192 research outputs found
Measurement of the Neutral Weak Form Factors of the Proton
We have measured the parity-violating electroweak asymmetry in the elastic
scattering of polarized electrons from the proton. The kinematic point
(theta_lab = 12.3 degrees and Q^2=0.48 (GeV/c)^2) is chosen to provide
sensitivity, at a level that is of theoretical interest, to the strange
electric form factor G_E^s. The result, A=-14.5 +- 2.2 ppm, is consistent with
the electroweak Standard Model and no additional contributions from strange
quarks. In particular, the measurement implies G_E^s + 0.39G_M^s = 0.023 +-
0.034 (stat) +- 0.022 (syst) +- 0.026 (delta G_E^n), where the last uncertainty
arises from the estimated uncertainty in the neutron electric form factor.Comment: 10 pages, 4 figures, submitted to Phys. Rev. Let
Dynamics of the quasielastic ^(16)O(e,e'p) reaction at Q^2≈0.8 (GeV/c)^2
The physics program in Hall A at Jefferson Lab commenced in the summer of 1997 with a detailed investigation of the ^(16)O(e,e′p) reaction in quasielastic, constant (q,ω) kinematics at Q^2≈0.8(GeV/c)^2, q≈1GeV/c, and ω≈445MeV. Use of a self-calibrating, self-normalizing, thin-film waterfall target enabled a systematically rigorous measurement. Five-fold differential cross-section data for the removal of protons from the 1p-shell have been obtained for 0<p_miss<350MeV/c. Six-fold differential cross-section data for 0<E_miss<120MeV were obtained for 0<p_miss<340MeV/c. These results have been used to extract the ALT asymmetry and the R_L, R_T, R_LT, and R_(L+TT) effective response functions over a large range of E_miss and p_miss. Detailed comparisons of the 1p-shell data with Relativistic Distorted-Wave Impulse Approximation (RDWIA), Relativistic Optical-Model Eikonal Approximation (ROMEA), and Relativistic Multiple-Scattering Glauber Approximation (RMSGA) calculations indicate that two-body currents stemming from meson-exchange currents (MEC) and isobar currents (IC) are not needed to explain the data at this Q^2. Further, dynamical relativistic effects are strongly indicated by the observed structure in ALT at p_miss≈300MeV/c. For 25<E_miss<50MeV and p_miss≈50MeV/c, proton knockout from the 1s1/2-state dominates, and ROMEA calculations do an excellent job of explaining the data. However, as p_miss increases, the single-particle behavior of the reaction is increasingly hidden by more complicated processes, and for 280<p_miss<340MeV/c, ROMEA calculations together with two-body currents stemming from MEC and IC account for the shape and transverse nature of the data, but only about half the magnitude of the measured cross section. For 50<E_miss<120MeV and 145<p_miss<340MeV/c, (e,e′pN) calculations which include the contributions of central and tensor correlations (two-nucleon correlations) together with MEC and IC (two-nucleon currents) account for only about half of the measured cross section. The kinematic consistency of the 1p-shell normalization factors extracted from these data with respect to all available O16(e,e′p) data is also examined in detail. Finally, the Q^2-dependence of the normalization factors is discussed
A First Comparison of the responses of a He4-based fast-neutron detector and a NE-213 liquid-scintillator reference detector
A first comparison has been made between the pulse-shape discrimination
characteristics of a novel He-based pressurized scintillation detector
and a NE-213 liquid-scintillator reference detector using an Am/Be mixed-field
neutron and gamma-ray source and a high-resolution scintillation-pulse
digitizer. In particular, the capabilities of the two fast neutron detectors to
discriminate between neutrons and gamma-rays were investigated. The NE-213
liquid-scintillator reference cell produced a wide range of scintillation-light
yields in response to the gamma-ray field of the source. In stark contrast, due
to the size and pressure of the He gas volume, the He-based
detector registered a maximum scintillation-light yield of 750~keV to
the same gamma-ray field. Pulse-shape discrimination for particles with
scintillation-light yields of more than 750~keV was excellent in the
case of the He-based detector. Above 750~keV its signal was
unambiguously neutron, enabling particle identification based entirely upon the
amount of scintillation light produced.Comment: 23 pages, 7 figures, Nuclear Instruments and Methods in Physics
Research Section A review addresse
Pion emission in 2H, 12C, 27Al, gamma pi+ reactions at threshold
The first data from MAX-lab in Lund, Sweden on pion production in
photonuclear reactions at threshold energies, is presented. The decrease of the
total yield of pi+ in gamma + 12C, 27Al reactions below 200 MeV as well as
differential, dsigma/dOmega, cross sections follow essentially predictions from
an intranuclear cascade model with an attractive potential for pion-nucleus
interaction in its simplest form. Double differential, d2sigma/dOmegadT, cross
sections at 176 MeV show, however, deviations from the model, which call for
refinements of nuclear and Coulomb potentials and possibly also for coherent
pion production mechanisms.Comment: 19 pages, 7 figure
Monte Carlo Simulation of the Photon-Tagger Focal-Plane Electronics at the MAX IV Laboratory
Rate-dependent effects in the electronics used to instrument the tagger focal
plane at the MAX IV Laboratory have been investigated using the novel approach
of Monte Carlo simulation. Results are compared to analytical calculations as
well as experimental data for both specialized testing and production running
to demonstrate a thorough understanding of the behavior of the detector system.Comment: 31 pages, 17 figures, modified after submission to Nuclear
Instruments and Methods in Physics Research Section
New Measurement of Compton Scattering from the Deuteron and an Improved Extraction of the Neutron Electromagnetic Polarizabilities
The electromagnetic polarizabilities of the nucleon are fundamental
properties that describe its response to external electric and magnetic fields.
They can be extracted from Compton-scattering data --- and have been, with good
accuracy, in the case of the proton. In contradistinction, information for the
neutron requires the use of Compton scattering from nuclear targets. Here we
report a new measurement of elastic photon scattering from deuterium using
quasimonoenergetic tagged photons at the MAX IV Laboratory in Lund, Sweden.
These first new data in more than a decade effectively double the world
dataset. Their energy range overlaps with previous experiments and extends it
by 20 MeV to higher energies. An analysis using Chiral Effective Field Theory
with dynamical \Delta(1232) degrees of freedom shows the data are consistent
with and within the world dataset. After demonstrating that the fit is
consistent with the Baldin sum rule, extracting values for the isoscalar
nucleon polarizabilities and combining them with a recent result for the
proton, we obtain the neutron polarizabilities as \alpha_n = [11.55 +/-
1.25(stat) +/- 0.2(BSR) +/- 0.8(th)] X 10^{-4} fm^3 and \beta_n = [3.65 -/+
1.25(stat) +/- 0.2(BSR) -/+ 0.8(th)] X 10^{-4} fm3, with \chi^2 = 45.2 for 44
degrees of freedom.Comment: 6 pages, 3 figures, comments from Physical Review Letters Referees
addresse
A measurement of the 4He(g,n) reaction from 23 < Eg < 70 MeV
A comprehensive set of 4He(g,n) absolute cross-section measurements has been
performed at MAX-lab in Lund, Sweden. Tagged photons from 23 < Eg < 70 MeV were
directed toward a liquid 4He target, and neutrons were identified using
pulse-shape discrimination and the Time-of-flight Technique in two
liquid-scintillator detector arrays. Seven-point angular distributions have
been measured for fourteen photon energies. The results have been subjected to
complementary Transition-coefficient and Legendre-coefficient analyses. The
results are also compared to experimental data measured at comparable photon
energies as well as Recoil-Corrected Continuum Shell Model, Resonating Group
Method, and Effective Interaction Hyperspherical-Harmonic Expansion
calculations. For photon energies below 29 MeV, the angle-integrated data are
significantly larger than the values recommended by Calarco, Berman, and
Donnelly in 1983.Comment: 16 pages, 14 figures, some more revisions, submitted to Physical
Review
Simulation of the Response of the Solid State Neutron Detector for the European Spallation Source
The characteristics of the Solid-state Neutron Detector, under development
for neutron-scattering measurements at the European Spallation Source, have
been simulated with a Geant4-based computer code. The code models the
interations of thermal neutrons and ionising radiation in the 6Li-doped
scintillating glass of the detector, the production of scintillation light and
the transport of optical, scintillation photons through the the scintillator,
en route to the photo-cathode of the attached multi-anode photomultiplier.
Factors which affect the optical-photon transport, such as surface finish,
pixelation of the glass sheet, provision of a front reflector and optical
coupling media are compared. Predictions of the detector response are compared
with measurements made with neutron and gamma-ray sources, a collimated alpha
source and finely collimated beams of 2.5 MeV protons and deuterons.Comment: Preprint 22 pages, 12 figures, published in NIM
Tagging fast neutrons from an 241Am/9Be source
We report on an investigation of the fast-neutron spectrum emitted by
241Am/9Be. Well-understood shielding, coincidence, and time-of-flight
measurement techniques are employed to produce a continuous, polychromatic,
energy-tagged neutron beam.Comment: 17 pages, 7 figures, submitted to Journal of Applied Radiation and
Isotope
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