From matter to radioactive nuclei

Atoms at the heart of matter

Seen through a microscope, matter looks like a collection of atoms which may appear to be disorganized, for instance in the case of a water drop, or well organized according to highly regular structures, as in the case of kitchen salt.

Whereas an atom is very small, with more than 99% of its mass being concentrated within a volume which is even 100,000 times smaller, called the atomic nucleus, the remainder of the atom's mass consists of very light and negatively charged particles, called electrons ([-]).

The atomic nucleus itself is comprised of two types of particle: the proton ([+]), which is a positively charged particle, and the neutron ([0]), which owes its name to the fact that it has no electric charge.

Since an atom is electrically neutral, the number of negatively charged electrons orbiting the nucleus is exactly the same as the number of protons in the nucleus: [+] + [-] = [0]. When an electron is withdrawn from, or added to an atom, the latter is transformed into an ion, in other words a negatively or positively charged atom.

From the atom to the nucleus

The number of protons in an atomic nucleus makes it possible to determine the chemical type, or element, this atom belongs to.
There are about one hundred different chemical elements in the Universe:

	The lightest of all, hydrogen, is an atom whose nucleus has only one proton, with a single orbiting electron.	The heaviest, uranium, has a nucleus composed of exactly 92 protons and a variable number of neutrons. Exactly 92 electrons orbit the nucleus.

Stable nuclei and radioactive nuclei

At first sight, the nucleus of an atom appears to be a very simple object, made of only two different types of particle. However, it is an extremely dense object, which may have many protons whose identical electrical charges could normally  be expected to drive them apart.

The cohesion of an atomic nucleus results only from the nuclear forces, which act as a powerful glue on the protons and neutrons. This glue is rather capricious since, for a given number of protons, it is efficient only when a certain number of neutrons are present: if they are too scarce or too numerous, the glue no longer functions and the nucleus disintegrates. It then becomes what is called a radioactive nucleus.

The phenomenon of radioactivity is therefore associated with the transformation of a nucleus, which "wants" to change into something more stable. A radioactive nucleus will choose among the various possible modes of radioactivity, that or those which provide it with the fastest return to stability. Thus, the lifetime of a radioactive nucleus depends only on the ease with which it can mutate into a more stable form.

The concept of isotopes

For nearly all chemical elements, there are several combinations of protons and neutrons which provide stability to the atom's nucleus.
 

At GANIL, all possible combinations one could think of have been tested. Thus, oxygen nuclei have been produced, with a number of neutrons varying from 5 to 16. Their lifetime varies from 13 thousandths of a second to nearly 2 minutes, which is amply sufficient to study their properties. Oxygen isotopes with less than 5 neutrons or more than 16 neutrons are so unstable that they decay instantaneously: for the physicist who does not have the time to observe them, it is as if they did not exist.

Producing radioactive nuclei

A simple method for producing radioactive nuclei consists in fragmenting stable nuclei into several pieces. This is the technique used in the GANIL facility.
In practice, a large quantity of atoms of a given chemical element are prepared, and transformed into ions by tearing off several of their electrons. This is the role of an ion source.

Since ions are charged particles, they may be accelerated by means of particle accelerators, and fired onto a piece of matter which itself contains atoms. From time to time, the nucleus of an accelerated ion strikes the nucleus of an atom in the target and breaks up into several fragments: this will produce ions having either a stable or a radioactive nucleus, according to their composition.
All that remains to be done is to select the radioactive ions of interest: this sorting operation is performed by instruments called spectrometers, which control the ions according to their electric and magnetic fields.

The study of radioactive nuclei

In the most simple case, the scientists only need to determine the life span or the decay mechanism of the radioactive nucleus which has been produced. It is then simply guided into an experimental chamber, where detectors are used to observe its transformation.
If the characteristic of interest is its shape, or the way protons and neutrons are arranged, within the radioactive nucleus, it is preferable to make the latter interact with the nuclei of another target, surrounded by detectors.