C.Y. Prescott

The Stanford linear accelerator polarized electron source

The Stanford 3-km linear accelerator at SLAC has operated exclusively since early 1992 using a polarized electron beam for its high-energy physics programs. The polarized electron source now consists of a diode-type gun with a strained-lattice GaAs photocathode DC biased at high voltage and excited with circularly polarized photons generated by a pulsed, Ti:sapphire laser system. The electron polarization at the source is > 80%. To date the source has met all the beam requirements of the SLC and fixed target programs with < 5% downtime.

Systematic study of trace radioactive impurities in candidate construction materials for EXO-200

The Enriched Xenon Observatory (EXO) will search for double beta decays of 136Xe. We report the results of a systematic study of trace concentrations of radioactive impurities in a wide range of raw materials and finished parts considered for use in the construction of EXO-200, the first stage of the EXO experimental program. Analysis techniques employed, and described here, include direct gamma counting, alpha counting, neutron activation analysis, and high-sensitivity mass spectrometry.

Performance of the SLD central drift chamber prototype

A two-cell prototype of the SLD Central Drift Chamber has been tested using CO2-isobutane (92%-8%) at one atmosphere. Average single wire resolution of 55 ¿m was achieved. Charge division tests indicate a resolution for the final design of ¿ 0.5% of wire length. dE/dx separation of ¿ - e should be useful up to at least 7 GeV/c.

Performance of the SLD Central Drift Chamber

We report for the first time on the performance of the Central Drift Chamber (CDC) of the Stanford Linear Collider Large Detector (SLD), which has been recording data since 1992. The low mass of the chamber and the use of a gas characterized by both a low drift velocity and low diffusion constant help to minimize the drift-distance measurement errors. We describe some of the calibrations and corrections applied to the data, and report on the resolutions achieved thus far. We measure an intrinsic drift resolution of 55-110 /spl mu/m in the region of uniform field. Analysis of the full drift-pulse waveform allows for efficient double-hit resolution of about 1 mm. Momentum resolution is characterized by the formula (dp/sub t//p/sub t//sup 2/)/sup 2/=0.0050/sup 2/+(0.010/p/sub t/)/sup 2/, where p/sub t/ is expressed in units of GeV/c. Used in conjunction with the SLD vertex detector, the CDC permits measurements of impact parameters of high-momentum tracks to the level of 10 /spl mu/m in the r-/spl phi/ plane and 36 /spl mu/m in the r-z plane. A resolution of 6.4% is achieved in the measurement of dE/dx for the electrons in Bhabha scattering events. >

A Linear RFQ Ion Trap for the Enriched Xenon Observatory

The design, construction, and performance of a linear radio-frequency ion trap (RFQ) intended for use in the Enriched Xenon Observatory (EXO) are described. EXO aims to detect the neutrinoless double-beta decay of {sup 136}Xe to {sup 136}Ba. To suppress possible backgrounds EXO will complement the measurement of decay energy and, to some extent, topology of candidate events in a Xe filled detector with the identification of the daughter nucleus ({sup 136}Ba). The ion trap described here is capable of accepting, cooling, and confining individual Ba ions extracted from the site of the candidate double-beta decay event. A single trapped ion can then be identified, with a large signal-to-noise ratio, via laser spectroscopy.

Performance of the SLD central drift chamber

Abstract The central drift chamber (CDC) of the Stanford Linear Collider Large Detector (SLD) has been recording data since 1992. The low mass of the chamber and the use of a gas characterized by both a low drift velocity and low diffusion constant help to minimize multiple scattering and drift-distance measurement errors. We describe some of the calibrations and corrections applied to the data, and report on the resolutions achieved thus far. We measure an intrinsic drift resolution of 55–110 μm in the region of uniform field. Analysis of the full drift-pulse waveform allows for efficient double-hit resolution of about 1 mm. Transverse momentum resolution is given by the formula ( σ(p t ) p t 2 ) 2 = 0.0050 2 + ( 0.010 p t ) 2 , where p t is in GeV/ c . Used in conjunction with the SLD vertex detector, the CDC permits measurements of impact parameters of high-momentum tracks to 10 μm in the r - φ plane and 36 μm in the r - z plane.

A Liquid Xenon Ionization Chamber in an All-fluoropolymer Vessel

A novel technique has been developed to build vessels for liquid xenon ionization detectors entirely out of ultra-clean fluoropolymer. We describe the advantages in terms of low radioactivity contamination, provide some details of the construction techniques, and show the energy resolution achieved with a prototype all-fluoropolymer ionization detector.

Mobility of thorium ions in liquid xenon

Abstract We present a measurement of the 226 Th ion mobility in LXe at 163.0xa0K and 0.9xa0bar. The result obtained, 0.240 ± 0.011 (stat) ± 0.011 (syst) cm 2 /(kV-s), is compared with a popular model of ion transport.

The high peak current polarized electron source of the Stanford Linear Collider

Abstract The Stanford Linear Collider injector requires two 2 ns pulses of 4.5–5.5 × 10 10 electrons, separated by 61 ns at 120 Hz, from its source. Since 1992, these currents have been provided by a polarized electron source based on GaAs photocathodes. A beam polarization of 76 ± 4% has been measured at the end of the 50 GeV linac. At low photocathode quantum efficiencies, and for excitation near threshold, the maximum current delivered by the source is constrained, not by the space charge limit of the gun, but by a “charge limit” of the photocathode. The charge limited current is proportional to the photocathode quantum efficiency, but the proportionality varies for different photocathode types. Experience with high polarization strained GaAs photocathodes on a test beamline and on the SLC is presented.

Performance of the SLC polarized electron source with high polarization

For the 1992 operating cycle of the SLAC Linear Collider (SLC), the polarized electron source (PES) during its maiden run successfully met the pulse intensity and overall efficiency requirements of the SLC. However, the polarization of the bulk GaAs cathode was low (/spl sim/27%) and the pulse-to-pulse stability was marginal. We have shown that adequate charge for the SLC can be extracted from a strained layer cathode having P/sub espl sim/80% even though the quantum efficiency (QE) is <1%. The recent addition of a separate chamber to the PES-which allows cathodes to be loaded into the gun after the vacuum bake and after high voltage (HV) processing without breaking vacuum-increases the reliability for achieving an adequate photoelectron yield. A new SLAC-built pulsed Ti:sapphire laser permits operation of the PES at the required wavelength with sufficient power to fully saturate the yield, and thus improve the e/sup -/ beam stability. The performance of the PES during the 1993 SLC operating cycle with these and other improvements is discussed.<<ETX>>