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The study of the processes occurring in a matter when ionizing radiation passes through is important for solving various problems. Examples of such problems are applied and fundamental problems in the field of radiation physics, chemistry, biology, medicine and dosimetry. This work is dedicated to computer modeling of the parameters of a tungsten converter for studying the processes of radiation damage during the interaction of ionizing radiation with solutions of organic dyes. Simulation was carried out in order to determine the optimal thickness of the converter under predetermined experimental conditions. Experimental conditions include: energies and type of primary particles, radiation intensity, target dimensions, relative position of the radiation source and target. Experimental studies of the processes of radiation damage occurring in solutions of organic dyes are planned to be carried out using the linear electron accelerator "LINAC-300" of the National Scientific Center "Kharkov Institute of Physics and Technology". Electrons with 15 MeV energy are chosen as primary particles. The interaction of electrons with the irradiated target substances is planned to be studied in the first series of experiments. Investigations of the interaction of gamma quanta with the target matter will be carried out in the second series of experiments. The tungsten converter is used to generate a flux of bremsstrahlung gamma rays. One modeling problem is determination of the converter thickness at which the flux of bremsstrahlung gamma will be maximal in front of the target. At the same time, the flow of electrons and positrons in front of the target should be as low as possible. Another important task of the work is to identify the possibility of determining the relative amount of radiation damage in the target material by the Geant4-modeling method. Radiation damage of the target substance can occur due to the effect of bremsstrahlung, as well as electrons and positrons. Computational experiments were carried out for various values of the converter thickness – from 0 mm (no converter) to 8 mm with a step of 1 mm. A detailed analysis of the obtained data has been performed. As a result of the data analysis, the optimal value of the tungsten converter thickness was obtained. The bremsstrahlung flux in front of the target is maximum at a converter thickness of 2 mm. But at the same time, the flux of electrons and positrons crossing the boundaries of the target does not significantly affect the target. The computational experiment was carried out by the Monte Carlo method. A computer program in C++ that uses the Geant4 toolkit was developed to perform calculations. The developed program operates in a multithreaded mode. The multithreaded mode is necessary to reduce the computation time when using a large number of primary electrons. The G4EmStandardPhysics_option3 model of the PhysicsList was used in the calculations. The calculations necessary for solving the problem were carried out using the educational computing cluster of the Department of Physics and Technology of V.N. Karazin Kharkiv National University.

Featured Application:  Pulsed X-ray source based on sub-microsecond intense pulsed electron beam accelerators.  X-ray sources based on pulsed electron accelerators stimulate the development of bremsstrahlung converter designs. The numerical optimization of transmission-type X-ray targets for maximum X-ray output by pulsed electron beams was carried out in the present work. The targets featured a combination of a heavy element (tungsten or molybdenum) X-ray conversion layer and a titanium membrane that served as the vacuum window, thermal shielding for converter heat, and an electron dump. The energy spectrum of the electron beam generated via explosive emission was analyzed via the space-charge effect, and was utilized for the source sampling algorithm for electron transportation simulation with a Monte Carlo method for X-ray emission analysis. It was revealed that the transmission photon intensity of a mono-material target is primarily affected by the thickness of the target, and there exists an optimal target thickness within which the photon fluence is restricted by insufficient electron stopping; when exceeded, the extra thickness of the X-ray converter target imposes absorption and attenuates the generated X-ray. Analysis on dual-layer targets proved that this optimized converter target thickness, combined with a proper titanium window, produces the highest X-ray photon emissions. [ABSTRACT FROM AUTHOR]

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.

X-ray sources based on pulsed electron accelerators stimulate the development of bremsstrahlung converter designs. The numerical optimization of transmission-type X-ray targets for maximum X-ray output by pulsed electron beams was carried out in the present work. The targets featured a combination of a heavy element (tungsten or molybdenum) X-ray conversion layer and a titanium membrane that served as the vacuum window, thermal shielding for converter heat, and an electron dump. The energy spectrum of the electron beam generated via explosive emission was analyzed via the space-charge effect, and was utilized for the source sampling algorithm for electron transportation simulation with a Monte Carlo method for X-ray emission analysis. It was revealed that the transmission photon intensity of a mono-material target is primarily affected by the thickness of the target, and there exists an optimal target thickness within which the photon fluence is restricted by insufficient electron stopping; when exceeded, the extra thickness of the X-ray converter target imposes absorption and attenuates the generated X-ray. Analysis on dual-layer targets proved that this optimized converter target thickness, combined with a proper titanium window, produces the highest X-ray photon emissions.

The four-layer converter consisting of Ta+H 2 O(2 mm)+Fe(2 mm)+Pb is considered. The influence of the Ta and Pb layers thicknesses on the penetration power of bremsstrahlung (BR) produced by 7.5 MeV electrons in the converter has been studied. The average BR energy increases with increase of Ta and Pb layers thickness due to the absorption of low-energy photons in Ta and Pb. Absorbed doses in an object caused by a single element of a small size converter (SEC) are calculated by the Monte Carlo method. The total absorbed doses in the object are determined by summing the doses from the SEC placed along the extended converter. The densities of the electron beam along the converter are determined for two- and four-sided irradiation of objects of given sizes, which provide the minimum Dose Uniformity Ratio (DUR) magnitudes. The polyethylene thickness is shown to be by the factor 1.6 larger in the case of four-sided irradiation by electron beam with optimal density and DUR < 1.5 than in the case of two-sided irradiation. • Pb layer in the Ta converter increases the average energy of the Ta converter. • A modeling BR from extended converter using a small single element proposed. • A single converter element of small size calculated by the Monte Carlo method. • The beam are determined for 2- and 4-sided irradiation for minimum values of DUR. [ABSTRACT FROM AUTHOR]

The design of a bremsstrahlung converter for intense pulsed electron beam with total electron absorption and separated vacuum seal is investigated. The target of the converter is made of molybdenum with a thickness of 250 μm, isolated in vacuum by a titanium membrane of 60 μm thick. The article presents the experimental data of the converter tests, such as the dependence of the dose on the distance, the curve of dose rate, the image of the X-ray source obtained by the pinhole camera, and the thermal image of the converter exit window from the infrared camera. A dose of 10 mGy per pulse was demonstrated, with an instantaneous pulse dose rate of 0.1 MGy/s at a distance of 3 cm from the target plane. Reliable continuous operation for more than 1 million pulses at 10 pps has been experimentally achieved. • Titanium membrane of 60 μm thick isolates the molybdenum bremsstrahlung converter of 250 μm thick in vacuum, which frees the converter from atmospheric pressure forces and reduces heat transfer from the converter target to the irradiated object. • The converter demonstrated the average dose rate of 0.1 Gy/s (10 pps) at distance of 3 cm from target plane while the pulsed dose rate was 0.1 MGy. • Long term 10 pps operation (> 106 pulses) was achieved while the energy flux was ~6.73 W/cm2 in the area of X-ray generation. [ABSTRACT FROM AUTHOR]