Angular Distribution Of Photons Research Articles

Correlated histogram representation of Monte Carlo derived medical accelerator photon-output phase space.

We present a method for condensing the photon energy and angular distributions obtained from Monte Carlo simulations of medical accelerators. This method represents the output as a series of correlated histograms and as such is well-suited for inclusion as the photon-source package for Monte Carlo codes used to determine the dose distributions in photon teletherapy. The method accounts for the isocenter-plane variations of the photon energy spectral distributions with increasing distance from the beam central axis for radiation produced in the bremsstrahlung target as well as for radiation scattered by the various treatment machine components within the accelerator head. Comparison of the isocenter energy fluence computed by this algorithm with that of the underlying full-physics Monte Carlo photon phase space indicates that energy fluence errors are less than 1% of the maximum energy fluence for a range of open-field sizes. Comparison of jaw-edge penumbrae shows that the angular distributions of the photons are accurately reproduced. The Monte Carlo sampling efficiency (the fraction of generated photons which clear the collimator jaws) of the algorithm is approximately 83% for an open 10x10 field, rising to approximately 96% for an open 40x40 field. Data file sizes for a typical medical accelerator, at a given energy, are approximately 150 kB, compared to the 1 GB size of the underlying full-physics phase space file.

Collision Spectroscopy of Highly Charged Ions at the ESR Storage Ring

With the advent of the relativistic storage ring ESR, a new generation of highly precise collision experiments has become accessible even for the heaviest ion species. At the ESR, the interaction of brilliant beams of cooled high-Z ions with low-dense gaseous matter can be studied without any beam collimation guaranteeing background-free experimental conditions. Furthermore, a unique feature of the ring is its capability to decelerate even bare uranium ions to moderate energies close to about 10 MeV/u, i.e. it allows to scan the beam energies from highly relativistic encounters to almost slow collisions. Currently, the main experimental activities focus on final state resolved electron capture, photon angular distribution and alignment investigations.

Excitation of lower autoionization states of the helium-like ions in x-ray scattering

The process of inelastic scattering of high-frequency photons leading to excitation of singlet autoionization states (AIS) with total angular momentum L = 0 located below the first excited level of the respective one-electron ion are considered. For the amplitude of the process analytical expressions are derived, which include one-dimensional integrals, the latter being evaluated numerically. The scattered photon angular distributions and the absolute cross section of the considered process are obtained. The transitions into two lower AIS are investigated. The approximation is employed, in which the electrons are described by Coulomb functions, and their interaction is taken into account in the first order of the perturbation theory. The correct AIS functions of the zero approximation are used which are obtained as solutions of the secular equation.

Study of deformation states in metals exposed to intense-pulsed-ion beams (IPIB)

Abstract One of the non-linear long-range effects of the interaction of intense-pulsed-ion beams (IPIB) with metals is their anomalous hardening in non-radiated regions. In experiments reported here, the IPIB parameters were: ion energy (70% C + , 30% H + ) E =0.2 to 0.6 MeV; current density j =20 to 250 A cm −2 ; pulse duration 60 to 100 ns; and fluence 1 to 6 J cm −2 pulse −1 . For j >90 A cm −2 , together with the known increase in microhardness ( H 0 ) observed in the irradiated region, a second peak at H z =(0.6 to 0.8) H 0 was found at the depth x = z =(40 to 100) R 0 , where R 0 is the ion-beam range (∼1 μm). H z and z values depend on the type of metal and irradiation regime. This anomalous hardening induces different local deformation states, which are the result of the generation and interaction of point (interstitials, vacancies) and linear (e.g., dislocations) defects in the shock-wave field. We used the recoil nucleus method for analysis of the deformation states after irradiation. Non-radiated and irradiated lead samples were saturated by hydrogen gas. Lead was selected because it should not react with hydrogen. The magnitude of hydrogen saturation, n h , is low in regions of compressive stress (because of the high concentration of interstitials) and high in tensile stress regions (because of the high concentration of vacancies). A primary 4 He 2+ -ion beam with an energy of 3.0 MeV was scanned on the transverse cross-section of the lead samples. The excited hydrogen recoil nucleus was knocked forward and registered by the detector. The hydrogen concentration n h ( x ) was uniform for non-irradiated samples, but had maxima and mimima for irradiated ones. In addition, the f -parameter distribution (obtained from annihilation photon angular distribution curves) with depth has been studied. A correlation is observed for all three kinds of H ( x ), n h ( x ) and f -parameter distribution that proves the proposed mechanism of hardening.

Angular distribution of radiation emitted after resonant transfer and excitation

We have calculated the angular distribution of photons emitted after dielectronic recombination of highly charged uranium ions and obtained differential cross sections for photon emission in resonant transfer and excitation (RTE) in collisions of with a graphite target using the impulse approximation. The results are compared with experimental data.

Radiative tau lepton pair production as a probe of anomalous electromagnetic couplings of the tau

We calculate the squared matrix element for the process e + e − → τ + τ − γ allowing for anomalous magnetic and electric dipole moments at the ττγ vertex. No interferences are neglected and no approximations of light fermion masses are made. We show that anomalous moments affect not only the cross section, but also the shape of the photon energy and angular distributions. We also demonstrate that in the case of the anomalous magnetic dipole moment, the contribution from interference involving Standard Model and anomalous amplitudes is significant compared to the contribution from anomalous amplitudes alone. A program to perform the calculation is available and it may be employed as a Monte Carlo generator.

Angular distribution of photons in dielectronic recombination

We have calculated the angular distribution of photons emitted after dielectronic recombination of highly charged uranium ions and obtained differential cross sections for resonant transfer and excitation in collisions of U90+ with C atoms using the impulse approximation.

Radiative electron capture and the photoelectric effect at high energies.

In high-energy atomic collisions between bare high-Z projectiles and low-Z target atoms, an electron may be captured radiatively into the projectile [radiative electron capture (REC)]. The photon angular distributions can be very well represented by radiative recombination (RR) of the projectile with free electrons. This process is the inverse of the photoelectric effect. In this paper, we present exact differential RR cross sections for ${\mathrm{Au}}^{79+}$ and ${\mathrm{U}}^{92+}$ at projectile energies of 0.2, 0.5, 1, 2, 5, and 10 GeV/u. We also show the differential cross sections for the photoelectric effect at x-ray energies corresponding to the former projectile energies. It is seen that because of the dramatic forward-peaking of the cross sections at high energies, measurements of REC from low-Z targets are the most practical way to study the photoelectric effect. In one particular example, 10.8 GeV/u ${\mathrm{Au}}^{79+}$ on Au targets, we show that the RR cross section multiplied with the number of target electrons is very close to the REC cross section calculated within the impulse approximation. \textcopyright{} 1996 The American Physical Society.

Ionization-excitation of He atoms by electron impact: alignment of )

We have measured the alignment of after simultaneous ionization and excitation by electron impact via the non-isotropic angular distribution of photons of the decay . The range of incident electron energy was between and . Within experimental errors we obtained a constant value of except in the range - where the alignment has values between -1.2 and -1.3. These larger negative values can be explained by the decay of two-electron excitations into . The alignment value of -0.92(14) indicates that in the investigated range of energy not only two-electron partial waves with L = 0 but also with L>0 contribute to the ionization-excitation cross section. Our value of -0.92(14) agrees reasonably well with a nearly constant theoretical value of obtained by a combined DWBA + R-matrix calculation.

Monte Carlo simulation of a miniature, radiosurgery x-ray tube using the ITS 3.0 coupled electron-photon transport code.

A miniature, interstitial x-ray generator has recently been developed and is currently undergoing clinical trials for the treatment of brain tumors. The maximum photon energy from this x-ray tube is 50 keV, although most of the initial testing has been carried out at 40 keV. Dose rates of up to 2 Gy/min in a water phantom at a distance of 10 mm from the tube tip are produced. In this paper we describe the modeling and simulation of x-ray production from this device using the ITS 3.0 Monte Carlo code. Verification of the simulation of x-ray production in the device was carried out by comparing predictions of spatial photon distribution, energy spectrum, and dose versus depth in water with experimentally obtained measurements. Agreement between the simulated results and experimental measurements was fairly good when comparing the angular distribution of photons emitted from the x-ray tube and very good when comparing dose rate versus depth in a water phantom. Discrepancies observed when comparing the calculated and measured estimates of characteristic line radiation were reduced by incorporation of a modification to the ITS code. Possible causes of the remaining discrepancy in bremsstrahlung intensity are discussed.

Effect of an Overhead Shield on Gamma-Ray Skyshine

A hybrid Monte Carlo and integral line-beam method is used to determine the effect of a horizontal slab shield above a gamma-ray source on the resulting skyshine doses. A simplified Monte Carlo procedure is used to determine the energy and angular distribution of photons escaping the source shield into the atmosphere. The escaping photons are then treated as a bare, point, skyshine source, and the integral line-beam method is used to estimate the skyshine dose at various distances from the source. From results for arbitrarily collimated and shielded sources, the skyshine dose is found to depend primarily on the mean-free-path thickness of the shield and only very weakly on the shield material.

Theoretical treatment of the bremsstrahlung process in the vicinity of giant atomic resonances.

The frequency and the angular distributions of photons emitted in the bremsstrahlung process of a 0.2--10 keV electron in collision with Ba are calculated for the photon frequencies above the 4d threshold: \ensuremath{\omega}=100--200 eV. The polarizational bremsstrahlung is taken into account. A simple method is described for the approximate calculation of the polarizational bremsstrahlung cross section. This method is particularly efficient in the case, when the many-electron correlations are important. \textcopyright{}1996 The American Physical Society.

New method for the polarizational bremsstrahlung calculation

A new algorithm is introduced for the calculation of the polarizational bremsstrahlung cross section. This method is particularly efficient for carrying out the slowly converging partial-wave series, which characterize the cross section of the process. The frequency and the angular distributions of photons emitted in a collision of a 0.2-10 keV electron with Eu are calculated for the photon frequencies above the 4d-threshold: omega =100-200 eV. The polarizational bremsstrahlung is taken into account.

Angular distribution and polarization of photons in the inclusive decay Λb → Xsγ

We study the angular distribution of photons produced in the inclusive decay of a polarized Λ b , Λ b → X sγ , using the technique of heavy quark effective theory. Finite non-perturbative corrections are obtained relative to the free quark decay b → sγ. These corrections affect significantly the intensity and polarization of photons emitted at small angles relative to the Λ b spin direction.

Program for computer simulation on photoproduction of mesons decaying into two photons

Experiments carried out using multi-detector arrays and wide-band taggers in general lead to large amounts of data which can only be meaningfully analysed if the experiment is accompanied by detailed computer simulations, being at least ten times as precise as the result of the experiment. For this reason we have developed a computer code for the simulation of photoproduction of mesons decaying into two photons which avoids Monte-Carlo procedures, and have applied this code to studies of possibilities to separate events of proton Compton scattering from events of the p( γ, π 0)p reaction. Requiring only a very moderate computer time, our new code leads to predictions of photon spectra and angular distributions of recoil protons without the fluctuations typical for a Monte Carlo result.

Photon angular distributions from radiative electron capture in relativistic atomic collisions.

A detailed discussion is given of the general behavior of differential cross sections for radiative electron capture in relativistic and nonrelativistic heavy-ion--atom collisions. From angular-momentum conservation and a simple approximate treatment, the basic qualitative features can be derived in an analytic form. Subsequently, the results of rigorous relativistic calculations are presented graphically for a sequence of projectile charges and projectile energies. It is shown that in radiative electron capture, the influence of the electron spin manifests itself in a particularly clear-cut way.

Radiative electron capture studied in relativistic heavy-ion-atom collisions.

The process of radiative electron capture (REC) in relativistic collisions of high-Z ions with low-Z gaseous and solid targets is studied experimentally and theoretically. The observed x-ray spectra are analyzed with respect to photon angular distributions as well as to total K-REC cross sections. The experimental results for angle-differential cross sections are well reproduced by exact relativistic calculations which yield significant deviations from standard ${\mathrm{sin}}^{2}$\ensuremath{\theta} distributions. Total cross sections for K-REC are shown to follow a simple scaling rule obtained from exact relativistic calculations as well as from a nonrelativistic dipole approximation. The agreement between these different theoretical approaches must be regarded as fortuitous, but it lends support to the use of the nonrelativistic approach for practical purposes.

Photon angular distribution of Radiative Electron Capture into the M shell of He-like uranium ions at 110?140 MeV/u

Measurements of the photon angular distribution of Radiative Electron Capture into the M shell have been performed with He-like uranium ions in the range 110–140 MeV/u. In addition, L REC was studied at a projectile energy of 140 MeV/u. In both cases, the experimental data show an asymmetry around 90° and agree well with a fully relativistic theory.

Simulation of the angular distribution of resonance radiation from a positive column discharge

A Monte Carlo simulation of radiation transport is used to determine the angular distribution of resonance radiation emanating from a positive column discharge. With the specification of the oscillator strength, absorption/emission profile, gas density, spatial distribution of radiating atoms, and boundary geometry, the angular distribution of photons arriving at the wall of the discharge is obtained. The ratio of the integral of the angular distribution to that for a diffuse, Lambertian surface is on the order of one-half for the parameter space studied here. When combined with a measurement of the absolute intensity at one angle, this distribution provides a means of determining the radiant emittance of the source when measurement of the actual distribution is problematic.

L-Subshell Resolved Photon Angular Distribution of Radiative Electron Capture into He-like Uranium.

The photon angular distributions for radiative electron capture (REC) into the $j=\frac{1}{2}$ and $j=\frac{3}{2}$ $L$ subshell levels were measured and calculated for ${\mathrm{U}}^{90+}$ \ensuremath{\rightarrow} C collisions at 89 MeV/u. The experiment provides the first study of the photon angular distribution of REC into a projectile $p$ state ($j=\frac{3}{2}$) which was found to exhibit a slight backward peaking in the laboratory frame. For radiative capture to the $j=\frac{1}{2}$ states, the measured angular distribution deviates considerably from symmetry around 90\ifmmode^\circ\else\textdegree\fi{}. The results demonstrate that the usual ${sin}^{2}{\ensuremath{\theta}}_{\mathrm{lab}}$ distribution is not valid in the high-$Z$ regime.