Large Proton Research Articles

Galileo GIOVE-A MEORAD Results and Analysis

The Giove-A spacecraft carries two radiation monitors, CEDEX, built by the University of Surrey and Merlin, built by QinetiQ, to study the radiation environment encountered in the Galileo orbit. The two monitors have been functioning since the beginning of the mission and have measured protons, heavy ions and electrons. The electron environment has been found to be highly variable and driven by solar interactions. Comparisons with AE-8 indicate that the electron energy spectrum for the period measured was somewhat harder than that expected from the model. A series of large solar proton events were detected in December 2006, registering as enhanced fluxes of protons, heavy ions and also triggering a large enhancement in the outer electron belt. Comparisons with POLE and INTEGRAL/IREM show an improved spectral match over AE-8.

Comparative Study of Membrane Humidifier and Enthalpy Wheel Humidifier for Large Power Fuel Cell System

The performance of membrane humidifier (MH) and enthalpy wheel humidifier (EWH) for a large power proton exchange membrane fuel cell (PEMFC) system is compared using simulations and experiments. The MH model is based on one dimensional diffusion equation and the EWH model is based on the porous media surface diffusion equation. Simulation results agree well with experimental data. According to the results, the effect of intake air temperature has a lower effect on the MH performance than it does on the EWH performance while the air mass flow has a much higher effect on the MH performance than it does on the EWH performance. MH performs better than EWH at a low flow rate but worse at a high flow rate. Vapor transfer mechanism in the humidifiers is also studied.

Large Amplitude, Proton- and Cation-Activated Latch-Type Mechanical Switches: O-Protonated Amides Stabilized by Intramolecular, Low-Barrier Hydrogen Bonds within Macrocycles

Large amplitude molecular switches have been developed using oxonium ions as the novel switching mechanism. Macrocycles that contain a polyether ring that are preorganized and of optimum geometry such that strong, linear Low-Barrier Hydrogen Bonds (LBHB, 2.4 to 2.6 A in length) are formed between a protonated amide oxygen and a cyclic ether, that lend significant iminol character to the amide. Deprotonation yields a large conformational change between closed and open forms, mindful of a new hinged, latch-type mechanical proton switch. Numerous open and closed forms have been characterized by X-ray crystallography, and the intramolecular hydrogen bond that forms between the protonated amide oxygen and the cyclic polyether oxygen accounts for the stability of these new acids. The open form of the deprotonated adducts persist in solution as indicated by the magnitude of coupling constants and other Nuclear Overhauser Effect experiments. Different saturated and unsaturated solid acids have been characterized including products derived from acetonitrile, propionitrile, caprylonitrile, acrylonitrile and adiponitrile, and also by reaction with primary amides in the case of phenyl and norbornene derivatives. We have also demonstrated that metal cations can replace the proton in the switching mechanism, characteristic of nascent synthetic pores.

Alternate HMQC experiments for recording HN and HC-correlation spectra in proteins at high throughput

Alternate implementations of the SOFAST-HMQC experiment are described. In these alternate SOFAST-HMQC experiments (ALSOFAST-HMQC) excitation of the magnetization of interest is achieved by non-selective rf pulses while preserving the equilibrium polarization of passive spins. This alternate excitation scheme also allows the incorporation of a novel sensitivity enhancement protocol which has been most recently developed by Brutscher and coworkers and which permits simultaneous detection of both the x- and y-components of the indirectly detected t(1)-interferograms without the need to introduce additional rf pulses and delays. We show that the ALSOFAST HC-HMQC experiment, which implements an alternate means of frequency selection, enables the detection of methyl resonances with large secondary proton chemical shifts. This is achieved by selecting coherences of interest via a frequency selective carbon inversion pulse. Detailed comparisons between SOFAST- and the presented ALSOFAST-HMQC experiment reveals a considerable degree of mutual complementarity.

Publisher's Note: Large proton contribution to the2+excitation inMg20studied by intermediate energy inelastic scattering [Phys. Rev. C 78, 024306 (2008)

Publisher's Note: Large proton contribution to the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mn>2</mml:mn><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>excitation in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Mg</mml:mi><mml:mprescripts /><mml:none /><mml:mrow><mml:mn>20</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>studied by intermediate energy inelastic scattering [Phys. Rev. C 78, 024306 (2008)

A Nickel Hydride Complex in the Active Site of Methyl-Coenzyme M Reductase: Implications for the Catalytic Cycle

Methanogenic archaea utilize a specific pathway in their metabolism, converting C1 substrates (i.e., CO2) or acetate to methane and thereby providing energy for the cell. Methyl-coenzyme M reductase (MCR) catalyzes the key step in the process, namely methyl-coenzyme M (CH3-S-CoM) plus coenzyme B (HS-CoB) to methane and CoM-S-S-CoB. The active site of MCR contains the nickel porphinoid F430. We report here on the coordinated ligands of the two paramagnetic MCR red2 states, induced when HS-CoM (a reversible competitive inhibitor) and the second substrate HS-CoB or its analogue CH3-S-CoB are added to the enzyme in the active MCR red1 state (Ni(I)F430). Continuous wave and pulse EPR spectroscopy are used to show that the MCR red2a state exhibits a very large proton hyperfine interaction with principal values A((1)H) = [-43,-42,-5] MHz and thus represents formally a Ni(III)F430 hydride complex formed by oxidative addition to Ni(I). In view of the known ability of nickel hydrides to activate methane, and the growing body of evidence for the involvement of MCR in "reverse" methanogenesis (anaerobic oxidation of methane), we believe that the nickel hydride complex reported here could play a key role in helping to understand both the mechanism of "reverse" and "forward" methanogenesis.

A Rigorous Framework To Interpret Water Relaxivity. The Case Study of a Gd(III) Complex with an α-Cyclodextrin Derivative

We present a general theoretical framework suitable for an economical, but rigorous, analysis of the relaxivity and EPR data of paramagnetic metal complexes. This framework is based on the so-called Grenoble method that properly accounts for the fluctuations of the "static" zero-field splitting Hamiltonian and avoids the misinterpretation of experimental data, which occurs with the Solomon, Bloembergen, and Morgan (SBM) formalism and may lead to erroneous conclusions. The applicability of the SBM approximation is discussed. Our approach is implemented in the case of a new Gd(3+) chelate with a cyclodextrin derivative ligand hexakis(2-O-carboxymethyl-3,6-anhydro)-alpha-cyclodextrin (ACX), designed to obtain lanthanide complexes of enhanced stability in comparison to natural cyclodextrins. The introduction of carboxymethyl units on the six residual hydroxyl groups of an alpha-per-3,6-anhydro cyclodextrin leads to mono- and binuclear Ln(3+) complexes with log beta(110) approximately = 7.5. The GdACX complex induces large water proton relaxivity in 0.1 M KCl aqueous solution. The molecular parameters governing the longitudinal (r1) and transverse (r2) relaxivities above 1 T are obtained through simple SBM-like theoretical expressions and complementary experimental techniques. The metal hydration state, the translational diffusion coefficient of the complex, and its rotational correlation time are derived from luminescence lifetime studies, pulse-field gradient NMR, and deuteron quadrupolar relaxation, respectively. The high relaxivity induced by the GdACX complex is attributed to its high hydration state in the presence of potassium ions and to a rotational correlation time lengthened by the hydrophilic character of the ACX scaffold.

Shell states of neutron-rich matter

The equation of state (EOS) for nuclear and neutron rich matter is investigated in a relativistic mean field (RMF) model. New shell states are found that minimize the free energy per baryon, calculated in a spherical Wigner-Seitz (WS) approximation, over a significant range of baryon densities. These shell states, that have both inside and outside surfaces, minimize the Coulomb energy of large proton number configurations at the expense of a larger surface energy. This is related to a possible depression in the central density of super heavy nuclei. As the baryon density increases, we find the system changes from normal nuclei, to shell states, and then to uniform matter.

Multimeric nature of voltage-gated proton channels

Voltage-gated potassium channels are comprised of four subunits, and each subunit has a pore domain and a voltage-sensing domain (VSD). The four pore domains assemble to form one single central pore, and the four individual VSDs control the gate of the pore. Recently, a family of voltage-gated proton channels, such as H(V) or voltage sensor only protein (VSOP), was discovered that contain a single VSD but no pore domain. It has been assumed that VSOP channels are monomeric and contain a single VSD that functions as both the VSD and the pore domain. It remains unclear, however, how a protein that contains only a VSD and no pore domain can conduct ions. Using fluorescence measurements and immunoprecipitation techniques, we show here that VSOP channels are expressed as multimeric channels. Further, FRET experiments on constructs with covalently linked subunits show that VSOP channels are dimers. Truncation of the cytoplasmic regions of VSOP reduced the dimerization, suggesting that the dimerization is caused mainly by cytoplasmic protein-protein interactions. However, these N terminus- and C terminus-deleted channels displayed large proton currents. Therefore, we conclude that even though VSOP channels are expressed mainly as dimers in the cell membrane, single VSOP subunits could function independently as proton channels.

Very fast prediction and rationalization of pKa values for protein–ligand complexes

The PROPKA method for the prediction of the pK(a) values of ionizable residues in proteins is extended to include the effect of non-proteinaceous ligands on protein pK(a) values as well as predict the change in pK(a) values of ionizable groups on the ligand itself. This new version of PROPKA (PROPKA 2.0) is, as much as possible, developed by adapting the empirical rules underlying PROPKA 1.0 to ligand functional groups. Thus, the speed of PROPKA is retained, so that the pK(a) values of all ionizable groups are computed in a matter of seconds for most proteins. This adaptation is validated by comparing PROPKA 2.0 predictions to experimental data for 26 protein-ligand complexes including trypsin, thrombin, three pepsins, HIV-1 protease, chymotrypsin, xylanase, hydroxynitrile lyase, and dihydrofolate reductase. For trypsin and thrombin, large protonation state changes (|n| > 0.5) have been observed experimentally for 4 out of 14 ligand complexes. PROPKA 2.0 and Klebe's PEOE approach (Czodrowski P et al. J Mol Biol 2007;367:1347-1356) both identify three of the four large protonation state changes. The protonation state changes due to plasmepsin II, cathepsin D and endothiapepsin binding to pepstatin are predicted to within 0.4 proton units at pH 6.5 and 7.0, respectively. The PROPKA 2.0 results indicate that structural changes due to ligand binding contribute significantly to the proton uptake/release, as do residues far away from the binding site, primarily due to the change in the local environment of a particular residue and hence the change in the local hydrogen bonding network. Overall the results suggest that PROPKA 2.0 provides a good description of the protein-ligand interactions that have an important effect on the pK(a) values of titratable groups, thereby permitting fast and accurate determination of the protonation states of key residues and ligand functional groups within the binding or active site of a protein.

The TOTEM experiment: total cross-section measurement and soft diffraction at LHC

The TOTEM experiment intends to measure the total proton proton cross-section at 14 TeV and the LHC luminosity with a precision of ≤ 5 % in short runs with a special β* = 90 m optics at an early stage of LHC operation. Furthermore, this will allow a measurement of the elastic proton proton scattering cross-section in a |t|-range from 0.04 to 1 GeV2, a study of soft proton proton diffraction with a large diffractive proton acceptance as well as provide precise information on the horizontal distribution of the proton proton interaction vertices.

Non‐photochemical loss in PSII in high‐ and low‐light‐grown leaves of Vicia faba quantified by several fluorescence parameters including LNP, , a novel parameter

Using the expression of fluorescence originated from the PSII open reaction center in the light by Oxborough and Baker (1997), we obtained a formula that expresses relationships between the quantum efficiency of PSII photochemistry in the dark (Phi(m)= F(v)/F(m)) and in the light Phi'(m)=F'(v)/F'(m):Phi'(m)=Phi(m)+L(NP), where L(NP)(=F(0)/F'(m)) denotes the quantum yield of light induced non-photochemical losses (heat dissipation and fluorescence de-excitation) in PSII. Using L(NP) and other conventional fluorescence parameters, we conducted quenching analyses with leaves of broad bean plants (Vicia faba L.) grown at 700 (high light; HL) and 80 mumol photons m(-2) s(-1) (low light; LL). We also examined whether behavior of q(0) quenching (q(0)=1-F'(0)/F(0)) is related to the reaction center quenching. When the actinic light (AL) was strong, Stern-Volmer quenching [NPQ=(F(m)-F'(m))/F'(m)] and L(NP) increased rapidly and then decreased slowly in HL leaves, while, in LL leaves, they increased slowly. It is probable that rapid formation of a large proton gradient was responsible for sharp rises in both parameters in HL leaves. The steady-state 'excess' parameter [Phi(Ex)= (1 - qP) Phi(m)/(Phi(m)+ L(NP))], fraction of energy migrating to closed PSII centers, increased with the photon flux density of AL in LL leaves. In contrast, in HL leaves, Phi(Ex) did not increase markedly. The examination of the relationship between Phi(Ex) and L(NP) obtained at various AL revealed that in LL leaves the increase in (1 - qP) with the increase in AL prevailed, while, in HL leaves, the increase in L(NP) suppressed the increase in (1 - qP). Using the difference between L(NP) and L(D) (Phi(ND)= L(NP)- L(D), where L(D)= F(0)/F(m)), q(0) and qN (=1-F'(v)/F(v)) were calculated without using measured F'(0). The relationships between q(0) and qN thus obtained for various AL levels were almost identical for both HL and LL leaves, implying no difference in the fluorescence origin between the HL and LL leaves. Usefulness of these equations expressing non-photochemical loss is discussed.

Short- and medium-term atmospheric constituent effects of very large solar proton events

Abstract. Solar eruptions sometimes produce protons, which impact the Earth's atmosphere. These solar proton events (SPEs) generally last a few days and produce high energy particles that precipitate into the Earth's atmosphere. The protons cause ionization and dissociation processes that ultimately lead to an enhancement of odd-hydrogen and odd-nitrogen in the polar cap regions (&gt;60° geomagnetic latitude). We have used the Whole Atmosphere Community Climate Model (WACCM3) to study the atmospheric impact of SPEs over the period 1963–2005. The very largest SPEs were found to be the most important and caused atmospheric effects that lasted several months after the events. We present the short- and medium-term (days to a few months) atmospheric influence of the four largest SPEs in the past 45 years (August 1972; October 1989; July 2000; and October–November 2003) as computed by WACCM3 and observed by satellite instruments. Polar mesospheric NOx (NO+NO2) increased by over 50 ppbv and mesospheric ozone decreased by over 30% during these very large SPEs. Changes in HNO3, N2O5, ClONO2, HOCl, and ClO were indirectly caused by the very large SPEs in October–November 2003, were simulated by WACCM3, and previously measured by Envisat Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). WACCM3 output was also represented by sampling with the MIPAS averaging kernel for a more valid comparison. Although qualitatively similar, there are discrepancies between the model and measurement with WACCM3 predicted HNO3 and ClONO2 enhancements being smaller than measured and N2O5 enhancements being larger than measured. The HOCl enhancements were fairly similar in amounts and temporal variation in WACCM3 and MIPAS. WACCM3 simulated ClO decreases below 50 km, whereas MIPAS mainly observed increases, a very perplexing difference. Upper stratospheric and lower mesospheric NOx increased by over 10 ppbv and was transported during polar night down to the middle stratosphere in several weeks past the SPE. The WACCM3 simulations confirmed the SH HALOE observations of enhanced NOx in September 2000 as a result of the July 2000 SPE and the NH SAGE II observations of enhanced NO2 in March 1990 as a result of the October 1989 SPEs.

Beam–beam dynamics in an electron–ion collider with detector-integrated dipole

To maximize luminosity in a collider, low- β quadrupoles need to be installed as close as possible to the interaction point (IP), where beams are focussed to small spot sizes. In an asymmetric collider such as an asymmetric B-factory [An Asymmetric B Factory Based on PEP: Conceptual Design Report, SLAC-0372 [1]; KEKB B-Factory Design Report, KEK Report 95-7 [2]] or an electron–ion collider [Hera—A Proposal for a Large Electron Proton Colliding Beam Facility at DESY, DESY HERA 81-10 [3]; M. Farkhondeh, V. Ptitsyn (Eds.), eRHIC Zeroth Order Design Report, BNL Note C-A/AP/142; L. Merminga, Y. Derbenev, ELIC: an electron-light ion collider at CEBAF, ICFA Beam Dynamics Newsletter No. 30, 2003, p. 22; J.B. Dainton, M. Klein, P. Newman, E. Perez, F. Willeke, Deep Inelastic Electron–Nucleon Scattering at the LHC, DESY 06-006, and Cockcroft-06-04], beams of different energy are brought into collision at the IP. The energy difference requires separate focusing systems for the two beams. Placing regular separator dipoles between the IP and the focussing system either reduces the available space for the detector and therefore the detector acceptance or forces the low- β quads to be placed further away from the IP, resulting in reduced luminosity. Combining the separation and focusing functions of the innermost elements by using off-center quadrupoles and/or additional dipole coils on superconducting low- β quadrupoles ameliorates the problem somewhat. However, even in this case the separation occurs rather far away from the interaction point, leading to a large width of the synchrotron radiation fan generated by the deflection of the lower-energy electron or positron beam. To overcome these limitations in the eRHIC design, we have proposed integration of dipole coils in the (superconducting) detector solenoid [C. Montag, et al., Interaction region design for the electron–ion collider eRHIC, in: Proceedings of the PAC 2005, 2005], a concept that had originally been proposed for the Linear Collider detector solenoid [B. Parker, A. Seryi, Phys. Rev. ST Accel. Beams 8 (2005) 041001]. This results in a curved trajectory of the lower-energy electron beam across the interaction point, and therefore in a considerable transverse offset of the head and tail of the long ion bunches with respect to the (short) electron bunches. This paper examines beam dynamics issues arising from the collision of a long high-energy ion beam with a lower-energy electron beam on a trajectory curved by the detector-integrated dipole.

Na+/H+ exchange and pH regulation in the control of neutrophil chemokinesis and chemotaxis

Large proton fluxes accompany cell migration, but their precise role remains unclear. We studied pH regulation during the course of chemokinesis and chemotaxis in human neutrophils stimulated by attractant peptides. Activation of cell motility by chemoattractants was accompanied by a marked increase in metabolic acid generation, attributable to energy consumption by the contractile machinery and to stimulation of the NADPH oxidase and the ancillary hexose monophosphate shunt. Despite the increase in acid production, the cytosol underwent a sizable alkalinization, caused by acceleration of Na(+)/H(+) exchange. The development of the alkalinization mirrored the increase in the rate of cell migration, suggesting a causal relationship. However, elimination of Na(+)/H(+) exchange by omission of external Na(+) or by addition of potent inhibitors was without effect on either chemokinesis or chemotaxis, provided the cytosolic pH remained near neutrality. At more acidic levels, cell motility was progressively inhibited. These observations suggest that Na(+)/H(+) exchange plays a permissive role in cell motility but is not required for the initiation or development of the migratory response. Chemokinesis also was found to be exquisitely sensitive to extracellular acidification. This property may account for the inability of neutrophils to access abscesses and solid tumors that have been reported to have inordinately low pH.

NOx enhancements in the middle atmosphere during 2003–2004 polar winter: Relative significance of solar proton events and the aurora as a source

In this study we combine odd nitrogen (NOx) observations from the GOMOS and POAM III instruments with a radio wave ionization index to provide a detailed description of the generation and descent of polar NOx into the upper stratosphere during the Northern Hemisphere winter of 2003–2004. The measurements are used to study the relative contributions of ionization due to solar proton events, energetic electron precipitation, and low‐energy (1–10 keV) electron precipitation on NOx production, and its subsequent downward transport to the upper stratosphere. We show that NOx generated from the large solar proton storm in October/November 2003 was transported into the upper stratosphere in agreement with model calculations, but that aurorally generated NOx also descended later in the winter. Both periods were highly significant and produced large long‐lived decreases in stratospheric ozone once it arrived at those altitudes. The observations made by GOMOS deep into the nighttime polar vortex are critical in differentiating between the stratospheric effects of these two events.

Bio-Inspired Membranes for Advanced Polymer Electrolyte Fuel Cells. Anhydrous Proton-Conducting Membrane via Molecular Self-Assembly

Abstract Bio-inspired materials or natural biopolymers have attracted much attention for industrial applications as biocompatible materials. Although these materials have been discarded as industrial waste around the world, there is an increasing interest in environmental, engineering, and electrical applications of the biomaterials because of their well-controlled structures and superior conducting properties. The controlled structures of the hydrogen-bonding network in biomolecules, such as bioenergetic proteins, are essential for efficient energy transduction in living systems and application of a biomolecular mechanism could make it possible to prepare efficient electrolytes with superior conducting properties. We report here a recent investigation on the new class and new concept of electrolyte materials with bio-inspired molecular architectures of acid–base pairs for anhydrous proton conduction and its application in intermediate temperature (100–200 °C) polymer electrolyte fuel cells (PEFC). Although some membranes were composed from entirely biomolecular materials, they showed large anhydrous proton conductivities from room temperature to 160 °C, and fuel cells employing biomembranes as a polymer electrolyte produced electrical power under non-humidified H2/O2 condition at 160 °C. These bio-inspired materials may have many potential applications not only because of its superior ion conducting properties, in particular, under anhydrous (water-free) or extremely low-humidity conditions, but also because of its biocompatibility.

Strange dibaryon in the bound kaon approach to the Skyrme model

The bound kaon approach to the Skyrme model is applied to exploring the possibility of deeply bound ppK - states. We derive the equation of motion for the kaon in the background field of baryon number 2 Skyrmion, which is expressed by product ansatz. Each Skyrmion is rotated in the space of SU(2) collective coordinate and the rotational motion is quantized to specify the spin and the isospin of the nucleon. We consider the spin-singlet proton–proton state. The equation of motion is numerically solved. We find that the s-wave kaon can acquire large binding energy for relatively large proton–proton distances. For instance, the binding energy for R = 2.0 fm is B K = 220 MeV . The kaon localizes in the region between the protons. The obtained results suggest the existence of the deeply bound ppK - states.

Production of Mesons and Baryons at High Rapidity and HighpTin Proton-Proton Collisions ats=200 GeV

We present particle spectra for charged hadrons pi(+/-), K(+/-), p, and p[over] from pp collisions at square root[s] = 200 GeV measured for the first time at forward rapidities (2.95 and 3.3). The kinematics of these measurements are skewed in a way that probes the small momentum fraction in one of the protons and large fractions in the other. Large proton to pion ratios are observed at values of transverse momentum that extend up to 4 GeV/c, where protons have momenta up to 35 GeV. Next-to-leading order perturbative QCD calculations describe the production of pions and kaons well at these rapidities, but fail to account for the large proton yields and small p[over]/p ratios.

TH‐C‐M100F‐06: Therapeutic Uniform Scanning Proton Beam Development and Characterization: Longitudinal Results

Purpose: To utilize a new depth‐dose detector and to construct, develop, characterize and quantify longitudinal beam profiles with an active uniform scanning proton beam delivery system. Method and Materials: The active uniform scanning beam delivery system is composed of two primary devices—wobbler magnet and binary range modulator. Through transverse scanning a large proton beam spot at about 10–15Hz, the wobbler magnet creates a flat integral transverse field. The binary range modulator constructs spread out Bragg peaks (SOBP) by incrementally stepping a pristine Bragg peak by 3mm or 6mm in depth. A depth‐dose detector was constructed to acquire all the longitudinal beam data. This detector, the Multi Ion Chamber Detector (MLIC), is composed of 122 parallel‐plate ion chambers in a 1D array. Results: Upon proper calibration and commissioning, the MLIC detector performed well for the acquisition of longitudinal beam data, providing adequate discrete data with 1.8mm spatial resolution. The full width half maximum of various energy pristine Bragg peaks was measured. This data necessitated changing the range modulator step size from 6mm to 3mm for all SOBPs below a certain proton energy (i.e. equivalent to 12.0cm range in water). The pristine peak and SOBP exhibit a range deficit that decreases as proton energy decreases. This effect is also correlated to the FWHM of the pristine peak. A library of various SOBP extents, from 14.5cm to 2.2cm, was constructed and shown to pass clinical specification. For a given SOBP, the skewness, defined as the slope of the 100% isodose or SOBP “flattop”, was correlated to proton range in water. Conclusion: The MLIC proved useful, efficient and satisfactory in measuring depth‐dose profiles for active scanning proton beams. The active scanning beam delivery system delivered therapeutically useable fields with increased modularity and flexibility compared to passive scattering techniques.