of secondary beams of exotic nuclei
of production rates
only 287 primordial nuclides exist. These are the nuclei
that can be found in natural sources. It is supposed that about 6000 nuclei are
supposed to "exist" in the sense that they live much longer than the time a
nucleon takes to cross the nucleus with its Fermi velocity (t >> 10-22
s). Only about half of them have
been observed up to now. Most of
the unobserved nuclides are neutron-rich.
The limitation to projectiles of primordial nuclides is a severe restriction to
the study of the properties of exotic nuclei.
Experiments with secondary
(or radioactive) beams are an important tool to explore
different properties of nuclei far off stability. Particular interest to
produce neutron-rich nuclei and to reach the nuclei involved in the
astrophysical r-process. The
scientific motivation for studying reactions with exotic
nuclei is described extensively in various reports in the context of
next-generation facilities (see e.g.
several reaction meachanisms that can be used to produce secondary beams:
A direct way to produce
neutron-rich nuclides is neutron capture. Most of the heavier nuclear species
in the universe have been produced in cosmic scenarios by this reaction
mechanism. Roughly two scenarios are considered according to the magnitude of
the neutron flux. The s process ("slow" process) evolves close to the valley
of beta stability. Since the time delay between two consecutive capture
reactions is longer than most of the beta half-lives, the beta-unstable
capture product decays, before the next neutron is captured. The r process
("rapid" process) evolves far from the valley of beta stability. The time
between two consecutive capture reactions is smaller than the beta half-lives
of the nuclei close to beta stability.
The application of neutron capture in laboratory under controlled conditions
for the production of very neutron-rich nuclei is not possible, because too
high neutron fluxes would be needed. A technical application, with or without
explicit intention, is the production of plutonium and minor actinides (actinides
with small production yields) in fission reactors. They are formed by
consecutive neutron capture and beta decay from 238U nuclei.
However, the production follows closely the valley of beta stability.
Two nuclei, gently brought into
contact, may fuse due to the attractive nuclear forces. For lighter systems,
this is the most important reaction mechanism at energies close to the Coulomb
barrier. In very heavy systems, however, the Coulomb force tends to
destabilise the merged system so that they re-separate immediately with higher
Fusion reactions, in particular using heavy-ion beams, have proven to be well
suited for the production of proton-rich nuclei up to the proton drip line.
They have been an important tool for exploring the properties of exotic nuclei
on the neutron-deficient side of the chart of the nuclides. Furthermore,
fusion is the only tool applied to produce super-heavy elements.
Since the nucleons of projectile and target just add up, the nuclear
composition of the fusion product is well defined. Only the evaporation
process leads to a loss of a few nucleons.
Due to the curvature of the line of beta-stability, fusion is not suited for
the production of neutron-rich nuclides. Another draw-back of fusion reactions
is the low beam energy required which only allows for the application of thin
targets, because the energy of the projectiles would become too low in a
thicker target due to electronic interactions. Therefore, only a very small
fraction of the projectiles goes into nuclear reactions, thus
resulting in low intensities of secondary beams.
Fission can be considered in some sense as the reverse of fusion. Here the
process starts from heavy rather neutron-rich nuclei. As a consequence, the
fission products are normally situated on the
neutron-rich side of the chart of nuclides. The production of nuclides which
are more neutron rich than the fissioning system is only possible due to
charge polarisation, that means that the two fission fragments are formed with
different N-over-Z ratios. Fluctuation phenomena as well shell effects might
be responsible for charge polarisation. However, the restoring force due to
the nuclear asymmetry energy is very strong. This limits the polarisation to
rather low values.
Most fission fragments are formed with appreciable excitation energies,
leading to neutron evaporation very shortly after fission. Therefore, the
average N-over-Z ratio of the fission fragments is below the value of the
Nuclear collisions at bombarding
energies well above the Fermi energy can be considered as quasi-free
nucleon-nucleon collisions. The collisions essentially remove a number of
nucleons from the projectile respectively target nucleus. Collisions at large
impact parameters are an interesting tool for the production of exotic
nuclides. We use the term fragmentation for these reactions in which a large
part of the projectile respectively target survives. This should not be
confounded with multi-fragmentation in which very light nuclei are produced at
more central collisions.
An important feature of fragmentation reactions is the strong
statistical fluctuation. This leads to large variations in the N-over-Z ratio
of the reaction products. Also the energy induced in the collision is subject
to a large fluctuation and extends to rather high values. Therefore, the
consecutive evaporation cascade has an important influence on the nuclear
composition of the fragmentation products observed.
Fragmentation is a mechanism which produces a large number of nuclides,
scattered over an extended region of the chart of the nuclides.
These different mechanisms are
used (or will be used) in two types of facilities: ISOL-type or in-flight-type.
In ISOL-type facilities, the secondary beams are produced as a target residues.
Consequently, the wanted beam particels have to be extracted from the target,
ionized and accelerated to needed energy. On the other hand, in in-flight-type
facilities, the secondary beams are produced as projectile residues, and,
therefore, the efforts for beam preparations are much smaller.
list of existing or planned radioactive-beam facilities
is given (a more
complete list is available on the EURISOL
I. Existing facilities
accélérateur national d'ions lourds, Caen, France)
für Schwerionenforschung GmbH, Darmstadt, Germany)
(Canada's National Laboratory for Particle and Nuclear Physics,
(National Superconducting Cyclotron Laboratory at Michigan State University,
(Oak Ridge National Laboratory / Holifield Radioactive Ion Beam Facility,
Beam Factory, Japan)
Texas A&M (Cyclotron
(Dual Solenoid Project, University of Notre Dame, USA)
future project) (An International Accelerator Facility, Darmstadt,
Official RIA website (Rare
Isotope Accelerator, USA) - RIA website of ANL
- RIA website of MSU
(European Isotope Separation On-Line Radioactive Nuclear Beam Facility,
II (Système de Production d'Ions Radioactifs en Ligne, Caen,
SIRIUS (Radioactive Beams for
Science and Medicine, Daresbury, Great Britain)
SPES (Study for the Production
of Exotic Species, Legnaro, Italy)
Our group is involved in
estimated the intensities of secondary beams at the future facilities:
at GSI and
Details can be found here:
Some of this work is also
summarized in the following publications:
PRODUCTION: PROTONS VERUS HEAVY IONS"
M. V. Ricciardi, S. Lukic,
K.-H. Schmidt, M. Veselsky
Contribution to the Proceedings The
Seventh International Conference on Radioactive Nuclear Beams RNB7, July 3-7,
2006, Cortina d'Ampezzo (Italy); to be
published by European Physical Journal A
"STUDIES ON THE BENIFIT OF EXTENDED
CAPABILITIES OF THE DRIVER ACCELERATOR FOR EURISOL"
K.-H. Schmidt, A. Kelic, S. Lukic, M. V. Ricciardi, M. Veselsky
accepted in Phys. Rev. ST AB
COMPARISON OF ISOLDE-SC YIELDS WITH CALCULATED IN-TARGET PRODUCTION RATES"
S. Lukic, F. Gevaert, A. Kelic, M. V. Ricciardi, K.-H. Schmidt, O. Yordanov
Nucl. Instrum. Methods A
565 (2006) 784-800,
"MODEL CALCULATIONS OF A TWO-STEP
REACTION SCHEME FOR THE PRODUCTION OF NEUTRON-RICH SECONDARY BEAMS"
J. Benlliure, M. V. Ricciardi, K.-H. Schmidt
Eur. Phys. J. A 17 (2003) 181-193
/ arXiv nucl-ex/0302008