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    Proton Scattering of Neutron-Rich He-Isotopes in Inverse Kinematics A New Setup for the High Momentum Transfer Measurements

    F. Aksouh1, O.A. Kisselev1,2, A. Bleile1, O.V. Bochkarev3, L.V. Chulkov3, D. Cortina-Gil1, A.V. Dobrovolsky1,2, P. Egelhof1, H. Geissel1, M. Hellström1, N.B. Isaev2, B.G. Komkov2, M. Màtos1, F.V. Moroz2, G. Münzenberg1, M. Mutterer4, V.A. Mylnikov2,

    S.R. Neumaier1, V.N. Pribora3, D.M. Seliverstov2, L.O. Sergueev2, A. Shrivastava1, K. Sümmerer1, H. Weick1, M. Winkler1 and V.I. Yatsoura2 1 GSI, 2 PNPI Gatchina, 3 Kurchatov Institute Moscow, 4 TU Darmstadt

    Proton elastic scattering at intermediate energies of around 700 MeV/u is well suited for determining radii and nuclear matter distributions of halo nuclei such as 6,8He [1] and 11Li [2]. The first measurement on the 6,8He nuclei in inverse kinematics has been performed at low momentum transfer up to |t| = 0.05 (GeV/c)2 using gaseous hydro- gen as proton target. From theoretical investigations [3] a high sensitivity on the inner part of the nuclear matter distributions is predicted when extending the p6He and p8He elastic cross section measurement to the higher mo- mentum transfer region (|t| = 0.1 - 0.4 (GeV/c)2). Cal- culations based on Glauber theory show that the position of the first diffraction minimum depends on the shape of the density distribution, and is within the model strongly correlated to the core radius of the halo nucleus . We con- clude, that a measurement of the angular distribution at higher momentum transfer should yield unambiguous in- formation about the intrinsic structure of these nuclei. In addition, also inelastic reaction channels may be investi- gated. A recent experiment has been carried out at GSI in Oc-

    tober 2000. The 6,8He beams were obtained via fragmen- tation of an 18O primary beam of about 1010 ions/spill. The projectile fragments were separated by the FRS yield- ing secondary beam intensities of 5× 103-105 ions/s with an energy of about 700 MeV/u. The experimental setup installed in cave B is shown in fig.1. A forward spectrom-

    MWPCs

    beam counters

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    scintillator wall ALADIN magnet

    veto

    P2 P3P1 S1

    from FRS He beams

    6,8

    S2

    P4

    S3

    scintillator TOF wall

    P5, P6

    liquid H target2

    protons

    He6,8

    Figure 1: Schematic drawing of the S174 experimental setup.

    eter has been used for tracking and identifying the pro- jectile nuclei and for separating the elastic events from the inelastic and break-up channels. It also provided the signal for the first level trigger. This spectrometer con- sisted of four X-Y position sensitive multiwire proportional chambers (MWPC) P1-P4 with cathode strip channel-by- channel readout, several beam scintillators (S1-S3, Veto), the ALADIN magnet and a position sensitive scintillator wall behind. The proportional chambers had rather high position resolution of 100-150 µm and an efficiency close to 100%. A second position sensitive scintillator wall was used to measure ∆E and time-of-flight of the recoil pro-

    tons. The second level trigger used the signals from two proportional chambers (P5, P6) detecting and tracking the recoil protons. The essential difference with respect to the previous 6,8He experiment [1] was, that the high recoil pro- ton energies up to Ep = 160 MeV allowed to replace the gaseous H2 target by a 600 mg/cm

    2 liquid H2 target. This target and the cryogenic device used for the experiment were constructed at CEA, Saclay. The cylindrical target had a total length of 120 mm and an inner diameter of 30 mm. The use of a liquid hydrogen target allows to de- crease significantly the background as compared to similar experiments with CH2 targets. The data analysis is still in progress. To illustrate the

    quality of the acquired data, X-position spectra obtained with the MWPC P4 in the field free region before the AL- ADINmagnet for two different experimental conditions are displayed in fig.2. The spectrum on the left side was taken

    Position MWPC4, cm

    0

    100

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    -20 -15 -10 -5 0 5 10 15 20

    Position MWPC4, cm

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    Figure 2: X position spectra from the MWPC P4 obtained for p6He scattering for two different experimental conditions (see text).

    for the case the second level trigger (that demands recoil protons) being switched on. The one shown on the right side was obtained for the case the second level trigger was switched off, thus displaying the profile of the unscattered beam. The dominant peak in the spectrum on the left side clearly reflects the distribution of elastically and inelasti- cally scattered 6He projectiles, whereas the peak around X=0 is due to the unscattered beam particles contributing to the background. From these online raw spectra we con- clude that practically background free data on elastic and inelastic p6,8He scattering will be available after the final analysis taking into account all measured parameters.

    References

    [1] G.D. Alkhazov et al., Phys. Rev. Lett. 78 (1997) 2313.

    [2] S.F. Neumaier et al., to be published in Nucl. Phys. A, A.V. Dobrovolsky et al., this report.

    [3] L.V. Chulkov et al., Nucl. Phys. A 587 (1995) 291.

  • - 194 -

    Progress in the Optimization of the FOCAL Crystal Spectrometer

    H.F. Beyer9, J. Bojowald4, G. Borchert4, F. Bosch9, W. Brüchle9, M. Czanta9, R.D. Deslattes1, E. Förster3, A. Freund2, A. Gumberidze9, A. Hamacher4, J. Hoszowska2, P. Indelicato6, H.-J. Kluge9,

    Chr. Kozhuharov9, D. Liesen9, B. Lommel9, T. Ludziejewski8, X. Ma9, B. Manil6, I. Mohos4, D. Protíc4, A. Simionovici2, Th. Sẗohlker9, C. Striezel7 S. Toleikis9, N. Trautmann5, J. Tschischgale3,

    A.H. Walenta7, O. Wehrhan3

    1National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA 2European Synchrotron Radiation Facility (ESRF), F-38043 Grenoble, France

    3Institut für Optik und Quantenelektronik, Friedrich Schiller-Universität, D-07743 Jena, Germany 4FZ J̈ulich, Institut f̈ur Kernphysik, Germany

    5Institut für Kernchemie, Universität Mainz, D-55128 Mainz, Germany 6Universit́e P. et M. Curie, Lab. Kastler Brossel, F-75252 Paris Cédex, 05 France

    7Universiẗat Siegen, Fachbereich 7 Physik, D-57068 Siegen, Germany 8The Andrzej Soltan Institute for Nuclear Studies, 05-400 Swierk, Poland

    9GSI Darmstadt

    The FOCAL x-ray spectrometer is being developed for the accurate measurement of the 1s Lamb shift in one-electron heavy ions. In theFOcussingCompensatedAsymmetricLaue geometry part of the possible wavelength resolution is traded off in favor of an increased sensitivity through a broadening of the crystal rocking curve [1].

    source

    curved crystal

    scanning or position- sensitive detector

    R 0

    z ≈ λ R 2d

    Figure 1: Principle of the x-ray optical arrangement used for systematic tests of the FOCAL spectrometer. The x-ray path is shown for one wavelength at two different locations of the gamma-ray source.

    The spectrometer has been set up on a test bench where sev- eral systematic investigations were performed during the past year. Figure 1 schematically shows the x-ray optical arrange- ment with a movable x-ray source parked at two different posi- tions. The x rays are dispersed along the positive and negative z axis in a symmetric way where the displacement is approxi- mately proportional to the wavelength. Spectra obtained from a 169Yb source were recorded either with a scanner equipped with a narrow slit and a conventional Ge(i) detector or with the new micro-strip germanium detector under development [2]. Recording the x-ray spectra for a couple of different gamma-ray lines and for a range of source-to-crystal distances, it is possible to map out the x-ray optics and the performance and possible deviations of the crystal from it’s ideal cylindrical shape. Up to now such tests have solely been made with the scanner. A first test of the spectrometer in combination with the new micro- strip detector was made for a fixed source-to-crystal separation

    amounting to 300 mm compared to 2 m as thenominal radius of curvature of the crystal.

    0.0

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    R el

    at iv

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    ity

    Position (mm)

    Tm-Kα1 (50.7 keV)

    Wavelength (pm)

    Ge(i)

    FOCAL scanning

    FOCAL + µ-strip

    FWHM = 409 eV

    99 eV

    60 eV

    Figure 2: The wavelength profile of the Tm-Kα1 line measured as the pulse-height spectrum of a conventional solid-state Ge(i) detector or measured with the FOCAL spectrometer equipped either with a position-sensitive micro-strip detector or in scan- ning mode.

    Figure 2 compares measurements of the Tm-Kα1 line near 50.7 keV from the decay of169Yb with FOCAL operated in scanning mode or with the micro-strip detector attached. For reference the pulse-height spectrum of the conv