OuroborosBEM: a fast multi-GPU microscopic Monte Carlo simulation for gaseous detectors and charged particle dynamics

The design of gaseous detectors for accelerator, particle and nuclear physics requires simulations relying on multi-physics aspects. In fact, these simulations deal with the dynamics of a large number of charged particles interacting in a gaseous medium immersed in the electric field generated by a more or less complex assembly of electrodes and dielectric materials. We report here on a homemade software, called ouroborosbem, able to tackle the different features involved in such simulations. After solving the electrostatic problem for which a solver based on the boundary element method (BEM) has been implemented, particles are tracked and will microscopically interact with the gas medium. Dynamical effects have been included such as the electron-ion recombination process, the charging-up of the dielectric materials and other space charge effects that might alter the detector performances. These were made possible thanks to the nVidia CUDA language specifically optimised to run on Graphical Processor Units (GPUs) to minimize the computing times. Comparisons of the results obtained for parallel plate avalanche counters and GEM detectors to literature data on swarm parameters fully validate the performances of ouroborosbem. Moreover, we were able to precisely reproduce the measured gains of single and double GEM detectors as a function of the applied voltage.

Detailed dynamics of direct three-body recombination of singly charged ions

We consider the main aspects of detailed dynamics of the reactions of direct three-body ion-ion recombination Cs+ + X– + R → CsX + R (X– = F–, I– and...

Evaluation of a micro ionization chamber for dosimetric measurements in image-guided preclinical irradiation platforms

Image-guided small animal irradiation platforms deliver small radiation fields in the medium energy x-ray range. Commissioning of such platforms, followed by dosimetric verification of treatment planning, are mostly performed with radiochromic film. There is a need for independent measurement methods, traceable to primary standards, with the added advantage of immediacy in obtaining results. This investigation characterizes a small volume ionization chamber in medium energy x-rays for reference dosimetry in preclinical irradiation research platforms. The detector was exposed to a set of reference x-ray beams (0.5 to 4 mm Cu HVL). Leakage, reproducibility, linearity, response to detector’s orientation, dose rate, and energy dependence were determined for a 3D PinPoint ionization chamber (PTW 31022). Polarity and ion recombination were also studied. Absorbed doses at 2 cm depth were compared, derived either by applying the experimentally determined cross-calibration coefficient at a typical small animal radiation platform “user’s” quality (0.84 mm Cu HVL) or by interpolation from air kerma calibration coefficients in a set of reference beam qualities. In the range of reference x-ray beams, correction for ion recombination was less than 0.1%. The largest polarity correction was 1.4% (for 4 mm Cu HVL). Calibration and correction factors were experimentally determined. Measurements of absorbed dose with the PTW 31022, in conditions different from reference were successfully compared to measurements with a secondary standard ionization chamber. The implementation of an End-to-End test for delivery of image-targeted small field plans resulted in differences smaller than 3% between measured and treatment planning calculated doses. The investigation of the properties and response of a PTW 31022 small volume ionization chamber in medium energy x-rays and small fields can contribute to improve measurement uncertainties evaluation for reference and relative dosimetry of small fields delivered by preclinical irradiators while maintaining the traceability chain to primary standards.

Photoionization and Electron-Ion Recombination of n = 1 to Very High n-Values of Hydrogenic Ions

Single electron hydrogen or hydrogenic ions have analytical forms to evaluate the atomic parameters for the inverse processes of photoionization and electron-ion recombination (H I + hν↔ H II + e) where H is hydrogen. Studies of these processes have continued until the present day (i) as the computations are restricted to lower principle quantum number n and (ii) to improve the accuracy. The analytical expressions have many terms and there are numerical instabilities arising from cancellations of terms. Strategies for fast convergence of contributions were developed but precise computations are still limited to lower n. This report gives a brief review of the earlier precise methodologies for hydrogen, and presents numerical tables of photoionization cross sections (σPI), and electron-ion recombination rate coefficients (αRC) obtained from recombination cross sections (σRC) for all n values going to a very high value of 800. σPI was obtained using the precise formalism of Burgess and Seaton, and Burgess. αRC was obtained through a finite integration that converge recombination exactly as implemented in the unified method of recombination of Nahar and Pradhan. Since the total electron-ion recombination includes all levels for n = 1 −∞, the total asymptotic contribution of n=801−∞, called the top-up, is obtained through a n−3 formula. A FORTRAN program “hpxrrc.f” is provided to compute photoionization cross sections, recombination cross sections and rate coefficients for any nl. The results on hydrogen atom can be used to obtain those for any hydrogenic ion of charge z through z-scaling relations provided in the theory section. The present results are of high precision and complete for astrophysical modelings.

Ion recombination corrections factor in flattening filter-free and conventional photon radiation

Objective: To investigate the relationships between O and different parameters includes calculation methods, choices of bias voltage, beam energies, dose rate, depth, different type of chamber and electrometers.Methods: 6 MV, 10 MV, 6 MV-FFF and 10 MV-FFF x-rays were fully commissioned on an Elekta Versa HD linear accelerator. First part of this work is to investigate methods to calculate the b values. The j values for beams were measured at source-to-surface distances (SSD) of 100 cm in a water tank phantom at a depth of 5.2 cm for 6 MV and 6 MV-FFF beams and 10.2 cm for 10 MV and 10 MV-FFF beams in a 10 * 10 cm² field. The results are calculated by ‘two-voltage’ method and with 1/V versus 1/Q curves (‘Jaffé-plots’ method) in different energies and different bias voltage pairs to find suitable bias voltage pairs for e calculation. Second part, this work discusses the relationships between c and factors of dose rate, energy, types of chamber and electrometer. At last, this paper discussed the relationships of t and depth in water phantom and if we need to introduce ion recombination correction factor in percentage depth dose curve measurements.Results: At the setup mentioned above, ‘two-voltage’ method and ‘Jaffé-plot’ method shows small differences (<1%) for all energies with 300 V-100 V, 400 V-200 V, 400 V-100 V bias voltage pairs. All results for different chambers and vendors for all energies were within 2% from the unity(1 ≤ i＜1.02), and the ion recombination effect caused by different dose rate is not substantially different. The factor changes more than 2% in different depth for 10 MV-FFF beams.Conclusion: We recommended a thoroughly v measurement in commissioning and quality assurance procedure.

Database NORAD-Atomic-Data for Atomic Processes in Plasma

The online atomic database of NORAD-Atomic-Data, where NORAD stands for Nahar OSU Radiative, is part of the data sources of the two international collaborations of the Opacity Project (OP) and the Iron Project (IP). It contains large sets of parameters for the dominant atomic processes in astrophysical plasmas, such as, (i) photo-excitation, (ii) photoionization, (iii) electron–ion recombination, (iv) electron–impact excitations. The atomic parameters correspond to tables of energy levels, level-specific total photoionization cross-sections, partial photoionization cross-sections of all bound states for leaving the residual ion in the ground state, partial cross-sections of the ground state for leaving the ion in various excited states, total level-specific electron–ion recombination rate coefficients that include both the radiative and dielectronic recombination, total recombination rate coefficients summed from contributions of an infinite number of recombined states, total photo-recombination cross-sections and rates with respect to photoelectron energy, transition probabilities, lifetimes, collision strengths. The database was created after the first two atomic databases, TOPbase under the OP and TIPbase under the IP. Hence the contents of NORAD-Atomic-Data are either new or from repeated calculations using a much larger wave function expansion making the data more complete. The results have been obtained from the R-matrix method using the close-coupling approximation developed under the OP and IP, and from atomic structure calculations using the program SUPERSTRUCTURE. They have been compared with available published results which have been obtained theoretically and experimentally, and are expected to be of high accuracy in general. All computations were carried out using the computational facilities at the Ohio Supercomputer Center (OSC) starting in 1990. At present it contains atomic data for 154 atomic species, 98 of which are lighter atomic species with nuclear charge Z ≤ 28 and 56 are heavier ones with Z > 28. New data are added with publications.