Issue 3, 2022

Optimization techniques and parameters setting of the GAMOS code in the Monte Carlo simulation of a medical accelerator

M. Assalmi$^1$, E. Diaf$^1$

In this work, we studied the Monte Carlo optimization techniques for the GAMOS code using the high performance of the HPC-Marwan computing grid. We determined the configuration of the initial parameters, including the initial electron beam parameters, CUT and StepMax, as well as the optimization of the Monte Carlo calculation in the case of small irradiation fields, in the build-up and penumbra regions. The head geometry of the Elekta Synergy MLCi2 medical linear accelerator was modeled in this work under a non-disclosure agreement (NDA) with Elekta AB, Stockholm, Sweden. The Monte Carlo simulations of the GAMOS code are validated by experimental measurements in PDD and distributed dose profile in a water phantom by comparing the mean dose error ε, mean absolute error, D20/D10 ratio, dmax and penumbra width. After a series of tests, we obtain the best configuration of the initial electron beam parameters which are 6.3 MeV, 0.8 MeV and 2 mm for mean energy, sigma and spot size, respectively. The effect of CUT and StepMax parameters on dose is significant for low density media and the computation time is slowed by a factor of 9.4 for StepMax=0.005mm compared to StepMax=0.5mm. Voxel size affects the accuracy of the dose calculation in the accumulation and penumbra regions.

N. P. Balabukha$^1$, D. A. Konyaev$^{1,2}$, N. E. Shapkina$^{1,2}$

The paper presents mathematical simulation of RCS of perfectly conducting bodies based on electromagnetic near-field computations. The necessity to measure bistatic RCS in anechoic chambers makes the topic really important. Near fields are simulated by means of FEKO software package (method of moments) on a cylindrical finite-size scanning area which partially encircles the object under study. The paper deals with two methods of obtaining bistatic RCS patterns based on fields calculated on the scanning surface: the Stratton-Chu formulas and the cylindrical wave expansion of electromagnetic field. The possibility of obtaining bistatic patterns in the specified range of bistatic angles for the chosen truncated scanning surfaces with the radius of 3 – 4 wavelengths is investigated. The results of the both methods are recorded and compared. The scanning surface height required for the specified accuracy of the RCS pattern is determined for the scanning surface completely encircling the cylinder in the azimuthal plane.

E. F. Zagirdinova$^1$, K. A. Kouzakov$^2$

The generation of an entangled spin state as a result of a low-energy elastic collision of two protons is considered. A theoretical model is used to describe the scattering process, which is in good agreement with the available experimental data on the angular differential cross sections of proton-proton collisions at low energies. The entanglement of the spin state of the final protons as a function of the scattering angle is analyzed using the Bell's inequality and pair concurrence. The corresponding numerical results for different values of the incident proton energy are compared with the results without taking into account nuclear forces. It is shown that the presence of nuclear interaction significantly enhances the spin entanglement effects.

Yu. V. Balakshin$^{1,2}$, D. K. Minnebaev$^{1,2}$, E. A. Vorobyeva$^{2,3}$, A. P. Evseev$^{1,2}$, A. V. Nazarov$^{1,2}$, A. A. Shemukhin$^{1,2}$, V. S. Chernysh$^{1,2}$

A graphene coating deposited on a copper substrate was irradiated with helium and argon ions with an energy of 100 keV. The effect of the irradiation fluence on the structure of the graphene coating was studied by Raman scattering. Defect formation in a graphene coating for various parameters of ion irradiation was simulated by the Monte Carlo method in the approximation of binary collisions. It was shown that the transition to the "displacement per atom" parameter makes it possible to compare defect formation in a two-dimensional target for different irradiation parameters. The paper shows that during irradiation, point defects accumulate in the graphene coating up to values of the “displacement per atom” parameter close to 5×10-4, after which the process of combining defective regions begins with subsequent destruction of the coating structure at values of the “displacement per atom” in the range 5×10-3 - 10-2.

E. V. Anreevna$^{1,2}$, K. A. Bukunov$^1$, A. P. Evseev$^1$, D. K. Minnebaev$^1$, N. G. Chechenin$^1$

Raman spectrometry is one of the most widely used techniques for the analysis of oriented multi-walled carbon nanotubes (MWNTs), while the analyzing laser beam is also a source of local heating. The laser radiation power was increased and then decreased to the initial values, simultaneously the behavior of characteristic peaks in the MWCNT spectrum was monitored: D, G, 2D(G'), and also D'. An additional peak was introduced to compensate for the difference between the experimental spectrum and the sum of the decomposition components (around 1500 cm-1), the nature of the change of which differs from the behavior of the known peaks, which indicates the different nature of their origin. It is noted that an increase in the laser power leads to a decrease in the number of defects in carbon nanotubes.

P. R. Zapevalin$^1$, A. G. Rudnitskiy$^1$, M. A. Shchurov$^1$, T. A. Syachina$^1$

The paper describes the novel software for satellite orbit determination and the algorithms, that were used in the development of this software. Developed by the laboratory for ballistic and navigation support of the Astro Space Center of Lebedev Physical Institute, its role is to provide ballistic and navigation assistance for the Millimetron (Spektr-M) space observatory. As a part of this project, the laboratory is tasked with developing a reliable ballistic tracking tool for Spektr-M. This article describes the software being developed to determine and refine the orbit of Millimetron spacecraft. It briefly describes the Spektr-M project, the algorithms used to determine the orbit and presents the first results of the software. Additionally, the number of ground tracking stations have been estimated for optimal measurements of Millimetron orbital parameters in order to meet the required orbit accuracy and reduce the time, when the space observatory is invisible. The implemented algorithms were tested and verified on simulated and real observational data taken from Radioastron (Spektr-R) mission.

M. Yu. Reshetnyak

The magnetic field suppresses hydrodynamic turbulence. This leads to a weakening of a large-scale magnetic field generation by small-scale flows and, simultaneously, to a decrease in ohmic dissipation. It is shown that a decrease in dissipation is important for the process of changing the polarity of the geomagnetic dipole: both the frequency of reversals increases and their fine structure changes. In particular, the time for the decrease in the dipole strength becomes comparable with the time for its recovery after the reversal. This is consistent with the results of 3D modeling and most paleomagnetic observations.

I. I. Mokhov$^{1,2}$, N. N. Medvedev$^1$

The results of the analysis of the features of quasi-cyclic El Niño phenomena of various types and their changes using long-term data (1870-2020) are presented. To detect changes in the characteristic periods and amplitudes of El Niño phenomena, along with wavelet analysis, a method based on the analysis of phase portraits for quasi-cyclic processes was used. Cross-wavelet analysis was used to assess the coherence of El Niño events of various types. Significant differences in changes in El Niño phenomena of various types are noted. At the same time, significant differences were revealed for periods before and after the middle of the 20th century. Against the background of large interdecadal variations, secular trends for the amplitude and period of El Niño quasi-cyclic phenomena are statistically insignificant. The greatest amplitudes of El Niño phenomena have been noted in recent decades.

I. I. Mokhov

The results of data analysis and model simulations are presented, which testify to the significant role of the temporal periodicity of boundary conditions with periodic spatial scanning in the formation of atmospheric temperature features of the planetary system, as an object of geophysical thermodynamics, along with key physical processes, including radiative and convective heat transfer. In particular, the results of model simulations with a change in the length of the annual cycle confirm the correspondence of the height of the tropopause to the height characteristic of the temperature skin layer for the annual cycle of insolation.

L. V. Zotov, N. S. Sidorenkov$^2$, C. J. Bizouard$^3$

Earth's rotation is variable: the pole drifts, simultaneously wobbling with annual and chandler (433 days) periods, Earth's rotation velocity also changes. In early 2000-s the amplitude of the chandler wobble started to decrease and in 2017-2020 reached its historic minima, comparable only with the minima in the end of 1920-s. We extracted chandler wobble using different methods (subtracting the trend and annual mode, through Panteleev's filtering, etc.) from EOP C04 and EOP С01 time series and demonstrated that since 2010s its phase is also changing. Such phase drift was also happening in 1920-1940-s and reached $\pi$ radian. Contemporary changes are approaching 2 radians already. Possible interpretations of these phenomena, their representation in the polar motion spectra and causes are discussed.

V. A. Bagatinsky$^{1,2}$, N. A. Diansky$^{1,3,4}$

To find out the role of thermohaline factors in the formation of North Atlantic (NA) thermohaline circulation climatic trends,the contributions of changes in potential temperature and salinity to the streamfunction trends of the Atlantic meridional overturning circulation (AMOC) were determined. To do this, using the INMOM (Institute of Numerical Mathematics Ocean Model) and the data of the EN4 electronic atlas (MetOffice, UK), the NA circulation was simulated by the diagnosis-adjustment method for the characteristic periods of 1951-2017, 1951-1990.and 1991-2017. In two selected periods 1951-2017 and 1951-1990 all the main features of changes in the AMOC are explained mainly by temperature changes, which prevail over salinity. At the same time, the observed trend towards the weakening of the main core of the AMOC streamfunction from 1991 to 2017 is caused by changes in salinity, and to the strengthening of the main core of the AMOC streamfunction from 1951 to 1990 caused by changes in potential temperature. A relationship was found between the trends of the AMOC streamfunction maximum at 38.5° N and the differences between the zonal-averaged sea levels at 34.5° N and 57.5° N, which can possibly be used as an indexof the AMOC streamfunction variability.