Presence Of Small Field Research Articles

Chiral anomaly induced magnetoconductances in an irradiated type-I Weyl semimetal

Magneto conductivities in Weyl semimetals (WSMs) in presence of small fields are studied using quasi-classical Boltzmann transport equations. Following such formalism here we consider irradiation via circularly polarized light on a two-node time reversal breaking WSM already under a dc/static electric field and study the magneto-transport properties due to the presence of chiral anomaly. Chiral anomaly affects both longitudinal magnetoconductivity as well as planar Hall conductivity. As our field set-up causes continuous time variation in the relative orientation between the fields, one naturally expects interesting magneto-transport behavior for different field strengths and tilting. The type-I tilting that we study here displays both positive and negative magnetoconductances depending on the field strengths and time. Furthermore, we find that a direct temporal tuning of the irradiated field strengths can lead to fluctuating magneto-transport behavior which can be easily improvised and checked in the laboratories.

Monte Carlo Simulations Applied to Conjunctival Lymphoma Radiotherapy Treatment

Small radiation fields are increasingly applied in clinical routine. In particular, they are necessary for the treatment of eye tumors. However, available treatment planning systems do not calculate the absorbed dose with the desired accuracy in the presence of small fields. Absorbed dose estimations obtained with Monte Carlo methods have the required accuracy for clinical applications, but the exceedingly long computation times associated with them hinder their routine use. In this article, a code for automatic Monte Carlo simulation of linacs and an application in the treatment of conjunctival lymphoma are presented. Simulations of clinical linear accelerators were performed with the general-purpose radiation transport Monte Carlo code penelope. Accelerator geometry files, in electron mode, were generated with the program AutolinaC. The Monte Carlo simulation of an annular electron 6 MeV field used for the treatment of the conjunctival lymphoma yielded absorbed dose results statistically compatible with experimental measurements. In this simulation, 2% standard statistical uncertainty was reached in the same time employed by a hybrid Monte Carlo commercial code (eMC); however, eMC showed discrepancies of up to 7% on the absorbed dose with respect to experimental data. Results obtained with the analytic algorithm Pencil Beam Convolution differed from experimental data by 10% for this case. Owing to the systematic application of variance-reduction techniques, it is possible to accurately estimate the absorbed dose in patient images, using Monte Carlo methods, in times within clinical routine requirements. The program AutolinaC allows systematic use of these variance-reduction techniques within the code penelope.

Direct aperture optimization for IMRT using Monte Carlo generated beamlets

This work introduces an EGSnrc-based Monte Carlo (MC) beamlet does distribution matrix into a direct aperture optimization (DAO) algorithm for IMRT inverse planning. The technique is referred to as Monte Carlo-direct aperture optimization (MC-DAO). The goal is to assess if the combination of accurate Monte Carlo tissue inhomogeneity modeling and DAO inverse planning will improve the dose accuracy and treatment efficiency for treatment planning. Several authors have shown that the presence of small fields and/or inhomogeneous materials in IMRT treatment fields can cause dose calculation errors for algorithms that are unable to accurately model electronic disequilibrium. This issue may also affect the IMRT optimization process because the dose calculation algorithm may not properly model difficult geometries such as targets close to low-density regions (lung, air etc.). A clinical linear accelerator head is simulated using BEAMnrc (NRC, Canada). A novel in-house algorithm subdivides the resulting phase space into 2.5 X 5.0 mm2 beamlets. Each beamlet is projected onto a patient-specific phantom. The beamlet dose contribution to each voxel in a structure-of-interest is calculated using DOSXYZnrc. The multileaf collimator (MLC) leaf positions are linked to the location of the beamlet does distributions. The MLC shapes are optimized using direct aperture optimization (DAO). A final Monte Carlo calculation with MLC modeling is used to compute the final dose distribution. Monte Carlo simulation can generate accurate beamlet dose distributions for traditionally difficult-to-calculate geometries, particularly for small fields crossing regions of tissue inhomogeneity. The introduction of DAO results in an additional improvement by increasing the treatment delivery efficiency. For the examples presented in this paper the reduction in the total number of monitor units to deliver is approximately 33% compared to fluence-based optimization methods.

Kinetics of systems with continuous symmetry under the effect of an external field.

Universality in the kinetics of systems with O(N) continuous symmetry, under influence of an external time-dependent field, is studied. The exponents that characterize the process are obtained, showing that they are independent of N, for N\ensuremath{\ge}2. We also get analytical expressions for the time dependence of the order parameter (magnetization) in the presence of small fields, in the limit N\ensuremath{\rightarrow}\ensuremath{\infty}. Our results show that universality goes beyond the value of the exponents. For small fields, the full dynamical evolution of the order parameter is universal. Similar universality is expected for arbitrary N. Agreement with numerical solutions of the basic nonlinear equations is excellent.

Neutron spin echo and neutron spin depolarisation studies in Y9Co7

High-resolution, neutron spin echo and neutron spin depolarisation techniques have been used to study the dependence of the local fields on temperature and low applied magnetic field (up to 55 Oe) in both the 'hybrid' and superconducting states of Y9Co7. In zero field neither experiment shows neutron spin rotation effects in either state, whereas considerable effects have been observed in a small field (a few oersteds). For the hybrid state such effects can be reconciled with strong spin fluctuations of a kind that can easily be affected by even small fields. A comparison of the observed effects in the presence of small fields in the superconducting state with the theoretical expression has yielded information on the temperature dependence of the penetration depth lambda (T). In this state, the effects of spin fluctuations seem not to show up dramatically.