Optical absorption and thermoluminescence studies in 100 MeV swift heavy ion irradiated CaF 2 crystals C. Pandurangappa a,⇑ , B.N. Lakshminarasappa b , Fouran Singh c , K.R. Nagabhushana d a Department of Physics, RNS Institute of Technology, Channasandra, Bangalore 560 061, India b Department of Physics, Jnanabharathi Campus, Bangalore University, Bangalore 560 056, India c Inter University Accelerator Centre, P.O. Box No. 10502, New Delhi 110 067, India d Department of Physics, PES Institute of Technology, Banashankari, Bangalore 560 085, India article info Article history: Received 15 September 2010 Received in revised form 23 October 2010 Available online 2 November 2010 Keywords: Optical absorption Ion fluence Thermoluminescence TL intensity Color centers abstract Pure and Ytterbium (Yb) doped Calcium fluoride (CaF 2 ) single crystals were irradiated with 100 MeV Ni 7+ ions for fluences in the range 5  10 11 –2.5  10 13 ions cm 2 . The irradiated crystals were characterized by Optical absorption (OA) and Thermoluminescence (TL) techniques. The OA spectra of ion irradiated pure CaF 2 crystals showed a broad absorption with peak at 556 nm and a weak one at 220 nm, whereas the Yb doped crystals showed two strong absorption bands at 300 and 550 nm. From the study of OA spectra, the defect centers responsible for the absorption were identified. TL measurements of Ni 7+ ion irradiated pure CaF 2 samples indicated a strong TL glow with peak at 510 K. However, the Yb doped crystals showed two TL glows at 406 and 496 K. The OA and TL intensity were found to increase with increase of ion fluence upto 1  10 13 ions cm 2 and thereafter it decreased with further increase of flu- ence. The results obtained are discussed in detail. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Defects in alkali halides under various types of irradiations are studied extensively during the past few decades. A wide range of potential applications exists for these materials and the develop- ment of various new techniques is directly related to the progress in this field. Irradiations with energetic ion beams play a vital role in the field of defect studies in inorganic materials. The energetic ions lose their energy during their passage through the material. This energy is spent either in displacing atoms of the sample by elastic collisions or exciting the atoms by inelastic collisions. Low energy ions up to a few hundred KeV have been used in the mod- ification of surface and interface. The loss of energy by the ions in exciting or ionizing the atoms by inelastic collisions is called as electronic energy loss. Electronic stopping is dominant at high energies where the displacement of atoms due to elastic collisions is insignificant. Swift heavy ions (SHI) irradiation is a technique for modification of structural, optical and electrical properties of solids due to intense interaction of incident ions with the target atoms [1–3]. Inorganic fluoride materials are found to be sensitive to SHI irradiation with generation of metallic inclusions and induction of tracks [4,5]. Among the popularly known fluorides, CaF 2 single crystals are excellent optical window materials because of their high transmissivity over a wide range from UV to IR region of the electromagnetic spectrum besides their use in dosimetry [6,7]. Nevertheless, there is much current interest in crystalline CaF 2 due to its use in deep UV lithography [8]. It is well established that when ionic crystals are irradiated with energetic radiations, they give rise to color centers [9]. These centers have been identified from various experimental techniques like optical absorption, luminescence, Raman and electron spin resonance [10]. Rare earth doped CaF 2 exhibits a variety of novel properties. When CaF 2 is doped with trivalent rare-earth (RE 3+ ) ions, the excess of charge is compensated by interstitial F  ions, leading to several kinds of luminescent centers [11]. At low RE 3+ ion concentrations, the dop- ant ions mainly form isolated centers whereas at higher concentra- tions, they aggregate and form complex clusters [12–14]. In the present work, OA and TL behavior of pure and ytterbium (Yb) doped CaF 2 single crystals irradiated with 100 MeV Ni 7+ ions for fluences in the range 5  10 11 –1  10 13 ions cm 2 are studied at room temperature (RT) and the results obtained are discussed. 2. Experimental Pure and 3 mol% Yb doped CaF 2 single crystals were procured from Shanghai Institute of Optics and Fine Mechanics, China. CaF 2 crystal slices of about 1 mm thickness were cleaved from the big block grown along [1 1 1] direction. The cleaved slices were 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.10.022 ⇑ Corresponding author. Tel.: +91 28611880; fax: +91 28611882. E-mail address: cpandu@gmail.com (C. Pandurangappa). Nuclear Instruments and Methods in Physics Research B 269 (2011) 185–188 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb