monoenergetischer

Monoenergetischer, or monoenergetic, describes radiation or particle beams in which the emitted particles share an approximately single energy. In practice the energy distribution has a finite width ΔE, and the relative energy spread ΔE/E characterizes how close the beam is to truly monoenergetic. A completely monoenergetic source with ΔE = 0 is not attainable due to fundamental and instrumental broadening.

Photons from nuclear transitions produce nearly monoenergetic gamma rays, forming gamma-ray lines, while X-ray monochromators use crystal diffraction (Bragg diffraction) to select a narrow energy band. In particle beams, electrons, protons, or heavier ions can be prepared with a small energy spread by careful acceleration and energy filtering, yielding quasi-monoenergetic beams. For neutrons, monoenergetic spectra are obtained from specific nuclear reactions or by time-of-flight selection in pulsed sources, though practical beams often retain some spread due to moderator and geometry effects.

Characterizing monoenergeticity involves the energy resolution of detectors and the beamline. Limitations include Doppler broadening, thermal motion, energy loss in materials, and finite source bandwidth. Instrumentation such as crystal monochromators, magnetic spectrometers, and time-of-flight setups are used to measure and improve the effective energy width of the beam.

Applications of monoenergetic radiation and beams span experimental physics and applied research. They are essential for precise cross-section measurements, detector calibration, and materials science investigations via elastic scattering. In medical physics, quasi-monoenergetic beams are employed in imaging and therapy where controlled energy deposition is important.