Showing papers on "Photon energy published in 1994"

Generalized comptonization models and application to the recent high-energy observations

Abstract: The theory of spectral formation in thermal X-ray sources, where the effects of Comptonization and Klein-Nishina corrections are important, is presented. Analytical expressions are obtained for the produced spectrum as a function of such input parameters as the plasma temperature, the optical depth of the plasma cloud and the injected soft photon spectrum. The analytical theory developed here takes into account the dependence of the scattering opacity on the photon energy. It is shown that the plasma temperature as well as the asymptotic rate of photon escape from the plasma cloud determine the shape of the upscattered hard tail in the emergent spectra, even in the case of very small optical depths. The escape distributions of photons are given for any optical depth of the plasma cloud and their asymptotic dependence for very small and large optical depths are examined. It is shown that this new generalized approach can fit spectra for a large variety of hard X-ray sources and determine the plasma temperature in the region of main energy release in Cyg X-1 and the Seyfert galaxy NGC 4151.

Observation of Photon-Assisted Tunneling through a Quantum Dot

Abstract: We have measured dc transport through a GaAs/AlGaAs quantum dot in the presence of a microwave signal of frequency $f$. We find features related to the photon energy $\mathrm{hf}$ whose positions in gate voltage are independent of the microwave power buy vary linearly with frequency. The measurements demonstrate photon-assisted tunneling in the mesoscopic regime. A comparison is made with a model that extends Coulomb blockade theory to include photon-assisted tunneling.

Two-photon molecular excitation provides intrinsic 3-dimensional resolution for laser-based microscopy and microphotochemistry.

Abstract: With the development of sensitive and specific fluorescent indicators, modern laser scanning microscopies enable visualization and measurement of submicron, dynamic processes inside living cells and tissues. Here we describe the working principles of new, nonlinear laser microscopies based on two-photon molecular excitation. In these techniques, a pulsed laser produces peak photon densities high enough that when focused into an appropriate medium, excitation by photon energy combinations can occur. For example, two red photons interacting simultaneously with a fluorescent molecule can excite within it a UV electronic transition, one corresponding to twice the energy of each single photon. Because the amount of two-photon excitation depends on the square of the local illumination intensity, this process exhibits a unique localization to the diffraction-limited spot of the beam focus. Elsewhere along the beam, excitation of background and photodamage is virtually nonexistent. Focal point localization of two-photon excitation lends to all visualization, measurement, and photopharmacology studies an intrinsic, three-dimensional resolution. We describe some preliminary biological applications, specifically, imaging of vital DNA stains in developing cells and embryos, imaging of cellular metabolic activity from NADH autofluorescence, spatially resolved measurements of cytoplasmic calcium ion activity, and optically induced micropharmacology using caged bioeffector molecules.

Optical functions of silicon at elevated temperatures

Abstract: The optical functions of silicon have been measured accurately at elevated temperatures using the two‐channel spectroscopic polarization modulation ellipsometer. The wavelength region covered is 240–840 nm (5.16–1.47 eV), and the temperature region covered is room temperature to 490 °C. Using this data, the refractive index n and the extinction coefficient k are both parameterized as functions of temperature T and photon energy E for photon energies below the direct band edge of silicon (∼3.36 eV or 370 nm). In this range, n(E,T) can be fit with five parameters, and k(E,T) can be fit with six parameters.

Photon-assisted tunneling through a quantum dot

Abstract: We study single-electron tunneling in a two-junction device in the presence of microwave radiation. We introduce a model for numerical simulations that extends the Tien-Gordon theory for photon-assisted tunneling to encompass correlated single-electron tunneling. We predict sharp current jumps which reflect the discrete photon energy hf, and a zero-bias current whose sign changes when an electron is added to the central island of the device. Measurements on split-gate quantum dots show microwave-induced features that are in good agreement with the model.

Inner-shell excitation and site specific fragmentation of poly(methylmethacrylate) thin film

Abstract: Soft x‐ray excitations in the 250–600 eV photon energy range on poly(methylmethacrylate) (PMMA) result in ionic fragmentation of the original polymer with the most intense ions corresponding to CH+3, H+, CH+2, CH+, CHO+, and COOCH+3. The photon energy dependence of ion desorption from thin films of PMMA was measured to investigate the primary steps in radiation induced decomposition following carbon and oxygen 1s electron excitations using monochromatic pulsed‐synchrotron radiation. It was clearly found that the decomposition depends on the nature of the electronic states created in the excited species. The fragmentation pattern changes depending on the transitions of the 1s electron to a Rydberg orbital, an unoccupied molecular orbital or the ionization continuum. Moreover, the fragmentation occurs specifically around the site of the atom where the optical excitation takes place. Excitations from carbon and oxygen 1s to σ* states seem to be specially efficient for ion production as observed in the case o...

Implementation of the Doppler broadening of a Compton-scattered photon into the EGS4 code

Abstract: A modification to the general-purpose Monte Carlo electron-photon transport code EGS4 [1] was made in order to include Doppler broadening of Compton-scattered photon energy due to electron pre-collision motion. The Compton-scattered photon energy is sampled from a cross section formula based on the Compton profile, and the Compton scattering is sustained if the energy imparted to the electron is less than its binding energy. The electron binding effect modifies the scattered photon energy, angular distribution, and total cross section of the Compton scattering, and affects the photon mean free path used in the calculations. In the improved EGS4 code, all of these electron binding effects in Compton scattering are treated consistently. A simulation of 40 keV photon scattering by C and Cu samples was performed using the improved EGS4 code; the calculated scattered photon spectra agreed well with the measurements.

Highly sensitive nuclear spectrometer apparatus and method

Abstract: A nuclear spectrometer optimizes energy resolution and throughput with asymmetrical leading and trailing weighting functions. Photon detector current is amplified, converted to a voltage step, and fed to a fast analog-to-digital converter (ADC) for pile-up rejection using another pair of leading and trailing weighting functions with sharp rising and falling edges respectively, and to a slow ADC whose output is examined for low-energy pile-up, slope corrected, and buffered for photon energy measurement. Digital triangular shaping locates the step. The triangle response is tested for asymmetry to reject very low energy pile-up missed by the fast ADC, and the signal noise near the step is tested against treshold. Valid steps are asymmetrically weighted by equal-area cusp functions where the leading function peaks at the step gap and the trailing function peaks slightly after the step gap.

VUV photoionization of sputtered neutral silver clusters

Abstract: We report on a study of neutral Ag atoms and Agn clusters sputtered from a polycrystalline silver sample by keV rare gas ion bombardment. The ejected neutral species were photoionized by a pulsed laser and detected by time-of-flight mass spectrometry. In contrast to previous studies of sputtered neutral silver clusters, the ionization was performed by a VUV laser (λ = 157 nm) which, due to its high photon energy of 7.9 eV, permits nonresonant single photon ionization (SPI) of all investigated species. It is shown that the corresponding SPI ionization cross sections do not vary dramatically between silver atoms and the different clusters. As a consequence, fragmentation influences encountered in previous studies with longer wavelength lasers are practically eliminated from the determination of yields and kinetic energy distributions of the sputtered clusters. The resulting relative cluster sputtering yields (normalized to the yield of silver atoms) exhibit a power law dependence on the cluster size n according to n−δ with exponents δ ranging from 4.3 to 7.4 depending on the nature and the bombarding energy of the primary ions. The kinetic energy distributions of the sputtered neutral atoms and clusters are evaluated up to clusters containing seven atoms. It is found that the asymptotic decay of the energy distribution towards high emission energies becomes steeper from Ag to Ag3 and remains practically constant for larger clusters.

A comparison of photoelectron spectroscopy and two-photon ionization spectroscopy : excited states of Au2, Au3, and Au4

Abstract: Photoelectron spectra of Au−n with n=2–4 are reported. Due to the relatively high photon energy used in our experiment (hν=6.424 eV) and the energy resolution of about 50 meV, various transitions into excited states of the neutral clusters are resolved. It is demonstrated that photoelectron spectra can serve as a map of the electronic states of a cluster, while the high resolution of the resonant two‐photon ionization (R2PI) method gains information about the symmetry of the states. The comparison with similar data of Ag−n clusters indicates the influence of relativistic effects and the large spin–orbit splitting for Au.

Low-energy electron transport in alkali halides

Abstract: A model of electron transport in alkali halides, below 10 eV, is described. It is based on theoretically calculated microscopic cross sections of electron interactions with lattice phonons. Both acoustic and optical scatterings are taken into account, the former being also treated as a quasielastic process that randomizes the electron motion. Monte Carlo calculations based on the model simulate the UV‐induced photoelectron emission from CsI. The calculated quantum efficiency and energy spectra are in good agreement with experimental data, in the photon energy range of 6.3–8.6 eV. The probability for an electron to escape from CsI, NaCl, and KCl is provided as a function of its energy and creation depth. A comparison is made between our approach and other phenomenological models.

Laser-assisted elastic electron-atom collisions: low electron energy and small scattering angle

Abstract: The authors report experimental measurements of electron spectra resulting from the scattering of low energy (6.2-32 eV) electrons by helium atoms through an angle of 9 degrees in the presence of a high intensity ( approximately 108 W cm-2)CO2 laser. The intensities of the additional peaks caused by the presence of the laser which occur separated from the elastic scattering peak by multiples of the photon energy are much greater than expected on the basis of calculations, using the Kroll-Watson approximation (1973). The intensity of these free-free transitions also increases with decreasing electron energy, again in disagreement with the calculations. The authors propose that the disagreement between the experimental results and those calculated is due to the electrons scattering from a helium target polarized by the laser field.

Characterization of a windowless Si(Li) detector in the photon energy range 0.1-5 keV

Abstract: Synchrotron radiation from the electron storage ring BESSY has been used to determine the absolute detection efficiency (DE) and the response function of a windowless Si(Li) detector in the 0.1–5 keV photon energy range. The determination of the DE above 0.4 keV was based on an energy dispersive measurement of the undispersed calculable radiation emitted from the storage ring, which is a primary radiation standard. A new method based on the measurement of the detector photocurrent for a known monochromatic spectral photon flux was used to measure the DE down to 60 eV. We have introduced a model for the absolute DE including all parts of the response function of a Si(Li) detector which expresses the DE only as an absorption effect in the gold contact and contamination layers. The agreement between the measured efficiency and the model calculation is better than 2%. A model that allows lineshapes from 0.1 keV upwards to be simulated and which agrees well with our measurements in the range up to 5 keV is presented.

Instrument for research on interfaces and surfaces

Abstract: We describe an instrument designed for studying the electronic structure of bulk, surface, and deep solid–solid interface. The analysis is made by soft‐x‐ray emission spectroscopy induced by electron bombardment. The target is placed under ultrahigh vacuum and can be prepared and treated in situ. High resolution is achieved both as concerns the photon energy and the electron‐beam energy. Tests have been made in the dispersive mode and in the characteristic isochromat mode. In both cases experimental resolution is in good agreement with the expected one.

Linearly polarized photons at MAMI (Mainz)

Abstract: Coherent bremsstrahlung is produced using the 855 MeV electron beam of the Mainz three-stage racetrack microtron MAMI and a diamond crystal inserted as a radiator into the MAMI tagging system. By counting only the electrons on the tagger ladder an intensity spectrum ranging from 50 MeV to 800 MeV was obtained with good statistical accuracy in a running time of 2 s. This short running time makes it possible to measure a complete intensity pattern versus photon energy for different orientations of the diamond crystal in about 15 minutes. By comparing these measured intensity patterns with calculations it becomes possible to precisely align the crystal axes with respect to the axes of the goniometer. Relative intensities of coherent bremsstrahlung are determined experimentally by normalizing the diamond data to data obtained with an amorphous Ni radiator. Good agreement is obtained between experiment and prediction when taking into account the beam divergence, multiple scattering of electrons in the radiator, the Z dependence of the bremsstrahlung spectrum and the finite Debye temperature of diamond. With a collimated photon beam having an aperture of half the characteristic angle degrees of linear polarization appear to be possible ranging from 80% at 200 MeV over 65% at 300 MeV and 55% at 400 MeV to 35% at 500 MeV.

High resolution threshold photoelectron and photoion spectroscopy of oxygen in the 12-50 eV photon range

Abstract: High resolution threshold photoelectron and photoion spectra of the oxygen molecule have been measured in the photon energy range 12-50 eV using a penetrating field electrostatic analyser and a synchrotron radiation source. Besides the well known X 2 Pi g, a 4 Pi u, A 2 Pi u, b 4 Sigma g-, B 2 Sigma g-, c 4 Sigma u- O2+ states, several other series of vibrational structure are observed in the 21-24 eV range. One series is interpreted to result, not from an ion state, but from an autoionization process involving a neutral state and the nuclear continuum of an ion state. Above 26 eV, three relatively strong features are observed at approximately 27.3 eV, 32.5 eV and 39.5 eV, interspersed with several weaker ones. Dissociation processes from some O2+ states have been studied using a threshold photoelectron-photoion coincidence technique (TPEPICO). These observations identify the weak, broad feature at 24.96 eV as the nu =2 level of the c 4 Sigma u- state.

DNA Damage Induced by Vacuum and Soft X-ray Photons from Synchrotron Radiation

Abstract: The recent development of irradiation systems using synchrotron radiation (SR) as a source is enabling researchers to obtain intense monochromatic photons having a narrow bandwidth in the vacuum-UV (VUV) and soft X-ray regions. We can thus systematically study the photon energy dependence of DNA damage formation in these energy regions. The photon energy dependence provides useful information about how energy-absorbing modes--excitations, so-called superexcitations, outer- and inner-shell ionizations--affect the type and amount of DNA damage. Furthermore, low energy electrons produced by low energy photons through photoelectric interactions are useful for studying how the electron energy affects the induction of DNA damage. A report is given on the present status of the SR irradiation systems in Japan as well as some results concerning the formation of DNA damage, in vitro and in vivo, by monochromatic photons in the VUV and soft X-ray regions.

EPR Dosimetry of Cortical Bone and Tooth Enamel Irradiated with X and Gamma Rays: Study of Energy Dependence

Abstract: Previous investigators have reported that the radiation-induced EPR signal intensity in compact or cortical bone increases up to a factor of two with decreasing photon energy for a given absorbed dose. If the EPR signal intensity was dependent on energy, it could limit the application of EPR spectrometry and the additive reirradiation method to obtain dose estimates. We have recently shown that errors in the assumptions governing conversion of measured exposure to absorbed dose can lead to similar "apparent" energy-dependence results. We hypothesized that these previous results were due to errors in the estimated dose in bone, rather than the effects of energy dependence per se. To test this hypothesis we studied human adult cortical bone from male and female donors ranging in age from 23 to 95 years, and bovine tooth enamel, using 34 and 138 keV average energy X-ray beams and 137Cs (662 keV) and 60Co (1250 keV) gamma rays. In a femur from a 47-year-old male (subject 1), there was a difference of borderline significance at the alpha = 0.05 level in the mean radiation-induced hydroxyapatite signal intensities as a function of photon energy. No other statistically significant differences in EPR signal intensity as a function of photon energy were observed in this subject, or in the tibia from a 23-year-old male (subject 2) and the femur from a 75-year-old female (subject 3). However, there was a trend toward a decrease (12-15%) in signal intensity at the lowest energy compared with the highest energy in subjects 1 and 3. Further analysis of the data from subject 1 revealed that this trend, which is in the opposite direction of previous reports but is consistent with theory, is statistically significant. There were no effects of energy dependence in the tooth samples.

A study of fec2 and fec2h by anion photoelectron spectroscopy

Abstract: delivered by two pulsed molecular beam valves (R. M. Jordan Co.) and is synchronized with the vaporization laser pulse. The plasma reactions of CHq and Fe produce a variety of cluster species which, together with the He carrier gas, undergo a supersonic expansion and are skimmed twice to form a collimated beam. FeC2- and FeC2H- are the smallest Fecontaining anions, and the FeC2- signal is quite weak. FeC2H2is also present, but its abundance is rather weak and unstable. About 70 cm downstream from the cluster nozzle, the negative ion species are extracted by a 1 kV high-voltage pulse into a 130 cm long flight tube for mass analyses. The TOF mass spectrometer has a large extraction volume and a modified Wiley-McLaren extraction stack with an added free-flight region between the two acceleration stages? Theoretically, a very high mass resolution can be achieved in this setup with the large extraction volume. We have obtained a mass resolution (MI AM) of more than 300, sufficient to resolve all the isotopic species pertinent to the present study. A three-grid mass gate is used to select only the desired cluster, 56FeC2- and 56FeC2H-, to enter the PES interaction zone. The 56FeC2H- ions can be mass-gated cleanly while the 56FeC2signal may be contaminated by a negligible amount of %FeC2H2-. The mass-selected cluster packet is decelerated by a new momentum deceleration procedure7 down to a very low velocity before photodetachment. This minimizes the Doppler broadening on the photoelectron kinetic energy distribution, a crucial step in achieving high-energy resolution with the magnetic bottle-type analyzer. The third harmonic of a second Qswitched Nd:YAG laser is used in the current study for the photodetachment. There is a cutoff for the transmission of lowenergy electrons at about 0.4 eV. The spectrometer is calibrated with the known spectrum of Cu-. The energy resolution (AE) in a TOF spectrometer is not constant with respect to the electron kinetic energy (Ek). AE is proportional to Ek3/2. An energy resolution of 30 meV at 1 eV electron kinetic energy is achieved in the current spectrometer with the new deceleration procedure, as measured from the photoelectron spectrum of Cu- at 3.49 eV photon energy. This resolution is no longer limited by the Doppler broadening. When the ion signal is weak, full deceleration cannot be achieved to reach this resolution.

Phase-retardation and full-polarization analysis of soft-x-ray synchrotron radiation close to the carbon K edge by use of a multilayer transmission filter.

Abstract: We have produced a multilayer transmission filter with 100 periods of Cr/C to achieve a significant phase retardation while maintaining good transmission for photon energies just below the carbon K edge. This device was installed into a polarimeter behind the SX700/3 monochromator at the Berlin synchrotron radiation laboratory, BESSY. The phase-retardation properties were observed as theoretically predicted. Agreement between experiment and calculation could be obtained by introduction of a rather small interface roughness in the simulation code (σ = 0.65 nm rms). An observed phase retardation of 5° was sufficient to permit, for the first time we believe, a complete and unambiguous polarization analysis of soft-x-ray synchrotron radiation (265-eV photon energy) with primary standards.

Nonresonant two‐photon pulsed field ionization of CH3S formed in photodissociation of CH3SH and CH3SSCH3

Abstract: Threshold photoelectron (PE) spectra for CH3S formed in the photodissociation of CH3SH and CH3SSCH3 in the photon energy range of 36 850–38 150 cm−1 have been measured using the nonresonant two‐photon pulsed field ionization (N2P‐PFI) technique. Both spin–orbit states CH3S(X 2E3/2) and CH3S(2E1/2) are observed from CH3SH and CH3SSCH3 in this photodissociation energy range. However, negligible intensities of vibrationally excited CH3S radicals are produced from CH3SH. In the case of CH3S from CH3SSCH3, the population ratio CH3S(ν3=1)/CH3S(ν3=0) is estimated to be ≊0.18. The simulation of the N2P‐PFI‐PE spectra reveals that the rotational temperature for CH3S(X 2E3/2,1/2) formed by photodissociation of CH3SH is ≊200–250 K and the branching ratio CH3S(2E1/2)/CH3S(X 2E3/2) is 0.5±0.1. For CH3S(X 2E3/2,1/2) produced from CH3SSCH3, the rotational temperature for CH3S(X 2E3/2,1/2) is ≊800–900 K and the branching ratio CH3S(2E1/2)/CH3S(X 2E3/2) is 1.1±0.2. This experiment demonstrates that the PFI‐PE spectr...

A comparative study of dosimetric properties of Plastic Water and Solid Water in brachytherapy applications

Abstract: In this study the dosimetricproperties of Plastic WaterT M and Solid WaterT M phantom materials are evaluated using Monte Carlophotontransport simulations In particular, their water‐equivalence with respect to absorption and attenuation of photons in the brachytherapy energy range are examined For the given chemical compositions of the materials, the linear attenuation coefficients were calculated for photons of 1 keV–2 MeV Moreover, absorbed doses to water in each phantom material were calculated at distances of 05–120 cm from point sources of 20 keV to 60Co gamma rays These results show that at low photon energies (below 100 keV), there are substantial differences (up to a factor of 5) between the absorbed dose in Plastic Water and that in liquid water The differences decrease as photon energy increases, and they become insignificant at 60Co gamma rays, as claimed by the manufacturer In contrast, calculations show that the difference in absorbed dose in Solid Water from that in liquid water, over the entire range of photon energies employed in this study, is less than 25% The results of this study demonstrate the necessity of careful dosimetric evaluation of a new phantom material, before its clinical application, particularly in energy ranges outside those referred to by the manufacturer

Resonant Auger-decay process in solid SiO2 at the Si 1s edge.

Abstract: The Auger-decay processes in solid ${\mathrm{SiO}}_{2}$ after resonant and off-resonant excitations around the Si 1s edge have been investigated using synchrotron radiation. It was found that the Si ${\mathit{KL}}_{2,3}$${\mathit{L}}_{2,3}$ Auger line splits into two peaks when the photon energy is close to that of the Si 1s\ensuremath{\rightarrow}3p resonance. These two peaks are ascribed to the normal and resonant Auger-decay processes. It was also observed that the resonant Auger peak is shifted to higher energy in proportion to the photon energy. The resonant Auger peak is attributed to the spectator Auger decay following the Si 1s\ensuremath{\rightarrow}3p excitation. The energy shift of the resonant Auger electron is interpreted in terms of the energy transfer from the excited electron in the conduction band to the Auger electron in the course of the Auger-decay process.

Time‐Resolved Luminescence of Exciton‐Polaritons in Solid Xenon

Abstract: The decay of excitons in solid Xe is investigated with time and spectrally resolved luminescence as a function of temperature and photon energy of excitation. The luminescence of free excitons exceeds the self-trapped exciton luminescence by a factor of 50 (peak-to-peak). The FE-decay curves yield a non-exponential behaviour. They can be reproduced with Monte-Carlo simulations of the exciton-polariton transport to the sample/vacuum interface, taking into account phonon scattering and trapping. A new upper limit for the intrinsic self-trapping rate in the lower 107 s−1 range and a surprisingly large experimental value for the scattering length ( ≈ 100 nm at T = 5 K) are deduced. It is indispensable to discuss the decay of excitons in solid Xe in terms of the exciton-polariton model.

Optical Constants of 6H?SiC Single Crystals

Abstract: The complex dielectric functions of 6H–SiC for light polarized perpendicular and parallel to the optic axis c were measured using spectroscopic ellipsometry. The measurements were made on the (0001) and (010) surfaces of 6H–SiC in the photon energy range between 1.2 and 5.4 eV at room temperature. The measured dielectric-function spectra were fitted with a classical Lorentz-type oscillator model. The indices of refraction in the transparent region (E < 4.5 eV) were also analyzed using the first-order Sellmeier equation. From this analysis, we obtained the high-frequency dielectric constants e∞ of 6H–SiC as 6.59 for E ⊥c and 6.67 for E //c. Using the Lyddane-Sachs-Teller relation, we also obtained the static dielectric constants e s as 9.76 for E ⊥c and 9.98 for E //c.