Ditter, M.

Fujara, F.

Mansouri, A.

Meyer, K-C.

Orth, S.

Quittek, S.

Schwartz, K.

Trautmann, C.

GSI Repository

Analysis of radiation damage in heavy-ion irradiated ionic crystals by magnetic resonance*M. Ditter1,#, K-C. Meyer1, S. Orth1, S. Quittek1, A. Mansouri1, K. Schwartz2, C. Trautmann1,2 and F. Fujara11 TU Darmstadt; Germany, 2 GSI, Darmstadt, Germany.We explore and expand the capabilities of magneticresonance techniques, mainly nuclear magnetic resonance(NMR) but also electron spin resonance (ESR), as a toolfor the characterization of radiation damage in solids. Atpresent, we are focussing on ionic crystals, mainly lithiumfluoride (LiF), which offers two suitable probe nuclei forNMR. In contrast, ESR is directly susceptible to para-magnetic defects and conduction electrons e.g. in metalliccolloids. In 2012 our research concentrated on three top-ics: (a) Utilizing spatially resolved field-cycling (FC)NMR data to obtain information about the distribution ofion-beam induced defects, (b) using NMR spectroscopyto detect the presence of fluorine gas within irradiatedsamples, and (c) examining defect dynamics above room temperature by ESR and spatially resolved NMR after aseries of stepwise annealing experiments.(a) Common NMR relaxation theory in the presence ofparamagnetic centres in alkali halides predicts a strong B-field dependence of the relaxation rate, depending on thedistribution of defects within the sample [1]. This effect is supported by our data recorded by FC NMR. However, sofar literature provides only analytical and approximatesolutions of the resulting differential equation for homogeneous distributions. We are currently working ona numerical approach to calculate relaxation rates for anygiven distribution. By comparing our model with dataderived from samples with known distributions, we hopeto deduce from FC NMR the topology of unknown defectstructures.(b) The formation of molecular fluorine gas formed in neutron-irradiated LiF was first observed by CW-NMRspectroscopy some five decades ago [2]. Until now it wasjust speculated that fluorine gas also forms in samplesirradiated with heavy ions having a strong radial gradientof the dose deposited around the ion track. Recently, we identified for the first time F2 molecules in several LiF crystals irradiated with 6×1011 to 2×1012 ions/cm² by NMR spectroscopy. We have evidence that theappearance of fluorine molecules requires an energy lossabove 10 keV/nm where tracks consist of a heavilydamaged core zone of few nm in diameter [3]. At present,the nature of this track core is not yet fully understood.c) With CW-ESR spectroscopy we detected theformation of metallic Li colloids in LiF at temperaturesabove 600 K during annealing experiments. Li colloids inheavy-ion irradiated LiF samples were already identifiedby optical absorption spectroscopy [4]. However, ESRallows us to gain access to more heavily irradiatedsamples where the large color centers concentration leads to a high optical density preventing quantitative analysisof optical absorption spectra.By spatially resolved NMR relaxation rate measurements on a stepwise annealed LiF crystal, weobserved a remarkable drop of the relaxation rate belowthe level found in the unirradiated part of the crystal (Figure 1). This effect appeared at a temperature whichcorresponds to the recently observed ion-beam inducedformation of impurity magnesium colloids [4]. We thustentatively attribute this effect also to the formation ofimpurity clusters. The concentration of paramagneticimpurities is decisive for the relaxation rate inunirradiated crystals.0 200 400 600 800 1000 12001E-30,010,11T -1 1/ s-1Depth / PmAnnealing temp: 300 K 490 K 590 K 690 K 730 Kion rangeFigure 1: Depth profiles of nuclear spin-lattice relaxationrates recorded for a LiF crystal irradiated with 2.2-GeVions of 6×1011 ions/cm². The sample was stepwise an-nealed for 20 min at different temperatures up to 730 K. Note the drop of relaxation rates at 730 K (red arrow) to a level below the value of the unirradiated, crystal (depth>100 μm).References[1] I.J. Lowe and D. Tse, Phys. Rev. 166 (1968) 279.[2] C.D. Knutson, H.O. Hooper, P.J. Bray, J. Phys.Chem. Solids 27 (1966) 147.[3] C. Trautmann, K. Schwartz, J.M. Constantini, T. Ste-ckenreiter, M. Toulemonde, Nucl. Instr. Meth. B 146 (1998) 367.[4] K. Schwartz, A.E. Volkov, M.V. Sorokin, R. Neu-mann, C. Trautmann, Phys. Rev. B 82 (2010)144116.* Work supported by DFG, No. FU 308/12.# michael.ditter@physik.tu-darmstadt.deGSITemplate2007GSI SCIENTIFIC REPORT 2012 PNI-INHOUSE-EXP-24393

Analysis of radiation damage in heavy-ion irradiated ionic crystals by magnetic resonance

http://repository.gsi.de/record/52285/files/PNI-INHOUSE-EXP-24.pdf

Analysis of radiation damage in heavy-ion irradiated ionic crystals by magnetic resonance

Abstract

Similar works

Full text

Available Versions

GSI Repository