Search Results (21 total)

The acceleration of electrons to ≃0.8  GeV has been observed in a self-injecting laser wakefield accelerator driven at a plasma density of 5.5×10¹⁸  cm^-3 by a 10 J, 55 fs, 800 nm laser pulse in the blowout regime. The laser pulse is found to be self-guided for 1 cm (>10z_R), by measurement of a single filament containing >30% of the initial laser energy at this distance. Three-dimensional particle in cell simulations show that the intensity within the guided filament is amplified beyond its initial focused value to a normalized vector potential of a₀>6, thus driving a highly nonlinear plasma wave.

Results are reported on the measurement of the atmospheric neutrino-induced muon flux at a depth of 2 kilometers below the Earth’s surface from 1229 days of operation of the Sudbury Neutrino Observatory (SNO). By measuring the flux of through-going muons as a function of zenith angle, the SNO experiment can distinguish between the oscillated and unoscillated portion of the neutrino flux. A total of 514 muonlike events are measured between -1≤cos⁡θ_zenith≤0.4 in a total exposure of 2.30×10¹⁴  cm² s. The measured flux normalization is 1.22±0.09 times the Bartol three-dimensional flux prediction. This is the first measurement of the neutrino-induced flux where neutrino oscillations are minimized. The zenith distribution is consistent with previously measured atmospheric neutrino oscillation parameters. The cosmic ray muon flux at SNO with zenith angle cos⁡θ_zenith>0.4 is measured to be (3.31±0.01(stat)±0.09(sys))×10^-10  μ/s/cm².

Coherent Smith-Purcell (SP) radiation originating from three different gratings has been measured at End Station A, SLAC, and has been used to reconstruct the time profile of the electron bunches. The beam energy during these experiments was 28.5 GeV (γ≅55 773) and the number of electrons in the bunch was 0.9–1.4×10¹⁰. The spectral distribution of the radiated energy was measured by means of an array of 11 pyroelectric detectors. Typical values of the FWHM of the bunch length are about 2.5 ps, but sharper peaks with FWHM less than 2.0 ps have also been observed. The longitudinal profile also varies with accelerator conditions and can best be approximated by a superposition of 3–4 Gaussian curves. Some typical profiles are presented, together with a discussion of the limitations and strengths of coherent SP radiation as a diagnostic tool. It is concluded that SP radiation offers excellent prospects in this respect, not only in the picosecond range, but potentially in the femtosecond range as well.

The Sudbury Neutrino Observatory (SNO) used an array of ³He proportional counters to measure the rate of neutral-current interactions in heavy water and precisely determined the total active (ν_x) ⁸B solar neutrino flux. This technique is independent of previous methods employed by SNO. The total flux is found to be 5.54_-0.31^+0.33(stat)_-0.34^+0.36(syst)×10⁶  cm^-2 s^-1, in agreement with previous measurements and standard solar models. A global analysis of solar and reactor neutrino results yields Δm²=7.59_-0.21^+0.19×10^-5  eV² and θ=34.4_-1.2^+1.3 degrees. The uncertainty on the mixing angle has been reduced from SNO’s previous results.

This article provides the complete description of results from the Phase I data set of the Sudbury Neutrino Observatory (SNO). The Phase I data set is based on a 0.65 kiloton-year exposure of ²H₂O (in the following denoted as D₂O) to the solar ⁸B neutrino flux. Included here are details of the SNO physics and detector model, evaluations of systematic uncertainties, and estimates of backgrounds. Also discussed are SNO's approach to statistical extraction of the signals from the three neutrino reactions (charged current, neutral current, and elastic scattering) and the results of a search for a day-night asymmetry in the ν_e flux. Under the assumption that the ⁸B spectrum is undistorted, the measurements from this phase yield a solar ν_e flux of ϕ(ν_e)=1.76_-0.05^+0.05(stat.)_-0.09^+0.09(syst.)×10⁶ cm^-2 s^-1 and a non-ν_e component of ϕ(ν_μτ)=3.41_-0.45^+0.45(stat.)_-0.45^+0.48(syst.)×10⁶ cm^-2 s^-1. The sum of these components provides a total flux in excellent agreement with the predictions of standard solar models. The day-night asymmetry in the ν_e flux is found to be A_e=7.0±4.9(stat.)_-1.2^+1.3%(syst.), when the asymmetry in the total flux is constrained to be zero.

We have used coherent Smith-Purcell radiation in order to investigate the longitudinal (temporal) profile of the electron bunch at the FELIX facility. Detection of the far-infrared radiation was achieved by a simple and compact experimental arrangement, consisting of an array of 11 room-temperature pyroelectric detectors. Accurate determination of the background radiation, use of high quality optical filters, and an efficient light collection system are essential for this type of experiment. The radiated power is in good agreement with the predictions of the surface current description of this process. It is concluded that 90% of the bunch particles are contained within 5.5 ps, with a temporal profile that could be approximately triangular in shape.

Results are reported from the complete salt phase of the Sudbury Neutrino Observatory experiment in which NaCl was dissolved in the ²H₂O (“D₂O”) target. The addition of salt enhanced the signal from neutron capture as compared to the pure D₂O detector. By making a statistical separation of charged-current events from other types based on event-isotropy criteria, the effective electron recoil energy spectrum has been extracted. In units of 10⁶cm^-2s^-1, the total flux of active-flavor neutrinos from ⁸B decay in the Sun is found to be 4.94_-0.21^+0.21(stat)_-0.34^+0.38(syst) and the integral flux of electron neutrinos for an undistorted ⁸B spectrum is 1.68_-0.06^+0.06(stat)_-0.09^+0.08(syst); the signal from (ν_x,e) elastic scattering is equivalent to an electron-neutrino flux of 2.35_-0.22^+0.22(stat)_-0.15^+0.15(syst). These results are consistent with those expected for neutrino oscillations with the so-called large mixing angle parameters and also with an undistorted spectrum. A search for matter-enhancement effects in the Earth through a possible day-night asymmetry in the charged-current integral rate is consistent with no asymmetry. Including results from other experiments, the best-fit values for two-neutrino mixing parameters are Δm²=(8.0_-0.4^+0.6)×10^-5 eV² and θ=33.9_-2.2^+2.4 degrees.

A search has been made for sinusoidal periodic variations in the ⁸B solar neutrino flux using data collected by the Sudbury Neutrino Observatory over a 4-year time interval. The variation at a period of 1 yr is consistent with modulation of the ⁸B neutrino flux by the Earth’s orbital eccentricity. No significant sinusoidal periodicities are found with periods between 1 d and 10 years with either an unbinned maximum likelihood analysis or a Lomb-Scargle periodogram analysis. The data are inconsistent with the hypothesis that the results of the recent analysis by Sturrock et al., based on elastic scattering events in Super-Kamiokande, can be attributed to a 7% sinusoidal modulation of the total ⁸B neutrino flux.

The passage of a finely focused electron beam near the surface of a periodic metallic grating produces radiation known as Smith-Purcell radiation. This paper presents an analysis of the role of the grating profile in the case of echelle-type gratings whose period consists of two facets only. Particular emphasis is placed on the ultrarelativistic regime and a comparison is made with recent experimental results in this region. It is shown that the details of the profile of the grating play an important role in the optimization of the radiated energy. The behavior of higher order modes and the limitations of the surface current description of the radiative process are also discussed briefly.

Upper limits on the ν̅ _e flux at the Sudbury Neutrino Observatory have been set based on the ν̅ _e charged-current reaction on deuterium. The reaction produces a positron and two neutrons in coincidence. This distinctive signature allows a search with very low background for ν̅ _e’s from the Sun and other potential sources. Both differential and integral limits on the ν̅ _e flux have been placed in the energy range from 4–14.8 MeV. For an energy-independent ν_e→ν̅ _e conversion mechanism, the integral limit on the flux of solar ν̅ _e’s in the energy range from 4–14.8 MeV is found to be Φ_{ν̅ e}≤3.4×10⁴  cm^-2 s^-1 (90% C.L.), which corresponds to 0.81% of the standard solar model ⁸B ν_e flux of 5.05×10⁶  cm^-2 s^-1, and is consistent with the more sensitive limit from KamLAND in the 8.3–14.8 MeV range of 3.7×10²  cm^-2 s^-1 (90% C.L.). In the energy range from 4–8 MeV, a search for ν̅ _e’s is conducted using coincidences in which only the two neutrons are detected. Assuming a ν̅ _e spectrum for the neutron induced fission of naturally occurring elements, a flux limit of Φ_{ν̅ e}≤2.0×10⁶  cm^-2 s^-1 (90% C.L.) is obtained.

The Sudbury Neutrino Observatory has precisely determined the total active (ν_x) ⁸B solar neutrino flux without assumptions about the energy dependence of the ν_e survival probability. The measurements were made with dissolved NaCl in heavy water to enhance the sensitivity and signature for neutral-current interactions. The flux is found to be 5.21±0.27(stat)±0.38(syst)×10⁶   cm^-2  s^-1, in agreement with previous measurements and standard solar models. A global analysis of these and other solar and reactor neutrino results yields Δm²=7.1_-0.6^+1.2×10^-5   eV² and θ=32.5_-2.3^+2.4 degrees. Maximal mixing is rejected at the equivalent of 5.4 standard deviations.

Data from the Sudbury Neutrino Observatory have been used to constrain the lifetime for nucleon decay to “invisible” modes, such as n→3ν. The analysis was based on a search for γ rays from the deexcitation of the residual nucleus that would result from the disappearance of either a proton or neutron from ¹⁶O. A limit of τ_inv>2×10²⁹  yr is obtained at 90% confidence for either neutron- or proton-decay modes. This is about an order of magnitude more stringent than previous constraints on invisible proton-decay modes and 400 times more stringent than similar neutron modes.

Coherent enhancement of the Smith-Purcell radiation produced from the interaction of a 1.8 MeV electron beam with a grating has been observed. The emitted radiation has been measured at angles in the 40° to 120° range, which correspond to wavelengths from 0.65 to 4 mm, approximately. The radiated power was 320 mW at 90°. Its angular distribution agrees well with the description of the process in terms of induced surface currents and has been used to infer the longitudinal profile of the electron bunch. It is concluded that the bunch has an approximately triangular profile, with 85% of the bunch particles contained within 14 ps. The possibilities of the technique as a bunch-shape diagnostic tool are also discussed.

The Sudbury Neutrino Observatory (SNO) has measured day and night solar neutrino energy spectra and rates. For charged current events, assuming an undistorted ⁸B spectrum, the night minus day rate is 14.0%±6.3%_-1.4^+1.5% of the average rate. If the total flux of active neutrinos is additionally constrained to have no asymmetry, the ν_e asymmetry is found to be 7.0%±4.9%_-1.2^+1.3%. A global solar neutrino analysis in terms of matter-enhanced oscillations of two active flavors strongly favors the large mixing angle solution.

Observations of neutral-current ν interactions on deuterium in the Sudbury Neutrino Observatory are reported. Using the neutral current (NC), elastic scattering, and charged current reactions and assuming the standard ⁸B shape, the ν_e component of the ⁸B solar flux is φ_e  =  1.76_-0.05^+0.05(stat)_-0.09^+0.09(syst)×10⁶  cm^-2 s^-1 for a kinetic energy threshold of 5 MeV. The non- ν_e component is φ_μτ  =  3.41_-0.45^+0.45(stat)_-0.45^+0.48(syst)×10⁶  cm^-2 s^-1, 5.3σ greater than zero, providing strong evidence for solar ν_e flavor transformation. The total flux measured with the NC reaction is φ_NC  =  5.09_-0.43^+0.44(stat)_-0.43^+0.46(syst)×10⁶  cm^-2 s^-1, consistent with solar models.

Solar neutrinos from ⁸B decay have been detected at the Sudbury Neutrino Observatory via the charged current (CC) reaction on deuterium and the elastic scattering (ES) of electrons. The flux of ν_e’s is measured by the CC reaction rate to be φ^CC(ν_e)  =  1.75±0.07(stat)_-0.11^+0.12(syst)±0.05(theor)×10⁶ cm^-2 s^-1. Comparison of φ^CC(ν_e) to the Super-Kamiokande Collaboration’s precision value of the flux inferred from the ES reaction yields a 3.3σ difference, assuming the systematic uncertainties are normally distributed, providing evidence of an active non- ν_e component in the solar flux. The total flux of active ⁸B neutrinos is determined to be 5.44±0.99×10⁶ cm^-2 s^-1.

A theoretical analysis of Smith-Purcell radiation at very high energies is presented. The energy per unit frequency and solid angle is expressed in closed form as a function of the grating geometry, beam energy, and viewing angles. A certain choice of grating geometry is shown to optimize the output energy for a particular order of radiation. Scaling laws are derived for the energy emitted into all orders of radiation in the relativistic limit. It is shown that the total energy emitted into each order scales as the three-halves power of the beam voltage.

An analytic solution for the radiated intensity distribution produced by an electron beam passing over a metallic diffraction grating (the Smith-Purcell effect) is derived. The approach is based upon an expression for the current traveling over the grating surface and the method can deal with arbitrary grating profiles. Although collective behavior in the electron beam is neglected, very high power density is predicted if high energy, short electron bunches are employed. The electron beam characteristics of various accelerators are used to illustrate the potential of high energy, accelerator based Smith-Purcell radiation sources.

A beam of 3.6-MeV electrons has been used to study the generation of radiation in the far infrared (FIR) by the Smith-Purcell mechanism. The dependence of wavelength on angle of emission, over angles from 56° to 150° and wavelengths from 350 to 1860 μm, is in excellent agreement with the Smith-Purcell dispersion relation. Comparison of the yield with that from a 5000-K source suggests that the spontaneous Smith-Purcell effect offers an easily tunable alternative to the synchrotron as a coherent FIR source, and that it could also form the basis of an inexpensive, compact free-electron laser.