Nanocrystals produced by nuclear reactions in lithium fluoride crystals

Lately the global interest to Li-compounds has increased more, since Li is the source of generating Tritium (the long-living beta-emitting isotope) in blanket of ”pure” thermonuclear reactors by exothermic ⁶Li(n,a)³H reaction discovered by Rutherford. Now this reaction is applied also for diagnosis of Li-based batteries by detecting alpha- and beta-emission. The widest gap ionic LiF crystals have been used for 70 years as a slow neutron dosimeter and thermo-luminescence dosimeter of absorbed X- and gamma-ray, despite quite a low dose limit.

The goal of this research was to find out what chemical compounds and crystal structures appear in the tracks of a few MeV energy alpha and tritium. To study other possible nuclear reactions with ⁶Li, ⁷Li and ¹⁹F, we irradiated in the research water-cooled uranium fission reactor with the neutron flux 10¹⁴ n/cm²s to fluencies 10¹⁵ – 10¹⁸ n/cm² and monitored alpha-beta-gamma-activity for a few years. Since the neutron flux is accompanied by g–quanta emission of nuclear recoils (gamma-flux 37 Gy/s), the contribution of gamma-induced nuclear reactions was evaluated by means of irradiation in the shut-down reactor for 100 hours and also by ⁶⁰Co g–quanta (1.17 and 1.33 MeV) at dose rate ~10 Gy/s to high doses 10⁷ Gy. And the effect of beta-emitting recoils was studied by exposure to electron beams accelerated to 4-5 MeV at beam current density 0.4÷3 µA to fluencies 10¹⁴ – 10¹⁷ e/cm². No post-irradiation alpha-beta-gamma-emission was detected after the gamma- and electron irradiation.

Using SEM and SPM we found square pyramidal micro-size tracks (1-5 µm) also nano-size hillocks (10-300 nm) inside the tracks and on the exposed surface in all the examined conditions of irradiations, which size depends mostly on the impact neutron, electron and gamma-quanta energy, while the distance between the tracks depended on the flux and accumulated fluency. By means of energy dispersion spectroscopy (EDS) installed in SEM we studied micro-local element composition inside nuclear tracks and between them in the surface micro-layer and found irradiation induced impurity Boron, Carbon and Oxygen atoms. It should be noted that K-lines of B and C are not well resolved. These element ions, also Hydrogen, were detected in FTIR spectra as additional lines of chemical bonds Li-H, B-H, B-O, and no carbon bonds were found, while Li-F bond line decreased. Taking into account EDS and FTIR data, we carried out X-ray diffraction analysis of crystal structure and phase composition by using HighScore FullProf codes, and data base PDF–2013 and 2019. Nanocrystal phases of Li?, LiT, BH₄, LiBH₂ and complicate compound (LiF)₂(B₂O₃)₃ were identified.

Suggested nuclear reactions in the reactor: ¹⁹F₉ + t ® 2 ¹⁰B₅ ; ⁷Li₃ + ⁴a₂®¹¹B₅;  with 4-5 MeV electrons: ¹⁹F₉ + b® ³He₂ +¹⁶O₈ ; ⁷Li₃ + b®⁴He₂ + ³He₂; with g–quanta: nuclear excitation with electron-positron pair and nuclear reaction of transmutation via a compound-nucleus (LiFH)®(BO) followed by chemical compound B₂O₃ as nano-crystallites. Thus, the participation of nuclear reaction products in the mechanism of chemical composition changing and nanocrystalline phase transformation in irradiated crystals is confirmed experimentally.

Ibragimova Elvira Memetovna entered PhD scholarship in 1970 and joined the laboratory of Prof. Vakhidov Sh.A. at the Institute of Nuclear Physics, Uzbekistan Academy of Sciences, since then she has been working there at the radiation physics and material science department. She received her degrees PhD in 1975, then DrSci in 2000, Prof. in 2019 for supervising 4 PhD and obtained a position of Principal Researcher. She led many International Research projects on modifications in various dielectric, semiconducting and superconducting materialsinduced by irradiation andpublished ~90 research articles in international journals. She got a few awards for important researches.