Damage creation in LiF and NaCl crystals irradiated with swift heavy ions at 8 and 300 K

Abstract

Single crystals of LiF and NaCl were irradiated at 8 and 300 K with different ions ( 12 C to 238 U) of MeVGeV energy in the electronic stopping regime. The ion-induced damage was studied by optical absorption, thermo-stimulated luminescence, and scanning force microscopy. The concentration of the color centers depends on the energy loss of the ions. In both crystals, the irradiation with heavy ions (Au, Pb, U) results in a higher F center concentration at 8 K than at 300 K; for light ions (C, Ti, Ni) the opposite effect takes place. The observed results can be explained by ion induced local heating. Contrary to color-center creation, the formation of nano-sized surface hillocks does not depend on the irradiation temperature. 1 Ion-induced damage in LiF and NaCl crystals The interaction of swift heavy ions with dielectrics takes place mainly by ionization and electronic excitations, whereas elastic collisions are dominating only at the end of the ion path [1,2]. Damage creation with such ions strongly depends on their energy loss (dE/dx) [2][3][4][5][6]. In LiF and NaCl crystals, color centers -typically created by the exciton mechanismare formed along the ion path within a radial halo region of several ten nanometers. In addition to these point defects, more complex damage is produced in a core region of the ion track of only a few nanometers if a critical dE/dx of 10 keV/nm is surpassed [5,7,8]. At the crystal surface, the impact zone of each ion reveals as a nano-sized hillock for ions with dE/dx ≥ 5 keV/nm [8,9].At helium temperature, the creation of color centers in alkali halides exposed to X-rays or fast electrons is less efficient than that at room temperature due to increased recombination of electron and hole centers in the primary Frenkel pairs [1]. As evidenced in our earlier study, NaCl ion-irradiated at 8 K exhibits a different behavior depending on the energy loss of the ions [6].To understand the peculiarities of damage creation processes in alkali halides a detailed study in LiF and NaCl crystals was performed by irradiation with swift ions at 8 and 300 K. Low temperature irradiation allows to examine the primary hole centers (I, H) and analyze the transient local heating in the track.2 Irradiation and optical spectroscopy The irradiation experiments on LiF and NaCl single crystals were performed at 8 and at 300 K using the linear accelerator UNILAC of the GSI Darmstadt, Germany. A large number of different projectiles between 12 C and 238 U of MeV-GeV energy were applied. The thickness of the samples was always larger than the ion range (R) [10]. The mean electronic energy loss varied between 0.7 and 26 keV/nm, and the fluence (Φ) ranged from 10 8 to 10 12 ions/cm 2 . For the irradiation at 8 K, the samples were mounted in a cryostat which also allows in situ absorption spectroscopy

The microscopic model of material excitation in swift heavy ion tracks

“…This figure demonstrates that the largest part of the excess energy transferred into the lattice is localized within the distances r < 3 nm from the ion trajectory. The maximum lattice heating in this area does not exceed 100 K which is in a good agreement with the experimental results reported in [9]. This agreement proves an applicability of the presented model to description of material excitation in SHI tracks.…”

“…The obtained little heating (~100 K) agrees well with the experiment-based estimations of track heating in irradiated LiF crystals [9].…”

Effect of inter-atomic Auger processes on relaxation of electronic vacancies at deep levels of highly ionized atoms in swift heavy ion tracks

“…Its density q = 2.32 g/cm 3 corresponds to the atomic density n a = 6.9 Â 10 22 1/cm 3 maximum energy of a delta-electron created by such Ca ion is E max = 24.8 keV. It corresponds to the electron velocity V max e ¼ 9 Â 10 7 m=s < V SiO 2 c which is also less that the speed of light in quartz.…”

“…[3,4]. In this case numerical simulations can give principle information about major processes governing relaxation of electronic subsystem in SHI tracks.…”

Track creation after swift heavy ion irradiation of insulators

“…In order to explain the observed effects of SHI irradiation as formation of nano-hillocks on the surface, creation of stable point-like defects (color-centers), it was found that the parameters of TTM need a special treatment, which would confine the energy inside a narrow track halo for timescales orders of magnitude longer than those given by the usual TTM [4][5][6][7]. For these reasons, electronic confinement usually is introduced via a temperature-dependent electronic diffusivity [5,6] and by fitting an electron-phonon coupling constant [5].…”