Monday, March 11, 2002 - DPG-Tagungen

Nuclear Physics Wednesday
HK31 Nuclear Physics / Spectroscopy V
Time: Wednesday 14:00–15:30 Room: C
Group Report HK 31.1 Wed 14:00 C
Nuclear radii and moments of short-lived isotopes in the range
17−28 Ne — •W. Geithner 1 , S. Kappertz 1 , M. Keim 1 , R. Neugart 1 ,
S.Wilbert 1 , P. Lievens 2 , K. Marinova 3 , K.-M. Hilligsoe 4 , H.
Simon 5 , S. Franchoo 6 , L. Weissman 6 ,andISOLDE Collaboration
6 — 1 Inst. für Physik, Univ. Mainz — 2 K.U. Leuven — 3 Univ. Sofia
— 4 Univ. Aarhus — 5 TU Darmstadt — 6 CERN, Geneva
Laser spectroscopy experiments at the ISOLDE mass separator on
neon isotopes from 17 Ne at the proton drip line to the neutron-rich 28 Ne
have revealed interesting aspects of nuclear structure in the lower sdshell
region. These include the question of a proton halo structure of
17 Ne and the mirror properties in comparison with 17 N, the N = 8 neutron
shell closure as well as the development of single-particle and deformation
properties with the number of neutrons in the sd shell. The
changes of nuclear mean square charge radii, and the magnetic moments
and quadrupole moments are obtained from the measurement of isotope
shifts and hyperfine structure of optical spectral lines. For experiments
on light elements the required accuracy and sensitivity of collinear laser
spectroscopy was achieved by improving the determination of Doppler
shifts and by employing a β-activity detection in combination with stateselective
collisional ionization. Recently, these improvements were also
exploited to complement earlier data on argon isotopes and to investigate
73 Kr for which a remarkably large inverted odd-even staggering was
observed. (Supported by BMBF and EU.)
HK 31.2 Wed 14:30 C
New experimental campaign on the 20 Ne(4 + ) g factor + — •J.
Leske 1 , K.-H. Speidel 1 , O. Kenn 1 , S. Schielke 1 , G. Müller 1 , J.
Gerber 2 , N. Benczer-Koller 3 ,andG. Kumbartzki 3 — 1 Institut
für Strahlen- und Kernphysik, Univ. Bonn, D-53115 Bonn — 2 Institut
de Recherches Subatomiques, F-67037 Strasbourg, France — 3 Dept. of
Physics and Astronomy, Rutgers Univ., New Brunswick, NJ 08903, USA
In view of the eminent role of 20 Ne for nuclear structure calculations the
early measurements of magnetic moments on the first 2 + and 4 + states
have caused tremendeous turmoil (see e.g. [1]). All these experiments
in which the technique of transient magnetic fields was employed have
yielded surprisingly a much smaller g factor for the 4 + state compared
to the 2 + state. It was argued that in analogy to heavy nuclei the pronounced
backbending in 20 Ne might be responsible for this observation.
However, such a scenario would be in striking contradiction with isospin
conservation: for pure T=0 states the g factors of 2 + and 4 + should be
equal to g � +0.5. Sensible T=1 admixtures cannot explain the observed
large difference. New measurements were performed at the Cologne tandem
accelerator in substantially improved experimental conditions. The
states of interest were populated in the reaction 12 C( 12 C, α) 20 Ne using
a technically improved multilayered target with evaporated Gd. Deexcitation
γ rays were measured in coincidence with α particles emitted at
backward and forward angles implying two different Ne velocities. Experimental
details and preliminary results will be discussed.
+ supported by DFG
[1] K.-H. Speidel et al., Nucl. Phys. A378 (1982) 130
HK 31.3 Wed 14:45 C
First γ rays produced by radioactive beams at REX-ISOLDE —
•Heiko Scheit for the MINIBALL collaboration and the REX-ISOLDE
collaboration — Max-Planck-Institut für Kernphysik, Heidelberg
During the commissioning of the REX radioactive beam accelerator
at ISOLDE/CERN in October and November of 2001 one of the MINI-
BALL triple cluster detectors was placed near the target station of the
REX accelerator. The aim was to study the operation of such a detector
under realistic conditions. Especially the γ background due to β decay of
the radioactive beam nuclei, and due to the proximity of the RF cavities
of the accelerator to the experimental station was investigated together
with the influence of the macro- and micro-structure of the REX beam.
In addition to the triple cluster detector a double-sided Si strip detector
was used to detect scattered particles in coincidence with γ rays.
The commissioning of the accelerator went exceptionally well and stable
23 Na and radioactive 24,25 Na isotopes were accelerated to 2 A·MeV
and transmitted to the target station with intensities between 10 5 and 10 7
particles per second, where 58 Ni and Be targets of 1 mg/cm 2 were used
to populate excited states via Coulomb excitation and nuclear reactions.
Preliminary results will be presented and an outlook on the experiments
to be performed at REX-ISOLDE using MINIBALL in 2002 will
be given.
Partly supported by the BMBF.
HK 31.4 Wed 15:00 C
ATRAP on the way to cold antihydrogen — •D. Grzonka for
the ATRAP collaboration — Research Centre Jülich, Germany
The ATRAP experiment at the CERN antiproton decelerator AD aims
for a test of the CPT invariance by a comparison of the hydrogen to antihydrogen
atom spectroscopy. For high precision measurements of atomic
transitions cold atoms of antihydrogen are essential which requires trapping
techniques. The present first phase of the ATRAP experiment deals
with the production of antihydrogen. In the last year the trapping and
handling of antiprotons and positrons in the trap was studied and optimized
resulting in a loss free trapping over longer periods which is
crucial for any recombination experiments. Furthermore two schemes
for antihydrogen production the Pulsed Field Recombination [1] and the
Three Body Recombination were studied. An intense interaction between
the overlapping antiproton and positron clouds has been achieved which
was demonstrated by the clear observation of a positron cooling of the
antiprotons [2]. It is very likely that in such a configuration also Rydberg
antihydrogen has been produced but the observed signals in the
performed studies are not sufficient for a clear statement. Status and
further steps on the the experimantal studies at ATRAP are given.
[1] C. Wesdorp et al., Phys. Rev. Lett. 84 (2000) 3799.
[2] G. Gabrielse et al., Phys. Lett. 507 (2001) 1.
HK 31.5 Wed 15:15 C
Analyzing-power of pp-bremsstrahlung ∗ — •Andrea Wilms for
the COSY-TOF collaboration — Institut für Experimentalphysik I,
Ruhr–Universität Bochum
The high angular acceptance of the time of flight spectrometer COSY–
TOF and the good emittance of the extracted proton beam make it
possible to measure cross sections of two and three particle reactions
down to the µb region. COSY extracted its first polarized proton beam
with a beam momentum of p = 798.0 MeV/c in Dec. 98, which provides
the possibility to measure polarization observables like asymmetries and
analyzing–powers by using an unpolarized LH2-target.
The event selection and the results for the angular distributions of several
reaction channels in the COSY energy range particularly of �pp–
bremsstrahlung, are presented. Additionally, the evaluation of the beam
polarization by using the well–known analyzing–power Ay of the elastic
proton scattering is shown. ∗ supported by the BMB+F
HK32 Electromagnetic and Hadronic Probes IV
Time: Wednesday 14:00–15:30 Room: D
Group Report HK 32.1 Wed 14:00 D
Neutral meson photoproduction off nuclei — •M. Pfeiffer for
the TAPSand A2 collaboration — II. Physikalisches Institut, Heinrich-
Buff-Ring 16, 35392 Giessen
The photoproduction of mesons allows a detailed study of lower lying
nucleon resonances. A series of experiments has been carried out with
photon energies up to 820 MeV at the Mainz Microtron (MAMI) using
different production targets.
The 3 He experiment focuses on the coherent eta production. Former
experiments on other light targets ( 2 H, 4 He) show small coherent contributions
only. However, the quantum numbers of 3 He suggest a larger
coherent signal. Standard theoretical models of the coherent photoproduction
fail to explain the cross section in the vicinity of the production
threshold, but calculations by Shevchenko et. al. [1] take the possibility