The development of light sources (LS) capable of operating at sub-angstrom wavelengths remains one of the foremost challenges in modern physics. High-brilliance, tunable LSs are indispensable for a broad range of applications, including particle physics, photo-nuclear research, nuclear structure studies, solid state physics and medical imaging. Current LS, such as X-ray Free Electron Lasers (XFELs), face significant challenges in reaching wavelengths below the angstrom scale due to the demanding advancements required in both magnet and accelerator technologies. 

A promising alternative is provided by crystalline undulators (CUs), which utilizes the unique properties of crystals to confine electrons or positrons through a phenomenon known as channeling.

A charged particle passing through a crystal will begin to oscillate, consequently emitting high-intensity X-rays or gamma rays. When the particle's direction relative to a crystal plane or axis is smaller than the critical angle, it undergoes channeling, experiencing a coherent interaction with the entire plane or axis rather than individual atoms.

Negatively charged particles oscillate between atomic nuclei, as illustrated in the figure, while positively charged particles oscillate between rows of atoms, being repelled by the nuclei. The oscillatory motion of a charged particle in a periodic potential is intrinsically linked to the emission of radiation, making channeling a highly efficient mechanism for generating high-energy photons X and gamma. 

Crystalline gamma-ray sources

A crystalline undulator, for instance, leverages the periodic bending of the crystal lattice to induce a channeling motion of charged particles, leading to the emission of gamma rays. This approach holds promise for producing coherent superradiant radiation at wavelengths significantly shorter than those achievable with traditional methods.

Magnetic undualtor Vs. Crystalline undulator

Traditional gamma-ray Light Sources rely on magnetic undulators, where magnetic fields are used to force charged particles into a sinusoidal path, causing them to emit synchrotron radiation. While effective, this method has limitations in terms of the photon energy range and the wavelengths that can be produced.

Crystalline undulators, on the other hand, utilize the intrinsic properties of a periodically bent crystal to achieve similar effects. When a beam of ultra-relativistic charged particles passes through such a crystal, it undergoes a channeling motion that can result in the emission of high-intensity gamma-ray radiation. One of the major advantages of crystalline undulators is their ability to produce radiation with wavelengths that are orders of magnitude shorter than those obtainable with magnetic undulators, reaching well below 1 Angstrom. This makes them particularly valuable for applications requiring extremely high photon energies and very short wavelengths, which are beyond the capabilities of existing Light Sources.

Technological Output

The development of crystalline gamma-ray Light Sources is expected to have a wide array of technological applications. In addition to their use in fundamental research, these sources have potential practical applications in technology, medicine, and industry. For example, they could be used for the disposal of radioactive waste through advanced photo-transmutation techniques or the production of radioisotopes for nuclear medicine.

Theoretical, computational, and experimental advancements in this field are paving the way for key technological developments, with the promise of creating more efficient, versatile, and powerful Light Sources. The ongoing research and collaboration among international experts are crucial in overcoming the challenges and achieving the practical realization of these novel gamma-ray sources, thus opening new horizons for scientific exploration and technological innovation.