Monday, March 11, 2002 - DPG-Tagungen

Nuclear Physics Wednesday
Group Report HK 24.2 Wed 11:15 B
Search for the Spin-Dipole Resonance in 12 B — •M.A. de Huu 1 ,
A.M. van den Berg 1 , N. Blasi 2 , M. Hagemann 3 , M.N. Harakeh 1 ,
J. Heyse 3 , M. Hunyadi 1 , R. de Leo 4 , S. Micheletti 2 , H. Okamura
5 , and H.J. Wörtche 1 for the EuroSuperNova collaboration
— 1 Kernfysisch Versneller Instituut, Groningen, The Netherlands —
2 INFN, Milano, Italy — 3 Vakgroep Subatomaire en Stralingsfysica, Universiteit
Gent, Belgium — 4 INFN, Bari, Italy — 5 Saitama University,
Saitama, Japan
We report on an experiment performed at KVI to search for the spindipole
resonance in 12 B and to decompose it into its three different spin
components (J π =0 − ,1 − ,and2 − )byusingthe( � d, 2 He + n) reaction on
12 C. To populate states in 12 B, we used a purely tensor-polarized deuteron
beam at Ed = 170 MeV extracted from the AGOR-cyclotron. The two
outgoing protons of the unbound 2 He nucleus from the 12 C(d, 2 He) reaction
were measured with the Big-Bite Spectrometer and the EuroSuperNova
focal-plane detection system. In coincidence, neutrons emitted
from unbound states in 12 B were detected with the EDEN detector using
a time-of-flight technique for the energy determination of the neutrons.
Both the data obtained for the tensor analyzing power and the angular
correlations for the neutron-decay channels will be used to disentangle
the contributions of the different spin-dipole states. Preliminary results
of the analysis will be shown.
HK 24.3 Wed 11:45 B
Observation of microwave-induced transitions between hyperfine
levels of antiprotonic helium — •Eberhard Widmann —
Department of Physics, University of Tokyo
The ASACUSA collaboration at the Antiproton Decelerator of CERN
succeeded in the year 2001 for the first time to observe two microwaveinduced
transitions between hyperfine levels of antiprotonic helium. The
hyperfine levels are those of a highly excited state (principal quantum
nunber 37, angular quantum number 35) of the exotic three-body system
¯p–e − –He 2+ . The observed transitions at ∼ 12.91 GHz correspond
to an electron spin flip in the orbital magnetic field of the antiproton.
The experimental accuracy is ∼ 1.5×10 −5 , and the measured frequencies
agree with three-body QED calculations on the level a few 10 −5 . The
current precision of the theory is limited by the omission of terms of order
α 2 ≈ 5 × 10 −5 which is worse than the experimental resolution.
This constitutes the first precise measnurement of the orbital magnetic
moment of a composite particle. In addition to providing a benchmark
for three-body QED calculations, the hyperfine structure of antiprotonic
helium is sensitive to the magnetic moment of the antiproton, an important
quantity related to CPT invariance. An increased accuracy of both
experiment and theory may yield an improved value of the antiproton
magnetic moment which is only known with an accuracy of 3 × 10 −3 .
HK 24.4 Wed 12:00 B
Gamow-Teller matrix elements from the (d, 2 He) reaction at
170 MeV — •S .Rakers, C. Bäumer, D. Frekers, E. Grewe,
B. Junk, andR. Schmidt for the EUROSUPERNOVA collaboration
— Institut für Kernphysik, Westfälische Wilhelms-Universität Münster,
Wilhelm-Klemm-Str. 9, D-48149 Münster
The (d, 2 He) reaction, where 2 He denotes the unbound system of two
protons being in a 1 S0 state, is a charge-exchange reaction of (�n,�p) type.
HK25 Nuclear and Particle Astrophysics III
At extreme forward angles, the probe excites Gamow-Teller states with
high purity.
We have performed (d, 2 He) experiments on the N=Z nuclei 12 Cand
24 Mg at Ed=170 MeV. The protons from the 2 He decay were detected
in coincidence with the BBS-ESN spectrometer/detector setup at the
AGOR cyclotron in Groningen. Energy resolutions of 145 keV were
achieved, allowing a detailed spectroscopy of the final nuclei 12 Band
24 Na.
In the talk, we will present spectra, angular distributions, and DWBA
calculations. In the case of the 24 Mg, the 0 ◦ spectrum is compared with
(p,n) reaction data in which the same levels in the analog nucleus 24 Al are
populated. A detailed one-to-one correspondence is observed. Spectra
canalsobecomparedwith(p,p ′ ) data, yielding a complete level scheme
for 1 + states in the A=24 (T=1) isospin triplet.
HK 24.5 Wed 12:15 B
Exclusive measurement of breakup reactions with the oneneutron
halo nucleus 11 Be — •R. Palit for the LAND collaboration
— Institut für Kernphysik, Johann Wolfgang Goethe Universität,D-
60486 Frankfurt, Germany
One-neutron removal reactions of 520 MeV/u 11 Be projectiles impinging
on carbon and lead targets were studied in a kinematically complete
experiment using the LAND set-up at GSI. The 10 Be fragments, neutrons,
as well as gamma-rays from the excited states of the fragment
were detected in coincidence and the partial cross sections populating
the ground and excited states of the core nucleus were deduced. The
relative energy spectrum between the 10 Be core and a neutron in the
continuum was derived, yielding the spectroscopic factor for the neutron
in the 2s halo state. The comparison of electromagnetic and nuclear
contributions to the breakup will be presented. It has been found that
Coulomb breakup predominantly populates the core ground state, while
excited states account for only a few percent of the total cross section.
Model calculations interconnecting the structure of 11 Be and the experimental
cross sections will be presented.
Supported by BMBF (06OF112, 06MZ864) and by GSI (OF ELZ, MZ KRK).
HK 24.6 Wed 12:30 B
Systematic study of the structure of neutron rich oxygen isotopes
— •Kate Jones for the LAND/S188/S233 collaboration —
Gesellschaft für Schwerionenforschung (GSI), Planckstr. 1, D-64291
Darmstadt, Germany
The one neutron removal from isotopes of oxygen with A = 17 to
23 has been studied in complete kinematics using the LAND/ALADIN
setup at GSI at energies around 500 MeV/A. The radioactive beams
were produced via fragmentation using the fragment separator, FRS.
Targets of carbon and lead have been used to study the nuclear knockout
and Coulomb dissociation respectively. The charged (A-1) fragments
and neutrons were detected in coincidence with the gamma-rays emitted
from excited states of the residual nucleus. The single particle structure
of the projectile can be deduced from the low-lying E1 strength using a
direct break up model. Detailed structural information of the odd-mass
oxygen isotopes, including the halo nucleus candidate 23 O, has been derived
from Coulomb dissociation.
Supported by BMBF (06OF112, 06MZ864) and by GSI (OF ELZ, MZ KRK).
Time: Wednesday 10:45–12:15 Room: C
Group Report HK 25.1 Wed 10:45 C
Astrophysical S factor for 7 Be(p,γ) 8 B from precision cross section
measurements — •A.R. Junghans 1 , E.C. Mohrmann 1 , K.A.
Snover 1 , T.D. Steiger 1 , E.G. Adelberger 1 , J.M. Casandjian 1 ,
H.E. Swanson 1 , L. Buchmann 2 , S.H. Park 2 ,andA. Zyuzin 2 —
1 CENPA, University of Washington, Seattle WA, U.S.A. — 2 TRIUMF,
Vancouver B.C., Canada
Super-K and SNO are sensitive mainly to neutrinos from the decay of
8 B produced by the 7 Be(p,γ) 8 B reaction in the Sun. An improved determination
of this cross section (Sfactor) at solar energies is necessary
at the level of ±5% in order that this reaction rate not be the dominant
uncertainty in the νe flux predicted by solar model calculations 1 . We
have made new, direct measurements of the 7 Be(p,γ) 8 B cross section at
27 points from Ecm = 0.18 to 1.2 MeV using the van de Graaff accelerator
of the University of Washington with a Terminal Ion Source, and a
106 mCi 7 Be target produced at TRIUMF and deposited on a Mo backing.
The water-cooled target, mounted on the end of a rotating arm,
is irradiated in the proton beam and then rotated 180 degrees in front
of a Si-detector where the α particles following the β decay of 8 Bare
counted. Our technique involves a number of improvements over previous
measurements. In our measurements, we have achieved a precision
±5% or better per point. Results will be presented and compared with
model calculations and with other experiments. 1 J. Bahcall et al. Phys.
Lett. B433 (1998) 1