Superheavy, and yet stable

Measurements of the shell effect in atomic nuclei contribute to finding out above which mass extremely heavy artificial elements no longer decay

August 23, 2012

The heaviest element on earth is uranium, which has the atomic number 92 in the periodic table. Although superheavy elements up to number 118 have been produced artificially, their atomic nuclei rapidly decay. A subtle quantum effect means that even heavier atomic nuclei above element 120 could exist for years, however. Physicists have been searching for this hypothetical “island of stability” for a long time. An international team that includes Klaus Blaum’s group at the Max Planck Institute for Nuclear Physics in Heidelberg has now taken a further crucial step in the right direction. In a spectacular precision experiment at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, the cooperating team has been able to determine the strength of the shell stability in heavy nuclei with 152 neutrons for the first time. A breakthrough in the understanding of the physics of atomic nuclei.

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Penning trap of the SHIPTRAP device
Penning trap of the SHIPTRAP device

Most Germans associate the surname “von Weizsäcker” with the former German president Richard von Weizsäcker; physicists immediately think of his older brother, Carl Friedrich, however. He was a physicist who, in the 1930s and 40s, made a decisive contribution to the description of a very small, but eminently important building block of matter - the atomic nucleus. “Carl Friedrich von Weizsäcker would have been 100 years old this year,” says Klaus Blaum of the German physicist whose model of the atomic nucleus is still being used today in a modified form. In a large international cooperative effort, Blaum and his colleagues have now made a crucial breakthrough in what is known about the structure of the atomic nuclei - knowledge that has remained almost stagnant for a long time. A great many physicists had been working towards this goal for decades and Carl Friedrich von Weizsäcker, who died in 2007, would no doubt have been fascinated.

In simple terms, an atom consists of an extended electron cloud and an atomic nucleus that is tiny in comparison. Nevertheless, the nucleus contains the total mass of the atom. Not only that: an atomic nucleus also has an extremely complex structure. And the larger such a nucleus is, the more obscure the play of the forces which determine its existence.

Two forces battle it out in atomic nuclei

The actors in this play are the nuclear building blocks, the neutral neutrons and the electrically charged protons. The protons fiercely repel each other with the second most powerful force in physics, the electric force. The strongest force known in physics today, the strong force, overpowers this repulsion, however. With its superstrong grip it forces the nucleus together. But like any superhero - be it Achilles, Siegfried or Superman - it has a weak point: its range is small. Its superstrong arms, so to speak, have ended up being much shorter than those of the electric force. The consequence: the bigger an atomic nucleus becomes, the harder it is for the strong force to keep it together against the electric force. Above a certain size it becomes unstable and breaks apart.

This simple image is only a rough description, though. A large atomic nucleus with its more than one hundred protons and far more than one hundred neutrons forms an extremely complex many-particle system, which is structured like an onion with concentric layers of different numbers of protons and neutrons. In this concentrated accumulation of quantum particles an effect which originates from the ordering principles of the quantum world plays a decisive role: atomic nuclei with perfectly filled shells are more stable than others. This effect even causes comparatively huge atomic nuclei, which really ought to decompose, to be kept together.

Those who remember school lessons in chemistry or physics will recognize this shell effect in a slightly different form. The electron cloud can also be sub-divided into energy shells, and the electron shells of inert gases are particularly stable. The cause is again this ordering principle. Electrons resemble protons and neutrons in that they are also, in a way, individualists of the quantum world, claiming a quantum state for themselves alone. And each shell has only a limited number of places. With inert gases, these places in the outer shell are full, which is why they are extremely stable chemically. This quantum effect protects them against attack from other; chemically more aggressive elements which want to fill their not completely filled shells with electrons of other atoms, for example, come what may. “It’s like playing ring-a-ring-o’-roses with children, where the dancing ring is closed,” explains Blaum: “It then becomes more difficult for more children to join in.”

Measurements with the world’s most sensitive balance for atomic nuclei

This game of shells also takes place in the atomic nuclei. The shells of the large atomic nuclei have a much more complex structure than electron shells, however. The large numbers of nuclear components all influence each other. The theoreticians have therefore only been able thus far to give a very imprecise estimate of which shells are really filled at which “magic” number of components. Therefore, experimental physicists have to find this out with ingenious tests. And this is precisely where the cooperation in Darmstadt involving Blaum’s team succeeded for the first time with the elements 102, nobelium, and 103, lawrencium. In addition to the physicists from Darmstadt and Heidelberg, Germany was represented by groups from the Universities in Mainz, Gießen, Greifswald and the Ludwig-Maximilians Universität München.

First the researchers produced the two elements 102 and 103 with the heavy ion accelerator in Darmstadt. Elements this heavy are only very rarely produced in the process, however. These few electrically charged atoms are collected by a complex apparatus called SHIPTRAP, and even this is only successful in a small number of cases. SHIPTRAP is the world’s most sensitive balance for atomic nuclei, which are heavier than uranium. It can weigh these atomic nuclei with an incredibly accuracy. SHIPTRAP operates in a completely different way to a kitchen balance, of course. It catches the electrically charged atom (ion) in a trap of electromagnetic fields. In this floating cage the ion performs a complex oscillatory motion, the frequency of oscillation depending on the mass of the atomic nucleus.

The magic number is 152

The physicists thus use the frequency to extract highly precise information on the mass of the nucleus. But now the question is how they get from the mass of the nucleus to its internal structure. The key is Einstein’s famous equation E = mc2, according to which mass and energy are two sides of the same coin. The mass measured thus indicates the energy that is contained in the atomic nucleus. And a portion of this energy, the so-called “binding energy”, in turn provides the decisive information on the exact shell structure of the nucleus. By weighing the elements 102 and 103 with a varying number of neutrons the team has now applied this ingenious method to obtain a “magic” number: the outer neutron shell must contain 152 neutrons. It is then full and stabilises the nucleus.

“We could thus exclude some of the models for atomic nuclei used until then as being wrong,” says Blaum’s on the far-reaching consequences of this breakthrough. It has taken decades, but the picture of the inner structure of heavy atomic nuclei is now finally becoming clearer. Armed with this knowledge, the physicists can now look more specifically for the famous island of stability. “We expect it at around element 120,” says Blaum, “and to be more precise, in a nucleus with around 180 neutrons.”

If such long-lived, superheavy elements can be produced artificially, they could possibly also be produced in rare events in the universe. No such extreme element has yet been detected - but the universe is gigantic. In any case, such basic research expands our knowledge on what keeps the world together at its very core.


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