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  • Articles  (28)
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  • Articles: DFG German National Licenses  (28)
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  • 1990-1994  (12)
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  • Physics  (28)
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  • Articles  (28)
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  • 1
    Electronic Resource
    Electronic Resource
    Biophysics of the photoablation process (1991)
    Springer
    Lasers in medical science 6 (1991), S. 241-254 
    Details
    ISSN: 1435-604X
    Keywords: Laser angioplasty ; Photoablation ; Photohydraulic effect ; Fast thermal explosion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Physics , Technology
    Notes: Abstract Besides the coagulation, where the body digests the necrotic tissue and direct evaporation of tissue, the photoablation effect turns out to be very important in tissue removal. In the case of high tissue absorption the process channels in photoablation can either be photochemical (bond breaking) or fast thermal. In the case of transparent media, a plasma formation due to high irradiances and an optical breakdown is necessary for ablation or photodisruption. All the process channels lead to a fast microscale explosion and to Shockwaves. For soft tissue the main process channel is the fast thermal explosion. Assuming that tissue will be disintegrated, if the energy deposited within a single laser pulse is larger than a material specific threshold, the thresholds for the radiant exposure and ablation rates respectively can be calculated. There is a large difference, whether the laser radiation is applied to the tissue surface in noncontact or through a fibre in contact. In contact the ‘fast thermal explosion’ happens in a closed chamber and hence the photohydraulic effect will support the photoablation. The thermally damaged zone in the surrounding tissue depends on the optical penetration depth mainly in cases that the pulse duration is shorter than a critical time given by the heat conductivity. Pulsed lasers can be used ‘non-thermally’ only if the average power is less than a tenth of a watt. With a higher amount of average power a pulsed laser will act comparable to a cw laser.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02030877
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  • 2
    Electronic Resource
    Electronic Resource
    VUV laser application in medicine? Dosimetry and theory for laser-tissue interactions (1996)
    Springer
    Optical and quantum electronics 28 (1996), S. 315-326 
    Details
    ISSN: 1572-817X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract At present no medical applications are known for VUV radiation; there seems to be no convincing evidence for using such wavelengths in laser therapy. To predict future VUV applications in medicine, an overview of the problems of dosimetry and laser-tissue interactions from the UV up to the MIR are discussed. The most important parameters for the laser-tissue interaction are the effective optical penetration depth of tissue and the pulse duration and repetition rate of the laser radiation. For the VUV and UV region the risk of mutagenicity has to be taken into consideration. Photorefractive surgery of the cornea at 193 nm as an example of the photoablation effect gives a brief impression of medical laser applications at the border of the VUV region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00385863
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  • 3
    Electronic Resource
    Electronic Resource
    The cosmo-petrological significance of the coalescence of microchondrules with the decrease of volatiles in chondrites (1969)
    Springer
    Astrophysics and space science 4 (1969), S. 3-43 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The type I carbonaceous chondrites, with volatiles between 24 and 30% (at 1000 C, N2 atm.), contain the maximum percentage of the low-temperature ground mass, in which the high-temperature minerals are dispersed as ‘microchondrules’. In the type II carbonaceous chondrites (vol. 12–24%), the loosely cohering aggregates of microchondrules, ‘grape-bunch chondrules’, reach a maximum. The type III carbonaceous chondrites and some enstatite chondrites (vol. 2–12%) contain the maximum of the ‘partly coalesced chondrules’, in which microchondrules of olivine and nickel-iron appear. The ureilites are interpreted as impact shocked aggregates of microchondrules in differing states of coalescence. The ‘fully coalesced chondrules’ are characteristic for the ordinary chondrites with volatiles below 1%. It appears that the evolution of chondrules with the decrease of volatiles in meteorites subdivides into: (A) primary condensation of microchondrules with diameters of 0.01 mm; (B) secondary accretion of the former into the chondrules of diameter range ±1 mm. The observations may be explained through the hypothesis that at the highest-temperature stage of condensation of the asteroid-type parent body was an incandescent cloud (preserved through the solidification of chondrules at an early stage of degassing) covered with cosmic dust. The carbonaceous chondrites orginate from the marginal incandescent fog and the correspondingly deeper zones of the incandescent cloud mantle. The absence of typical chondritic rocks on earth may be explained by the slower cooling rate of this celestial body of relatively greater mass.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00651261
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  • 4
    Electronic Resource
    Electronic Resource
    Weitere Messungen zur Anisotropie der intermolekularen Wechselwirkung durch Streuung von Molekülen in definiertem Quantenzustand (1969)
    Springer
    The European physical journal 226 (1969), S. 139-155 
    Details
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract In a previous paper the anisotropy parameter of the attractive part of the intermolecular potential has been determined from total cross sections measured by scattering of TlF-molecules in selected rotational states by rare gases. In this series of papers, further measurements on the systems TlF-He, Ne, Ar, Xe, CH4, and CsF-He, Ar will be described. In the case of CsF-He, it has been possible to determine the anisotropy parameter of the repulsive part of the potential additionally, by comparing cross sections related to different rotational states over a large velocity range and by using the high energy approximation. The determination of the anisotropy parameter for the repulsive potential requires knowledge of the parameters of the angle averaged potential. Part I describes the determination of these parameters for a Lennard-Jones-potential from measurements of the velocity dependence and the absolute value of the total cross section by scattering of not state selected CsF-molecules by He and Ar. Since these systems show no glory undulations in the experimentally accessible velocity range, they require an evaluation procedure, which is independent of the appearance of such undulations. The procedure described below only uses the velocity dependence of the cross section itself by fitting of quantum mechanically calculated cross sections. The finite resolving power of the apparatus and the influence of velocity distributions are taken into account. The results are communicated, the parallel evaluation of an experiment on K-Ar serves as a test for the procedure described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01392017
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  • 5
    Electronic Resource
    Electronic Resource
    Weitere Messungen zur Anisotropie der intermolekularen Wechselwirkung durch Streuung von Molekülen in definiertem Quantenzustand (1969)
    Springer
    The European physical journal 226 (1969), S. 279-292 
    Details
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Improved measurements of the ratio of scattering cross sections for various molecular rotational states are reported for scattering of TlF in rotational states ¦J, M〉=¦1, 0〉 and ¦1, 1〉, and CsF in rotational states ¦2, 0〉 and ¦2, 2〉 by rare gases. The results are interpreted in terms of an angle dependent attractive potentialV=−2ε(r m /r) 6(1+q 6 P 2(cosΘ) in which the repulsive part of the interaction is neglected. The “high energy” approximation is used to calculate the cross section, which contains the velocity dependence and the state dependence as factors. The experiments show for all scattering partners with the exception of He and Ne, that the state dependence is velocity independent. In those cases this result provides a justification for the neglect of the repulsive potential term. The results for the anisotropy parameterq 6, which to a good approximation depends only on properties of the moleculus, are:q 6=0.23±0.01 for TlF,q 6=0.28±0.02 for CsF.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01392090
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  • 6
    Electronic Resource
    Electronic Resource
    Attenuation operators and complex wave velocities for scattering in random media (1996)
    Springer
    Pure and applied geophysics 148 (1996), S. 269-285 
    Details
    ISSN: 1420-9136
    Keywords: Wave scattering ; attenuation ; random media ; reflectivity method
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The concept of attenuation operators and complex velocities is applied to scattering attenuation in two and three dimensions, using the minimum-phase assumption for the attenuation operator. Acoustic 2D finite-difference computations of synthetic seismograms show, that the attenuation operator describes well the decay and lowpass filtering of the averaged wave form, which follows from averaging travel-time-corrected wave forms along the wave front. In the case of exponential random media, analytical forms of the attenuation operators and complex velocities are available. The complex velocities are incorporated into the reflectivity method. As an application, synthetic seismograms are presented for theS n wave, attenuated by lithospheric velocity and density fluctuations. The limitations of attenuation operators and complex velocities for scattering are also discussed. With these quantities it is not possible to model phenomena related to the scattered waves themselves, such as amplitude and travel-time fluctuations along the wave front, codas and precursors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00882063
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  • 7
    Electronic Resource
    Electronic Resource
    The effect of preparation technique on the optical parameters of biological tissue (1999)
    Springer
    Applied physics 69 (1999), S. 445-453 
    Details
    ISSN: 1432-0649
    Keywords: PACS: 42.10; 42.20; 78; 87
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. The absorption coefficient μa, the scattering coefficient μs, and the scattering anisotropy factor g of porcine liver were studied in vitro using the integrating sphere technique and inverse Monte Carlo simulation in the wavelength range 450 to 700 nm. A reference preparation technique was developed using a dermatome providing specimens of 200 to 800 μm thickness without pre-freezing the tissue. The optical parameters as measured applying the reference preparation were compared to those measured after cryo-homogenisation. We found significant deviations of the scattering coefficient and the anisotropy factor which were compensated when the reduced scattering coefficient μs ′ was calculated. We also compared the effects of freezing reference specimens at -20 °C and at 77 K without homogenisation. For both freezing protocols noticeable deviations were found in all three optical parameters as well as in μs ′. The impact of tissue storage at 4 °C was measured in the range 4 to 48 h post mortem and showed a clear reduction of μa and a significant increase of μs even after 24 h of storage. Short-time storage of the specimens in saline solution reduced all three optical parameters significantly. In conclusion, the tissue preparation must be controlled in order to provide in vitro optical parameters that sufficiently mimic the in vivo situation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003400050833
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  • 8
    Electronic Resource
    Electronic Resource
    Nonlinear Power Handling of YBa2Cu3O7−x Films and Microwave Devices (1999)
    Springer
    Journal of superconductivity 12 (1999), S. 343-351 
    Details
    ISSN: 1572-9605
    Keywords: YBa2Cu3O7−x ; power handling ; magnetic and thermal field limitations ; defects ; microwave quenches
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Physical mechanisms which limit the power handling of YBa2Cu3O7−x films and devices are discussed in terms of a quantitative classification scheme. The possible limitations are devided into magnetic or thermal, and global or local in nature. Analytical estimations are compared with measurements of YBa2Cu3O7−x films (Ø = 1″–2″) using a niobium-shielded sapphire resonator at 19 GHz, and disk resonators at 2 GHz. Magnetic effects are found to play an essential role in nonoptimized films in terms of weak-links, and in high-quality films if the lower critical field Bcl is reached. The majority of films and disk resonators appear to suffer from microwave heating. Global heating appears predominantly at CW operation. Local heating results mainly from defects in films of medium quality. Defect-induced quenches are observed at moderate field levels, sometimes resulting in an irreversible degradation of the power handling.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1007701332081
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  • 9
    Electronic Resource
    Electronic Resource
    Electromagnetic properties of electrophoretic YBa2Cu3O7−δ films (1990)
    Springer
    Journal of superconductivity 3 (1990), S. 323-329 
    Details
    ISSN: 1572-9605
    Keywords: Y1Ba2Cu3O7−δ ; electrophoretic films ; grain aligned
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract In order to apply highT c superconductivity to general microwave cavities and to shielding devices for quasistationary magnetic fields, we have developed an electrophoretic coating technique. This work reports about the continuation of our experiments. More than 60 samples of electrophoretic layers deposited on silver substrates were fabricated by systematically varying the processing parameters. The r.f. surface resistance measured at 21.5 GHz and 77 K at low excitation fields is found to be a sensitive measure of the quality of the samples and falls rapidly with increasing average grain size. Textured electrophoretic layers of large grain size show the smallest sensitivity ofR s against the r.f. surface magnetic fieldH s. The magnetic sensitivitydR s /dH s is found to be in direct proportion toR s (77 K). After optimizing the sintering procedure the electrodes of the prototype cavity for a compact hydrogen maser were coated. The cavity was successfully operated at 1.42 GHz, and a surface resistance of 1 mΩ was achieved at 77 K. This compares to 4 mΩ for copper at the same temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00625275
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  • 10
    Electronic Resource
    Electronic Resource
    Microwave surface resistance of epitaxial YBa2Cu3O7 thin films at 18.7 GHz measured by a dielectric resonator technique (1992)
    Springer
    Journal of superconductivity 5 (1992), S. 195-201 
    Details
    ISSN: 1572-9605
    Keywords: High-temperature superconductivity ; microwave cavities ; sample surface impedance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistanceR s of 1×1 cm2 superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE01δ oscillation mode the highly reproducible quality factor of about 105 results in a sensitivity of ±50μΩ forR s measurements. The temperature dependence ofR s can be measured up to values as high as 1 Ω. We have investigated several YBa2Cu3O7 thin films prepared by high oxygen pressure d.c. sputtering on LaAlO3 and NdGaO3. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivityρ(T) withρ(300K)/ρ(100K) values of about 3.7. Those films show, besides the initial fall-off just belowT c , a further strong decrease ofR s at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. ForT≪Tc/2 these films exhibit an exp (−αT c/T) dependence ofR s withα-values around 0.4. These observations may be explained by a superconducting energy gap with 2Δ/kT c≈0.8 for charge carriers localized in the CuO chains for YBa2Cu3O7.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00618067
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