E. L. Garwin

Parity Non-Conservation in Inelastic Electron Scattering

Abstract We have measured parity violating asymmetries in the inelastic scattering of longitudinally polarized electrons from deuterium and hydrogen. For deuterium near Q2 = 1.6 (GeV/c)2 the asymmetry is (−9.5 × 10−5)Q2 with statistical and systematic uncertainties each about 10%.

Further measurements of parity non-conservation in inelastic electron scattering☆

Measurements of the y-dependence of the parity nonconserving asymmetries for inelastic electron scattering from deuterium. The measurements cover a range of y values from 0.15 to 0.36 and show only a small y-dependence. The results are in good agreement with the Weinberg-Salam model for sin/sup 2/theta/sub W/ = 0.224 +- 0.020. 20 references.

Electron‐Phonon Interaction in Alkali Halides. I. The Transport of Secondary Electrons with Energies between 0.25 and 7.5 eV

The interaction between conduction electrons with energies from 0.25 to 7.5 eV and longitudinal optical phonons in alkali halides is studied in detail by time‐dependent perturbation theory. Expressions for the rate and angular distribution of scattering are obtained. The electron‐transport problem is then solved with the exact quantum mechanical scattering results by a direct simulation Monte Carlo method. Probabilities of escape and average energy losses for electrons generated isotropically at a certain depth in the material, with a given initial energy, are computed for CsI, KCl, NaF, and LiF. A simple theory shows the effective mass and temperature dependence. The effect of including scattering to angles other than forward is quite apparent in the results.

A Very High Charge, High Polarization Gradient-Doped Strained GaAs Photocathode

Abstract A high-gradient-doping technique is applied to strained polarized photocathodes. A 5.0– 7.5 nm p-type surface layer doped to 5×10 19 cm −3 is found sufficient to overcome the surface charge limit while maintaining high beam polarization. This technique can be employed to meet the charge requirements of the Next Linear Collider with a polarization approaching 80%.

Systematic study of polarized electron emission from strained GaAs/GaAsP superlattice photocathodes

Spin-polarized electron photoemission has been studied for GaAs∕GaAs1−xPx strained superlattice cathodes grown by gas-source molecular beam epitaxy. The superlattice structural parameters are systematically varied to optimize the photoemission characteristics. The heavy-hole and light-hole transitions are reproducibly observed in quantum efficiency spectra, enabling direct measurement of the band energies and the energy splitting. Electron-spin polarization as high as 86% with over 1% quantum efficiency has been observed.

Surface properties of metal‐nitride and metal‐carbide films deposited on Nb for radio‐frequency superconductivity

Various effects occur which can prevent attainment of the high Q’s and/or the high gradient fields necessary for the operation of radio‐frequency (rf) superconducting cavities. One of these effects, multipactor, both causes the cavity to detune during filling due to resonant secondary electron emission at the cavity walls, and lowers the quality factor (Q) by dissipative processes. TiN deposited onto the high‐field regions of room‐temperature Al cavities has been used at the Stanford Linear Accelerator Center to successfully reduce multipactor in the past. We have therefore studied TiN and its companion materials, NbN, NbC, and TiC, all on Nb substrates under several realistic conditions: (1) as‐deposited, (2) exposed to air, and (3) electron bombarded. The studied films (up to 14‐nm thickness) were sputter deposited onto sputter‐cleaned Nb substrates. Results indicate that all the materials tested gave substantially the same results. The maximum secondary electron yields for as‐deposited films were reduc...

Energy-dependence of inner potential in Fe from low-energy electron absorption (target current)

Abstract For unpolarized low-energy electrons (0–60 eV) incident on a Fe(0 0 1)_ surface, the measured current I absorbed by the crystal (target current) displays a considerable amount of fine structure depending on the angle of incidence. Dynamical calculations of the elastic reflection coefficient R e show that - d 2 R e /d E 2 is in good agreement with d 2 I /d E 2 for suitably chosen real and imaginary parts of the inner potential. The energy dependence of the latter is thereby determined. Comparison with a bulk band structure calculated using the same real potential reveals a correpondence between target current minima and band gaps as well as current maxima and critical points.

Properties of thin antimultipactor TiN and Cr2O3 coatings for klystron windows

Low secondary electron emission yield (1–1.3), low radio‐frequency (rf) loss films are of interest as coatings for alumina ceramic high power klystron windows. Very thin (15–50 A) Ti and TiN layers have been previously used with success in klystron tubes operating at lower power levels (<35 MW peak power) and short (2.5 μs) pulses but are subject to property changes during air exposure, tube bakeout (550 °C for up to 10 days) and in situ electron bombardment. Higher power tubes (≥50 MW) with longer (5 μs) pulse lengths require coatings that remain stable under these more rigorous conditions in order to avoid multipactoring and window failure due to overheating. Air‐oxidized Cr films offer an alternative to TiN. This work shows that they have the required resistivity for low rf loss, combined with stability in the secondary electron emission (SEE) yield under bakeout and electron bombardment. SEE, AES, XPS, ELS, oxidation, bakeout, and electron bombardment results are presented for Cr and TiN layers deposi...

Enhanced electron spin polarization in photoemission from thin GaAs

The polarization of photoemitted electrons from thin GaAs layers grown by molecular beam epitaxy has been measured. Polarization as high as 49% was observed for a 0.2‐μm‐thick GaAs sample at excitation photon wavelengths longer than 750 nm. The maximum polarization is dependent on the thickness of the GaAs layer, decreasing to about 41% for a 0.9‐μm‐thick GaAs sample.

Surface properties of Cr2O3

Thin (<5 nm) air‐oxidized Cr layers are deposited on the alumina output windows of radio‐frequency klystron tubes to prevent electron multipactor by reducing the secondary electron emission yield of the alumina surface. The top several nanometers of these layers appear to be Cr2O3. To compare the measured surface properties of these layers with those of clean stoichiometric Cr2O3, quasibulk Cr2O3 layers were produced by wet‐H2‐firing magnetron‐deposited Cr films on Cu substrates and characterized by x‐ray photoelectron‐, Auger electron‐, and electron energy loss spectra and secondary electron emission yield measurements. Other properties measured were x‐ray diffraction structure, sheet resistance, and optical reflectivity. In particular, the peak of the secondary electron yield was found to be ∼1.7, which is considerably higher than the <1 yield value reported earlier in the literature. The Cr2O3 Cr 2p x‐ray photoelectron core level spectrum was curve‐fit using Doniach–Sunjic line shapes and statistical f...