Unfocused Radiation Research Articles

Interaction between thin conductive fibers and microwave radiation

We study the effect of an anomalously strong interaction of electromagnetic radiation with very thin conducting fibers. Metal, semiconductor or graphite fibers, which diameter is several hundred times smaller than the radiation wavelength, strongly absorb and scatter the electromagnetic waves, which electric vector is parallel to the fiber axis. The efficiency factors of attenuation, absorption, scattering and pressure of radiation for fibers with a diameter of several micrometers in the centimeter and millimeter ranges reach several thousand. We determine the nature of this effect. It occurs when the transverse dimension of the fiber is comparable to the thickness of the skin layer. Then the electric field of the incident wave fills the entire volume of the fiber, and the absorbed power is the highest. We also provide a theoretical analysis of such effect, determine conditions for its existence: the ratio between the radiation wavelength and the diameter of the fiber, the value of conductivity. We also provide an experimental study of the effect, when unfocused radiation beam with a wavelength of 8 mm at the output of waveguide with a \(7.2 \times 3.4\) mm cross section transfers more than 10% of the beam power to a 12 μm diameter graphite fiber. We make an analysis of the mathematical model of the process of heating of graphite fiber by a microwave radiation beam. Then we solve a heat conduction equation and find the temperature distribution along the fiber, which is in agreement with the measured one.

LIGA fabrication of X-ray Nickel lenses

Nickel refractive planar lenses generating line and point X-ray spots were fabricated by LIGA technology. The optimum parameters of the lenses were simulated for a kinoform lens profile and an X-ray energy range from 100 keV to 1 MeV. The focusing elements of the lens are characterised by an aspect ratio of about 200 and a side-wall roughness of less than 5 nm r.m.s. The optical characteristics of the lenses were tested at the ESRF using a photon energy of 212 keV. A focal spot of 10 μm size was measured using a X-ray sensitive film as a width at a half maximum of the darkening of this detector. A ratio of appr. 7 of the detector responses of the focused spot to the unfocused radiation was measured. The correspondent characteristics of the focal spot of the lens for the revealed photon flux are FWHM = 6 μm and ~20 for the gain.

Sagittal focusing of synchrotron radiation diffracted on the walls of a longitudinal hole drilled into a single-crystal monochromator.

A very simple method of sagittal focusing of X-ray synchrotron radiation is presented. A special ray-tracing program which utilizes the diffraction-refraction effect is developed. It is demonstrated both by ray-tracing simulations and by an experiment whereby a reasonably good sagittal concentration of 8 keV synchrotron radiation may be achieved by diffraction on the walls of a cylindrical hole drilled into an Si crystal. The holes were drilled parallel to the (111) planes and their diameter, 1 mm, was chosen so that the focusing distance fits the geometrical arrangement of beamline BM5 at the ESRF. Two such crystals have been used in a dispersive and non-dispersive arrangement. The better result was achieved using the dispersive arrangement. The intensity at the centre of the focus is increased by five times with respect to unfocused radiation. Excellent agreement exists between the ray-tracing simulations and experimental results.

Detection of PO, Cl and P from the photodissociation of POCl 3 at 193 nm

The photodissociation of POCl 3 has been investigated using the unfocused radiation of an excimer operating at 193 nm. PO, P and Cl radicals were probed by resonance-enhanced multiphoton ionization, and signal intensities were studied as a function of excimer laser power. The probe laser was found to produce a non-resonant PO + background signal in addition to the resonant ionization signal of neutral PO. A set of possible dissociation pathways has been postulated.

Construction and performance of a bent crystal x-ray monochromator

We describe an x-ray monochromator which uses a sagittally bent crystal to focus 5.7 mrad of horizontal divergence. Complete construction details are given, enabling the reader to duplicate the device. The rocking curve width and diffraction efficiency differ insignificantly between the bent (focusing) and flat crystal geometries. The unit has been operated successfully at energies up to 18 keV, providing a seventyfold increase in x-ray flux over unfocused radiation.

ChemInform Abstract: INFRARED MULTIPHOTON PHOTOPHYSICS: DECOMPOSITION OF CNF2N+1I (N = 1, 2, 3)

The title compounds have been subjected to unfocused radiation from a high‐power CO2‐TEA laser under collisionless conditions in a mass spectrometric flow system (VLPO/ method). We report absolute photodissociation yields as a function of fluence for CF3I and C3F7I. In the case of CF3I, we find high yields (100%) at fluences ?4 J/cm2, whereas for C3F7I yields level off at 20%–30%, depending on the wavelength. Secondary photolysis of Ċ2F5 and Ċ3F7 at fluences ?2.5 J/cm2 yields CF2 and ĊF3, but no C2F4. Interpretation of data in terms of a log–normal cumulative distribution function of reaction times or fluence suggests that CF3I interacts homogeneously with the laser field at 1070 and 1083 cm−1, but possibly inhomogeneously at 1076 cm−1, whereas C3F7I represents an inhomogeneous interaction case at all wavelengths. Based upon our photodissociation yields and the validity of a master equation representation, we calculate the infrared absorption spectrum of C3F7I as a function of internal energy.

Infrared multiphoton photophysics: Decomposition of CnF2n+1I (n = 1, 2, 3)

The title compounds have been subjected to unfocused radiation from a high-power CO2-TEA laser under collisionless conditions in a mass spectrometric flow system (VLPO/ method). We report absolute photodissociation yields as a function of fluence for CF3I and C3F7I. In the case of CF3I, we find high yields (100%) at fluences ?4 J/cm2, whereas for C3F7I yields level off at 20%–30%, depending on the wavelength. Secondary photolysis of Ċ2F5 and Ċ3F7 at fluences ?2.5 J/cm2 yields CF2 and ĊF3, but no C2F4. Interpretation of data in terms of a log–normal cumulative distribution function of reaction times or fluence suggests that CF3I interacts homogeneously with the laser field at 1070 and 1083 cm−1, but possibly inhomogeneously at 1076 cm−1, whereas C3F7I represents an inhomogeneous interaction case at all wavelengths. Based upon our photodissociation yields and the validity of a master equation representation, we calculate the infrared absorption spectrum of C3F7I as a function of internal energy.

Homodyne detection of infrared radiation from a moving diffuse target

Experiments have been performed in which the radiation from a CO 2 laser was coherently detected after being scattered from a moving diffuse reflector. This is generally the configuration of an infrared laser radar. The power-spectral-density of the heterodyne signal was measured and its width was shown to agree with the calculated value based on a theoretical model for the process. Expressions are obtained for the ratio of heterodyne signal bandwidth to heterodyne frequency for the cases of focused radiation, unfocused radiation, and for a typical radar configuration. In most cases, the heterodyne signal is found to possess a narrow-band character. The probability density of the signal envelope was also measured for a known scatterer (providing Gaussian scattered radiation) and was found to be Rayleigh distributed, as expected. The power-spectral-density and envelope probability distribution provide information about a scattering medium or target which cannot be obtained from average-value measurements of the heterodyne signal-to-noise ratio. This information is useful for communications applications, infrared radar, and heterodyne spectroscopy experiments. Finally, a simple and direct method of obtaining information about the statistics of an incident radiation field (which does not involve photocounting) is discussed.

Performance of spectrally selective collector surfaces in a solar-driven carnot space-power system

A study was made of the theoretical performance of spectrally selective surfaces when used as the collectors of solar energy for driving heat-engine cycles in space. Selective surfaces were considered for which the monochromatic emissivity changed abruptly at specific wavelengths between 0.5 and 10 microns. The Carnot cycle was used as the heat engine, and waste heat was rejected from a black radiator. Systems using both focused and unfocused radiation were considered. It was found that spectrally selective surfaces hold considerable promise as collectors of solar energy in unfocused systems. The efficiency for these is several times greater than that obtainable from black or gray collectors. However, this advantage of spectral selectivity diminishes if the sunlight is focused on the collector. In highly focused systems, such as those using parabolic mirrors, black collectors give substantially the same efficiency as the most ideal spectrally selective surface.

Equilibrium temperatures of ideal spectrally selective surfaces

Abstract Spectrally selective surfaces were assumed for which the monochromatic emissivity changed abruptly at specific wavelengths. These surfaces could be completely characterized by two parameters: the height of the step change in emissivity, and the wavelength of the change. This characterization is more general than the widely used a e ratio. Equilibrium temperatures were calculated for such surfaces when exposed to both focused and unfocused solar radiation at the earth's orbit distance. Varying step heights at wavelengths between 0.3 and 20 microns were assumed. With unfocused radiation, temperatures as low as 40 K were found for surfaces for which the emissivity increased from zero to unity at long wavelengths, and temperatures as high as 2400 K when the change was from unity to zero at short wave-lengths. For focused radiation, much higher temperatures were found. Such surfaces may find application in power-generation cycles or in the storage of cryogenic propellants in space.