monoenergetic

Monoenergetic describes a beam or source in which the particles or photons have, in principle, a single energy value, seldom exactly realized in practice but achieved within a narrow energy spread. The defining characteristic is a small energy dispersion around a central energy E0, with the spread usually reported as the full width at half maximum (FWHM) or as a relative spread ΔE/E. A truly monoenergetic source would produce a delta-function energy distribution, while real sources exhibit finite broadening due to production mechanisms, scattering, and instrumentation.

In practice, monoenergetic beams are sought for precise measurements, calibration, and controlled experiments. Photons from nuclear transitions tend to be treated as nearly monoenergetic with energy equal to the transition energy, although Doppler broadening and recoil can introduce modest line broadening. Particle beams can be tuned to a specific kinetic energy using accelerators, with energy selection achieved through magnetic or electric dispersion, degraders, and collimation. Monoenergetic neutrons and gamma rays are produced by particular nuclear reactions, such as D-D or D-T fusion for neutrons at fixed energies, and gamma lines from radioactive decay or nuclear reactions for photons.

Applications include detector calibration, spectroscopy, and precision experiments in nuclear and particle physics. In medicine, monoenergetic beams are advantageous in radiotherapy and diagnostic imaging when energy control enhances dose distribution or image quality. Limitations arise from energy loss and straggling in materials, secondary reactions, and finite energy resolution of the producing or measuring equipment, which prevent perfect monoenergeticity.