A design for large-area fast photo-detectors with transmission-line readout and waveform sampling

We present a preliminary design and the results of simulation for a photo-detector module to be used in applications requiring the coverage of areas of many square meters with time resolutions less than 10 picoseconds and position resolutions of less than a millimeter for charged particles. The source of light is Cherenkov light in a radiator/window; the amplification is provided by panels of micro-pores functionalized to act as microchannel plates (MCPs). The good time and position resolution stems from the use of an array of parallel 50 Ω transmission lines (strips) as the collecting anodes. The anode strips feed multi-GS/sec sampling chips which digitize the pulse waveform at each end of the strip, allowing a measurement of the time from the average of the two ends, and a 2-dimensional position measurement from the difference of times on a strip, and, in the orthogonal direction, the strip number, or a centroid of the charges deposited on adjacent strips. The module design is constructed so that large areas can be `tiled' by an array of modules

Influence of Growth Method on K3Sb Photocathode Structure and Performance

Future high brightness photoelectron sources delivering gt;100 mA average current call for a new generation of photocathodes. Materials which qualify for this purpose should exhibit low intrinsic emittance, long lifetime and high quantum efficiency at photon energies in the visible range of the spectrum to relax drive laser requirements. A combination of material science techniques are used to determine the influence of the growth parameters on structure and performance of photocathode materials . In situ XRR, XRD and GiSAXS measurements were performed at the synchrotron radiation sources, NSLS and CHESS. The growth of K3Sb, a precursor material of one of the prime candidates CsK2Sb, was studied intensively to optimize this intermediate growth step in terms of quantum efficiency and roughness. Three methods, a layer by layer type and a super lattice type were examined. K3Sb exists in two crystallographic phases, namely cubic and hexagonal. The cubic phase exhibits a higher quantum efficiency at 532 nm than the hexagonal phase and transforms more easily into CsK2Sb, tuning this phase is believed to be one of the key parameters in the CsK2Sb growt

Orbital character of O 2p unoccupied states near the Fermi level in CrO2

The orbital character, orientation, and magnetic polarization of the O 2$p$ unoccupied states near the Fermi level ($E_F$) in CrO$_2$ was determined using polarization-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) from high-quality, single-crystal films. A sharp peak observed just above $E_F$ is excited only by the electric field vector ($\bf E$) normal to the tetragonal $c$-axis, characteristic of a narrow band ($\approx$ 0.7 eV bandwidth) constituted from O 2$p$ orbitals perpendicular to $c$ (O 2$p_y$) hybridized with Cr 3$d_{xz-yz}$ $t_{2g}$ states. By comparison with band-structure and configuration-interaction (CI) cluster calculations our results support a model of CrO$_2$ as a half-metallic ferromagnet with large exchange-splitting energy ($\Delta_{exch-split}$ $\approx$ 3.0 eV) and substantial correlation effects.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. B Rapid Com

Ce-L3-XAS study of the temperature dependence of the 4f occupancy in the Kondo system Ce2Rh3Al9

We have used temperature dependent x-ray absorption at the Ce-L3 edge to investigate the recently discovered Kondo compound Ce2Rh3Al9. The systematic changes of the spectral lineshape with decreasing temperature are analyzed and found to be related to a change in the $4f$ occupation number, n_f, as the system undergoes a transition into a Kondo state. The temperature dependence of $n_f$ indicates a characteristic temperature of 150K, which is clearly related with the high temperature anomaly observed in the magnetic susceptibility of the same system. The further anomaly observed in the resistivity of this system at low temperature (ca. 20K) has no effect on n_f and is thus not of Kondo origin.Comment: 7 pages, three figures, submitted to PR

Position Measurements with Micro-Channel Plates and Transmission lines using Pico-second Timing and Waveform Analysis

The anodes of Micro-Channel Plate devices are coupled to fast transmission lines in order to reduce the number of electronics readout channels, and can provide two-dimension position measurements using two-ends delay timing. Tests with a laser and digital waveform analysis show that resolutions of a few hundreds of microns along the transmission line can be reached taking advantage of a few pico-second timing estimation. This technique is planned to be used in Micro-channel Plate devices integrating the transmission lines as anodes

Hard and Soft X-MCD Studies of CrO2

Chromium dioxide is investigated by X-MCD measurements on the Cr L2,3 , Cr K-edges and on the O K-edge in combination with Ligand Field Multiplet Model (LFM) calculations and Linearized Augmented Plane Wave (LAPW) band-structure calculations. Applying so called Sum Rules [2, 3] to the Cr L2,3- edges one deduces an upper limit for the orbital-moment of 10-2µB which is in good agreement with magneto-meter-measurements. Both Cr K-edge and O K-edge X-MCD measurements reflect a significant polarization of the p-like bands and are well described by a spin-density calculation

Spin-dependent extended x-ray absorption fine structure in magnetic oxides

The spin-dependent extended x-ray absorption fine-structure (EXAFS), i.e., the magnetic counterpart to the conventional EXAFS when using circularly polarized light in a ferro(i)magnet, was measured at the L2,3L2,3 edges of Eu in Eu3Fe5O12Eu3Fe5O12 and EuO and at the KK edge of CrO2.CrO2. Complementary near-edge x-ray magnetic circular dichroism measurements at the O KK edge were made to directly access the oxygen magnetization. It is shown that there is an influence of the nearly nonmagnetic oxygen atoms on the magnetic EXAFS depending on the local symmetry of the crystal as well as on the relative direction of the magnetic moments of neighboring atoms. The observed influences at distances corresponding to the oxygen neighbors in EuO are interpreted as the harmonic focusing effect, i.e., over-next magnetic neighbors behind the oxygen are responsible for the observed contributions