Search Results (39 total)

We report the first measurement of an angular correlation parameter in neutron β decay using polarized ultracold neutrons (UCN). We utilize UCN with energies below about 200 neV, which we guide and store for ∼30  s in a Cu decay volume. The interaction of the neutron magnetic dipole moment with a static 7 T field external to the decay volume provides a 420 neV potential energy barrier to the spin state parallel to the field, polarizing the UCN before they pass through an adiabatic fast passage spin flipper and enter a decay volume, situated within a 1 T field in a 2×2π solenoidal spectrometer. We determine a value for the β-asymmetry parameter A₀=-0.1138±0.0046±0.0021.

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q²=0.15, 0.25  (GeV/c)². The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A_n provides a direct probe of the imaginary component of the 2γ exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments.

We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12≤Q²≤1.0  GeV². These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q² dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.

We report on a precision measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A_PV=[-131±14(stat)±10(syst)]×10^-9, leading to the determination of the weak mixing angle sin⁡²θ_W^eff=0.2397±0.0010(stat)±0.0008(syst), evaluated at Q²=0.026  GeV². Combining this result with the measurements of sin⁡²θ_W^eff at the Z⁰ pole, the running of the weak mixing angle is observed with over 6σ significance. The measurement sets constraints on new physics effects at the TeV scale.

We have used element-specific hysteresis measurements, based on the x-ray magnetic circular dichroism technique, to investigate magnetic trilayer structures composed of Fe and Ni layers. Within a critical regime we have discovered a class of structures in which the exchange interaction, the mechanism responsible for the macroscopic magnetism, can become vanishingly small. The experimental observations are supported by first principles theory and are explained as arising from a cancellation of several competing magnetic interactions. Hence, we have discovered a system with a novel exchange interaction between magnetic layers in direct contact that replaces the conventional exchange interaction in ferromagnets.

We report a measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A_PV=[-175±30(stat)±20(syst)]×10^-9. This first direct observation of parity nonconservation in Møller scattering leads to a measurement of the electron’s weak charge at low energy Q_W^e=-0.053±0.011. This is consistent with the standard model expectation at the current level of precision: sin⁡²θ_W(M_Z)_MS̅ =0.2293±0.0024(stat)±0.0016(syst)±0.0006(theory).

We report on a new measurement of the parity-violating asymmetry in quasielastic electron scattering from the deuteron at backward angles at Q²=0.038 (GeV/c)². This quantity provides a determination of the neutral weak axial vector form factor of the nucleon, which can potentially receive large electroweak corrections. The measured asymmetry A=-3.51±0.57  (stat)±0.58  (syst)  ppm is consistent with theoretical predictions. We also report on updated results of the previous experiment at Q²=0.091 (GeV/c)², which are also consistent with theoretical predictions.

VISA (Visible to Infrared SASE Amplifier) is a high-gain self-amplified spontaneous emission (SASE) free-electron laser (FEL), which achieved saturation at 840 nm within a single-pass 4-m undulator. The experiment was performed at the Accelerator Test Facility at BNL, using a high brightness 70-MeV electron beam. A gain length shorter than 18 cm has been obtained, yielding a total gain of 2×10⁸ at saturation. The FEL performance, including the spectral, angular, and statistical properties of SASE radiation, has been characterized for different electron beam conditions. Results are compared to the three-dimensional SASE FEL theory and start-to-end numerical simulations of the entire injector, transport, and FEL systems. An agreement between simulations and experimental results has been obtained at an unprecedented level of detail.

The visible-infrared self-amplified spontaneous emission amplifier (VISA) free electron laser (FEL) is an experimental device designed to show self-amplified spontaneous emission (SASE) to saturation in the near infrared to visible light energy range. It generates a resonant wavelength output from 800–600 nm, so that silicon detectors may be used to characterize the optical properties of the FEL radiation. VISA is designed to show how SASE FEL theory corresponds with experiment in this wavelength range, using an electron beam with emittance close to that planned for the future Linear Coherent Light Source at SLAC. VISA comprises a 4 m pure permanent magnet undulator with four 99 cm segments, each of 55 periods, 18 mm long. The undulator has distributed focusing built into it, to reduce the average beta function of the 70–85 MeV electron beam to about 30 cm. There are four FODO cells per segment. The permanent magnet focusing lattice consists of blocks mounted on either side of the electron beam, in the undulator gap. The most important undulator error parameter for a free electron laser is the trajectory walk-off, or lack of overlap of the photon and electron beams. Using pulsed wire magnet measurements and magnet shimming, we were able to control trajectory walk-off to less than ±50 μm per field gain length.

We report the first measurement of the vector analyzing power in inclusive transversely polarized elastic electron-proton scattering at Q²=0.1 (GeV/c)² and large scattering angles. This quantity must vanish in the single virtual photon exchange, plane-wave impulse approximation for this reaction, and can therefore provide information on two photon exchange amplitudes for electromagnetic interactions with hadronic systems. The observable we have measured is driven by the imaginary part of the two photon exchange amplitude, the hadronic side of which is simply the Compton amplitude for the proton with two virtual photons. We find a small but nonzero value of A=-15.4±5.4 ppm.

We report a new measurement of the parity-violating asymmetry in elastic electron scattering from the proton at backward scattering angles. This asymmetry is sensitive to the strange magnetic form factor of the proton as well as electroweak axial radiative corrections. The new measurement of A  =  -4.92±0.61±0.73 ppm provides a significant constraint on these quantities. The implications for the strange magnetic form factor are discussed in the context of theoretical estimates for the axial corrections.

Core-level x-ray absorption spectroscopy measurements have been performed at the L_3,2 edges of Fe and Co thin films. The degree of source coherence was varied using soft x rays from five different beam lines. We observe an increase of the L edge resonance intensities relative to the continuum states which we propose is due to the presence of transverse coherence in the exciting radiation.

In an x-ray free electron laser like the Linear Coherent Light Source being designed at SLAC, electron bunches about 70μm FWHM long are sent into a beam tube only 5 mm in internal diameter and more than 100 m in length. Because of the surface roughness of the beam tube, wakefields can be generated that catch up to the bunch and interact with it, causing energy spread and emittance growth. The strength of this effect depends on the details of the roughness of the surface. We present here a study in which the roughness of the beam tube was measured and the longitudinal impedance of the tube was calculated. Our result shows that a commercially available beam tube can be made smooth enough so the resulting wakefield effects are within the tolerance determined by the requirement that the induced relative energy spread of the beam be less than 5×10^-4.

Gamow-Teller transition strengths obtained in the ¹²⁷I(p,n)¹²⁷Xe reaction studied at 94, 159, and 197 MeV incident proton energies are presented, and used to evaluate the efficiency of ¹²⁷I as a solar neutrino detector. Excitation of the J^π=(3/2)⁺ first excited state at E_x=0.125 MeV in ¹²⁷Xe, sensitive to ⁷Be solar neutrinos, is evaluated. The sum of Gamow-Teller strength up to particle emission threshold and up to 20 MeV excitation energy are also reported. This paper also provides a new measurement of the Coulomb displacement energy ΔE_c, the excitation energy of the isobaric analog state, and the centroid of the Gamow-Teller resonance in ¹²⁷Xe.

We report measurements of very large output intensities corresponding to a gain larger than 10⁵ for a single pass free-electron laser operating in the self-amplified spontaneous emission (SASE) mode at 12 μm. We also report the observation and analysis of intensity fluctuations of the SASE radiation intensity in the high-gain regime. The results are compared with theoretical predictions and simulations.

Soft x-ray magnetic circular dichroism (MCD) data were collected at the Co L edges from a series of epitaxial R-Co (R=Pr, Nd, Sm, Dy, and Ho) intermetallic compound films grown by sputter deposition. The Co orbital-to-spin moment ratios were extracted from the data using the MCD sum rules. An enhanced Co orbital moment, as compared to that of bulk hcp-Co, is seen in all but one of the films. The enhancement is dependent on both the average R-Co bond length and on the R species. These results suggest that a significant transition-metal (TM) orbital moment is the origin of the TM sublattice contribution to the magnetocrystalline anisotropy energy in R-TM compounds.

We report the first measurement of the parity-violating asymmetry in elastic electron scattering from the proton. The asymmetry depends on the neutral weak magnetic form factor of the proton which contains new information on the contribution of strange quark-antiquark pairs to the magnetic moment of the proton. We obtain the value G_M^Z  =  0.34±0.09±0.04±0.05 n.m. at Q²  =  0.1 (GeV/c)².

We present the results of a measurement of the spin-dependent asymmetry in ³→(e→,e′) inelastic scattering at kinematics on the low-energy transfer side of the quasielastic peak, including the region near the breakup threshold. Comparison with existing calculations based upon the plane wave impulse approximation shows significant deviation between the data and the model near the breakup threshold. Good agreement between data and theory is seen at higher energy transfer.

We measured Faraday rotation of linearly polarized soft x rays across the Fe L_2,3 edges transmitted through an Fe/Cr multilayer in an applied magnetic field; the maximum rotation of 6×10⁴ deg/mm is larger than that observed for Fe in other spectral regions. Rotation data agree well with Kramers-Kronig analysis of absorption data measured using left and right circular polarization. A tunable elliptically polarizing undulator source provided linearly and circularly polarized x-ray beams. The tunable multilayer polarizer used extends optical rotation techniques into the 50–900-eV region, with element specificity from core resonances.

The transverse-longitudinal asymmetry A_TL^′ in ³He→(e→, e^′) quasielastic scattering at momentum transfer Q²=0.14 (GeV/c)² has been measured to be 1.52 ± 0.55(stat) ± 0.15(syst)%. The plane wave impulse approximation (PWIA) prediction for this measurement ranges from 2.1% to 2.9%, where the variation is due to uncertainty in the initial state wave function, nucleon form factors, and off-shell prescription. The data may suggest a suppression with respect to the PWIA, which has also been observed for the unpolarized longitudinal response function.

We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized ³He target. The neutron magnetic form factor G_Mⁿ has been extracted from the measured asymmetry based on recent PWIA calculations using spin-dependent spectral functions. Our determination of G_Mⁿ at Q²=0.19 (GeV/c)² agrees with the dipole parametrization. This experiment represents the first measurement of the neutron magnetic form factor using spin-dependent electron scattering.

Measurements of the spin-dependent asymmetry in scattering longitudinally polarized electrons from polarized He³ at quasielastic kinematics are reported. The measurements were made at two kinematics and spin angles, one sensitive to the helicity-dependent transverse-longitudinal interference response function, R_TL′, and the other to the helicity-dependent transverse response function, R_T′. For the experiment a metastability exchange optically pumped polarized He³ target was used; a general discussion of the technique used to polarized the He³, along with the details of the design and operation of the target system, are presented here. A comparison is made of the world’s data on the He→³(e→,e′) quasielastic asymmetry with several theoretical predictions, including calculations that use the plane wave impulse approximation and a fully spin-dependent spectral function. There is good agreement between data and theory at the current level of experimental precision.

We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized ³He gas target. The asymmetry is measured at kinematics sensitive to the transverse-longitudinal response function R_TL(Q²,ω). The value of the neutron electric form factor G_Eⁿ(Q²=0.16 (GeV/c²))=+0.070±0.100±0.035 is extracted from the asymmetry using a Faddeev calculation of the ³He wave function.

We have measured the carbon K near-edge x-ray-absorption fine structure of graphite by transmission, electron yield, and fluorescence yield. A rounded and prominent peak is identified in the near-edge spectrum, among the other well-known spectral features. With the aid of a full-potential linearized augmented-plane-wave band calculation, this feature is interpreted as originating from interlayer states in low-symmetry regions of the Brillouin zone. This finding is relevant to interpreting the near-edge structure of graphite intercalation compounds specially of the alkali-metal type.

A recent technique, based on differential subshell ionization by tuned synchrotron radiation, has been applied to measurements of L-subshell Coster-Kronig yields of Ni, Cu, and Mo from L₂ and L₃ Auger spectra. Results for Ni are f₂₃=0.6+0.2, f₁₂=0.4+0.2, and f₁₃=0.5+0.2; for Cu, f₂₃=0.8+0.1, f₁₂=0.44+0.06, and f₁₃=0.3+0.2; for Mo, f₂₃=0.15+0.02, f₁₂=0.15+0.02, and f₁₃=0.61+0.06. Measured transition probabilities are compared with previously available information. The results are generally consistent with the body of theoretical and experimental data. For Ni and Cu, the present measurements of f₂₃ confirm that the L₂-L₃M_4,5 Coster-Kronig channel is accessible in the metals, even though it is energetically cut off in free atoms. Further improvements in synchrotron-radiation sources are likely to make it possible to throw light on several critical questions by means of the present method.