Transfer reactions
The advent of a new generation of 4 pi gamma-spectrometers aiming at
a substantially improved detection sensitivity opens new perspectives
for nuclear structure studies employing direct reactions. Here we report
on two experiments, one performed during the Euroball-Cluster campaign at GSI
and the other performed at the accelerator laboratory of the INFN in Legnaro
(Italy) using the Euroball I spectrometer GASP,
both using particle-gamma coincidence techniques.
Enhanced Neutron-pair Transfer between superfluid heavy nuclei
In this study multiple Coulomb-excitation and neutron-transfer reactions were
investigated in the system 206Pb+118Sn using two
different bombarding energies below the Coulomb-barrier.
The experiment was performed at the accelerator UNILAC at the Gesellschaft
für Schwerionenforschung in Darmstadt (GSI) with the experimental
set-up consisting of five EUROBALL-Cluster detectors (EB) and the
Heidelberg-Darmstadt crystal-ball (CB) for measuring gamma-rays and
in addition three parallel plate counters for measuring the recoil nuclei.
The experimental technique was to detect in coincidence
one backscattered Sn-isotope and a photon in at least one EB-detector.
Gamma-transitions following multiple inelastic excitation as well as
one- and two-neutron transfer could be observed. The intensities of the
transitions were used in a semiclassical approach to determine absolute
reaction-probabilities as a function of the distance of closest approach
of the two nuclei in the reaction.
The probabilities of the gamma-transitions following inelastic excitation
could be explained with Coulomb-excitation calculations within the accuracy
of the in the calculation used reduced matrix elements. By comparison of the
probabilities of the transitions following one- and two-neutron transfer,
enhanced two-neutron transfer could be observed, expressed in terms of the
enhancement factor EF.
The high efficiency of the CB in measuring the total gamma-multiplicity and
gamma-sum energy of a reaction made it possible to select
"super-cold" transfer and thus to determine for the first time an enhancement
factor referring to well defined states of both partner nuclei.
The measured
enhancement factor was EF=900 and independent of the bombarding energy.
Furthermore for the first time a direct comparison with theoretical work of
Broglia et al. [bro78] was possible, which explains an enhancement of this
order with the superfluid properties, specifically the configuration mixing,
of the nuclei in their ground states and first excited states.
For further details, see also PhD-Thesis
of I.Peter (in german) and I. Peter ,W. von Oertzen et al., EPJ A , (1999) accepted for publication.
Investigations of Multi-Nucleon Transfer-Reactions using gamma-ray spectroscopy
The system 110Pd + 52Cr was chosen to study the mechanism of multinucleon
transfer at energies in the vicinity of the Coulomb barrier (170MeV, 185MeV,
200MeV and 215MeV). It is characterized by well adjusted Q-values for neutron
and proton transfer and large nucleon pairing energies. The high efficiency
of GASP enabled the identification of 10 different transfer channels by their
characteristic gamma-decay. The most exotic 4p6n transfer leading to 100Mo
has a cross section of only sigma(tr) 40ub. Considering the similarity of the
effective Q-values the observed population pattern for exotic channels seems
to be in favour of correlated multinucleon transfer.
With the inner BGO ball of GASP it was possible to measure total excitation
energies as a function of the gamma-multiplicity. The correlation for the most
prominent transfer channels show a 'cold' component - representing collective
excitations along the yrast line - which is another indication for correlated
pair transfer. For the 2n and 2p channel this fraction is 30(3) percent,
respectively 18(9) percent. From the Q(eff)-values and the pairing energies of the
corresponding donor and acceptor nuclei it may be concluded that 'cold'
(correlated pair) transfer is strong if the pairing energies are larger then
the Q-value. This would also explain previous results on cold transfer
probabilities in other scattering systems.
For further details, see also PhD-Thesis
of K.Vetter (in german).
