The ABCs of Nuclear Research at HRIBF

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General descriptions

All matter is comprised of atoms which contain an inner core called the nucleus around which negatively charged electrons orbit. The nucleus is comprised of positively charged particles called protons and neutral particles called neutrons. In matter, the atom has no net charge which means that the number of electrons is equal to the number of protons. Elements are atoms which have the same number of protons and are represented by the atomic number Z. Isotopes of an element have different numbers of neutrons (represented by the symbol N) and are represented by pairing the atomic mass (A = N + Z) with the symbol for the element.

For example, a coin made out of copper contains one element and it has 29 protons (Z = 29). The coin contains two isotopes of copper with 34 and 36 neutrons (N = 34 and 36). Thus the atomic masses for these isotopes are 63 (A = 29 + 34) and 65 (A = 29 + 36). These isotopes are represented by the symbol for copper, Cu, in the format ⁶³Cu and ⁶⁵Cu. All other known isotopes of copper: ^{52-62,64,66-80}Cu are unstable and will transform over time into different elements.

The smallest nucleus is hydrogen which contains a single proton and no neutrons. Nuclei heavier than hydrogen would not be able to form if it did not contain neutrons because the positive charge of the protons would repel each other. The neutrons help overcome this by contributing to what is called the strong force, a short-ranged attractive force between both protons and neutrons. However, as the number of protons increase, the nucleus needs more neutrons than protons to hold together. The heaviest stable nucleus with equal numbers of protons and neutrons is ⁴⁰Ca which has 20 protons and 20 neutrons. By the time a nucleus has 82 protons, ie., Pb, it needs at least 122 neutrons to be stable. If a nucleus does not have the right balance of protons and neutrons, then it decays by one of several processes that are usually called radioactivity.

Figure 1 - On this chart of the nuclides (isotopes), black squares represent stable nuclei and the yellow squares indicate unstable nuclei that have been produced and studied in the laboratory. The many thousands of these unstable nuclei yet to be explored are indicated in green. The red vertical and horizontal lines show the magic numbers, reflecting regions where nuclei are expected to be more tightly bound and have longer half-lives.

Much like electrons, nucleons, the general term for protons and neutrons, organize themselves into shells. The nucleus is a quantum system which means that it can only exist in specific (or quantized) energy levels described by various quantum numbers. The orbitals which make up the various shells of the nucleus can be described by these quantum numbers and follow specific rules. For example, any one orbital can contain at most two protons and two neutrons.

Nuclear research studies the effects the quantum rules have on the properties of the nucleus. One probe into the nucleus is to study its decay, the emission of energy from the nucleus. There are many methods for the nucleus to decay; the more common modes (radioactivity) are

beta decay - the transmutation or change of one type of nucleon into the other type
alpha decay - the emission of 4-helium nucleus (2 protons and two neutrons bound together)
gamma decay - the emission of high frequency light as the nucleus loses energy
fission process - the division of a nucleus into (usually two) large fragments
proton (neutron) emission - the emission of a proton (neutron) from the nucleus

It is important to note that we are surrounded at all times by radioactivity and in small amounts it is not harmful to us. Things as diverse as cement walls and bananas all contain naturally occuring radioactive nuclei. For the nuclear physicist, however, most of the nuclei around us are stable or close to it, i.e., they are in the lowest energy state possible. Therefore, in order to study nuclei and their many energy states not easily found in nature, we need special tools to produce them from the stable ones around us.

Figure 2 - This sketch is the same as figure 1 but shows the energy relationship between elements and thus, emphasizes Einstein's famous equation that mass is energy times the speed of light squared (E=Mc²). The isotopes in the valley are the stable isotopes (black squares in figure 1) and the elements along the side are the radioactive isotopes which move transform down the sides of the valley (decay toward stability).

At HRIBF, we use two accelerators which are able to produce a beam of energetic ions. An ion is an atom where the number of electrons no longer equal the number of protons. Negatively charged ions have more electrons than protons. These ions bombard a target and interact with the nuclei of the atoms which make up the target. Many things can happen when nuclei interact:

In almost all cases, the resulting nuclei are left in excited states which decay by emitting some form of radiation. Prior to 1996, HRIBF used beams and targets which consisted of only stable nuclei. Today, beams of radioactive nuclei are commonly used for experimentation. Stable hydrogen or helium nuclei are accelerated by the Oak Ridge Isochronous Cyclotron (ORIC) and strike a thick target such as uranium carbide or hafnium oxide. New nuclei are formed which can be extracted, ionized, and formed into a beam. This new beam of ions can then be accelerated by the 25 UEC tandem accelerator, a Guiness World Record holder, and used to strike another target which is where the actual experiment takes place. An animiated (svg) sketch of the radioactive beam production and delivery process is available.

The final target is typically surrounded by various experimental instruments which detect the radiation produced during the collision. Any new nuclei produced may be studied as they decay toward the stable nuclei. Follow the links to learn more about the various sub-fields of nuclear science studied at HRIBF.

Decay spectroscopy - the study of radioactive nuclei via typically slow decay processes such as beta decay, alpha decay, proton emission; typical time scale greater than 0.000001 second (10^-6 s or microsecond).
In-beam spectroscopy - the study of short-lived nuclear states primarily by gamma transitions emitted within the first moments of formation; typical time scale 0.000000000001 second (10^-12 s or picosecond) and less.
Reaction spectroscopy - the study particle transitions and resonances; typical time scale 0.000000000000001 second (10^-15 s or femtosecond) and less.
Nuclear astrophysics and nuclear reaction studies - these fields utilize data from nuclear structure such as half-lives and the energies of excited states to analyze their data and determine various rates and interactions. For example, isotope half-lives are direct input into astrophysical processes that cook new elements in stars.
Nuclear structure theory - the study of the quantum many-body problem and the predictive arm of the field.
Nuclear Data Project - our collective knowledge of the structure of the nucleus.