needed for nuclear physics, detectors of greatly varying characteristics have been and continue to be developed for a spectrum of experimental observables. These range from the high-resolution detection of optical radiation at eV energies in studies of nuclear hyperfine structures, to sophisticated multidetector arrays needed to disentangle thousands of reaction products in high-energy nuclear collisions, and to scintillation detectors of thousands of tons buried deep underground to register the most elusive particles in nature, the neutrinos coming from the cosmos, the Sun, or from accelerators. Detectors are used in stand-alone mode, as in the underground neutrino experiments; as single detectors and small arrays at low-energy facilities; and in vast assemblies of complex particle detection systems at the facilities with the highest-energy beams.

The continuous advancement of accelerators, detectors, and data acquisition techniques provides a rich milieu for training and innovation over a wide spectrum of technical areas, including electronics, vacuum technology, large-scale data acquisition and computer systems with corresponding software development, novel detector materials and sensors, automated high-level control systems, ion-beam and accelerator technology, and superconductivity.

The experimental work in nuclear physics goes hand in hand with the development of theoretical understanding. The theoretical effort is undertaken by a number of researchers at universities and laboratories. An important part of the infrastructure of the field is the Institute of Nuclear Theory, described in Box 7.1.

Nuclear physics needs primary beams of electrons, protons, and heavy ions over a wide energy range. Each serves a complementary class of experiments. Secondary beams of other particles, such as neutrons, pions, muons, neutrinos, and radioactive ions, can be derived principally from intense proton and heavy-ion beams. Accelerator facilities for nuclear science fall into two major categories: larger facilities that operate for substantial outside-user communities, and smaller facilities that mainly serve local groups of scientists.

A variety of accelerators was invented, built, and used in the 1930s to begin the exploration of nuclei. During the 1950s and 1960s, needs from nuclear physics experiments led to major improvements in these technologies and a number of accelerators, cyclotrons, and Van de Graaffs (now considered small) were constructed and used for research at university laboratories. As the requirements for beam energies, intensities, and especially beam species grew, larger dedicated facilities were built at universities and at the national laboratories. It was also during this period that high-energy physics started, and the accelerator developments of nuclear physics formed the basis of the first high-energy facilities—greatly