Search

The first result on a dramatic improvement of the photocathode lifetime of the polarized electron source at NIKHEF is presented. The improvement was obtained after replacing the original DC power supply with a pulsed power supply for the photocathode gun high voltage. The pulsed high voltage power supply provides a negative Gauss-like pulse, with an amplitude of 100 kV, and a full-width of 600 μs, with repetition rates up to 10 Hz. Contrary to using DC high voltage, no deterioration of the vacuum in the acceleration chamber is observed. A photocathode lifetime of 180 h has been measured, using a strained layer InGaAsP photocathode. The lifetime is independent of whether or not the photocathode gun is operated at the pulsing rate of 1 Hz.

We are developing a dark matter detector for Weakly Interacting Massive Particles (WIMPs) using Series Arrays of Superconducting Tunnel Junctions (SASTJs) to read out phonon signals from absorber crystals. We discuss the factors which have determined the choice of our detector scheme. As an improvement over previously produced SASTJs using a shadow mask technique, a new fabrication process of high-quality Al/Al-oxide/Al junctions and SASTJs is being developed using photolithography to define the structures. The processing and the electrical characterisation of the first devices produced in this way are described

We discuss the design of a cryogenic detector for a WIMP dark matter search based on single crystal absorbers and using Series Arrays of Superconducting Tunnel Junctions (SASTJs). The distribution of recoil vectors of target nuclei from WIMP interactions are affected by the motion of the laboratory through the dark matter halo. The angular distribution of recoil directions is skewed due to the motion of the solar system around the galaxy and is modulated by the diurnal and annual rotation of the earth. We discuss the kinematics of the recoil events and how a directional signal might be identified in our cryogenic detectors using the fast response of SASTJs to the ballistic phonons arising in the absorber from WIMP interactions. We consider how the anisotropy of a dark matter recoil distribution can be used to place statistical limits on its component relative to the isotropic background signal. We also consider how the dark matter limit is altered if only the axis of the nuclear recoil, rather than the full recoil direction is available. We also briefly consider the effect of phonon focusing within single crystal absorbers. Focusing will modulate strongly the signal detected by the SASTJs, on the crystal surface, as the position of the interaction within the crystal varies. A comparison is made between the behaviour of phonons in strongly focusing crystals, such as Nb, Si and LiF, and their near isotropic propagation in BaF2.

A new nuclear microprobe comprising of a quintuplet lens system is being constructed at the Ion Beam Facility of the "Vrije Universiteit" Amsterdam in collaboration with the Microanalytical Research Centre of the University of Melbourne. An overview of the Amsterdam set-up will be presented.Detailed characterisation of the individual lenses was performed with the grid shadow method using a 2000 mesh Cu grid mounted at a relative angle of 0.5degrees to the vertical lens line focus. The lenses were found to have very low parasitic aberrations equal or below the minimum detectable limit for the method, which was approximately 0.1% for the sextupole component and 0.2% for the octupole component. We present experimental and theoretical grid shadow patterns, showing results for all five lenses. Crown Copyright (C) 2003 Published by Elsevier B.V. All rights reserved.

AmPS is a 300-900 MeV electron ring. The machine was designed as a pulse stretcher to provide continuous external beams. After years of successful operation in this mode the nuclear physics program now concentrates on experiments with stored beams interacting with internal gas targets. These experiments require currents between 50 and 200 mA through 60 cm long 15 mm wide cylindrical target cells. Gas pressures in the open-ended cells correspond to target thickness of 10/sup 14/ to 10/sup 16/ atoms/cm/sup 2/. Strong stray fields from a spectrometer and from target holding fields disturb the closed orbit. Under operational conditions the linac provides a maximum energy of 700 MeV, so ramping is required to achieve 900 MeV. To further increase the luminosity at the target a reduced emittance lattice based on the existing magnet configuration has been designed. Late 1996 a polarized electron injector as well as a "Siberian Snake" to control the spin on the internal target became operational. The polarized injector delivers a few mA to the linac so stacked injection in the ring is required. With strained InGaAsP cathodes a polarization of 80% was obtained. Test results, performance and prospects are presented.

We report on first measurements with polarized electrons stored in a medium-energy ring and with a polarized internal target. Polarized electrons were injected at 442 MeV (653 MeV), and a partial (full) Siberian snake was employed to preserve the polarization. Longitudinal polarization at the interaction point and polarization lifetime of the stored electrons were determined with laser backscattering. Spin observables were measured for electrodisintegration of polarized He-3, with simultaneous detection of scattered electrons, protons, neutrons, deuterons, and He-3 nuclei, over a large phase space.

We report on the results of studies of the longitudinal electron polarization in the AmPS storage ring at NIKHEF. The ring was operated using a partial Siberian snake at the first magic energy, and a full Siberian snake between 440 MeV and 720 MeV. We have investigated the effect of high beam currents in the ring, and found that some of the electron polarization is lost if the beam current becomes larger than ≈120 mA.

Quasi-elastic electron scattering may provide precise information on the S′ and the D-wave parts of the 3He ground-state wave function, the neutron form factors, and the role of spin-dependent reaction mechanism effects. An experiment is being performed at the AmPS storage ring at NIKHEF (Amsterdam, the Netherlands), where polarized electrons (up to 900 MeV) are used in combination with large acceptance electron and hadron detectors. Preliminary results from data at four-momentum transfer squared Q2=0.15 GeV2 are presented.

An overview of the polarized electron source used at the NIKHEF accelerator facility is given. Since its first operation in September 1996 we have obtained currents in the source larger than 150 mA, and polarization degrees up to 88%. Approximately 350 hours have been dedicated to physics with a polarized 3He internal target. Using one InGaAsP photocathode, 2 μs long pulses were produced with a current of 2 mA in the local linac, with a polarization degree of 82.5%, at a repetition rate of 1 Hz. Source operations had to be interrupted about every eleventh day for approximately an hour for photocathode reactivation. A photocathode lifetime (1/e) of 180 hours has been achieved.

Compton backscattering polarimetry provides a fast measurement of the polarization of an electron beam in a storage ring. Since the method is nondestructive, the polarization of the electrons can be monitored during internal target experiments. At NIKHEF a Compton polarimeter has been constructed to measure the polarization of the longitudinally polarized electrons stored in the AmPS ring. First results obtained with the polarimeter, the first Compton polarimeter to measure the polarization of a stored longitudinally polarized electron beam, are presented in this paper.

We are setting up a dedicated neutron-scattering facility in order to study the response of cryogenic detectors to nuclear recoils in preparation for dark matter searches. The design and status of the facility are presented.