Transmission of megawatt relativistic electron beams through millimeter apertures.

High-power, relativistic electron beams from energy-recovering linacs have great potential to realize new experimental paradigms for pioneering innovation in fundamental and applied research. A major design consideration for this new generation of experimental capabilities is the understanding of the halo associated with these bright, intense beams. In this Letter, we report on measurements performed using the 100 MeV, 430 kW cw electron beam from the energy-recovering linac at the Jefferson Laboratory's Free Electron Laser facility as it traversed a set of small apertures in a 127 mm long aluminum block. Thermal measurements of the block together with neutron measurements near the beam-target interaction point yielded a consistent understanding of the beam losses. These were determined to be 3 ppm through a 2 mm diameter aperture and were maintained during a 7 h continuous run.

Charge form factor of the neutron at low momentum transfer from the 2H-->(e-->,e'n)1H reaction.

We report new measurements of the neutron charge form factor at low momentum transfer using quasielastic electrodisintegration of the deuteron. Longitudinally polarized electrons at an energy of 850 MeV were scattered from an isotopically pure, highly polarized deuterium gas target. The scattered electrons and coincident neutrons were measured by the Bates Large Acceptance Spectrometer Toroid (BLAST) detector. The neutron form factor ratio GEn/GMn was extracted from the beam-target vector asymmetry AedV at four-momentum transfers Q2=0.14, 0.20, 0.29, and 0.42 (GeV/c)2.

-Measurement of the proton's electric to magnetic form factor ratio from 1H(over -->)(e(over -->),e'p).

We report the first precision measurement of the proton electric to magnetic form factor ratio from spin-dependent elastic scattering of longitudinally polarized electrons from a polarized hydrogen internal gas target. The measurement was performed at the MIT-Bates South Hall Ring over a range of four-momentum transfer squared Q2 from 0.15 to 0.65 (GeV/c)(2). Significantly improved results on the proton electric and magnetic form factors are obtained in combination with existing cross-section data on elastic electron-proton scattering in the same Q2 region.

Longitudinal-transverse separations of deep-inelastic structure functions at low Q2 for hydrogen and deuterium.

We report on a study of the longitudinal to transverse cross section ratio, R=sigmaL/sigmaT, at low values of x and Q2, as determined from inclusive inelastic electron-hydrogen and electron-deuterium scattering data from Jefferson Laboratory Hall C spanning the four-momentum transfer range 0.06

The role of tip plasmons in near-field Raman microscopy.

The enhancement in electric field strength in the vicinity of a metal tip, through the excitation of plasma modes in the tip, is investigated using the finite difference time domain method; such tip enhancement has significant potential for application in scanning near-field Raman microscopy. To represent an experimentally realistic geometry the near-field probe is described by a conical metal tip with a spherical apex, with radii 20 nm and 200 nm considered, in close proximity to a glass substrate. Illumination through the substrate is considered, both at normal incidence and close to the critical angle, with the polarization in the plane of incidence. By modelling the frequency dependent dielectric response of the metal tip we are able to highlight the dependence on the scattering geometry of the nature of the electromagnetic excitations in the tip. In particular, the strongest electric field enhancement with the greatest confinement occurs for the excitation of modes localized at the tip apex, excited only for off-normal incidence. Bulk modes excited in the tip also produce enhancement, although over a larger area and with significantly less enhancement than that of the localized modes; however, the excitation of bulk modes is independent of the angle of incidence.

Fluorescence scanning near-field optical microscopy of polyfluorene composites.

Fluorescence scanning near-field optical microscopy (SNOM) is used to investigate binary polyfluorene-based composites of varying composition. The samples investigated contain blends of the polymer poly(9,9'-dioctylfluorene-cobenzothiadiazole), F8BT, with similar polyfluorenes of wider band gap. Images acquired from a film containing 50% by weight F8BT exhibit a high degree of correlation between the topography and fluorescence, with an F8BT-rich phase which protrudes from the surface of the film forming isolated regions with sizes from hundreds of nanometres to several micrometres. A film containing 10% by weight F8BT also has micrometre-size F8BT-rich regions, but also present are small and locally varying proportions of F8BT in the other polyfluorene component phase, indicating a hierarchy of phases within this sample. The fluorescence and topographic images of a third sample studied, containing 90% by weight F8BT, display no correlation, demonstrating that it is not always appropriate to use topographic information to determine the phase structure within polymer blends. The fluorescence SNOM images acquired from these samples are able to assist our understanding of the photovoltaic efficiency of devices fabricated from these films, which are governed by the extent of the interfacial area between these two constituent polymers.