Showing papers on "Photon energy published in 2004"

Full Valence Band Photoemission from Liquid Water Using EUV Synchrotron Radiation

Abstract: The valence band photoelectron spectra of liquid water (H2O and D2O) are studied in the photon energy range from hν = 60 to 120 eV. The experiments use a 6 μm diameter liquid-jet free vacuum surface at the MBI undulator beamline of the synchrotron radiation facility BESSY. Photoelectron emission from all four valence molecular orbitals (MOs) is observed. In comparison to those of the gas phase, the peaks are significantly broadened and shifted to lower binding energies by about 1.5 eV. This is attributed primarily to the electronic polarization of the solvent molecules around an ionized water molecule. Energy shifts, peak broadening, and relative peak intensities for the four MOs differ because of their specific participation in the hydrogen bonding in liquid water. Relative photoionization cross sections for MOs were measured for hν = 60, 80, and 100 eV. The main difference for liquid water, as compared to the gas phase, is the relative intensity decrease of the 1b2 and 3a1 orbitals, reflecting changes o...

Combined long and short time-scale X-ray variability of NGC 4051 with RXTE and XMM-Newton

Abstract: We present a comprehensive examination of the X-ray variability of the narrow-line Seyfert 1 (NLS 1) galaxy NGC 405 1, one of the most variable active galactic nuclei (AGN) in the sky. We combine over 6.5 years of frequent monitoring observations by RXTE with a > 100-ks continuous observation by XMM-Newton and so present an overall 2-10 keV power spectral density (PSD) covering an unprecedented frequency range of over 6.5 decades from 10(-2) Hz. The combined RXTE and XMM-Newton PSD is a very good match to the PSD of the galactic black-hole binary system (GBH) Cygnus X-1 when in a 'high', rather than 'low', state, providing the first definite confirmation of an AGN in a 'high' state. We also find that a bending power law, rather than a sharply broken power law, besides being more physical, is a much better description of the high-state PSD of Cygnus X-1 and is also a better description of the PSD of NGC 4051.At low frequencies the PSD of NGC 4051 has a slope of -1.1 bending, at a frequency nu(B) = 8(-3)(+4) x 10(-4) Hz, to a slope Of alpha(H) similar to -2. Although nu(B) does not depend on photon energy, alpha(H) is steeper at lower energies. If nu(B) scales with mass, we imply a black-hole mass of 3(-1)(+2), x 10(5) Mcircle dot in NGC 4051, which is consistent with the recently reported reverberation value of 5(-3)(+6) x 10(5) MCcircle dot. Hence NGC 4051 is emitting at similar to30 per cent L-Edd.NGC 4051 follows the same rms-flux relationship as GBHs, consistent with higher Fourier frequencies being associated with smaller radii.From the cross-power spectra and cross-correlation functions between XMM-Newton light curves in different energy bands, we note that the higher-energy photons lag the lower-energy ones. We also note that the lag is greater for variations of longer Fourier period and increases with the energy separation of the bands. Variations in different wavebands are very coherent at long Fourier periods but the coherence decreases at shorter periods and as the energy separation between bands increases. This behaviour is again similar to that of GBHs, and of MCG-6-30-15, and suggests a radial distribution of frequencies and photon energies with higher energies and higher frequencies being associated with smaller radii.Combining our observations with observations from the literature we find it is not possible to fit all AGN to the same linear scaling of break time-scale with black-hole mass. However, broad-line AGN are consistent with a linear scaling of break time-scale with mass from Cygnus X-1 in its low state and NLS1 galaxies scale better with Cygnus X-1 in its high state. We suggest that the relationship between black-hole mass and break time-scale is a function of at least one other underlying parameter which may be accretion rate or black-hole spin or both.

Photoluminescence in quantum-confined SnO2 nanocrystals: Evidence of free exciton decay

Abstract: Nanocrystalline SnO2 quantum dots were synthesized at room temperature by hydrolysis reaction of SnCl2. The addition of tetrabutyl ammonium hydroxide and the use of hydrothermal treatment enabled one to obtain tin dioxide colloidal suspensions with mean particle radii ranging from 1.5 to 4.3 nm. The photoluminescent properties of the suspensions were studied. The particle size distribution was estimated by transmission electron microscopy. Assuming that the maximum intensity photon energy of the photoluminescence spectra is related to the band gap energy of the system, the size dependence of the band gap energies of the quantum-confined SnO2 particles was studied. This dependence was observed to agree very well with the weak confinement regime predicted by the effective mass model. This might be an indication that photoluminescence occurs as a result of a free exciton decay process.

Measurement of gamma p ---> K+ Lambda and gamma p ---> K+ Sigma0 at photon energies up to 2.6-GeV

Abstract: The reactions $\gamma p \rightarrow K^ + \Lambda$ and $\gamma p \rightarrow K^ + \Sigma^0$ were measured in the energy range from threshold up to a photon energy of 26 GeV The data were taken with the SAPHIR detector at the electron stretcher facility, ELSA Results on cross-sections and hyperon polarizations are presented as a function of the kaon production angle and the photon energy The total cross-section for $\Lambda$ production rises steeply with energy close to threshold, whereas the $\Sigma^0$ cross-section rises slowly to a maximum at about $E_\gamma = 145$ GeV Cross-sections together with their angular decompositions into Legendre polynomials suggest contributions from resonance production for both reactions In general, the induced polarization of $\Lambda$ has negative values in the kaon forward direction and positive values in the backward direction The magnitude varies with energy The polarization of $\Sigma^0$ follows a similar angular and energy dependence as that of $\Lambda$ , but with opposite sign

Electrodeless time-resolved microwave conductivity study of charge-carrier photogeneration in regioregular poly(3-hexylthiophene) thin films

Abstract: The electrodeless flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study the photogeneration of charge carriers in spin-coated films of regioregular poly(3hexylthiophene) sP3HTd, over the photon energy range from 1.9 to 5.2 eV for incident light intensities from 10 13 to 10 16 photons/ cm 2 per s 3n sd pulse. The initial, single-photon quantum yield of photoionization, f, has been estimated from the low-intensity limit to the photoconductivity based on a charge carrier mobility of 0.014 cm 2 /V s(determined in separate pulse-radiolysis TRMC experiments on bulk P3HT). The value of f is constant at s1.7± 0.4 d% within the range 1.9‐ 3.0 eV, which encompasses the first electronic absorption band of P3HT. Above 3.0 eV, f increases, up to a value of s 7±2 d% at 5.2 eV. The activation energy of the photoconductivity was found to be approximately 50 meV at all photon energies. The high-intensity, sublinear dependence of the photoconductivity can be described by the occurrence of either exciton-exciton annihilation or diffusional charge recombination with rate coefficients of 2.33 10 ˛8 cm 3 / s and 1.1 3 10 ˛8 cm 3 /s.

Curvature Effects in Gamma-Ray Burst Colliding Shells

Abstract: An elementary kinematic model for emission produced by relativistic spherical colliding shells is studied. The case of a uniform blast-wave shell with jet opening angle θj 1/Γ is considered, where Γ is the Lorentz factor of the emitting shell. The shell, with comoving width Δr', is assumed to be illuminated for a comoving time Δt' and to radiate a broken-power-law νLν spectrum peaking at comoving photon energy . Synthetic gamma-ray burst (GRB) pulses are calculated, and the relation between energy flux and internal comoving energy density is quantified. Curvature effects dictate that the measured νFν flux at the measured peak photon energy pk be proportional to in the declining phase of a GRB pulse. Possible reasons for discrepancies with observations are discussed, including adiabatic and radiative cooling processes that extend the decay timescale, a nonuniform jet, and the formation of pulses by external shock processes. A prediction of a correlation between prompt emission properties and times of the optical afterglow beaming breaks is made for a cooling model, which can be tested with Swift.

Prompt Gamma-Ray Burst Spectra: Detailed Calculations and the Effect of Pair Production

Abstract: We present detailed calculations of the prompt spectrum of γ-ray bursts (GRBs) predicted within the fireball model framework, in which emission is due to internal shocks in an expanding relativistic wind. Our time-dependent numerical model describes cyclo-synchrotron emission and absorption, inverse and direct Compton scattering, and e± pair production and annihilation (including the evolution of high-energy electromagnetic cascades). It allows, in particular, a self-consistent calculation of the energy distribution of e± pairs produced by photon annihilation and hence, a calculation of the spectra resulting when the scattering optical depth due to pairs, τ±, is high. We show that emission peaks at ~1 MeV for moderate-to-large τ±, reaching τ± ~ 102. In this regime of large compactness we find that (1) a large fraction of shock energy can escape as radiation even for large τ±; (2) the spectrum depends only weakly on the magnetic field energy fraction; (3) the spectrum is hard, e2 dN/de ∝ eα with 0.5 < α < 1, between the self-absorption (essa = 100.5±0.5 keV) and peak (epeak = 100.5±0.5 MeV) photon energy; (4) the spectrum shows a sharp cutoff at ~10 MeV; and (5) thermal Comptonization leads to emission peaking at epeak 30 MeV and cannot, therefore, account for observed GRB spectra. For small compactness, spectra extend to higher than 10 GeV with flux detectable by GLAST, and the spectrum at low energy depends on the magnetic field energy fraction. Comparison of the flux at ~1 GeV and ~100 keV may therefore allow the determination of the magnetic field strength. For both small and large compactness, the spectra depend only weakly on the spectral index of the energy distribution of accelerated electrons.

Non-contact pumping of light emitters via non-radiative energy transfer

Abstract: A light emitting device is disclosed including a primary light source having a defined emission photon energy output, and, a light emitting material situated near to said primary light source, said light emitting material having an absorption onset equal to or less in photon energy than the emission photon energy output of the primary light source whereby non-radiative energy transfer from said primary light source to said light emitting material can occur yielding light emission from said light emitting material.

Photoion mass spectrometry of five amino acids in the 6-22 eV photon energy range

Abstract: A photoionization mass spectrometry study in the 6–22 eV photon energy region of five amino acids, glycine-h5 and its -d5 isotopologue, a-alanine, b-alanine, a-aminoisobutyric acid and a-valine, revealed VUV-induced degradation pathways of these important biological molecules. The fragmentation patterns, ionization energies and ion appearance energies are reported, many for the first time, and are compared with results of electron impact and other studies. Assignment of ion peaks and determination of fragment ion formation channels were assisted by mass spectral data on deuterated isotopologues of the three proteinaceous amino acids studied. Thermochemical data, coupled with the observed ion appearance energies, was also useful in clarifying dissociative photoionization pathways. Ion pair formation appears to occur in certain low energy dissociation processes. Isomeric interconversion between a-alanine and b-alanine cations does not occur up to 20 eV excitation energy. Some astrophysical implications concerning the prospects for amino acid observation and survival in the interstellar medium and in meteorites are briefly discussed. 2003 Elsevier B.V. All rights reserved.

Photon mass attenuation coefficients, effective atomic numbers and electron densities of some thermoluminescent dosimetric compounds

Abstract: Photon mass attenuation coefficients of some thermoluminescent dosimetric (TLD) compounds, such as LiF, CaCO3, CaSO4, CaSO4.2H2O, SrSO4, CdSO4, BaSO4, C4H6BaO4 and 3CdSO4.8H2O were determined at 279.2, 320.07, 514.0, 661.6, 1115.5, 1173.2 and 1332.5 keV in a well-collimated narrow beam good geometry set-up using a high resolution, hyper pure germanium detector. The attenuation coefficient data were then used to compute the effective atomic number and the electron density of TLD compounds. The interpolation of total attenuation cross-sections of photons of energyE in elements of atomic numberZ was performed using the logarithmic regression analysis of the data measured by the authors and reported earlier. The best-fit coefficients so obtained in the photon energy range of 279.2 to 320.07 keV, 514.0 to 661.6 keV and 1115.5 to 1332.5 keV by a piece-wise interpolation method were then used to find the effective atomic number and electron density of the compounds. These values are found to be in agreement with other available published values.

Probability of the resistive state formation caused by absorption of a single-photon in current-carrying superconducting nano-strips

Abstract: We have studied supercurrent-assisted formation of the resistive state in nano-structured Nb and NbN superconducting films after absorption of a single photon. In amorphous narrow NbN strips the probability of the resistive state formation has a pronounced spectral cut-off. The corresponding threshold photon energy decreases with the bias current. Analysis of the experimental data in the framework of the generalized hot-spot model suggests that the quantum yield for near-infrared photons increases faster than the photon nergy. Relaxation of the resistive state depends on the photon energy making the phenomenon feasible for the development of energy resolving single-photon detectors.

Polymer gel water equivalence and relative energy response with emphasis on low photon energy dosimetry in brachytherapy.

Abstract: The water equivalence and stable relative energy response of polymer gel dosimeters are usually taken for granted in the relatively high x-ray energy range of external beam radiotherapy based on qualitative indices such as mass and electron density and effective atomic number. However, these favourable dosimetric characteristics are questionable in the energy range of interest to brachytherapy especially in the case of lower energy photon sources such as 103Pd and 125I that are currently utilized. In this work, six representative polymer gel formulations as well as the most commonly used experimental set-up of a LiF TLD detector–solid water phantom are discussed on the basis of mass attenuation and energy absorption coefficients calculated in the energy range of 10 keV–10 MeV with regard to their water equivalence as a phantom and detector material. The discussion is also supported by Monte Carlo simulation results. It is found that water equivalence of polymer gel dosimeters is sustained for photon energies down to about 60 keV and no corrections are needed for polymer gel dosimetry of 169Yb or 192Ir sources. For 125I and 103Pd sources, however, a correction that is source-distance dependent is required. Appropriate Monte Carlo results show that at the dosimetric reference distance of 1 cm from a source, these corrections are of the order of 3% for 125I and 2% for 103Pd. These have to be compared with corresponding corrections of up to 35% for 125I and 103Pd and up to 15% even for the 169Yb energies for the experimental set-up of the LiF TLD detector–solid water phantom.

Optical properties of thermally evaporated tin-phthalocyanine dichloride thin films, SnPcCl2

Abstract: The optical properties of tin-phthalocyanine dichloride thin films have been studied. The films used in the characterisation studies were thermally evaporated. The spectral and optical parameters have been investigated using spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2100 nm. The absorption spectra recorded in UV–VIS region for the as-deposited and annealed samples showed two absorption bands, namely the Q- and Soret band. No remarkable effect was observed after annealing. A structure with energy separation of magnitude 0.2 eV is seen on the Q- and Soret bands. A transition involving d-electrons of the central metal atom was indicated in the high photon energy region. The dispersion curve of the refractive index showed an anomalous dispersion in the absorption region and a normal one in the transparent region. The band-model theory was applied to determine the optical parameters. The fundamental and the onset of the indirect energy gaps were determined to be 2.79 and 1.51 eV respectively.

PLEIADES: A picosecond Compton scattering x-ray source for advanced backlighting and time-resolved material studies

Abstract: The PLEIADES (Picosecond Laser-Electron Inter-Action for the Dynamical Evaluation of Structures) facility has produced first light at 70 keV. This milestone offers a new opportunity to develop laser-driven, compact, tunable x-ray sources for critical applications such as diagnostics for the National Ignition Facility and time-resolved material studies. The electron beam was focused to 50 μm rms, at 57 MeV, with 260 pC of charge, a relative energy spread of 0.2%, and a normalized emittance of 5 mm mrad horizontally and 13 mm mrad vertically. The scattered 820 nm laser pulse had an energy of 180 mJ and a duration of 54 fs. Initial x rays were captured with a cooled charge-coupled device using a cesium iodide scintillator; the peak photon energy was approximately 78 keV, with a total x-ray flux of 1.3×106 photons/shot, and the observed angular distribution found to agree very well with three-dimensional codes. Simple K-edge radiography of a tantalum foil showed good agreement with the theoretical divergence-...

Intense narrow-bandwidth extreme ultraviolet laser system tunable up to 20 eV

Abstract: Narrow-bandwidth, broadly tunable vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) radiation has been generated up to 20 eV by resonance-enhanced four-wave mixing in argon using the five two-photon resonances of argon between 105 000 and 109 000 cm−1 corresponding to the optically accessible fine-structure components of the 3p54p electron configuration. These two-photon resonances were reached by using the output of an F2 excimer laser (157 nm) and the tripled output of a dye laser. The highest VUV/XUV intensities were obtained using the 3p54p′[1/2]0←3p6(1S0) two-photon resonance at 108 722.62 cm−1 in combination with the main excimer line. The conversion efficiency reaches an optimum for photon energies around 16 eV and slowly decreases when the photon energy rises to 20 eV. The use of the argon resonances also facilitates the generation of intense VUV laser radiation around 90 000 cm−1, a region that is not easily accessible by four-wave mixing with the commonly used two-photon resonances of krypt...

Electronic photodissociation spectroscopy of Aun−⋅Xe (n=7–11) versus time-dependent density functional theory prediction

Abstract: Electronic (one-photon) photodepletion spectra were recorded for gold cluster anions complexed with one xenon atom over the photon energy range 2.1–3.4 eV. Clusters were generated by pulsed laser vaporization and probed under collisionless molecular beam conditions. The spectra obtained are highly structured with the narrowest features—assigned to individual electronic transitions—having bandwidths of less than 40 meV. Time-dependent density functional theory predictions of optically allowed transitions for the most stable—planar—isomers of the corresponding bare metal cluster anions are generally consistent with the experimental observation.

Structural origin of dipole x-ray resonant scattering in the low-temperature phase ofNd0.5Sr0.5MnO3

Abstract: We have investigated the low temperature phase of a Nd0.5Sr0.5MnO3 single crystal by x-ray resonant scattering at the Mn K-edge of the (3 0 0), (0 3 0) and (0 5/2 0) reflections. Strong resonances were observed for the s-s' channel in the (3 0 0) and (0 3 0) reflections and for the s-p' channel in the (0 5/2 0) reflection. These resonances show a p-periodicity on the azimuthal angle, having the intensity at the minimum position almost zero. The intensity dependence on the photon energy, azimuthal angle and polarisation dependencies has been analysed using a semi-empirical structural model. Contrary to previous claims of charge (Mn3+-Mn4+) and orbital ordering in this compound, our results show that the dipole resonant superlattice reflections can be explained by the presence of two types of Mn sites with different local geometric structure. One of the Mn sites is surrounded by a tetragonal-distorted oxygen octahedron whereas the other site has a symmetric octahedral environment. This model also establishes that no real space charge ordering is needed to explain the experimental data. Intermediate valence states according to a fractional charge-segregation Mn+3.42-Mn+3.58 were deduced.

Band spectroscopy of colloidal photonic crystal films

Abstract: Here we report on the optical properties associated with photonic bands of three-dimensional photonic colloidal crystals. Optical spectroscopy analysis shows fluctuations of the transmitted and reflected light intensity in photon frequency regions where no stop bands open up. The different optical features observed at low and high photon energy ranges are analyzed in terms of the band structure of the crystal. A relationship is found between dispersion of the bands and the features observed experimentally. On these premises, we show it is possible to map the higher-energy band region along nonprincipal directions of the first Brillouin zone by transmission spectroscopy.

Observation of two-photon above-threshold ionization of rare gases by XUV harmonic photons

Abstract: We have observed two-photon above-threshold ionization in Ar, Xe and He by the fifth harmonic (25 eV photon energy) of a KrF laser.

Induced Photon Emission from Quark Jets in Ultrarelativistic Heavy-Ion Collisions ¶

Abstract: We study the induced photon bremsstrahlung from a fast quark produced in AA collisions due to multiple scattering in quark-gluon plasma For RHIC and LHC conditions, the induced photon spectrum is sharply peaked at a photon energy close to the initial quark energy In this region, the contribution of the induced radiation to the photon fragmentation function exceeds the ordinary vacuum radiation Contrary to previous analyses [4–7], our results show that, at RHIC and LHC energies, the final-state interaction effects in quark-gluon plasma do not suppress the direct photon production and may even enhance it at p T ∼5–15 GeV

Experimental check of the Gerasimov-Drell-Hearn sum rule for H-1

Abstract: For the first time we checked the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the proton experimentally in the photon energy range from 0.2-2.9 GeV with the tagged photon facilities at MAMI (Mainz) and ELSA (Bonn). New data of the doubly polarized total cross section difference are presented in the energy range from 1.6 to 2.9 GeV. The contribution to the GDH integral from 0.2-2.9 GeV yields [254+/-5(stat)+/-12(syst)] mub with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule.

Fully differential cross sections for photo-double-ionization of D2.

Abstract: We report the first kinematically complete study of the four-body fragmentation of the D2 molecule following absorption of a single photon. For equal energy sharing of the two electrons and a photon energy of 75.5 eV, we observed the relaxation of one of the selection rules valid for He photo-double ionization and a strong dependence of the electron angular distribution on the orientation of the molecular axis. This effect is reproduced by a model in which a pair of photoionization amplitudes is introduced for the light polarization parallel and perpendicular to the molecular axis.

Ultra Low-Power All-Optical Switching

Abstract: Using analytical modeling and detailed numerical simulations, we investigate properties of hybrid systems of Photonic Crystal micro-cavities which incorporate a highly non-linear Ultra Slow Light medium. We demonstrate that such systems, while being miniature in size (order wavelength), and integrable, could enable ultra-fast non-linear all-optical switching at single photon energy levels.

Photodissociation of protonated amino acids and peptides in an ion storage ring. Determination of Arrhenius parameters in the high-temperature limit

Abstract: We have measured the time dependence of the fragmentation of protonated amino acids and peptides upon UV excitation in an electrostatic ion storage ring. After absorption of a 266 nm photon, protonated Trp (TrpH+) has a lifetime of 10–20 μs but also a component with a millisecond lifetime is present. The long lifetime may be due to fluorescence, which leads to ions with lower excitation energy, or to the decay of the Trp+˙ radical cation formed after prompt hydrogen loss in the laser interaction region. Only one component with a lifetime of about 10 μs was detected for TyrH+. The lifetime of photoexcited PheH+ is even shorter with an upper limit of a few microseconds. For the singly protonated tripeptides (LysTrpLysH+ and LysTyrLysH+), the decay curves are found to consist of a single component that can be reproduced with an assumption of statistical decay after equilibration of the photon energy among all vibrational modes. The rate constant is expressed in the Arrhenius form in terms of the microcanonical temperature, and the decay rate is obtained by integration over the energy distribution, which has a spread corresponding to the canonical energy distribution at room temperature. The resulting deviation from exponential decay makes it possible to determine the decay parameters from a measurement at a single photon wavelength. Activation energies of Ea = 1.24 ± 0.07 and 1.5 ± 0.4 eV were determined for LysTrpLysH+ and LysTyrLysH+, respectively, with pre-exponential factors of Ad = 1011.1±0.5 and 1012.9±2.6 s−1.

Monte Carlo modeling of a high-sensitivity MOSFET dosimeter for low- and medium-energy photon sources.

Abstract: Metal–oxide–semiconductor field effect transistor(MOSFET)dosimeters are increasingly utilized in radiation therapy and diagnostic radiology. While it is difficult to characterize the dosimeter responses for monoenergetic sources by experiments, this paper reports a detailed Monte Carlo simulation model of the High-Sensitivity MOSFETdosimeter using Monte Carlo N-Particle (MCNP) 4C. A dose estimator method was used to calculate the dose in the extremely thin sensitive volume. Efforts were made to validate the MCNP model using three experiments: (1) comparison of the simulated dose with the measurement of a Cs-137 source, (2) comparison of the simulated dose with analytical values, and (3) comparison of the simulated energy dependence with theoretical values. Our simulation results show that the MOSFETdosimeter has a maximum response at about 40 keV of photon energy. The energy dependence curve is also found to agree with the predicted value from theory within statistical uncertainties. The angular dependence study shows that the MOSFETdosimeter has a higher response (about 8%) when photons come from the epoxy side, compared with the kapton side for the Cs-137 source.

Photon-induced Kondo satellites in a single-electron transistor.

Abstract: We measure the differential conductance of a single-electron transistor (SET) irradiated with microwaves. The spin-entangled many-electron Kondo state produces a zero-bias peakin the dc differential conductance if the quantum dot in the SET contains an unpaired electron. When the photon energy hf is comparable to the energy width of the Kondo peak and to e (the charge on the electron) times the microwave voltage across the dot, satellites appear in the differential conductance shifted in voltage by ±hf/e from the zero-bias resonance. We also observe an overall suppression of the Kondo features with increasing microwave voltage.

Near surface photon energy spectra outside a 6 MV field edge.

Abstract: The purpose of this study was to investigate the difference between a 6 MV linear accelerator x-ray energy spectrum outside the field edge near a phantom surface, and the corresponding spectrum on the central axis. The Monte Carlo code MCNP-4A was used to calculate the spectra on the central axis and at 1, 2, 5 and 10 cm from the edge of a 4 x 4 cm2, 10 x 10 cm2 and 15 x 15 cm2 field. Compared to the spectrum on the central axis, the spectra outside the field edge showed two distinct regions: a broad peak below about 0.5 MeV, and a lower amplitude, less rapidly changing region at higher energies from 0.5 to 6 MeV. The lower energy peak was due to scattered photons, and the higher energy component was due mainly to primary photons transmitted through the jaws of the secondary collimator. The potential impact of these spectral differences on critical organ photon dosimetry was determined by calculating the ratio of the sensitivity of a Scanditronix EDD-5 diode and of a LiF:Mg:Ti thermoluminescent dosimeter (TLD) outside the field edge to their respective sensitivity at the calibration position on the central axis. The lower energy peak combined with the non-uniform energy sensitivity of each detector produced up to a two-thirds overestimate of x-ray dose outside the field by the diode, whereas the response ratio of the TLD was about unity. These results indicated that a similar evaluation was required for profile measurements of a dynamic wedged field and measurements in an intensity modulated beam with either type of detector.

Determination Of Electron Flux Spectra In A Solar Flare With An Augmented Regularization Method: Application To Rhessi Data

Abstract: Kontar et al. (2004) have shown how to recover mean source electron spectra in solar flares through a physical constraint regularization analysis of the bremsstrahlung photon spectra that they produce. They emphasize the use of non-square inversion techniques, and preconditioning combined with physical properties of the spectra to achieve the most meaningful solution to the problem. Higher-order regularization techniques may be used to generate ${overline F}(E)$ forms with certain desirable properties (e.g., higher order derivatives). They further note that such analyses may be used to infer properties of the electron energy spectra at energies well above the maximum photon energy observed. In this paper we apply these techniques to data from a solar flare observed by RHESSI on 26 February, 2002. Results using different orders of regularization are presented and compared for various time intervals. Clear evidence is presented for a change in the value of the high-energy cutoff in the mean source electron spectrum with time. We also show how the construction of the injected (accelerated) electron spectrum $F_0(E_0)$ (assuming that Coulomb collisions in a cold target dominate the electron energetics) is facilitated by the use of higher-order regularization methods.