Angular Distribution Of Bremsstrahlung Photons Research Articles

Computer Simulation of the Angular Distribution of Electrons and Bremsstrahlung Photons in Tantalum Converter

The present work is dedicated to study of the possibilities of improving the production method of 11C and 18F medical isotopes using a linear electron accelerator. Experimental studies of 11C and 18F isotopes production by the photonuclear technique were carried out in [1]. Various targets were irradiated by bremsstrahlung photons following by measurements targets’ activities. This experimental research has been carried out in the energy range from 10 to 40 MeV. The current work studies an angular distribution of bremsstrahlung photons in order to estimate the possibilities of producing maximum achievable levels of medical isotopes 18F and 11С activities. The angular distribution of bremsstrahlung photons is an important property of the irradiation stand which contains an assembly of targets. This important property allows designing necessary changes in the target assembly setup used to produce medical isotopes. The research presented in the current work was done as a computer simulation. The computer software package ‘KIPT’ was developed in С++ programming language, using the Geant4 toolkit in order to obtain the angular distribution of bremsstrahlung photons. Experimental setup materials as well as structural elements positions were defined in the DetectorConstruction class of our program. The parameters of an electron beam were defined in the PrimaryGeneratorAction class. The electron beam diameter was defined as 8 mm, energy Ее=36.7 MeV, corresponding to the real experiment carried out at the “Accelerator” Science and Research Establishment of Kharkiv Institute of Physics and Technology [1]. Models of physical processes occurring while the electron beam crosses the target assembly were defined in the PhysicsList class. All classes and modules necessary for the analysis of simulation results were included to our program together with visualization modules. Visualization modules use the OpenGL graphics library and the Qt5 software to represent the relative position of experimental setup parts and to visualize particle trajectories. As a result of this work, the angular distributions were obtained for the beam of electrons and bremsstrahlung photons directly before the target. This result will allow target assembly parameter optimization for optimal production of medical isotopes by the photonuclear technique.

Experimental evidence of molecular coherence effects in the bremsstrahlung radiation processes

The first experimental evidence of Young-type interferences is presented in bremsstrahlung (BS) processes by measuring the double differential cross sections (DDCS) of BS photons emitted from a two-centre N2 molecule under impact of 4.0 keV electrons. The spectra were recorded for emission of photons in the angular range of 45°–120° and the energy range of 2.0–3.8 keV using a Si PIN photodiode detector (FWHM = 165 eV at 5.9 keV). The measured variation of DDCS ratio of N2 to Ar as a function of photon energy shows no obvious signature of interference structure. However, the measured angular distribution of the intensity ratio of BS photons of N2 to Ar is found to exhibit the noticeable oscillations. Furthermore, a clear oscillatory structure is observed in the angular distribution of BS photons if the intensity of photons emitted from N2 molecules at angle θ to the intensity of photons emitted at angle 90°, i.e. I(θ)/I(90°) is plotted as a function of emission angle. Noting the fact that, only one theoretical work on coherence effects in BS processes under low-energy electron impact with homonuclear diatomic molecules is reported in the literature by Borie et al (1970 Phys. Rev. A 2 1443) which fails to explain the observed oscillations and shows orders of magnitude difference in the amplitude and mismatch in the shape of oscillations. The presence of observed oscillations is suggested to arise due to coherent superposition of BS photons of a given energy emitted in scattering of 4.0 keV electrons with a two-centre N2 molecule. Strong disagreement between our measurements and the calculations from Borie et al theory clearly suggests that a more refined and adequate theoretical model together with additional experimental investigations are needed to verify the present observations.

Study of anisotropy of bremsstrahlung radiation emitted from 4.0 keV electrons in scattering by CH4 molecule

A study of anisotropy of bremsstrahlung (BS) photons emitted from 4.0 keV electrons in scattering by a free CH4 molecule is made by performing the measurements of cross sections of BS photons differential in energy and emission angle, that is, the doubly differential cross sections (DDCS). The DDCS spectra of BS photons were recorded using a Si -PIN photodiode detector in the energy range of 2.0–3.8 keV and angular detection range of 45–120° relative to the incident beam direction of electrons. The measured angular distribution of BS photons suggests that the radiation emerging from the considered electron-molecule collisions is anisotropic and its relative magnitude depends on the energy of emitted photons. The experimental DDCS spectra of CH4 are compared with DDCS spectra calculated from the atomic field bremsstrahlung formulations of Kissel- Quarles-Pratt (KQP) using additivity hypothesis. A good agreement is observed between the measured DDCS of CH4 molecule and those obtained from KQP theory. The anisotropy of the radiation, that is, its angular asymmetry is found to compare reasonably well with the predictions of KQP. The anisotropy of the dipole radiation as often defined by a polarisation parameter P (or degree of polarisation) is found to vary from (29.0 ± 9)% to (70.0 ± 9)% for photons with energy ranging from 2.0 keV to 3.8 keV, respectively.

BREMS: A program for calculating spectra and angular distributions of bremsstrahlung at electron energies less than 3 MeV

This work describes a program that implements the method developed by Tseng and Pratt in the 1970s for exact screened calculations of atomic-field bremsstrahlung. The calculation method is based on the relativistic partial-wave formulation describing interaction of the incident electron with an arbitrary central potential of the neutral target atom. The set of Fortran-90 codes BREMS has been written with the aim of creating a comprehensive library of spectra and shape functions of unpolarized atomic-field bremsstrahlung for all chemical elements and for the approximate range of the incident electron energy from 10 eV to 3 MeV on a dense energy grid, using the best theory available. Several examples of using this software and its verification against the published data calculated using the same theory are provided. BREMS can be run on modern personal computers, with processing times from several minutes to several hours, depending on the user-specified values of the atomic number, incident electron energy, and photon energy, as well as on the accuracy requested. Program summaryProgram title: BREMSProgram Files doi: http://dx.doi.org/10.17632/mvd57skzd9.1Licensing provisions: GNU General Public License v3Programming language: Fortran 90Nature of problem: The atom is described as a static spherically symmetrical charge distribution of infinite mass. The elementary process of bremsstrahlung is described as a one-electron transition in the field of the neutral atom with a point-like nucleus. The atom is described by a central potential. The singly and doubly differential cross sections of bremsstrahlung are calculated using the relativistic partial wave formulation, which is currently the best theory for calculating the cross sections of unpolarized atomic-field bremsstrahlung.Solution method: The set of codes BREMS solves the Dirac equation with one of three possible types of the interaction potential: 1) the point-Coulomb potential (unscreened nucleus), 2) the Thomas–Fermi–Czavinsky potential, 3) the Kohn–Sham potential. It is also possible to use an arbitrary interaction potential specified by the user in tabular format. The wave functions are expanded in partial wave series and the radial integrals are calculated numerically. The photon energy spectrum and the angular distribution of bremsstrahlung photons are expressed in terms of the mentioned radial matrix elements, resulting in an infinite series, which must be truncated at a predefined value of the absolute value of the quantum number κ. A partial-wave interpolation method is applied in order to reduce the number of radial integrals that have to be calculated. The truncation error is decreased by extrapolation of the differential cross sections as functions of the number of terms retained in the truncated series.Additional comments: (i) Restrictions: Although there is no explicit limitation on the incident electron energy in the code of BREMS, it is currently applicable only at electron energies that are not greater than approximately 3 MeV. At the low-energy end of the spectrum, the single-electron approximation may not be sufficiently accurate due to an increased importance of the many-electron effects at electron energies of the order of 100 eV or lower.(ii) Unusual features: Multiple precision (MP) and quadruple precision (QP) arithmetic are used in several stages of the calculation in order to eliminate the rounding errors due to strong cancellation of terms in the alternating series. The capability of the MP system to change the precision level at runtime is used to optimize the precision level automatically and thus to decrease the computation time.

Measurement of the angular distribution of thick target bremsstrahlung produced by 10–25 keV electrons incident on Ti and Cu targets

Angular distributions of bremsstrahlung photons produced by 10–25 keV electrons incident on thick Ti (Z = 22) and Cu (Z = 29) targets using a Si-PIN photodiode detector have been measured and new results are reported. The incidence angle, defined as the angle between the direction of the incident electrons and the normal of the target surface, varied between 15° and 75°. The intensity of the bremsstrahlung radiation, differential in angle and energy, that is, the double differential bremsstrahlung yield, was obtained by varying the incidence angle, while the photon detector was kept fixed perpendicular to the electron beam direction in reflection geometry. The experimental bremsstrahlung yields were normalized so that the values measured at 45° coincide with the results of Monte Carlo simulations. For the other incidence angles, the measured yields are found to be in good agreement with simulation results, within the experimental uncertainties. Our results suggest that the bremsstrahlung radiation emerging from the considered electron-target collision reactions is anisotropic in general; a relatively larger anisotropy is observed for all energy photons produced by high-energy electrons compared to that by low energy electrons. Furthermore, there is an indication of a weak dependence of the angular variation of the bremsstrahlung yield on atomic number (Z) of the bombarded targets.

Angular distributions of bremsstrahlung photons from ECR plasma

High-energy bremsstrahlung photon emission beyond a critical energy from electron cyclotron resonance (ECR) heating has long attracted much attention, and its nature has yet been unsolved. We have measured bremsstrahlung photons from the 28-GHz ECR ion source at Busan Center of Korean Basic Science Institute. The gamma-ray detection system consists of three NaI(Tl) scintillation detectors placed 62 cm radially from the beam axis and a NaI(Tl) scintillation detector at the extraction port for monitoring the photon intensity along the beam axis. Bremsstrahlung photon energy spectra were measured at nine azimuthal angles at the RF power of 1 kW. Azimuthal angular distributions of bremsstrahlung photons were found to be in coincidence with the structure the ECR ion source and the shape of ECR plasma.

A combined approach of variance-reduction techniques for the efficient Monte Carlo simulation of linacs

A method based on a combination of the variance-reduction techniques of particle splitting and Russian roulette is presented. This method improves the efficiency of radiation transport through linear accelerator geometries simulated with the Monte Carlo method. The method named as ‘splitting-roulette’ was implemented on the Monte Carlo code and tested on an Elekta linac, although it is general enough to be implemented on any other general-purpose Monte Carlo radiation transport code and linac geometry. Splitting-roulette uses any of the following two modes of splitting: simple splitting and ‘selective splitting’. Selective splitting is a new splitting mode based on the angular distribution of bremsstrahlung photons implemented in the Monte Carlo code . Splitting-roulette improves the simulation efficiency of an Elekta SL25 linac by a factor of 45.

Angular Distribution of Bremsstrahlung from Copper and Tungsten Targets Bombarded by 18, 28, and 38MeV Electrons

We measured the angular distribution of bremsstrahlung photons between 0 and 120 degrees from copper and tungstentargets bombarded by 18, 28, and 38MeV electrons at the electron linac of Hokkaido University. Activation detectors and two types of dosimeters were applied for the measurement. The measured results were compared with those calculated with the Monte Carlo code MCNP5 to verify the calculated results. The calculated results of both the reaction rates and ambient dose equivalent rates gave good agreement with the measured ones in the case of 18MeV electrons. A good agreement between the measurement and calculation was also observed for relative angular distributions of dose rates from 38MeV electrons; however, the calculation overestimated the measured data by a factor of 1.2 to 2.0. To achieve further accuracy of calculation, improvement of photonuclear reaction data and absorbed dose measurement are necessary for bremsstrahlung radiation dosimetry.

Angular Distribution of Bremsstrahlung from Copper and Tungsten Targets Bombarded by 18, 28, and 38 MeV Electrons

We measured the angular distribution of bremsstrahlung photons between 0 and 120 degrees from copper and tungstentargets bombarded by 18, 28, and 38MeV electrons at the electron linac of Hokkaido University. Activation detectors and two types of dosimeters were applied for the measurement. The measured results were compared with those calculated with the Monte Carlo code MCNP5 to verify the calculated results. The calculated results of both the reaction rates and ambient dose equivalent rates gave good agreement with the measured ones in the case of 18MeV electrons. A good agreement between the measurement and calculation was also observed for relative angular distributions of dose rates from 38MeV electrons; however, the calculation overestimated the measured data by a factor of 1.2 to 2.0. To achieve further accuracy of calculation, improvement of photonuclear reaction data and absorbed dose measurement are necessary for bremsstrahlung radiation dosimetry.

Experimental results on 2–30 keV bremsstrahlung from thick and thin targets

The recent experimental investigations on electron bremsstrahlung produced from impact of 2–30 keV electrons with thick solid and thin gaseous targets are reviewed. The theoretical models describing the energy and angular distributions of bremsstrahlung photons are discussed with their brief outlines and formulations to explain the experimental data. The results on thick target bremsstrahlung (TTB) spectra produced by keV electrons have suggested that there is a need to develop a comprehensive theory for accounting the solid state effects. It is further noted that the prediction of the modified KKD formula gives a reasonable agreement with the TTB data, whereas a semi-empirical formula gives a better fit to the data for thick targets. The available experimental data for dependence of double differential cross-sections of emitted photons on impact energy and their emission angles for gaseous atoms and molecules exhibit a good agreement with the theoretical calculations of Kissel et al., [1983. Shape functions for atomic-field bremsstrahlung from electrons of kinetic energy 1–500 keV on selected neutral atoms 1< Z<92. Atom. Data Nucl. Data Tables 28, 381–460].

Simulation of x-ray spectra generated by electron impact on solids

A simulation algorithm that generates x-ray emission spectra of samples irradiated with kilovolt electron beams was developed. The algorithm is based on a Monte Carlo package which performs simulations of both electron and photon transport for arbitrary materials and geometries. The code uses semi-empirical ionization cross-sections obtained from an optical data model and a simple model to describe the angular distribution of bremsstrahlung photons. The simulation efficiency is optimized by the use of a variance reduction technique. The reliability of the simulation code was analyzed by comparing the results with electron probe measurements for different specimens and measurement conditions, including thin films and oblique incidence. In general, there was good agreement between the simulation results and experiences. Copyright © 1998 John Wiley & Sons, Ltd.