Evidence for topological nonequilibrium in magnetic configurations

We use direct numerical simulations to study the evolution, or relaxation, of magnetic configurations to an equilibrium state. We use the full single-fluid equations of motion for a magnetized, non-resistive, but viscous fluid; and a Lagrangian approach is used to obtain exact solutions for the magnetic field. As a result, the topology of the magnetic field remains unchanged, which makes it possible to study the case of topological nonequilibrium. We find two cases for which such nonequilibrium appears, indicating that these configurations may develop singular current sheets.Comment: 10 pages, 5 figure

Topological structure of two-dimensional magnetostatic equilibria

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Synthesis of minimal cost nonlinear feedback shift registers

This paper is concerned with the determination of the "minimal" nonlinear feedback shift register (NLFSR) that generates a given sequence. A total ordering between structural vectors that takes into account the implementation cost is specified. Minimality is defined with respect to this ordering. A block as well as a sequential algorithm are developed that combine a linear dependence test implemented by an extension of the Fundamental Iterative Algorithm (EFIA) with a minimality procedure that traces the totally ordered structural vectors. © 2002 Elsevier Science B.V. All rights reserved

Marginal value-based pricing of additional spectrum assigned to cellular telephony operators

This paper deals with the development of a framework for estimating the spectrum price when additional spectrum is assigned to cellular telephony operators. Pricing is based on the spectrum marginal value and expresses the cost savings achieved over infrastructure via the use of the additional spectrum. The resulting values reflect the economic/engineering value of the spectrum and do not take account of its strategic value. © 2002 Elsevier Science B.V. All rights reserved

Monte Carlo validation in the diagnostic radiology range

The present paper aimed to the validation of Monte Carlo simulation codes already developed by the reporting team, for the study of photon transport and absorption in scintillator materials. Comparisons are reported between the developed codes, MCNP developed codes and other published data. First, the mean number of interactions for each incident photon was determined and compared to the published data. Agreement to within ±1.5% was achieved. Second, the depth of energy deposition in water was assessed for three monoenergetic X-ray beams (15, 20, 30 keV). The energy deposited in slabs of water phantoms of varying depths was tallied. Excellent agreement within ±2% was achieved, except that for 15 keV were a more rapid drop with increasing depth was found. Second, spatial distribution of the energy absorption due to scatter radiation was assessed. Good agreement (below ±5%) with published data was achieved. Third, a water slab with thickness 5, 10, 15, 20 cm was modeled, irradiated by a monoenergetic narrow beam of photons of various energies. Last, the lateral spread of energy deposition was assessed in a 1 cm thick slab in the center of an 8 cm thick water phantom, irradiated by a 50 keV narrow beam. Again, good agreement (below ±2%) was achieved. © 2006 Elsevier B.V. All rights reserved

Response of Lead Fluoride (PbF2) Crystal under X-ray and Gamma Ray Radiation

Background: In this research, the response of a 10 × 10 × 10 mm3 commercially available PbF2 crystal was experimentally assessed under X-ray and gamma ray radiation to verify the possible application of this material in X-ray medical imaging. Methods: The measurements were performed under X-ray from 50 to 130 kVp and gamma ray excitation (Tc-99m 140 keV and I-131 365 keV). The PbF2 response was experimentally assessed by the determination of the absolute luminescence efficiency (AE), X-ray luminescence efficiency (XLE), and the stopping power of this scintillating crystal in terms of the energy absorption efficiency (EAE). The results were compared with bismuth germanate (BGO) crystal, which is commonly used in medical imaging modalities. Results: The AE of PbF2 gradually decreased from 50 kVp up to 130 kVp. The maximum value was 0.61 efficiency units (EU) at 140 keV, and the minimum value was 0.03 EU at 71 keV (130 kVp). Similarly, low values appeared for the XLE, where the maximum value was 16.9 × 10−5 at 140 keV. Conclusions: These findings show that the PbF2 scintillator has unacceptably low luminescence efficiency. Although PbF2 can effectively absorb radiation, the scintillation light, at room temperatures, is negligible, and, thus, it could not be used in medical imaging applications in the examined energy range. © 2023 by the authors

Evaluation of the GSO : Ce scintillator in the X-ray energy range from 40 to 140 kV for possible applications in medical X-ray imaging

The purpose of the present study was to evaluate, under X-ray medical imaging conditions, the X-ray luminescence efficiency (XLE) and the optical quantum gain (OQG) of the Gd2SiO5:Ce scintillator in single crystal form, suitable for tomographic applications. Intrinsic physical properties and light emission characteristics of the Gd2SiO5:Ce scintillator, were also studied. Both experimental and Monte Carlo techniques were used. Various X-ray tube voltages (40-140 kV), currently employed in X-ray imaging applications, were used. XLE was found to vary slowly with X-ray tube voltage from (0.021 +/- 0.003) to (0.017 +/- 0.003). OQG varied from (317 +/- 18) to (466 +/- 23) light photons per incident X-ray. These values were adequately high for imaging applications using the particular energy range. Additionally, it was found by Monte Carlo simulations that for crystal thicknesses higher than 0.5 cm both XLE and OQG reached saturation levels, indicating that higher thickness crystals are of no practical use in X-ray medical imaging. (c) 2006 Elsevier B.V. All rights reserved