The experimental search for the direct interaction of exotic new
particles in underground detectors is entering a new phase in
which the experimental sensitivities are beginning to probe into
the favoured parameter space for the neutralinos expected within
supersymmetric extensions to the standard particle physics model
(SUSY). This is happening at a time when the evidence for cold
dark matter is stronger than ever. This review of the field gives
a summary of current status, both the evidence and the underground
searches, and looks forward to expected progress in the coming
The accretion of electrostatic charge in the isolated LISA test masses due to energetic particles in the space environment hinders the drag-free operation of the gravitational inertial sensors. Robust predictions of charging rates and associated stochastic fluctuations are therefore required for the exposure scenarios expected throughout the mission. We report on detailed charging simulations with the
Geant4 toolkit, using comprehensive geometry and physics models, for
galactic cosmic-ray protons and helium nuclei. These predict net charging rates of up to +100 elementary charges per second during the solar minimum period, decreasing by half at solar maximum. Charging from sporadic solar events involving energetic protons was also investigated. Other physical processes hitherto overlooked as potential charging mechanisms have been assessed. Significantly, the kinetic emission of very low-energy secondary electrons due to bombardment of the inertial sensors by primary cosmic rays and their secondaries can produce charging currents comparable with the Monte Carlo rates.
We report on a Monte Carlo simulation of electrostatic charging of the
LISA proof masses by cosmic-ray protons and alpha particles, developed
using the Geant4 toolkit. A positive charging rate of 58+/-17 +e/s
(proton charges per second) was obtained with the minimum Geant4
energy threshold for the production of secondary particles by
electromagnetic processes. This charging rate does not seem to depend
strongly on the tracking of low-energy secondary electrons, and is
some 5 times larger than that found in previous simulations. The
difference is only partly explained by the slightly larger proof mass
considered in this study. This figure is used to place new limits on
the required discharge time of the LISA test masses.
LISA employs a capacitive sensing and positioning system to maintain the drag free environment of the test masses acting as interferometer mirror elements. The need for detailed electrostatic modelling of the test mass environment arises because any electric field gradient or variation associated with test mass motion can couple the test mass to its housing, and ultimately the spacecraft. Cross-couplings between components in the system can introduce direct couplings between sensing signals, sensing axes and the drive signal. A variation in cross-couplings or asymmetry in the system can introduce capacitance gradients and second derivatives, giving rise to unwanted forces and spring constant modifications. These effects will vary dependent on the precise geometry of the system and will also tend to increase the sensitivity to accumulated charge on the test-mass. Presented are the results of a systematic study of the effect of the principal geometry elements (e.g. machining imperfections, the caging mechanism) on the test mass electrostatic environment, using the finite element code ANSYS. This work is part of an ongoing ESA study into drag-free control for LISA and the LTP on SMART 2 and ultimately aims to eliminate geometries that introduce too large a disturbance and optimise the electrostatic design.
Theoretically, gallium arsenide detectors offer an attractive alternative to silicon for the high energy x-ray astronomer, due to the greater absorption power of the material. However, in practice, GaAs detectors made from bulk-grown crystals have a spectral resolution which falls well short of both expectation and that achieved by silicon detectors of comparable thickness. In contrast, a detector fabricated from GaAs grown by the liquid phase epitaxial (LPE) process displays full charge collection with a resolution in agreement with that expected from measurements of leakage current and device capacitance. Experimental results are presented showing the level of spectral resolution possible in a variety of GaAs detectors, including Liquid Enhancement Czochralski material from various manufacturers, Vertical Bridgeman and LPE material. Both the detector performance and the electrical characteristics (voltage- current, noise, etc) have been explored for each device.
Silicon bolometers are currently under development for milliKelvin operation; these devices are being produced using Si wafer fabrication technology. The design and performance of individual bolometers, using doped layers with a thickness in the range 0.1 to 3 (mu) , are described. The use of epitaxial growth to replace ion implantation for improved performance is described and initial results reported.
Two types of drift device, namely photodiodes and position sensitive drift chambers with segmented anode and cathode structures, have been studied at room temperature and below. Leakage current and electron mobility have been investigated at low temperature for the drift photodiodes. Self-triggering has been achieved for the position sensitive drift chambers using 60 keV photons, and differences in arrival time between the prompt trigger signal from the cathode and the delayed anode signal have been studied as a function of drift distance and temperature. The response of the photodiodes when coupled to a CsI scintillator at room temperature has been assessed.
Recent developments in the manufacture of GaAs detectors for high energy physics applications and dark matter searches have resulted in working devices made from LEC and HB starting material This offers the promise of routine manufacture of reproducible devices at a modest cost. The most advanced of the techniques is that of Schottky diodes on LEC material. Results are presented demonstrating the performance of such devices (3.0 mm X 5.0 mm X 200 micrometers ) as x-ray detectors, including their low temperature operation. An alternative technique using charge collection in bulk HB material at temperatures down to liquid helium also is briefly described.
The main design features and the early findings of the Rosat XUV wide field camera (WFC) are discussed. The most important data on the WFC telescope and detectors are presented. The WFC operational features, observing efficiency, filter performance, thermal performance star tracker performance, and single-event upsets are discussed. The first WFC images are compared with preflight calibration data.
The background in x-ray telescopes in near-Earth orbits from low-energyelectrons (<100 keV) haslong been a sourceofconcern. Recent data on low-energy electron populations and improvements in the modeling of the propagation of low-energy electrons through Wolter I grazing- incidence optical systems now allow better quantitative estimates of the electron throughput of such systems. The modeling that has recently been done with respect to the U.K. soft x-ray telescope (Wide Field Camera) on ROSAT is described. The results from the modeling are then applied to several other missions using grazing-incidence optics, and the implications are discussed.
The ROSAT project is an international collaboration between the Federal Republic of Germany, the United Kingdom, and the United States. The satellite, due to be launched in June 1990, carries a payload of two coaligned imaging telescopes: the German X-Ray Telescope (XRT), which operates in the soft x-ray band (0.1 to 2 keV or 6 to 100 A), and the UK Wide Field Camera (WFC), which operates in the XUV band (0.02 to 0.2 keV or 60 to 600 A). ROSAT will perform two main tasks in its anticipated two to four year lifetime: a six-month all-sky survey in the soft x ray and XUV bands followed by a program of pointed observations for detailed studies of thousands of individual targets. In this paper we review the
design and performance of the WFC. The instrument is a grazing incidence telescope comprising a set of three nested, Wolter-Schwarzschild Type I, gold-coated aluminum mirrors with a microchannel plate detector at their common focus. Thin plastic and metal film filters define the wavelength passbands.