Boson peak measurements in neutron-irradiated quartz crystals

We have measured the low-frequency Raman scattering in neutron-irradiated quartz crystals with four different irradiation doses from 4.7x10(19) n/cm(2) to 1x10(20) n/cm(2) and for 2 different crystallographic directions. For the used doses the range of density change of the investigated samples was 12% (the maximum change during amorphization is 14%) and the amorphous fraction varied from 35% to 100%. The same measurement was done in neutron-irradiated amorphous silica with a maximal dose 2x10(20) n/cm(2). In all cases we observed the boson peak in the Raman spectra. The position of the peak, at 67 +/- 3 cm(-1), was found to be the same for all the investigated samples independent of the dose. The shape of the peak for doses 6.8x 10(19) n/cm(2) and higher was also found to be the same for 5 investigated samples (including irradiated vitreous silica). We found that the position of the boson peak in neutron-irradiated quartz crystals and vitreous silica corresponds to the Ioffe-Regel crossover frequency for phonons. The origin of the boson peak in neutron-irradiated quartz and vitreous silica can be attributed to local soft optic modes, which are analogous to the soft optic mode that drives the alpha-beta transition in quartz.