121591364
Electron motion in counting gases — New answers and new questions
Abstract An experimental method is described which allows the simultaneous measurement of drift velocity, longitudinal and transverse diffusion coefficients of electrons in counting gas mixtures. Results in CF 4 and argon/CF 4 are presented. In CH 4 and Ar/CH 4 mixtures the problems in the calculation of the transverse diffusion in molecular gases are discussed. An empirical formula which can be used for practical purposes has been derived. A comparison with recently published theoretical results suggests that a multiterm Boltzmann code could be the adequate formalism to calculate the motion of electrons in counting gas mixtures.
119881774
Anisotropic low energy electron collision cross sections for methane derived from transport coefficients
Based on a comprehensive set of transport parameters of electrons in methane derived from single electron spectra in the range of field 0.01-15 Td the authors determined a set of elastic and inelastic scattering cross sections up to 3 eV. The necessity to include anisotropic elastic scattering in a Boltzmann analysis of methane is demonstrated. In addition the drift velocity in a mixture of methane with helium was used to enhance the reliability of the derived cross sections. The low energy part of the cross sections for elastic scattering is subject to a MERT analysis and is compared with total cross section data from single scattering experiments.
123101687
Luminosity determination at HERA-B
Abstract A detailed description of an original method used to measure the luminosity accumulated by the HERA-B experiment for a data sample taken during the 2002–2003 HERA running period is reported. We show that, with this method, a total luminosity measurement can be achieved with a typical precision, including overall systematic uncertainties, at a level of 5% or better. We also report evidence for the detection of δ -rays generated in the target and comment on the possible use of such delta rays to measure luminosity.
55568152
Ultrabroadband terahertz source and beamline based on coherent transition radiation
Coherent transition radiation (CTR) in the THz regime is an important diagnostic tool for analyzing the temporal structure of the ultrashort electron bunches needed in ultraviolet and x-ray free-electron lasers. It is also a powerful source of such radiation, covering an exceptionally broad frequency range from about 200 GHz to 100 THz. At the soft x-ray free-electron laser FLASH we have installed a beam transport channel for transition radiation (TR) with the intention to guide a large fraction of the radiation to a laboratory outside the accelerator tunnel. The radiation is produced on a screen inside the ultrahigh vacuum beam pipe of the linac, coupled out through a diamond window and transported to the laboratory through an evacuated tube equipped with five focusing and four plane mirrors. The design of the beamline has been based on a thorough analysis of the generation of TR on metallic screens of limited size. The optical propagation of the radiation has been computed taking into account the effects of near-field (Fresnel) diffraction. The theoretical description of the TR source is presented in the first part of the paper, while the design principles and the technical layout of the beamline are described in the second part. First experimental results demonstrate that the CTR beamline covers the specified frequency range and preserves the narrow time structure of CTR pulses emitted by short electron bunches.
124862478
BUNCH COMPRESSION MONITOR
The longitudinal charge distribution in the electron bunches has a strong impact on the lasing process in a Free-Electron Laser based on the principle of Self Amplified Spontaneous Emission of radiation. For the ultraviolet and soft X ray FEL FLASH at DESY, structures in the range of ten to hundred micrometers play a crucial role. The investigation of the longitudinal charge distribution in the electron bunches on a bunch-by-bunch basis is an important issue for optimizing the bunch compression and improving the performance of the machine. This paper introduces a new tool for longitudinal diagnostics based on THz spectroscopy of coherent radiation. A novel spectrometer has been designed which permits to analyze the radiation of single electron bunches in a broad spectral range and with high resolution. Preliminary measurements with this spectrometer are presented. A strong variation of the CTR intensity in the 40 μm regime is observed when the off-crest phase in the accelerating cavities preceding the first magnetic bunch compressor is changed in small steps. This variation in the CTR intensity is found to be correlated with a similar variation in the pulse energy of the FEL light. Hence the THz spectrometer appears to be a very sensitive device for controlling the parameters of the bunch compression scheme.
119297111
HATSouth: A Global Network of Fully Automated Identical Wide-Field Telescopes*
HATSouth is the world's first network of automated and homogeneous telescopes that is capable of year-round 24 hr monitoring of positions over an entire hemisphere of the sky. The primary scientific goal of the network is to discover and characterize a large number of transiting extrasolar planets, reaching out to long periods and down to small planetary radii. HATSouth achieves this by monitoring extended areas on the sky, deriving high precision light curves for a large number of stars, searching for the signature of planetary transits, and confirming planetary candidates with larger telescopes. HATSouth employs six telescope units spread over three prime locations with large longitude separation in the southern hemisphere (Las Campanas Observatory, Chile; HESS site, Namibia; Siding Spring Observatory, Australia). Each of the HATSouth units holds four 0.18 m diameter f/2.8 focal ratio telescope tubes on a common mount producing an 8.2° × 8.2° field of view on the sky, imaged using four 4 K × 4 K CCD cameras and Sloan r filters, to give a pixel scale of 3.7'' pixel-1. The HATSouth network is capable of continuously monitoring 128 square arc degrees at celestial positions moderately close to the anti-solar direction. We present the technical details of the network, summarize operations, and present detailed weather statistics for the three sites. Robust operations have meant that on average each of the six HATSouth units has conducted observations on ~500 nights over a 2 years time period, yielding a total of more than 1 million science frames at a 4 minute integration time and observing ~10.65 hr day-1 on average. We describe the scheme of our data transfer and reduction from raw pixel images to trend-filtered light curves and transiting planet candidates. Photometric precision reaches ~6 mmag at 4 minute cadence for the brightest non-saturated stars at r ≈ 10.5. We present detailed transit recovery simulations to determine the expected yield of transiting planets from HATSouth. We highlight the advantages of networked operations, namely, a threefold increase in the expected number of detected planets, as compared to all telescopes operating from the same site.
119385175
SN 1991T: Reflections of past glory
We have obtained photometry and spectra of SN 1991T which extend more than 1000 days past maximum light, by far the longest of SN Ia has been followed. Although SN 1991T exhibited nearly photometric behavior in the first 400 days following maximum, by 600 days its decline had slowed, and by 950 days the supernova brightness was consistent with a constant apparent magnitude of m(sub B) = 21.30. Spectra near maximum showed minor variations on the SN Ia theme which grew less conspicuous during the exponential decline. At 270 days the nebular spectrum was composed of Fe and Co lines common to SN Ia. However, by 750 days past maximum light, these lines had shifted in wavelength, and were superposed on a strong blue continuum. The luminosity of SN 1991T at 950 days is more than (9.0 x 10(exp 38)(D/13 Mpc)(2) ergs/s with a rate of decline of less than 0.04 mag per 100 days. We show that this emission is likely to be light that was emitted by SN 1991T near maximum light which has reflected from foreground dust, much like the light echos observed around SN 1987A.
110313521
LHCb online system technical design report: Data acquisition and experiment control
40306381
The bright optical afterglow of the nearby gamma-ray burst of 29 March 2003.
Past studies of cosmological gamma-ray bursts (GRBs) have been hampered by their extreme distances, resulting in faint afterglows. A nearby GRB could potentially shed much light on the origin of these events, but GRBs with a redshift z <or= 0.2 have been estimated to occur only rarely, about once per decade. Here we report the discovery of the bright optical afterglow emission from the burst of 29 March 2003 (GRB030329; ref. 2). The brightness of the afterglow and the prompt report of its position resulted in extensive follow-up observations at many wavelengths, along with the measurement of the redshift, z = 0.169 (ref. 4). The gamma-ray and afterglow properties of GRB030329 are similar to those of GRBs at cosmological redshifts. Observations have already identified the progenitor as a massive star that exploded as a supernova.
122048635
Time resolution limits of the MWPCs for the LHCb muon system
Abstract Detailed simulations of the timing properties of the Multi-Wire-Proportional-Chambers of the LHCb muon system are compared with actual measurements. The simulated time resolution is in good agreement with the measurement which allows to study the resolution limiting parameters like diffusion, charge deposit fluctuations, cluster position fluctuations, electronics noise, track position and wire spacing to show how they affect the performance of the system. Methods to improve the time resolution of the muon chambers are discussed.
58921873
Study of electron pair production below the Z mass at the CERN pp collider
Abstract Results on the cross section for the production of electron pairs in p p collisions at √ s = 630 GeV are presented. The measured value is σ = 405 ± 51 (syst.) ± 84 (syst.) pb, in the invariant mass interval 10 m α s 2 ) QCD contributions. The comparison of these data with those of lower energy experiments show approximate scaling as a function of the variable √τ = m √s .