Nuclear Spectroscopy

a branch of nuclear physics that is concerned with the study of the discrete spectrum of nuclear states, namely, with the determination of energy, spin, parity, isotopic spin, and other quantum characteristics of the nucleus in the ground and the excited states. A knowledge of these numbers is necessary to determine the structure of nuclei and to ascertain the forces that act between nucleons (see).

The enumerated characteristics are established by measuring the energies, intensities, angular distributions, and polarizations of the radiation emitted by the nucleus either during radioactive decay or in the course of nuclear reactions. The acquisition of spectroscopic data as a result of the study of radioactive decay is frequently called spectroscopy of radioactive emissions, which is differentiated according to the type of emission into alpha-, beta-, and gamma-ray spectroscopy.

In nuclear spectroscopic studies that are based on the use of nuclear reactions, three trends are clearly delineated: the use of direct nuclear reactions, the use of Coulomb excitation of the nucleus, and the use of resonance reactions. Neutron spectroscopy is of particular importance in the third area, that is, the investigation of the energy dependences of the probabilities of nuclear reactions caused by neutrons (seeNEUTRON SPECTROSCOPY).

The technical means used in modern nuclear spectroscopy are extremely diverse. They include magnetic spectrometers for measuring the energies of charged particles, crystal-diffraction spectrometers for measuring the energies of gamma radiation, and various nuclear radiation detectors for registering and measuring the energy of particles and gamma quanta from the effects of the interaction of particles with the atoms of a substance (excitation and ionization of atoms). Solid-state detectors have assumed considerable importance among such spectrometric devices;

Figure 1. Block diagram of measurement complex (based on the synchrocyclotron at the Joint Institute for Nuclear Research) for studying decay schemes of neutron-deficient nuclei formed upon the bombardment of target nuclei, such as Ta, by protons with energies of 680 megaelectron volts

they combine comparatively good energy resolution (relative accuracy of energy measurement, ~1–10 percent) with high “luminosity” (percentage of effectively utilized radiation), with the luminosity of some instruments approaching values close to 1 (the energy resolution of the best magnetic spectrometers is 0.1 percent at a luminosity of about 10 ^–3).

Owing to the development of semiconductor detectors and accelerator equipment, as well as the use of computers for collecting and processing experimental data and for monitoring experiments, it has become possible to set up automated measurement complexes to obtain large volumes of systematized precision information on the properties of nuclei (see Figure 1).

The methods of nuclear spectroscopy are used in nearly all areas of nuclear research, as well as in areas outside the realm of physics, such as biology, chemistry, medicine, and engineering. For example, activation analysis is based on data about decay schemes of radioactive elements; the Mössbauer spectroscopy is applied to the study of, for example, the electronic structure of solids and the structure of molecules (Mössbauer effect). Nuclear spectroscopic data are also necessary in chemical, biological and other studies by isotope tracer techniques.