Monte Carlo investigation of electron fluence perturbation in MRI-guided radiotherapy beams using six commercial radiation detectors

With the integration of MRI-linacs to the clinical workflow, the understanding and characterization of detector response in reference dosimetry in magnetic fields are required. The magnetic field perturbs the electron fluence (Fe), and the degree of perturbation depends on the irradiation conditions and the detector type. This work evaluates the magnetic field impact on the electron fluence spectra in several detectors to provide a deeper understanding of detector response in these conditions. Monte Carlo calculations of Fe are performed in six detectors (solid-state: PTW60012 and PTW60019, ionization chambers: PTW30013, PTW31010, PTW31021, and PTW31022) placed in water and irradiated by an Elekta Unity 7 MV FFF photon beam with small and reference fields, at 0 T and 1.5 T. Three chamber-axis orientations are investigated: parallel or perpendicular (two possibilities: FL towards the stem or the tip) to the magnetic field and perpendicular to the beam. One orientation for the solid-state detector is studied: parallel to the beam and perpendicular to the magnetic field. Additionally, Fe spectra are calculated in modified detector geometries to identify the underlying physical mechanisms behind the fluence perturbations. The total Fe is reduced up to 1.24% in the farmer chamber, at 1.5 T, in the parallel orientation. The interplay between the gyration radius and the farmer chamber cavity length significantly affects Fe in the perpendicular orientation; the total fluence varies up to 5.12% in magnetic fields. For the small-cavity chambers, the maximal variation in total Fe is 0.19%, for the reference field, in the parallel orientation. . In contrast, significant small-field effects occur; the total Fe is reduced between 9.86% to 14.50% at 1.5T (with respect to 0T) depending on the orientation. The magnetic field strongly impacted the solid-state detectors in both field sizes, probably due to the high-density extracameral components. The maximal reductions of total Fe are 15.06±0.09% (silicon) and 16.00±0.07% (microDiamond). This work provides insights into detector response in magnetic fields by illustrating the interplay between several factors causing dosimetric perturbation effects: 1) chamber and magnetic field orientation, 2) cavity size and shape, 3) extracameral components, 4) air gaps and their asymmetry, 5) electron energy. Low-energy electron trajectories are more susceptible to change in magnetic fields, and generally, they are associated with detector response perturbation.

Evolution of an arched magnetized laboratory plasma in a sheared magnetic field

Arched magnetized structures are a common occurrence in space and laboratory plasmas. Results from a laboratory experiment on spatio-temporal evolution of an arched magnetized plasma ( $\beta \approx 10^{-3}$ , Lundquist number $\approx 10^{4}$ , plasma radius/ion gyroradius $\approx 20$ ) in a sheared magnetic configuration are presented. The experiment is designed to model conditions relevant to the formation and destabilization of similar structures in the solar atmosphere. The magnitude of a nearly horizontal overlying magnetic field was varied to study its effects on the writhe and twist of the arched plasma. In addition, the direction of the guiding magnetic field along the arch was varied to investigate its role in the formation of either forward- or reverse-S shaped plasma structures. The electrical current in the arched plasma was well below the current required to make it kink unstable. A significant increase in the writhe of the arched plasma was observed with larger magnitudes of overlying magnetic field. A forward-S shaped arched plasma was observed for a guiding magnetic field oriented nearly antiparallel to the initial arched plasma current, while the parallel orientation yielded the reverse-S shaped arched plasma.

Room temperature XFEL crystallography reveals asymmetry in the vicinity of the two phylloquinones in photosystem I

Photosystem I (PS I) has a symmetric structure with two highly similar branches of pigments at the center that are involved in electron transfer, but shows very different efficiency along the two branches. We have determined the structure of cyanobacterial PS I at room temperature (RT) using femtosecond X-ray pulses from an X-ray free electron laser (XFEL) that shows a clear expansion of the entire protein complex in the direction of the membrane plane, when compared to previous cryogenic structures. This trend was observed by complementary datasets taken at multiple XFEL beamlines. In the RT structure of PS I, we also observe conformational differences between the two branches in the reaction center around the secondary electron acceptors A1A and A1B. The π-stacked Phe residues are rotated with a more parallel orientation in the A-branch and an almost perpendicular confirmation in the B-branch, and the symmetry breaking PsaB-Trp673 is tilted and further away from A1A. These changes increase the asymmetry between the branches and may provide insights into the preferential directionality of electron transfer.

Molecular Dynamics Simulations on the Chain Fold During the Isothermal Orientation of n-Alkanes on Graphene

The orientation of hydrocarbon chains plays a key role in the applications of organic materials. And chain folding in the process of molecular orientation is also of great significance for the design of organic molecular thin films. The effect of chain length and simulation temperature on the isothermal orientation of n-alkanes on graphene surface is studied by molecular dynamics simulation in this paper. And the chain folding is also described. The n-alkanes can form perpendicular ordered structure, parallel ordered structure or perpendicular orientation at relative low temperature and parallel orientation at relative high temperature on graphene surface. The chain fold happens when long n-alkanes form perpendicular ordered structure on graphene surface. And the simulation results show the interactions of n-alkane−graphene and n-alkane−n-alkane affect chain fold.

Epoxy and Polyester Composites’ Characteristics under Tribological Loading Conditions

This research examines the friction and dry wear behaviours of glass fibre-reinforced epoxy (GFRE) and glass fibre-reinforced polyester (GFRP) composites. Three fibre orientations—parallel orientation (P–O), anti-parallel orientation (AP–O), and normal orientation (N–O)—and various sliding distances from 0–15 km were examined. The experiments were carried out using a block-on-ring configuration at room temperature, an applied load of 30 N, and a sliding velocity of 2.8 m/s. During the sliding, interface temperatures and frictional forces were captured and recorded. Worn surfaces were examined using scanning electron microscopy to identify the damage. The highest wear rates for GFRE composites occurred in those with AP–O fibres, while the highest wear rates for GFRP composites occurred in those with P–O fibres. At longer sliding distances, composites with P–O and N–O fibres had the lowest wear rates. The highest friction coefficient was observed for composites with N–O and P–O fibres at higher sliding speeds. The lowest friction coefficient value (0.25) was for composites with AP–O fibres. GFRP composites with P–O fibres had a higher wear rate than those with N–O fibres at the maximum speed.

Acoustic Properties of Larch Bark Panels

The potential of tree bark, a by-product of the woodworking industry, has been studied for more than seven decades. Bark, as a sustainable raw material, can replace wood or other resources in numerous applications in construction. In this study, the acoustic properties of bark-based panels were analyzed. The roles of the particle size (4–11 mm and 10–30 mm), particle orientation (parallel and perpendicular) and density (350–700 kg/m3) of samples with 30 mm and 60 mm thicknesses were studied at frequencies ranging from 50 to 6400 Hz. Bark-based boards with fine-grained particles have been shown to be better in terms of sound absorption coefficient values compared with coarse-grained particles. Bark composites mixed with popcorn bonded with UF did not return the expected results, and it is not possible to recommend this solution. The best density of bark boards to obtain the best sound absorption coefficients is about 350 kg/m3. These lightweight panels achieved better sound-absorbing properties (especially at lower frequencies) at higher thicknesses. The noise reduction coefficient of 0.5 obtained a sample with fine particles with a parallel orientation and a density of around 360 kg/m3.

Adhesion dynamics in the neocortex determine the start of migration and the post-migratory orientation of neurons

The neocortex is stereotypically organized into layers of excitatory neurons arranged in a precise parallel orientation. Here we show that dynamic adhesion both preceding and following radial migration is essential for this organization. Neuronal adhesion is regulated by the Mowat-Wilson syndrome-associated transcription factor Zeb2 (Sip1/Zfhx1b) through direct repression of independent adhesion pathways controlled by Neuropilin-1 (Nrp1) and Cadherin-6 (Cdh6). We reveal that to initiate radial migration, neurons must first suppress adhesion to the extracellular matrix. Zeb2 regulates the multipolar stage by transcriptional repression of Nrp1 and thereby downstream inhibition of integrin signaling. Upon completion of migration, neurons undergo an orientation process that is independent of migration. The parallel organization of neurons within the neocortex is controlled by Cdh6 through atypical regulation of integrin signaling via its RGD motif. Our data shed light on the mechanisms that regulate initiation of radial migration and the postmigratory orientation of neurons during neocortical development.

Time Domain Reflectometric Studies on Methylcellosolve – Acetylacetone Binary Solutions

Time Domain Reflectometric studies has been applied on the binary solutions of methylcellosolve (MCS) with acetylacetone (ACACT) in the entire concentration range at 298 K in the frequency range of 10 MHz – 32 GHz. The values of relaxation time (τ), effective Kirkwood correlation factor (geff ), corrective Kirkwood correlation factor (gf ) and excess permittivity (εE ) have been calculated. The relaxation time of MCS slowly decreases with increase in ACACT concentration suggesting that the size of the heteroassociated rotating unit decreases. The rate at which τ value decreases is more in MCS rich solutions which means that the formation of heteromolecular H – bonds dominantly occur in MCS rich solutions. Parallel orientation among the dipoles occur as suggested by the g^eff values which are greater than unity in all the solutions. The excess permittivity ε^E values calculated using mole and volume fractions qualitatively exhibit the same deviation from ideal behaviour.

Interfacial ferroelectricity by van der Waals sliding

Despite their partial ionic nature, many layered diatomic crystals avoid internal electric polarization by forming a centrosymmetric lattice at their optimal van-der-Waals stacking. Here, we report a stable ferroelectric order emerging at the interface between two naturally-grown flakes of hexagonal-boron-nitride, which are stacked together in a metastable non-centrosymmetric parallel orientation. We observe alternating domains of inverted normal polarization, caused by a lateral shift of one lattice site between the domains. Reversible polarization switching coupled to lateral sliding is achieved by scanning a biased tip above the surface. Our calculations trace the origin of the phenomenon to a subtle interplay between charge redistribution and ionic displacement, and provide intuitive insights to explore the interfacial polarization and its unique “slidetronics” switching mechanism.

Trim9 and Klp61F promote polymerization of new dendritic microtubules along parallel microtubules

Axons and dendrites are distinguished by microtubule polarity. In Drosophila, dendrites are dominated by minus-end-out microtubules while axons contain plus-end-out microtubules. Local nucleation in dendrites generates microtubules in both orientations. To understand why dendritic nucleation does not disrupt polarity, we used live imaging to analyze the fate of microtubules generated at branch points. We found that they had different rates of success exiting the branch based on orientation: correctly oriented minus-end-out microtubules succeeded in leaving about twice as often as incorrectly oriented microtubules. Increased success relied on other microtubules in a parallel orientation. From a candidate screen, we identified Trim9 and kinesin-5 (Klp61F) as machinery that promoted growth of new microtubules. In S2 cells, EB1 recruited Trim9 to microtubules. Klp61F promoted microtubule growth in vitro and in vivo, and could recruit Trim9 in S2 cells. In summary, the data argue that Trim9 and kinesin-5 act together at microtubule plus ends to help polymerizing microtubules parallel to pre-existing ones resist catastrophe.