Mean Photon Energy Research Articles

Monte Carlo simulation of dynamic wedged fields

This thesis studied the dosimetric characteristics of the dynamic wedge (DW) for a linear accelerator using a Monte Carlo technique based on the EGS4/BEAM system. The geometry of a linear accelerator was simulated. For the DW technique, the segmented treatment table (STT) contains information on the collimator position versus cumulative weighting of monitor units. The simulation of DW was accomplished by weighting the incidence electron fluence with the STT. Calculations were performed for DW and physical wedge (PW) with wedge angles ranging from 15° to 60° for both 6 and 18 MV photon beams and for a whole range of field sizes. The calculated percentage depth doses and beam profiles agreed with the corresponding measurements within 2%. Our calculations show that the effects of a DW on the spectrum and angular distributions (as well as electron contamination) are much less significant than with a PW. A 45° PW in the 6 MV photon beam results in a 30% increase in mean photon energy due to beam hardening. It also results in a 5% dose reduction in the build‐up region due to filtration of contaminated electrons. Neither this mean photon energy increase nor dose reduction is found for a DW. Field size dependence of a DW was also investigated thoroughly. The mean photon energy on the beam axis was reduced by 12.3% as the field size increases from 4×4 to 20×20 for both DW and open fields. The dose in the build‐up region is increased up to 10% due to the increase of the contaminated electrons for a large field size. The results of this work should improve radiotherapy treatment planning with dynamic wedges.

Technique factors and image quality as functions of patient weight at abdominal CT.

To investigate how changes in kilovolt peak and milliampere second settings, and patient weight affect transmitted x-ray energy fluence and the image contrast-to-noise ratio (CNR) at abdominal computed tomography (CT). Cylinders of water were used as patient models, and x-ray spectra, including x-ray tube potentials of 80-140 kVp, were investigated. The mean photon energy and energy fluence transmitted through water cylinders with varying diameters and the image contrast for fat, muscle, bone, and iodine relative to water were determined. The effect of changing the x-ray tube potential on CNR also was investigated. At a constant kVp, increasing patient weight from 10 kg to 120 kg reduced the transmitted energy fluence by two orders of magnitude. Changing the x-ray tube potential from 80 kVp to 140 kVp increased the mean photon energy from approximately 52 keV to approximately 72 keV and thus reduced the image contrast relative to water by 12% for muscle, 21% for fat, 39% for bone, and 50% for iodine (approximate reduction values). Increasing the x-ray tube potential from 80 kVp to 140 kVp increased the CNR by a factor of 2.6 for muscle and by a factor of 1. 4 for iodine. With changes in patient weight at abdominal CT, x-ray tube potentials must be varied to maintain a constant detector energy fluence. Increasing the x-ray tube potential generally improves CNR.

Properties of a-Se for use in flat panel X-ray image detectors

The present paper examines why a-Se has become the key photoconductor material in flat panel direct conversion X-ray image detectors. We critically examine the X-ray sensitivity of stabilized a-Se layers as a function of electric field and mean photon energy. The radiation energy W ± absorbed per free electron–hole pair liberated for photoconduction in both a-Se and a-Si:H seem to follow the Que–Rowlands rule of W ±≈2.2 E g +E phonon for amorphous semiconductors.

In vivo measurement of radius calcium/phosphorus ratio by X-ray absorptiometry

We describe a new method for assessing the skeletal Ca/P ratio in vivo using X-ray absorptiometry. By placing cerium (Ce) and samarium (Sm) filters in the X-ray beam from a commercial X-ray source (Norland), mean photon energies of 39 and 89 keV were obtained. The instrument was designed to take measurements of the forearm, at a site located at the distal 1/3 of the radius. The system was calibrated with three bone phantoms: Ca 10(PO 4) 6(OH) 2, Ca(HPO 4)(H 2O) 2 and Ca(HPO 4) 2(H 2O)). The precision for measuring the Ca/P ratio in the human radius was 2.3% CV for a skin dose to the forearm ranging from 0.3 to 0.4 mGy, depending on the width of the arm. The Ca/P ratio of the radius was significantly lower in patients with postmenopausal osteoporosis than in premenopausal controls.

X Ray Spectrometry of Low Energy Photons for Determining Conversion Coefficients from Air Kerma, Ka, to Personal Dose Equivalent, Hp(10), for Radiation Qualities of the ISO Narrow Spectrum Series

The pulse height spectra of the radiation qualities N-10 to N-120 of the ISO narrow spectrum series (N) were measured using a commercial Ge spectrometer and a specially designed lead collimator. For the unfolding of these spectra, three response matrices of the Ge spectrometer for different maximum energies and energy resolutions were calculated by the EGS4 (Electron Gamma Shower version 4) code incorporating the PRESTA and LSCAT options. Then, the pulse height spectra were unfolded with two different algorithms to check the correctness. To verify the measuring and unfolding method, the first half value layers (HVLs) were determined using small ionisation chambers and the fluence spectra. The mean photon energies were calculated, also on the basis of the fluence spectra, and compared with published values. For each radiation quality N-10 to N-120 the conversion coefficients hpK(10;N,a) from air kerma Ka to personal dose equivalent at 10mm depth, Hp (10,a), were calculated for angles of incidence a of 15, 30, 45, 60(and 75(between the unidirectional photon field and the normal to the slab phantom surface. Because the fluence spectra were influenced by the air density, in particular the low energy spectra, the hpK(10;N,a) values were normalised to reference conditions. Differences of up to about 88% between the hpK(10;N,a) values presented in this work and those given in ISO/FDIS 4037-3 were found.

X-ray irradiation induced bulk space charge in stabilized a-Se x-ray photoconductors

The time-of-flight (TOF) transient photoconductivity technique is used with an appropriate method and algorithm, based on the time dependence of the transient photocurrent i(t), to probe the spatial distribution of x-ray induced net bulk space charge in stabilized a-Se (a-Se:0.3%As doped with 10 to 20 ppm Cl) x-ray photoconductors with long carrier lifetimes (hole lifetime of ∼500 μs and electron lifetime of ∼750 μs from interrupted TOF). It is observed that x-ray irradiation of biased a-Se photoconductors results in a net negative space charge distribution. The spatial variation of the space charge density ρ(x) was found to decay exponentially from the irradiation surface. The spatial dependence of ρ(x) did not follow the photon absorption profile as closely as observed previously when only unbiased a-Se layers were irradiated. However, the changes in the spatial distribution of ρ(x) with the x-ray mean photon energy exhibited the same tendency as the photon absorption profile. The distribution of the trapped charge and rate of detrapping was examined as a function of temperature, photon energy, and with infrared illumination. The total negative space charge was found to decrease with time following a power law dependence of the form N∼t−n, where n≈0.3 and was independent of the irradiation intensity, which indicates that electron detrapping occurs from an energy distribution of deep localized states. Using an infrared (IR) filter that allows photon energies smaller than 1.1 eV to pass, it is shown that IR illumination increases the rate of detrapping and confirms that electrons are trapped in deep electron trapping states between 0.6 and 1.1 eV in the density of states diagram of a-Se [M. Abkowitz Philos. Mag. Lett. 58, 53 (1988)]. The mean detrapping current was found to be ∼10 pA, orders of magnitude smaller than the expected dark current. However, the trapped charge modifies the electric field at the top metal electrode causing it to become more injecting and leading to an increase in the dark current. The increase in the dark current is a possible origin for the persistent x-ray photocurrents recently reported for a-Se x-ray photoconductors.

X-ray sensitivity of a-Se for x-ray imaging with electrostatic readout

Currently there is much interest in stabilized amorphous selenium (a-Se alloyed with 0.2–0.3% As and doped with 5–20 ppm Cl) as an x-ray photoconductor in various x-ray imaging systems. Although optical photogeneration in a-Se is well characterized, experiments on its x-ray sensitivity have been limited and, further, have resulted in widely differing conclusions especially for the electron hole pair (EHP) creation energy EEHP. In this work, we first introduce absolute and incremental sensitivity (S and s, respectively) concepts within the electrostatic readout mode of operation and then establish models linking these two sensitivities to the energy EEHP absorbed to create a free EHP. X-ray irradiation induced xerographic discharge experiments have been carried out on vacuum deposited stabilized a-Se x-ray photoconductor films to examine how the sensitivity depends on the charge carrier schubwegs (μτF where μ is the drift mobility, τ is the lifetime, and F is the electric field), the mean photon energy, and the detector thickness L. Experiments indicate that unless both electron and hole schubwegs are much longer than the photoconductor thickness, the sensitivity is reduced. The efficiency of EHP generation per absorbed x-ray photon, i.e., the apparent energy absorbed per free electron hole pair EEHP has been determined as a function of electric field and mean photon energy. EEHP decreases with increasing applied field and energy. By examining EEHP vs 1/F and extrapolating to 1/F=0, a saturation value EEHP0 for EEHP is predicted and estimated (4 to 6 eV) which, within experimental errors, is close to the theoretical expectation of ∼2.2Eg and substantially smaller than previously reported values for EEHP0 (18–29 eV).

The determination of the effective attenuation coefficient from effective organ depth and modulation transfer function in gamma camera imaging

Absolute measurement of activity implies a determination of effective depths and effective attenuation coefficients. In order to define restoration filters, it is necessary to measure the transfer function, i.e. position a line source at an effective depth for the specific measurement situation. A phantom was designed which can simulate an organ with a certain thickness at a certain depth. The phantom was used to measure transfer functions and a comparison was made with transfer functions from a line source to determine effective depths. Effective attenuation coefficients were calculated for , and for different organ thicknesses and depths of simulated organs. The effective attenuation coefficient for was found to be , in good agreement with previously published values. For , the attenuation coefficient decreased with the depth of an organ due to the use of two energy windows in the measurements and a corresponding change in mean photon energy by depth. For , the attenuation coefficient decreased with increasing organ thickness due to the increasing fraction of scattered radiation in the 40% energy window used. Using attenuation coefficients of 0.124, 0.184 and for , and respectively, the derived equations can be used to calculate the position of a conventional line source for measurements of transfer functions for a specific organ with a certain thickness at a certain depth for definition of different types of restoration filter.

Instabilities in Photoionized Interstellar Gas

We present a linear analysis of acoustic and thermo-reactive instabilities in a diffuse gas, photoionized and heated by a radiation field, cooled by collisional excitation of hydrogen and metal lines. The hydrogen recombination reaction has a stabilizing effect on the thermal mode found by Field (1965) since the condensation instability occurs in a narrower region of the parameter space and grows on longer time scales due to its oscillatory character. This effect is stronger when the mean photon energy is not much larger than the hydrogen ionization energy. Moreover, for fixed values of thermal pressure and photoionization rate, there are thermo- reactive unstable equilibria for which no transition to a stable phase is possible. By extending our analysis of the thermo-reactive modes to the nonlinear regime we show that when no phase transition is possible the medium evolves through a series of nonequilibrium states characterized by large amplitude, nonlinear periodic oscillations of temperature, density and hydrogen ionization fraction. We find also unstable acoustic waves which, for solar metal abundances, are the fastest growing modes in two temperature intervals: around $T\sim 100$~K and $T\sim 8000$~K (i.e. cold and warm phase, respectively), independent of the mean photon energy. Possible implications for the interstellar medium and intergalactic medium are briefly outlined.

Investigation of the40Ca(γ,N) reaction at 60 MeV

The values of the angle-integrated cross section for the decay to various low-lying states in39K following the40Ca(γ,π) reaction at a mean photon energy of 60 MeV are reported on, together with the values of the reduced Legendre coefficientsa i . The40Ca(γ,gp) and40Ca(γ,n) reactions yield the same angular cross-section values and these agree with the corresponding RPA-calculations.

Thermoluminescence Efficiency of LiF:Mg,Cu,P (MCP-N) Detectors to Photons, Beta-Electrons, Alpha Particles and Thermal Neutrons

The thermoluminescence efficiency, ?, of LiF:Mg,Cu,P (MCP-N detectors, commercially produced at the INP, Krakow, Poland), relative to 662 keV 137Cs gamma rays, has been measured for (i) 1250 keV 60Co gamma rays, (ii) filtered X ray beams of average energies in the range 15-300 kVp, (iii) 147Pm beta electrons, (iv) thermal neutrons and (v) stopping alpha particles of initial energies in the range 0.5-5 MeV. A rapid decrease of TL efficiency with decreasing mean photon energy from ? = 1.04 ± 0.02 for 60Co, ? = 0.93 ± 0.02 for 300 kVp X rays to ? 0.59 ± 0.016 for 15 kVp X rays was observed. The measured value of relative efficiency for 147Pm beta electrons was ? = 0.90 ± 0.02. The relative efficiency for alpha particles decreased from ? = 0.06 ± 0.004 to ? = 0.03 ± 0.007 for particles of initial energies of 5 MeV and 1 MeV, respectively. The measured response of MCP-N detectors after doses of thermal neutrons was equal to 0.72 x 1010 Gy.n-1.cm2, which corresponds to ? = 0.10 ± 0.01. The efficiency for 2.73 MeV 3H tritons was found to be ? = 0.155 ± 0.02. The rapid decrease of sensitivity of LiF:Mg,Cu,P with increasing ionisation density is a microdosimetric effect, resulting from the saturation of the TL signal from high energy deposits. An empirical relationship between the mean lineal energy, yF, and ? has been found which can be used to predict the TL efficiency of MCP detectors for photons and electrons. However, TL efficiency ? is not a unique function of yF so this relationship cannot be used to predict the value of ? over the whole range of ionisation densities (LET) of heavy charged particles stopping in the detector.

Thermoluminescence glow curves of CaF2: Dy crystals irradiated by soft X-rays

Applying a deconvolution of the thermoluminescence glow curves, parameters of single glow peaks of CaF2: Dy TLD 200 dosemeters irradiated by soft X-rays were determined. A dependence of the height ratio of low temperature (T≅393, 413 and 473 K) single peaks on energy of absorbed photons was measured in a region of 1–22.2 keV. Standard radionuclides109Cd,238Pu,55Fe and iodine laser produced aluminium plasma (T e ∼500 eV) were used as soft X-ray sources. The ratios of the heights of different single peaks are discussed with respect to high local doses. The decreasing ratio of the heights of the first and third and/or second and third peak with increasing photon energy allows to determine reversely a mean photon energy of absorbed soft X-ray radiation.

Model computations for x-ray scattering by the Boltzmann equation

This paper extends the previous work on scatter problems by Barret and Swindel [ Radiological Imaging, Vols 1 and 2. Academic Press, New York (1981)], using formal transport theory based on the Boltzmann equation: we focus on the various aspects of photon scattering of a thin pencil beam incident on a uniform slab of a scattering medium (water, soft tissue, bone, etc.), where the observational plane (image plane in radiology) is positioned behind the exit face of the slab. Expressions are derived for the number of scattered photons, the scatter fraction and the scattered mean photon energy, and numerically evaluated for various input parameters.

Results of the First National Intercomparison for Individual Monitoring Services in the Netherlands

In 1990 an intercomparison programme for individual monitoring services was developed and carried out by the National Institute of Public Health and Environmental Protection. Six services participated in this first national intercomparison in the Netherlands, all using TLD systems for whole-body monitoring. The dosemeters were irradiated with photons on the surface of a 30 cm diameter sphere of polymethylmethacrylate (PMMA). The range of the mean photon energy was 48 keV to 1250 keV. The range of the doses was 0.055 mSv to 12.3 mSv

Photoproduction of the Λ c charmed baryon

In a photoproduction experiment using a mean photon energy of 100 GeV we have observed 29±8 Λ c ( Λ c ) charmed-baryon and antibaryon decays in the pK −π + ( p K +π −) final state. Quasi two-body final states do not contribite significantly to this channel. The mass of the Λ c was measured to be 2281.7±2.7±2.6 MeV/ c 2 and its lifetime 0.18±0.03±0.03 ps. The ratio of Λ c D production, measured in this experiment, is significantly greater than that predicted by photon-gluon fusion and using a Lund model to describe the hadronization. This excess cannot be completely accounted for in this model, even using a Λ c branching fraction in pK π as high as 5%.

Self-consistent models of spherical accretion onto black holes. I - One-temperature solutions. II - Two-temperatures solutions with pairs

view Abstract Citations (37) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Self-consistent Models of Spherical Accretion onto Black Holes. I. One-Temperature Solutions Park, Myeong-Gu Abstract Spherically symmetric, steady state accretion onto a black hole is considered for various dimensionless accretion rates m^dot^ (=M^dot^c^2^/L_E_, where M^dot^ is the accretion rate and L_E_ is the Eddington luminosity). Models are constructed by iteratively integrating relativistic hydrodynamic and radiation moment equations with bremsstrahlung and Comptonization as the main radiative processes. Electrons and ions are assumed to be coupled completely. The effects of preheating, and the shock that may result, are also considered. At least one type of self-consistent model is found for any m^dot^, self- consistency meaning that the gas at all radii is heated by the radiation it produces. Two sets of models (characterized by different temperatures) were found in the range 3 <~ m^dot^ <~ 100. The dimensionless luminosity l (=L/L_E_) is 5.8 x 10^-5^m^dot^^2^ for m^dot^ < I. Low-temperature models (T ~ 10^4^ K), which exist for m^dot^ >~ 0.1, have l ~ 7 x 10^-10^m^dot^ln(10^8^/m^dot^); high-temperature models, which exist for 3 <~ m^dot^ <~ 100, have 3 x 10^-4^ <~ l <~ 3 x 10^-2^. The m^dot^ = 100 model has the highest luminosity of all: l = 2.6 x 10^-2^. High-m^dot^ models have correct mean photon energies for AGNs. Because of the effects of preheating, no steady state high-temperature self-consistent solution exists for 0.1 <~ m^dot^<~ 3 without a shock, and none exists for m^dot^ >~ 100 regardless of the presence or absence of a shock. The calculations also suggest that the high-temperature m^dot^ > 1 model is unstable at a fixed accretion rate. Publication: The Astrophysical Journal Pub Date: May 1990 DOI: 10.1086/168668 Bibcode: 1990ApJ...354...64P Keywords: Accretion Disks; Astronomical Models; Black Holes (Astronomy); Computational Astrophysics; Quasars; Temperature Effects; Active Galactic Nuclei; Coulomb Collisions; Emission Spectra; Iterative Solution; Luminosity; Pair Production; Radiative Transfer; Stellar Mass Accretion; black holes; hydrodynamics; radiation mechanisms; quasars; Astrophysics; BLACK HOLES; HYDRODYNAMICS; RADIATION MECHANISMS; QUASARS full text sources ADS | Related Materials (1) Part 2: 1990ApJ...354...83P

The photon energy dependence of radiophotoluminescent glass dosimeters over the range 17 keV to 6.8 MeV

A detailed knowledge of the photon energy dependence of a dosimeter is essential for the interpretation of measurements obtained with it in complex photon spectra. This paper describes the measurement of the photon energy dependence of Toshiba FD-P10-7 and SEI radiophotoluminescent glass dosimeters embedded in aluminium sufficiently thick to establish transient electron equilibrium. A combination of heavily filtered bremsstrahlung, radionuclide and positive ion accelerator sources were used to cover the mean photon energy range 17 keV to 6.8 MeV. Particular emphasis is given to the description of the 6.8 MeV calibration field based on the 19F(p, αγ) 16O reaction, which was established for these measurements.

RPA correlation effects in radiative pion capture

A sum rule approach to radiative pion capture is given in terms of the energy-weighted and inverse-energy weighted sum rules. They are evaluated with RPA accuracy for Skyrme-like interactions. The static polarizabilities of the dipole, quadrupole and octupole isovector modes turn out to be the relevant quantities to determine the total branching ratios and mean photon energies.

Sum rule approach to radiative pion capture: A full hamiltonian calculation for 1p shell nuclei

A sum-rule approach to radiative pion capture is given for the self-conjugated nuclei in the 1p shell. The non-energy-weighted and the energy-weighted sum rules are evaluated for the full effective hamiltonian. Total branching ratios and mean photon energies are determined from these sum rules. A comparison with the Kroll-Rudermann approximation shows that the mean photon energies are practically unaffected, while the total branching ratios increases of the order of 10%.

How Accurately do Thermoluminescence and Photoluminescence Dosimetry Systems Indicate the New ICRU Dose Quantities for Individual Monitoring?

Abstract In 1985, the German-Swiss radiation protection association and the PTB organised an intercomparison programme for individual monitoring systems to find out whether these systems are able to measure the new dose equivalents H'(10) and H'(0.07) specified in ICRU Report 39. Twelve laboratories from Switzerland, Austria, England and the Federal Republic of Germany participated with 30 dosimetry systems including TLD systems for whole-body monitoring (13), TLD systems for extremity dosimetry (5), PLD systems (2), and film dosimetry systems (6). The dosemeters were irradiated with photons on the surface of an Alderson human phantom or an ICRU sphere. The range of the mean photon energy was 26 keV to 662 keV. It was observed that there are already some dosimetry systems of every type (TLD, PLD, etc) whose response, with respect to H'(10) and H'(0.07), does not vary appreciably with photon energy. However, in general, special effort is needed to implement the new dose quantities in routine monitoring.