433 research outputs found
Measurement of the Induced Proton Polarization P_n in the 12C(e,e'\vec{p}) Reaction
The first measurements of the induced proton polarization, P_n, for the 12C
(e,e'\vec{p}) reaction are reported. The experiment was performed at quasifree
kinematics for energy and momentum transfer (\omega,q) \approx (294 MeV, 756
MeV/c) and sampled a recoil momentum range of 0-250 MeV/c. The induced
polarization arises from final-state interactions and for these kinematics is
dominated by the real part of the spin-orbit optical potential. The
distorted-wave impulse approximation provides good agreement with data for the
1p_{3/2} shell. The data for the continuum suggest that both the 1s_{1/2} shell
and underlying l > 1 configurations contribute.Comment: 5 pages LaTeX, 2 postscript figures, accepted by Physical Reveiw
Letter
Quantum Effects in the Mechanical Properties of Suspended Nanomechanical Systems
We explore the quantum aspects of an elastic bar supported at both ends and
subject to compression. If strain rather than stress is held fixed, the system
remains stable beyond the buckling instability, supporting two potential
minima. The classical equilibrium transverse displacement is analogous to a
Ginsburg-Landau order parameter, with strain playing the role of temperature.
We calculate the quantum fluctuations about the classical value as a function
of strain. Excitation energies and quantum fluctuation amplitudes are compared
for silicon beams and carbon nanotubes.Comment: RevTeX4. 5 pages, 3 eps figures. Submitted to Physical Review Letter
Intrinsic thermal vibrations of suspended doubly clamped single-wall carbon nanotubes
We report the observation of thermally driven mechanical vibrations of
suspended doubly clamped carbon nanotubes, grown by chemical vapor deposition
(CVD). Several experimental procedures are used to suspend carbon nanotubes.
The vibration is observed as a blurring in images taken with a scanning
electron microscope. The measured vibration amplitudes are compared with a
model based on linear continuum mechanics.Comment: pdf including figures, see:
http://www.unibas.ch/phys-meso/Research/Papers/2003/NT-Thermal-Vibrations.pd
A Measurement of the Interference Structure Function, R_LT, for the 12C(e,e'p) reaction in the Quasielastic Region
The coincidence cross-section and the interference structure function, R_LT,
were measured for the 12C(e,e'p) 11B reaction at quasielastic kinematics and
central momentum transfer of q=400 MeV/c. The measurement was at an opening
angle of theta_pq=11 degrees, covering a range in missing energy of E_m = 0 to
65 MeV. The R_LT structure function is found to be consistent with zero for E_m
> 50 MeV, confirming an earlier study which indicated that R_L vanishes in this
region. The integrated strengths of the p- and s-shell are compared with a
Distorted Wave Impulse Approximation calculation. The s-shell strength and
shape are compared with a Hartree Fock-Random Phase Approximation calculation.
The DWIA calculation overestimates the cross sections for p- and s-shell proton
knockout as expected, but surprisingly agrees with the extracted R_LT value for
both shells. The HF-RPA calculation describes the data more consistently, which
may be due to the inclusion of 2-body currents in this calculation.Comment: 8 Pages LaTex, 5 postscript figures. Submitted to Phys. Rev.
Quasielastic 12C(e,e'p) Reaction at High Momentum Transfer
We measured the 12C(e,e'p) cross section as a function of missing energy in
parallel kinematics for (q,w) = (970 MeV/c, 330 MeV) and (990 MeV/c, 475 MeV).
At w=475 MeV, at the maximum of the quasielastic peak, there is a large
continuum (E_m > 50 MeV) cross section extending out to the deepest missing
energy measured, amounting to almost 50% of the measured cross section. The
ratio of data to DWIA calculation is 0.4 for both the p- and s-shells. At w=330
MeV, well below the maximum of the quasielastic peak, the continuum cross
section is much smaller and the ratio of data to DWIA calculation is 0.85 for
the p-shell and 1.0 for the s-shell. We infer that one or more mechanisms that
increase with transform some of the single-nucleon-knockout into
multinucleon knockout, decreasing the valence knockout cross section and
increasing the continuum cross section.Comment: 14 pages, 7 figures, Revtex (multicol, prc and aps styles), to appear
in Phys Rev
A Comparison of Polarization Observables in Electron Scattering from the Proton and Deuteron
Recoil proton polarization observables were measured for both the p(,e) and d(,en reactions at two values of Q using a newly commissioned proton
Focal Plane Polarimeter at the M.I.T.-Bates Linear Accelerator Center. The
hydrogen and deuterium spin-dependent observables and
, the induced polarization and the form factor ratio
were measured under identical kinematics. The deuterium and
hydrogen results are in good agreement with each other and with the plane-wave
impulse approximation (PWIA).Comment: 9 pages, 1 figure; accepted by Phys. Rev. Let
Measurement of the recoil polarization in the p (\vec e, e' \vec p) pi^0 reaction at the \Delta(1232) resonance
The recoil proton polarization has been measured in the p (\vec e,e'\vec p)
pi^0 reaction in parallel kinematics around W = 1232 MeV, Q^2 = 0.121 (GeV/c)^2
and epsilon = 0.718 using the polarized c.w. electron beam of the Mainz
Microtron. Due to the spin precession in a magnetic spectrometer, all three
proton polarization components P_x/P_e = (-11.4 \pm 1.3 \pm 1.4) %, P_y =
(-43.1 \pm 1.3 \pm 2.2) %, and P_z/P_e = (56.2 \pm 1.5 \pm 2.6) % could be
measured simultaneously. The Coulomb quadrupole to magnetic dipole ratio CMR =
(-6.4\pm 0.7_{stat}\pm 0.8_{syst}) % was determined from P_x in the framework
of the Mainz Unitary Isobar Model. The consistency among the reduced
polarizations and the extraction of the ratio of longitudinal to transverse
response is discussed.Comment: 5 pages LaTeX, 1 table, 2 eps figure
Instrumental performance and results from testing of the BLAST-TNG receiver, submillimeter optics, and MKID arrays
Polarized thermal emission from interstellar dust grains can be used to map
magnetic fields in star forming molecular clouds and the diffuse interstellar
medium (ISM). The Balloon-borne Large Aperture Submillimeter Telescope for
Polarimetry (BLASTPol) flew from Antarctica in 2010 and 2012 and produced
degree-scale polarization maps of several nearby molecular clouds with
arcminute resolution. The success of BLASTPol has motivated a next-generation
instrument, BLAST-TNG, which will use more than 3000 linear polarization
sensitive microwave kinetic inductance detectors (MKIDs) combined with a 2.5m
diameter carbon fiber primary mirror to make diffraction-limited observations
at 250, 350, and 500 m. With 16 times the mapping speed of BLASTPol,
sub-arcminute resolution, and a longer flight time, BLAST-TNG will be able to
examine nearby molecular clouds and the diffuse galactic dust polarization
spectrum in unprecedented detail. The 250 m detector array has been
integrated into the new cryogenic receiver, and is undergoing testing to
establish the optical and polarization characteristics of the instrument.
BLAST-TNG will demonstrate the effectiveness of kilo-pixel MKID arrays for
applications in submillimeter astronomy. BLAST-TNG is scheduled to fly from
Antarctica in December 2017 for 28 days and will be the first balloon-borne
telescope to offer a quarter of the flight for "shared risk" observing by the
community.Comment: Presented at SPIE Millimeter, Submillimeter, and Far-Infrared
Detectors and Instrumentation for Astronomy VIII, June 29th, 201
The reaction dynamics of the 16O(e,e'p) cross section at high missing energies
We measured the cross section and response functions (R_L, R_T, and R_LT) for
the 16O(e,e'p) reaction in quasielastic kinematics for missing energies 25 <=
E_miss <= 120 MeV at various missing momenta P_miss <= 340 MeV/c. For 25 <
E_miss < 50 MeV and P_miss \approx 60 MeV/c, the reaction is dominated by
single-nucleon knockout from the 1s1/2-state. At larger P_miss, the
single-particle aspects are increasingly masked by more complicated processes.
For E_miss > 60 MeV and P_miss > 200 MeV/c, the cross section is relatively
constant. Calculations which include contributions from pion exchange currents,
isobar currents and short-range correlations account for the shape and the
transversity but only for half of the magnitude of the measured cross section.Comment: 6 pages, 4 figures, submitted to Phys Rev Lett, formatting error
fixe
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