# State of the Art of High-Flux Compton/Thomson X-rays Sources
## 4 Discussion of Different Paradigms: High Energy Sources Operating in Quantum Regime  
Increasing the electron energy and the pump photon energy \(\gamma\)-rays can be generated. The recoil factor, that in the case of low energy Compton sources was order or below \(10^{-3}\) and therefore completely negligible, in this case is larger than this value, and begins to influence the spectrum emitted, shifting the peak towards lower energy value. At the same time, the cross section diminishes, lowering the flux. One of the most interesting example of applications with \(\gamma\)-rays is the non-destructive isotope imaging by measuring nuclear resonance fluorescence (NRF) [90]. In the experiment, done at the TERAS facility, then damaged by the 2011 hearthquake, the quasi-three dimensional distribution of \({}^{208}\)Pb was identified by measuring the amount of 5512 MeV \(\gamma\)-rays which were emitted from \({}^{208}\)Pb via the NRF process. As a matter of fact, high energy photons generated by ICS can be useful for testing nuclear transitions with MeV-class energies, even in excess of 20 MeV. Many pioneering experiments shown the generation of \(\gamma\)-rays of few / tens MeV via Compton back-scattering [91; 92; 93; 94] between Infrared lasers and electrons in storage rings. At the New SUBARU line at SPRING-8 the interaction between a GeV-class electron beam with Nd and CO\({}_{2}\) lasers produced up to 5\(\cdot\)10\({}^{6}\) photon at 17-37 MeV [95].The UVSOR-III ICS gamma source works also in this regime, providing collisions between the electron beam produced with the homonym storage ring and a 1.94 \(\upmu\)m laser, with an output of up to 10\({}^{7}\) gamma photons at 5.4 MeV [30]. This paradigm have been exploited even to generate GeV-class photons. At the LEPS beamline, ultraviolet (UV) laser light with a wavelength of 355 nm is injected into SPring-8 to produce a 10\({}^{6}\) photons/s beam in the energy range up to 2.4 GeV [96], with the idea to increase further photon energy and flux with a new 266 nm laser line [97]. Free-electron laser oscillators for Ultra-Violet and Vacuum Ultra-Violet have also been employed as Compton photon sources for MeV \(\gamma\)-rays. An interesting option is the interaction between the Free-electron laser light and a positron beam, proved at ACO [98]. A flux of 2.5\(\cdot\)10\({}^{5}\) ph/s of \(\gamma\)-rays at 3 MeV has been generated at New SUBARU [26]; gamma rays of 14.6 and 25.2 MeV have been generated at UVSOR-III [99], while 2\(\cdot\)10\({}^{5}\) ph/s at 12 MeV have been observed at DUKE [92; 100]. A nearly monochromatic beam of 100% linearly polarized \(\gamma\) rays has been produced via Compton backscattering inside a free electron laser optical cavity. 12.2 MeV \(\gamma\) rays were obtained by backscattering 379.4 nm free-electron laser photons from 500 MeV electrons circulating in a storage ring [101]. The experience at Duke has evolved in the high intensity \(\gamma\)-ray source (HI\(\gamma\)S), representing, with its flux of 10\({}^{10}\) ph/s, indeed, the state of the art of \(\gamma\) Compton sources. HI\(\gamma\)S is based on Compton scattering of UV photons generated in a FEL oscillator by electrons at a 1.2 GeV storage ring. The \(\gamma\)-ray source routinely provides quasi-monochromatic photons at 1-100 MeV [12]. A different concept of gamma ray source is based instead on the first paradigm. A warm linac implemented up to 700 MeV and a Nd-Yag laser empowered by a laser recirculator [69] increasing the repetition rate up to \(3.2\cdot 10^{3}\) Hz were at the basis of the EuroGammas proposal for the ELI_NP project [27]. The peculiar property of this approach was that the high quality of the electron beam allowed to reduce the relative bandwidth down to 3\(\cdot\)10\({}^{-3}\), satisfying the constraints of the nuclear photonics applications. The successive project for ELI_NP, named VEGA [102], follows instead the same line as CLS. A plasma accelerator can also provide gamma rays using the fundamental or the second harmonics of the laser [103; 104].