Nuclear physics
Experiments by Otto Hahn in 1911 and by James Chadwick in 1914 discovered that the beta decay spectrum was continuous rather than discrete.
The 1903 Nobel Prize in Physics was awarded jointly to Becquerel, for his discovery and to Marie and Pierre Curie for their subsequent research into radioactivity.
"[4] Hans Geiger expanded on this work in a communication to the Royal Society[5] with experiments he and Rutherford had done, passing alpha particles through air, aluminum foil and gold leaf.
But Rutherford instructed his team to look for something that shocked him to observe: a few particles were scattered through large angles, even completely backwards in some cases.
This was a particularly remarkable development since at that time fusion and thermonuclear energy, and even that stars are largely composed of hydrogen (see metallicity), had not yet been discovered.
The Rutherford model worked quite well until studies of nuclear spin were carried out by Franco Rasetti at the California Institute of Technology in 1929.
In the Yukawa interaction a virtual particle, later called a meson, mediated a force between all nucleons, including protons and neutrons.
The center of the atom contains a tight ball of neutrons and protons, which is held together by the strong nuclear force, unless it is too large.
The liquid-drop model is able to reproduce many features of nuclei, including the general trend of binding energy with respect to mass number, as well as the phenomenon of nuclear fission.
Superimposed on this classical picture, however, are quantum-mechanical effects, which can be described using the nuclear shell model, developed in large part by Maria Goeppert Mayer[28] and J. Hans D.
Ab initio methods try to solve the nuclear many-body problem from the ground up, starting from the nucleons and their interactions.
[30] Much of current research in nuclear physics relates to the study of nuclei under extreme conditions such as high spin and excitation energy.
Experimenters can create such nuclei using artificially induced fusion or nucleon transfer reactions, employing ion beams from an accelerator.
Beams with even higher energies can be used to create nuclei at very high temperatures, and there are signs that these experiments have produced a phase transition from normal nuclear matter to a new state, the quark–gluon plasma, in which the quarks mingle with one another, rather than being segregated in triplets as they are in neutrons and protons.
Plotted on a chart as a function of atomic and neutron numbers, the binding energy of the nuclides forms what is known as the valley of stability.
A frontier in current research at various institutions, for example the Joint European Torus (JET) and ITER, is the development of an economically viable method of using energy from a controlled fusion reaction.
For a neutron-initiated chain reaction to occur, there must be a critical mass of the relevant isotope present in a certain space under certain conditions.
The conditions for the smallest critical mass require the conservation of the emitted neutrons and also their slowing or moderation so that there is a greater cross-section or probability of them initiating another fission.
In two regions of Oklo, Gabon, Africa, natural nuclear fission reactors were active over 1.5 billion years ago.
[32] Measurements of natural neutrino emission have demonstrated that around half of the heat emanating from the Earth's core results from radioactive decay.
[33] According to the theory, as the Universe cooled after the Big Bang it eventually became possible for common subatomic particles as we know them (neutrons, protons and electrons) to exist.
The most common particles created in the Big Bang which are still easily observable to us today were protons and electrons (in equal numbers).
Since the creation of heavier nuclei by fusion requires energy, nature resorts to the process of neutron capture.
