S. Platchkov

Deuteron A(Q2) structure function and the neutron electric form factor

We present new measurements of the deuteron A(Q2) structure function in the momentum transfer region between 1 and 18 fm−2. The accuracy of the data ranges from 2 % to 6 %. We investigate the sensitivity of A(Q2) to the nucleon-nucleon interaction and to the neutron electric form factor GEn. Our analysis shows that below 20 fm−2 GEn can be inferred from these data with a significantly improved accuracy. The model dependence of this analysis is discussed.

3H and 3He electromagnetic form factors

Abstract We report the results of three experiments on elastic electron scattering from 3 H and 3 He. A detailed description of the experiments and the data obtained is given. We have performed a combined analysis of the world data on 3 H and 3 He. This analysis gives a complete experimental information on the trinucleon electromagnetic form factors up to q 2 = 30 fm −2 , and also provides a separation into the isoscalar and isovector form factors. The results are compared to selected calculations that include nucleonic and mesonic degrees of freedom.

Micromegas as a large microstrip detector for the COMPASS experiment

Abstract Recent results on the gaseous microstrip detector Micromegas which will be used to track particles in the COMPASS experiment at CERN are presented. Developments concerning its mechanical and electrical design, associated readout electronics and gas mixture were carried out. Particular attention was paid to the discharge phenomenon which affects this type of microstrip detector. The adequacy of the options finally retained, especially the SFE16 readout and the use of a Ne–C 2 H 6 –CF 4 gas mixture, was demonstrated in a set of beam tests performed on a 26×36 cm 2 prototype. Operating at a gain of ∼6400, full efficiency is reached along with a spatial resolution of ∼50 μm and a timing accuracy of 8.5 ns . Discharges are kept at a low rate, less than one per SPS spill in a COMPASS-like environment. Via a decoupling of the strips through individual capacitors their impact is greatly reduced. They generate a dead time on the full detector of ∼ 3 ms , affecting marginally the detection efficiency given their rate. The probability of discharge, at a given value of efficiency, is found to decrease with the mean value of the gas mixture atomic number. In view of these results, the commissioning of Micromegas for COMPASS is foreseen in the near future.

CHARGE DENSITY OF 40Ca

A large number of theoretical calculations [1-8] are available for the 40Ca ground-state wavefunction. This particular attention paid to 40Ca results from the fact that it is the heaviest N=Z magic nucleus. HartreeFock (HF) calculations yield quite similar results for the density in the nuclear surface region, mainly because many of the effective nucleon-nucleon forces used have been adjusted to fit the nuclear rms-radii. Consequently, calculations differ mainly at small radii, by different amounts of structure in the charge density p (r). A number of possible physical mechanisms that influence this fine structure have been discussed in the literature [9-14].

Radial Distribution of Valence Neutrons from Elastic Electron Scattering

Cross sections for elastic scattering of 175--300-MeV electrons by the nuclear magnetization current density of /sup 87/Sr have been measured. The results are used to determine the shape of the radial wave function of the valence neutron shell. The experiment yields a rms radius of 1g/sub 9///sub 2/ neutrons that is 0.31 +- 0.04 fm smaller than predicted by density-dependent Hartree-Fock theory.

On the radial distribution of the 1f72 proton orbit

Abstract Cross sections for magnetic elastic electron scattering of high multipolarity from 51 V and 59 Co have been measured. They are interpreted in terms of the rms radius of the 1f 7 2 proton orbit. The results for 51 V and 59 Co 1f 7 2 rms radii are smaller by 3% and 9%, respectively, than predicted by the Hartree-Fock-Bogoliubov theory.

A precise determination of the 3s proton orbit

Abstract Elastic electron scattering cross sections have been measured for 205 Tl and 206 Pb at an incident energy of 502 MeV, covering a momentum transfer range from 1.4 to 3.2 fm −1 . The charge density difference of the two isotones is accurately determined. We show that the shape of the 3s proton orbit can be extracted from this difference without ambiguity. The experimental shape is compared to mean field theory predictions.

Development of a fast gaseous detector : "micromegas"

Abstract Several 15×15 cm 2 gaseous Micromegas chambers (MICROMEsh GAseous Structure) which consist of a conversion gap and an amplification gap separated by a thin grid have been extensively tested in low-intensity 10 GeV /c pion beam and high-intensity (up to 5×10 5 Hz / mm 2 ) 100 GeV /c muon beam. The detector behaviour has been studied with respect to many parameters: conversion gaps of 1 and 3xa0mm, amplification gaps of 50 and 10xa0μm, an external magnetic field and many different filling gases. So far no effect of the magnetic field up to 1.3xa0T has been observed. The gas mixture argon + cyclohexane appears to be very suitable with gains above 10 5 and a full-efficiency plateau of 50xa0V at 340xa0V. With a conversion gap as small as 1xa0mm and an electronics with a threshold at 5000 electrons the efficiency reaches 96%. With the addition of CF 4 a time resolution of 5xa0ns (RMS) has been obtained. A spatial resolution better than 60 μ m has been observed with anode strips of 317 μ m pitch and was explained by transverse diffusion in the gas. Simulations show that with a pitch of 100 μ m and the appropriate gas a resolution of 10 μ m is within reach. This development leads to a new generation of cheap position-sensitive detectors which would permit high-precision tracking or vertexing close to the interaction region, in very high-rate environments.

Compton scattering off polarized electrons with a high-finesse Fabry–Pérot Cavity at JLab

Abstract We built and commissioned a new type of Compton polarimeter to measure the electron beam polarization at the Thomas Jefferson National Accelerator Facility (Virginia, USA). The heart of this polarimeter is a high-finesse monolithic Fabry–Perot cavity. Its purpose is to amplify a primary 300 mW laser beam in order to improve the signal to noise ratio of the polarimeter. It is the first time that a high-finesse Fabry–Perot cavity is enclosed in the vacuum of a particle accelerator to monitor the beam polarization by Compton polarimetry. The measured finesse and amplification gain of the cavity are F=26 000 and G =7300. The electron beam crosses this high-power photon source at an angle of 23 mrad in the middle of the cavity where the photon beam power density is estimated to be 0.85 MW / cm 2 . We have used this facility during the HAPPEX experiment (April–July 1999) and we give a preliminary measurement of Compton scattering asymmetry.

The gaseous microstrip detector micromegas for the COMPASS experiment at CERN

Abstract The measurements foreseen in the COMPASS experiment at CERN, require high resolution tracking detectors, with low radiation length and high rate capability. For this purpose we have developed and optimized a gaseous microstrip detector ‘Micromegas’. Twelve planes with 1024 strips each, assembled in 3 stations of 4 views XYUV, have been operated with success in the summer of 2002 in the COMPASS environment. We describe here the performances and results obtained.