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Linné on line arrow Physics and the Cosmos arrow Physics and the Cosmos arrow The atomic nucleus – a new world with new forces

The atomic nucleus – a new world with new forces


The atomic nucleus consists of protons with positive charge, which were discovered in 1919 by Rutherford, and neutrons that are electrically neutral, as discovered by Chadwick in 1932. The protons and neutrons (or nucleons as they are both called) are kept together by a new kind of force that we do not encounter in our everyday life, the so called strong force. It is called so because it is so strong that it can keep the atomic nucleus together even though the protons are repelled by each other due to the electromagnetic force. With the help of only three building blocks, the proton, the neutron, and the electron, as well as their electromagnetic and strong interactions we can form all the atoms in the periodic system.

Protons and neutrons are equally affected by the strong force, but not all particles are. One such example is the electron. The particles that are affected by the strong force are called hadrons and today we know of several hundred different kinds of hadrons. Today we also know that the hadrons are not fundamental particles, but instead they are built up of quarks. The first proofs that the proton has an inner structure came in the end of the 1960's from an experiment done at Stanford Linear Accelerator Center in the USA. By irradiating protons in a fixed target with high energy electrons and studying how the electrons were scattered one concluded that protons have an inner structure, the quarks. Already earlier one had hints that protons and neutrons have an inner structure based on there magnetic dipole moments which do not at all agree with what one expects from a pointlike particle. For the proton one had measured the electric dipole moment to be 2.79 instead of 1 and for the neutron -1.91 instead of 0.

The structure of the atomic nucleus

In the same way as the electrons surrounding the atomic nucleus are in different energy levels or shells, the nucleons in the atomic nucleus are also in different shells. Among the first to study the energy levels of the atomic nucleus was Helge Tyrén in Uppsala. Together with his collaborators he studied the energy required to knock out protons from the atomic nucleus. The results confirmed a theory for the shells of the atomic nucleus which had been put forward earlier. At the The Svedberg Laboratory in Uppsala one is still doing research on the shell structure of different atomic nuclei and collective phenomena such as vibrational and rotational excitations.

The Gustaf Werner cyclotron at TSL (the The Svedberg Laboratory)
  The Gustaf Werner cyclotron was built in the late 1940's and was for a short while the most powerful accelerator in Europe. It has since then been upgraded and is today used for medical and nuclear physics research. It is also used as an injector for the so called CELSIUS ring, where research is done both in nuclear and particle physics.

Based on fundamental research in nuclear physics a lot of applications have been developed, such as nuclear power and medical applications. Dating of archaeological material can also be made with the help of nuclear technology by exploiting the lifetime of radioactive isotopes.

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