## 3 Results and Discussion  
### Raman Spectra of Neutron-Irradiated Diamonds  
Raman scattering is an informative nondestructive method for the analysis of a wide class of carbon materials. Figure 1 shows the Raman spectra of irradiation-modified natural and CVD diamonds. In the unirradiated diamond, the maximum of Raman line was at 1332.4 cm\({}^{-1}\), and FWHM (full width at half maximum) is \(\approx\)2 cm\({}^{-1}\). The irradiation with fast neutrons with fluence \(F\) = 3 \(\times\) 10\({}^{18}\) cm\({}^{-2}\) leads to a decrease in the intensity of diamond Raman peak, an increase in FWHM up to \(\approx\)6 cm\({}^{-1}\), and a shift of the diamond line to 1328.8 cm\({}^{-1}\). The maximum of the band near 1630 cm\({}^{-1}\), which is characteristic of the Raman spectra of radiation damaged diamonds [54; 55], was at 1634 cm\({}^{-1}\). The spectra also contain a rather sharp defect-induced band at 1420 cm\({}^{-1}\), which is attributed to intrinsic defect containing interstitial atoms [56]. In the low-frequency region of the spectrum of diamond irradiated with fast neutrons with fluence \(F\) = 3 \(\times\) 10\({}^{18}\) cm\({}^{-2}\), a wide structural band dominates with singularities 990, 1008, 1120, and 1235 cm\({}^{-1}\), which exactly coincide with phonon frequencies at the singular points of the Brillouin zone LA(\(L\)), LA(\(K\)) and TO(\(W\)), TO(\(K\)), LO(\(K\)), and LO(\(L\)), respectively [56; 57; 58]. In the range of 900-1300 cm\({}^{-1}\), the spectrum is close to the spectrum of the phonon density of states (PDOS) of diamond [59]. The appearance of these singularities in the Raman spectra is attributed to phonon confinement effect [60; 61] due to the high concentration of intrinsic irradiation-induced defects and consequently a decrease in the phonon mean free path \(L_{\rm ph}\) in radiation damaged diamond [49]. The broad low-frequency band with a maximum at about 400 cm\({}^{-1}\) (Figure 1) coincides with the band observed in natural diamonds implanted with helium ions [54; 62], as well as in nanodiamonds [61; 63]. Historically, such a peak in disordered materials has been called the boson peak [64; 65]. According to our studies [62], amorphous diamond is unstable under normal conditions and due to the pressure from the diamond matrix can exist only in the form of nanoinclusions. The coexistence of amorphous and crystalline regions in radiation-damaged diamond is also noticeable in the Raman spectra, in which there is simultaneously a diamond peak, a  
Figure 1: Raman spectra of natural and CVD diamonds unirradiated and irradiated with fast reactor neutrons with fluences \(F\) from 3 \(\times\) 10\({}^{18}\) to 5 \(\times\) 10\({}^{20}\) cm\({}^{-2}\) (see also [49]) or uniformly implanted with multiple-energy keV helium ions with a total dose of 1 \(\times\) 10\({}^{17}\) and 2.5 \(\times\) 10\({}^{17}\) cm\({}^{-2}\). All spectra were recorded with excitation at wavelength \(\lambda\) = 473 nm except for the spectra at \(F\) = 3 \(\times\) 10\({}^{18}\) cm\({}^{-2}\), which was recorded with excitation at \(\lambda\) = 532 nm. wide band with a spectrum close to PDOS of diamond, as well as a boson peak characteristic of amorphous diamond. A further increase in the neutron fluence (up to 1 \(\times\) 10\({}^{19}\) cm\({}^{-2}\) and higher) leads to broadening, weakening, and low-frequency shift of the diamond peak in the Raman spectra (Figure 1), which is typical for radiation-disordered diamonds [41; 54; 66; 67; 68] with the phonon mean free path \(L_{\rm ph}\)\(<\) 5 nm [49]. The absence of a diamond peak in the Raman spectra is partly explained by the fact that the cross section for Raman scattering of light in diamond is much smaller than in other allotropic forms of carbon. As the fluence (level of radiation damage) increases, the maximum of the boson peak shifts slightly towards higher frequencies, and its relative intensity increases, which indicates an increase in the volume fraction of amorphized diamond in the samples.