Spatially resolved nuclear spin relaxation, electron spin relaxation and light absorption in swift heavy ion irradiated LiF crystals

Abstract

Spatially resolved (19)F and (7)Li spin-lattice relaxation rates are measured for LiF single crystals after irradiation with two kinds of swift heavy ions ((12)C of 133 MeV and (208)Pb of 1.78 GeV incident energy). Like in earlier studies on (130)Xe and (238)U irradiated LiF crystals, we found a strong enhancement of the nuclear spin-lattice relaxation rate within the ion penetration depth and a slight--but still significant--enhancement beyond. By evaluating the nuclear relaxation rate enhancement within the ion range after irradiation with different projectiles, a universal relationship between the spin-lattice relaxation rate and the dose is deduced. The results of accompanying X-band electron paramagnetic resonance relaxation measurements and optical absorption spectroscopy are included in a physical interpretation of this relationship. Also the reason for the enhanced relaxation rate beyond the ion range is further discussed.

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“…The resulting dose profile depends on the projectile. In [3] a close relation between the dose profile and the spin-lattice relaxation rate has been reported. The projectile range, the mean range of the ions calculated with the TRIM code [11], is well defined within a typical straggling length of 3 lm, only.…”

“…11 the red profile was measured without, the black one after optimizing the alignment of the crystal. The transition region 2 observed in all previous measurements has virtually disappeared while the rate increase at the end of the ion range (''Bragg peak''), so far only observed at higher fluences [3] where this effect is more pronounced, becomes visible. Note an increased relaxation rate close to the crystal surface which had not been recognized before.…”

“…For the test measurements we chose LiF single crystals (Korth) partly already investigated in one of our previous studies [3]. Most irradiations were performed at the UNILAC linear accelerator of GSI Darmstadt.…”

“…In previous studies [1][2][3] we presented such NMR relaxation rate profiles measured on Lithium Fluoride (LiF) samples irradiated with various projectiles. Ionic crystals such as LiF are very sensitive for the creation of colour centres through electron excitation.…”

“…The strength of this effect was found to depend strongly on projectile and fluence. We assigned the enhanced relaxation rate in region 3 to F-centres produced by secondary X-rays [3]. However, we still lack a quantitative understanding of this effect.…”