339 research outputs found
Atomic Hydrogen Cleaning of Polarized GaAs Photocathodes
Atomic hydrogen cleaning followed by heat cleaning at 450C was used
to prepare negative-electron-affinity GaAs photocathodes. When hydrogen ions
were eliminated, quantum efficiencies of 15% were obtained for bulk GaAs
cathodes, higher than the results obtained using conventional 600C heat
cleaning. The low-temperature cleaning technique was successfully applied to
thin, strained GaAs cathodes used for producing highly polarized electrons. No
depolarization was observed even when the optimum cleaning time of about 30
seconds was extended by a factor of 100
Surface Analysis of OFE-Copper X-Band Accelerating Structures and Possible Correlation to RF Breakdown Events
X-band accelerator structures meeting the Next Linear Collider (NLC) design
requirements have been found to suffer vacuum surface damage caused by radio
frequency (RF) breakdown, when processed to high electric-field gradients.
Improved understanding of these breakdown events is desirable for the
development of structure designs, fabrication procedures, and processing
techniques that minimize structure damage. RF reflected wave analysis and
acoustic sensor pickup have provided breakdowns localization in RF structures.
Particle contaminations found following clean autopsy of four RF-processed
travelling wave structures, have been catalogued and analyzed. Their influence
on RF breakdown, as well as that of several other material-based properties,
will be discussed.Comment: 21 pages, 8 figures, 4 tables, Submitted to JVST A as a proceeding of
the 50th AVS conference (Baltimore, MD, 2-7 Nov 2003
Beam Test of a Segmented Foil SEM Grid
A prototype Secondary-electron Emission Monitor (SEM) was installed in the 8
GeV proton transport line for the MiniBooNE experiment at Fermilab. The SEM is
a segmented grid made with 5 um Ti foils, intended for use in the 120 GeV NuMI
beam at Fermilab. Similar to previous workers, we found that the full
collection of the secondary electron signal requires a bias voltage to draw the
ejected electrons cleanly off the foils, and this effect is more pronounced at
larger beam intensity. The beam centroid and width resolutions of the SEM were
measured at beam widths of 3, 7, and 8 mm, and compared to calculations.
Extrapolating the data from this beam test, we expect a centroid and width
resolutions of 20um and 25 um, respectively, in the NuMI beam which has 1 mm
spot size.Comment: submitted to Nucl. Instr. Meth.
Nuclear Magnetic Quadrupole Moments in Single Particle Approximation
A static magnetic quadrupole moment of a nucleus, induced by T- and P-odd
nucleon-nucleon interaction, is investigated in the single-particle
approximation. Models are considered allowing for analytical solution. The
problem is also treated numerically in a Woods-Saxon potential with spin-orbit
interaction. The stability of results is discussed.Comment: LATEX, 9 pages, 1 postscript figure available upon request from
"[email protected]". BINP 94-4
Pion-Muon Asymmetry Revisited
Long ago an unexpected and unexplainable phenomena was observed. The
distribution of muons from positive pion decay at rest was anisotropic with an
excess in the backward direction relative to the direction of the proton beam
from which the pions were created. Although this effect was observed by several
different groups with pions produced by different means, the result was not
accepted by the physics community, because it is in direct conflict with a
large set of other experiments indicating that the pion is a pseudoscalar
particle. It is possible to satisfy both sets of experiments if helicity-zero
vector particles exist and the pion is such a particle. Helicity-zero vector
particles have direction but no net spin. For the neutral pion to be a vector
particle requires an additional modification to conventional theory as
discussed herein. An experiment is proposed which can prove that the asymmetry
in the distribution of muons from pion decay is a genuine physical effect
because the asymmetry can be modified in a controllable manner. A positive
result will also prove that the pion is NOT a pseudoscalar particle.Comment: 9 pages, 3 figure
Direct inversion of S-P differential arrival-times for Vp/Vs ratio in SE Asia
Open Access via Jisc Wiley agreementPeer reviewedPublisher PD
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Test of the electronic structure of Fe(100) by absorbed current spectroscopy
The absorbed electron current for a clean Fe(100) surface as a function of energy rises step-like at the vacuum-energy cutoff with an absorption close to 1. The smooth decrease of absorbed current at higher electron energy due to secondary electron emission is superimposed by a considerable amount of fine structure, the amplitude of which decreases with increasing energy. These features are found in good agreement with the results of a calculation of the elastic part of the electron reflection coefficient. Further, they are compared with the ferromagnetic electronic bulk bandstructure calculated above the vacuum energy. From the comparison with the experimental data, the energy dependence of the real and imaginary parts of the inner potential is determined
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A polarization study of strained GaAs photocathode structures
The polarized electron source at SLAC has performed extremely well during recent years supplying electrons having a spin polarization of 78% (85%) for high (low) current operation with beam current limited primarily by experimental requirements. However, there is room for improvement in the electron polarization. The less-than-ideal polarizations are a result of both imperfections and depolarizing mechanisms within the photocathode. The structure of the photocathode used at SLAC in the polarized electron source is a single-strained emitting layer structure grown atop a GaAs substrate. Here, the properties of several types of strained GaAs and GaAsP photocathodes have been studied using x-ray diffraction and photoemission
Recent Progress at SLAC Extracting High Charge from Highly-Polarized Photocathodes for Future-Collider Applications
Future colliders such as NLC and JLC will require a highly-polarized
macropulse with charge that is more than an order of magnitude beyond that
which could be produced for the SLC. The maximum charge from the SLC
uniformly-doped GaAs photocathode was limited by the surface charge limit
(SCL). The SCL effect can be overcome by using an extremely high (>1019 cm-3)
surface dopant concentration. When combined with a medium dopant concentration
in the majority of the active layer (to avoid depolarization), the surface
concentration has been found to degrade during normal heat cleaning (1 hour at
600 C). The Be dopant as typically used in an MBE-grown superlattice cathode is
especially susceptible to this effect compared to Zn or C dopant. Some relief
can be found by lowering the cleaning temperature, but the long-term general
solution appears to be atomic hydrogen cleaning.Comment: 11 pages, 3 figures, 1 table, contributed to 10th Workshop on
Polarized Sources and Targets, Novosibirsk, Sept. 22-26, 2003, to be
submitted to Nucl. Instrum. and Meth.
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