The distribution of the intensity of particles (or radiation) emitted in a nuclear process as a function of energy. The nuclear spectrum is a unique signature of the process.
For example, when very slow neutrons (with speeds less than 0.5% of the speed of light) hit nitrogen nuclei, there is a high probability that they will be captured and that the nuclear system which is formed will emit a set of gamma rays (electromagnetic radiation) of very precise energies. The 24 gamma rays have energies ranging from 1.68 to 10.83 MeV, and their relative intensities are well known. A spectrum of these gamma rays, that is, the number of gamma rays having a particular energy, versus that energy can provide a unique signature of the presence of nitrogen. An application is the passing of a beam of slow neutrons through luggage at an airport: the presence of unusual amounts of nitrogen indicates that a plastic explosive may be present. This testing is nondestructive: relatively few neutrons are needed to produce the characteristic spectrum, and the luggage and its contents are not harmed. See Gamma rays
The methods used to measure nuclear spectra depend on the nature of the particles (radiation) involved. The most accurate energy measurements are those of gamma rays. Gamma-ray spectra can be measured by determining the energy deposited by the gamma rays in a crystal, often made of sodium iodide, containing thallium impurities [NaI(Tl)], or of germanium, containing lithium impurities [Ge(Li)]. In a NaI(Tl) detector, the gamma-ray energy is transferred to electrons within the crystal, and these charged particles in turn produce electromagnetic radiation with frequencies in the visible range. The crystal is surrounded by detectors (photomultipliers) that are sensitive to the visible light. The intensity of the signal in the photomultipliers is proportional to the energy of the gamma rays that entered the NaI(Tl) crystal. The signal pulse is amplified electronically, and the pulse heights (pulse sizes) are displayed in a pulse-height multichannel analyzer in a histogram. Usually the number of pulses having a certain height (strength) is plotted versus the height. What results is a plot showing the number of gamma rays having a certain energy versus the energy of the gamma rays, a spectrum.
Neutron spectra are often determined by measuring their velocities. This is done by a time-of-flight technique in which an electronic timer measures the time interval between the emission of the neutron from a nucleus and its arrival at a detector a known distance away. This measurement uniquely determines the velocity, and thus the kinetic energy, of the neutrons. See Neutron spectrometry
Measurements of nuclear spectra involving charged particles, such as pions, protons and alpha particles, are often made by determining their momenta (mass × velocity) and then calculating the corresponding kinetic energy. Momentum measurements are made by passing the beam of charged particles through a region in which a magnetic field exists. A magnetic field that is constant in time will not cause a change in a charged particle's speed, but it will cause a charged particle to deviate in its path. See Particle accelerator
Modern magnetic spectrometers use sophisticated counter telescopes and multiwire proportional counters, which permit not only the registering of the particles characterized by a certain value of the radius of curvature (and therefore of momentum) but enable the particular particle (proton, alpha particle, or whatever) that caused the signal to be identified. Contemporary magnetic spectrometer systems not only utilize complex arrangements of magnetic fields, detectors, and electronics but also generally require powerful computers to monitor and analyze the results. See Particle detector