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Creating Super Heavy Elements
Uranium
with the proton number Z=92 is the heaviest element which can be found
in nature. All elements with larger proton numbers have been created
artificially in nuclear reactions. While elements up to Z=100 can be
reached in neutron-capture processes with subsequent beta decays, the
elements with more than 100 protons are created in nuclear fusion
reactions.
In order to fuse two nuclei they have to come in contact with their surfaces. Only then the nuclear
forces can act which is the basic requirement for the fusion process.
Before the surfaces can touch the repulsive electric forces (Coulomb
repulsion) due to the charged protons in both nuclei have to be
overcome. For this, one of the nuclei (projectile) is accelerated up to
a velocity which is just high enough to overcome the Coulomb forces and
then collides with a target nucleus which is provided as a thin foil.
Usual projectile velocities for the synthesis of heavy elements are
≈10% the speed of light. Such beams are provided by the UNILAC
accelerator.
Figure
1: As first step to fusion the projectile and target nucleus have to
come in contact with their surfaces. A di-nuclear system is formed
which sticks together via the nuclear force. The di-nuclear system can
break up after some time or the nuclei can fuse and form a compound
nucleus with a certain excitation energy. Due to this excitation energy
the compound nucleus can undergo fission, preferentially in two pieces
with approximately the same mass. Or is can emit some neutrons and go
into the energetic ground state (fusion product). (Image source: GSI Helmholtzzentrum für Schwerionenforschung, S.Heinz)
For
the synthesis of heavy elements usually the lighter of both nuclei is
provided as projectile. About 1012 projectiles impinge the target every
second. But a fusion reaction occurs rather rarely and becomes more and
more unlikely with increasing proton numbers of the nuclei since also
the Coulomb repulsion is increasing. For example, some isotopes of
element 102 can be produced with a rate of one nucleus per second,
while for the element 112 one nucleus per week is produced.
For
nuclei with more than 104 protons the repulsive Coulomb forces are
already so strong that they should prevent the binding of the neutrons
and protons in the nucleus. But a stabilizing effect is created by the
arrangement of the nucleons on discrete energy levels (shells) which
enables the existence of even heavier nuclei. All elements with
Z>104 exist merely due to these "shell effects". Each shell can be
occupied by a certain number of neutrons or protons. Nuclei where all
shells contain the maximum possible number of nucleons ("closed
shells") reveal a considerably enhanced stability in comparison to
other nuclei, which means that more energy has to be spent in order to
excite or decompose them. Also in the region of superheavy nuclei such
closed shells are expected but they have not yet been experimentally
identified. Predictions from different theoretical models expect these
closed shells for proton numbers Z=114 or 120-126 and for the neutron
number N=184. It is the major goal of present superheavy element
research to find this so-called "island of stability".
 Figure 2: Presently known isotopes of heavy and superheavy elements. The blue background shows the stability of the nuclei located in the respective region. Darker regions correspond to more stable nuclei.
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