Effect of electronic energy loss and irradiation temperature on color-center creation in LiF and NaCl crystals irradiated with swift heavy ions

LiF and NaCl crystals were irradiated at 8 K and 300 K with various light and heavy ions (C, Ti, Ni, Kr, Sm, Au, Pb, and U) of kinetic energy between about 50 and 2600 MeV, providing electronic energy losses from 0.7 to 26.4 keV/nm. A cryostat installed at the beamline allowed in situ absorption spectroscopy and thermostimulated luminescence (TSL) measurements from 8 K upward. Creation of electron and hole color centers is analyzed as a function of irradiation temperature, fluence, and thermal and optical bleaching. Anion interstitials ($I$ and $H$ centers) were only observed in crystals irradiated at 8 K. These defects are unstable and disappear in the temperature range 10–100 K. For heavy ions (Au, U), the $F$-center accumulation efficiency at low fluences is larger at 8 K than at room temperature. The opposite effect is observed for light ions (C, Ti, Ni). The results are discussed within the frame of transient heating influencing separation or annealing of point defects.

Figure 1

Scheme of cryostat for absorption and thermostimulated luminescence measurements at the UNILAC beamline.

Figure 2

Absorption spectra of LiF crystals irradiated at 8 K with Ti ions (555 MeV) of different fluence.

Figure 3

Absorption spectra of NaCl crystals irradiated at 8 K with Au ions (2186 MeV) of different fluence.

Figure 4

Optical density as a function of fluence for $F$ and $H$ centers $\left(\sim 330\text{nm}\right)$ in NaCl irradiated at 8 K with Au ions (2187 MeV). In the given fluence range, the ratio of $D_H\left(330\text{nm}\right)/D_F$ is about 0.22.

Figure 5

Absorption spectra of NaCl crystals irradiated at 8 K with (1) Ti ions (555 MeV) and (2) U ions (2640 MeV) at different fluence corresponding to a respective energy density of $7.6\times {10}^{20}\text{eV}/{\text{cm}}^3$ and $5.1\times {10}^{20}\text{eV}/{\text{cm}}^3$.

Figure 6

Absorption spectra of LiF crystals irradiated with U ions (2640 MeV) of $5\times {10}^9\text{ions}/{\text{cm}}^2$ at (1) 8 K $\left(n_F=1.1\times {10}^{16}{\text{cm}}^{-2}\right)$, (2) 300 K $\left(n_F=7.4\times {10}^{15}{\text{cm}}^2\right)$, and (3) sample 1 annealed to 300 K $\left(n_F=5\times {10}^{15}{\text{cm}}^{-2}\right)$.

Figure 7

Absorption spectra of NaCl crystals irradiated with U ions (920 MeV) of ${10}^{10}\text{ions}/{\text{cm}}^2$ at (1) 8 K $\left(n_F=2.5\times {10}^{15}{\text{cm}}^{-2}\right)$, (2) 300 K $\left(n_F=1.7\times {10}^{15}{\text{cm}}^2\right)$, and (3) sample 1 annealed to 300 K $\left(n_F=6\times {10}^{14}{\text{cm}}^{-2}\right)$.

Figure 8

Concentration of $F$ centers $\left(n_F\right)$ as a function of fluence in NaCl and LiF crystals irradiated at 8 and 300 K: (a) NaCl irradiated with 54-MeV C ions; (b) NaCl irradiated with 2187-MeV Au ions; (c) LiF irradiated with 555-MeV Ti ions; (d) LiF irradiated with 900-MeV U ions. For light ions (C, Ti) the concentration of $F$ centers at 8 K is lower than at 300 K and for heavy ions (Au, U) the situation is opposite.

Figure 9

Absorption data for NaCl crystals irradiated at RT with 2187-MeV Au ions: (a) Optical density of absorption band of $F$ and $V_3$ centers as a function of ion fluence; (b) absorption spectra of crystals exposed to a fluence of $4\times {10}^{11}$ and ${10}^1\text{ions}/{\text{cm}}^2$.

Figure 10

Absorption spectra of NaCl crystals irradiated at 300 K with 2187-MeV Au ions of fluence $4\times {10}^{10}\text{ions}/{\text{cm}}^2$ (a) before (solid line) and after 30 min bleaching (dashed line) and (b) difference of spectra of (a).

Figure 11

Absorption spectra of NaCl crystals irradiated at 8 K with 209-MeV Ni ions of fluence $5\times {10}^{11}\text{ions}/{\text{cm}}^2$ as irradiated (solid line) and after 15 min bleaching (dashed line).

Figure 12

(Color online) Thermostimulated luminescence of crystals irradiated at 8 K: (a) LiF exposed to 1400-MeV U ions of fluence ${10}^9\text{ions}/{\text{cm}}^2$ (1) and 54-MeV C ions of fluence $5\times {10}^{10}\text{ions}/{\text{cm}}^2$ (2); (b) NaCl exposed to 1400-MeV U ions of fluence ${10}^9\text{ions}/{\text{cm}}^2$ (1) and 54-MeV C ions of fluence $5\times {10}^{10}\text{ions}/{\text{cm}}^2$ (2).

Figure 13

Scanning force microscope image of LiF crystals irradiated at 8 K (top) and 300 K (bottom) with 1140-MeV Pb ions of fluence $5\times {10}^9\text{ions}/{\text{cm}}^2$. The size of the hillocks (diameter and height) does not depend on the irradiation temperature.