|
| Exotic Nuclei - the Key to Our Universe |
| |
Exotic Nuclei - the Key to Our Universe
Anything nature can cook up in her kitchens is now available to
researchers. GSI's accelerator systems can create exotic nuclei with
extremely high proton-to-neutron ratios, giving us deeper insight into the
origin and stability of the matter in our universe. Compared to other
nuclides, the so-called magic nuclei have a particular significance.
 |
| The supernova SN 1987A, observed in 1987 in the area of the Tarantula Nebula, was visible even to the naked eye. Supernovas are characterized by an increase in brightness of up to a hundred million times the original value within a few days. All elements above iron in the periodic table are created in the course of these stellar explosions. Photo: European Southern Observatory ESO |
The state of matter on our planet is not typical of the rest of the universe. In the interior of
stars and in supernova explosions, unimaginably high temperatures and pressures predominate,
creating particles and nuclei which do not naturally occur here below.
The number
of these exotic, unstable nuclei brewed up in the cauldrons of the universe goes far beyond the
manifold spectrum of stable isotopes ordinarily encountered on Earth.
Production and investigation of such exotic nuclei in modern accelerators is of great interest
for two reasons. First, we can refine our theoretical models of the properties of nuclei only by
looking beyond the range of isotopes available to us on Earth and thereby taking into account
the broadest possible spectrum of the nuclei present in the universe. Second, we know today
that the synthesis of elements in the stars takes place via exotic nuclei. These nuclei decay
principally through the emission of beta particles - i.e. high-speed electrons - until the stable
nuclei known to us on Earth are left. Thus, the formation of chemical elements and their
abundance are essentially determined by the properties of these exotic nuclei.
 |
| The chart of the nuclides today. Each of the almost 2500 known atomic nuclei is
classified here according to the number of protons (Z) and neutrons (N). Only the nuclei
marked in black, forming the "crest" of the "mountain range" in the diagram, are stably present
in nature. All the others were produced through nuclear reactions; some 150 of these were first
produced at GSI in Darmstadt. The lines Bn = 0 and Bp = 0 mark the limits of stability
according to current theory. The vertical and horizontal double lines indicate full shells which
lead to magic nuclei. The "high points" of 1994 - the new elements 110 and 111 discovered at
GSI, as well as the heaviest doubly magic nucleus, 100Sn, which is also symmetrical in its
number of neutrons and protons - are clearly marked. |
The Gesellschaft für Schwerionenforschung (Society for Heavy ion Research or GSI) in
Darmstadt is the only accelerator laboratory in the world with the capacity to investigate these
nuclei throughout the entire periodic table: from hydrogen, the lightest element, to the heaviest
element so far created artificially, the element with atomic number 111. Darmstadt has
equipment which can produce beams of any of these nuclei over a broad energy range.
In principle, the method of producing the nuclei is always the same: a beam of
accelerated particles from the linear accelerator UNILAC, or the heavy ion synchrotron SIS, is
directed against a foil or a piece of matter, called the target. Nuclear reactions in the target
produce exotic nuclei which are then separated by separators according to nuclear charge and
mass. They are then available for research.
At UNILAC there are two separators for low-energy nuclei, each of which functions in a
different way. In the On-Line Mass Separator, exotic nuclei beams of the lowest energy are
created by extracting the reaction products from a target located in an ion source, accelerating
them in a high-voltage field, and then separating them in a magnetic field into pure isotopes. By
contrast, as described in the discussion of element synthesis, the SHIP velocity filter uses
electrical and magnetic fields to divide the reaction products whilst still in flight and can thus
separate extremely unstable, short-lived nuclei.
Investigating the stability of exotic nuclei
The SIS accelerator is capable of creating ion beams at relativistic energies, i.e., at velocities
approaching that of light. This means that the spectrum of nuclei which can be investigated
experimentally is substantially widened. In conjunction with the experimental storage ring, a
fragment separator makes it possible to create secondary beams of these exotic nuclei. In this
way, entirely new kinds of experiments into nuclear structure and nuclear reactions can be
carried out.
 |
| Mass measurement with the help of the ESR storage ring. The two structures in the
detail of the frequency spectrum correspond to the ground state of the manganese nucleus
(right) and its isomeric excited state. They are distinguished by a small (but still detectable)
relative mass difference of around 7x10-6. |
The first important piece of information about the stability of an atomic nucleus is provided by
its lifespan, or half-life. As nuclei approach the limits of stability, they become increasingly
short-lived and exist for only a small fraction of a second. Extremely sensitive methods
developed at GSI even permit half-life values to be determined from the decay of the nuclei of
individual atoms. To date, no fewer than five new elements and some 150 new isotopes have
been discovered at GSI. Information about their properties is yielded in decay spectroscopy;
here beta decay, as the most common naturally occurring kind of decay, is the most important.
The beta half-lives of exotic nuclei are of fundamental importance for the understanding of the
material structure of the universe.
The masses of the nuclei provide further information about their stability. From the mass, it can
be determined how strongly the nucleus is bound internally. At GSI, a new method which
makes it possible to determine the masses of unstable atomic nuclei directly has been
developed and applied for the first time. The nuclei separated in the fragment separator are
transferred into the ESR storage ring where they are stored and then cooled by interaction with
an electron beam in such a way that all the ions in the ring circulate at exactly the same speed.
However, in as far as their masses are different, they travel along different paths in the ESR
and have different periods of circulation. The mass of a nucleus can thus be determined directly
from its period of circulation. The precision and resolution of this method is so good that it is
even possible to detect whether an atomic nucleus is in its ground state or in an excited state.
This is because the acquired energy of excitation causes an increase in mass.
Among the nuclides, the so-called "magic nuclei" are of an especial significance. Analogous to
the structure of electron shells which, when filled, give us the stable noble gases, the protons
and neutrons in the atomic nucleus also form filled shells that lead to particularly stable nuclei
- the magic nuclei. The correct prediction of shell filling is thus an important test of nuclear
models, particularly if the nuclei are highly unstable.
 |
| Confirmation of the doubly magic tin nucleus 100Sn (with 50 protons and 50
neutrons) in the secondary beam of the SIS fragment separator. The energy loss, which is
proportional to the nuclear charge, is plotted against the ratio of nuclear mass to charge. The
observed events are circled. |
In the region of the heavier elements there are only three stable, doubly magic nuclei, i.e.,
nuclei with full shells of protons as well as of neutrons. These are calcium with mass numbers
of 40 and 48, and lead with mass number of 208. If the unstable nuclei are included, four more
doubly magic nuclei are predicted: nickel with mass numbers of 56 and 78, and tin with mass
numbers of 100 and 132. The investigation of these nuclei and their neighboring nuclides is of
great importance for the testing and further development of theoretical nuclear models.
One nucleus which has been long sought-after is that of tin with a mass number of 100. This
nucleus not only has a doubly magic structure, it also possesses the same number of protons
and neutrons, a fact which means that it exhibits a high degree of symmetry. However, tin-100
is far off stability and its experimental synthesis therefore extremely difficult. Nevertheless, the
existence of this rare isotope was finally confirmed when it was produced for the first time in
1994 at Darmstadt.
The limits of stability
A fundamental issue for the understanding of our universe is the question concerning the limits
within which matter can exist, if even only for a very short time. Protons and neutrons are the
basic building-blocks of the nucleus. In the stable nuclei which surround us, they have a
particular ratio to one another. This means that if, for example, a nucleus is produced with an
excess of protons, and a certain boundary value is thereby crossed, the nucleus will
spontaneously decay through the emission of protons. When the proton decay from the ground
state of the element lutetium-151 was discovered in 1982 at GSI, this boundary line - also
referred to as the proton drip line - was attained and crossed experimentally for the first time.
For light nuclei near to the neutron drip line - i.e., with so many excess neutrons that an
additional added neutron will not bind to the nucleus - yet another effect can be observed: the
radius of the nucleus is "inflated" to form a halo. This new phenomenon occurs in certain
neutron-rich nuclei near the neutron drip line, such as beryllium and lithium with mass number
11. In these nuclei, the very weakly bound neutrons move far away from the nucleus, forming a
sort of thin "cloud" of neutrons around the nucleus. One question that arose from this
phenomenon was whether this halo effect also shows up in nuclei at the proton drip line. It
turns out that it does, as was recently confirmed for the first time for the proton-rich isotope
boron-8.
When two atomic nuclei fly past one another without touching, forces of repulsion operate
between them due to their positive charges. These forces are called Coulomb forces. Through
this electromagnetic interaction, atomic nuclei can be set into high-speed rotation or
oscillation. In extreme cases this can lead to nuclear fission - to the nucleus splitting. However,
for such Coulomb excitation or even Coulomb fission to take place, the nuclei must meet each
other at a very high speed. In the case of uranium the speed must be at least one-tenth the
speed of light, or 30,000 km/s. With such velocities attainable with the UNILAC accelerator,
GSI has been using the electromagnetic field of a passing nucleus to excite nuclei in its nuclear
spectroscopy experiments since the very beginning.
Experiments with Coulomb excitation can also yield information about nuclear structure. In
this respect, a series of exotic nuclear forms, including bi-axial and tri-axial ellipsoidal forms
and pear-shaped octopole deformations, was observed at GSI. Other GSI experiments showed
for the first time that the deformations of particular nuclei change dynamically when these
nuclei are set into rotation. In a rotating system, additional centrifugal and Coriolis forces,
which can play a role in shaping the inner structure of the nucleus, are at work.
Giant resonances and nuclear structure
Using the SIS heavy ion synchrotron, atomic nuclei can be accelerated to substantially higher
velocities - i.e., up to 80-90 percent of the speed of light. In reactions at these velocities, inner
oscillations known as giant resonances are induced. Such resonances are by no means new: in
particular physicists have long been aware of a dipole resonance in which protons and neutrons
collectively oscillate against one another. However, these oscillations can only arise in discrete
quanta, called phonons (not to be confused with photons).
Despite intensive research, until a few years ago, only the lowest of these phonons was known.
By lowest, we mean the phonon with the lowest energy. However, it then emerged that high-
energy heavy ion Coulomb excitation of the higher phonons is particularly effective.
Subsequently, the electromagnetic excitation of the second dipole resonance phonon, the
double dipole giant resonance, was confirmed by various methods in Darmstadt.
 |
| Installation of scintillation counters for the investigation of collective excitations in nuclei. |
The giant resonances and their higher phonons lie at such high excitation energies that the
nuclei are no longer stable and decay through the emission of nucleons. This is why we also
speak of Coulomb dissociation of nuclei in this context. At GSI, these dissociation processes
are used to investigate the inner structure of those radioactive nuclei which do not occur in
nature, but can be produced as secondary beams at the fragment separator.
In the easily fissionable nuclei of the actinide group, a giant resonance does not decay through
the emission of nucleons, but leads instead to fission of the nucleus. Thus, with the help of the
high-energy, heavy ion beams produced by SIS, new methods for further investigating nuclear
fission - one of the earliest research areas in nuclear physics - have now been developed. A
key point here is that such experiments can now be extended to beams of radioactive isotopes
which were previously inaccessible. Utilizing such secondary beams, the fission of an entire
new range of isotopes has recently been investigated in Darmstadt.
However, it is not only the fission process alone which is of great interest to nuclear physicists,
but also the fission products themselves. For example, the Coulomb fission of uranium beams
in SIS can produce extremely neutron-rich nuclei. With the help of this and other methods,
physicists have recently been able to produce the doubly magic nucleus nickel-78 for the first
time. This nucleus plays an important role in the synthesis of elements in stars.
|
|
©
Imprint, Copyright Notice and Disclaimer (in German language)
gsi.de