Santa's miniatures - 19 Aout 2008

An "alphabet" of shapes

The shape of an object is its ID card, and provides a host of information about its "personality" and its mode of interaction with its surroundings. A shape "alphabet" (as first suggested by the French mathematician Adrien-Marie Legendre) is used by scientists to describe such diverse objects as planets, teardrops or atomic nuclei.

The spherical shape, for example, of a soccer ball is very commonly found in nature. Unchanged when rotating around any axis passing through its center, it has a finite number of axes of symmetry. Other objects such as planet Earth, which is flatter at its poles, or a rugby ball, have a single axis of symmetry. From the infinitely large planetary systems to the infinitely small world of atomic nuclei, Nature generally chooses to give objects one of these two shapes.

Occasionally, objects may have a more complex shape, with no axis of symmetry. Among those, objects having a triaxial shape are obtained by deforming a sphere differently along three axes of symmetry. This is the particular case of the planetary object 2003 EL61, recently discovered in the Kuiper belt, a region of the solar system which extends beyond Neptune's orbit.
 

A recipe of protons and neutrons

In the infinitely small world of atomic nuclei, physicists probe the elementary constituents of matter, neutrons and protons, to apprehend the forces which keep them together. Just as we experiment with cooking recipes, different combinations of neutrons and protons can be tested. These may lead to nuclei which already occur naturally, as well as to other nuclei not found on Earth, but which can be created artificially for a very brief period of time. Changing the number and proportion of the constituents of the atomic nucleus may result in a suddenly different and unexpected behavior of the nucleus as a whole, for instance in terms of its degree of stability, size or shape.

A new form of nucleus

Recently, physicists at GANIL created a very uncommon species of Argon (Ar), with 48 nucleons (18 protons and 30 neutrons); that is, a nucleus having 8 more neutrons than the heaviest naturally occurring Ar on Earth. By using cutting-edge detector systems (such as the VAMOS spectrometer and the EXOGAM gamma radiation detection assembly), and studying the properties of these exotic nuclei, which only survive for a fraction of a second, they discovered that an unexpected phenomenon occurred, which is interpreted by theoreticians as reflecting a triaxially shaped nucleus. Such a result raises a number of fundamental questions about the mode of interaction between neutrons and protons, which can lead to this exotic shape.
 

”Santa”: nickname for the dwarf planet 2003 EL61 recently discovered in the Kuiper belt.