Thermal annealing behavior of a-Al 2 O 3 scintillation screens S. Lederer a,b,⇑ , S. Akhmadaliev c , P. Forck a,d , E. Gütlich a,d , A. Lieberwirth a,b , W. Ensinger b a GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstraße 1, 64291 Darmstadt, Germany b Technische Universität Darmstadt, Alarich-Weiss-Straße 2, 64287 Darmstadt, Germany c Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany d Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, 60438 Frankfurt am Main, Germany article info Article history: Received 22 May 2015 Received in revised form 4 August 2015 Accepted 10 August 2015 Available online 1 September 2015 Keywords: Alumina Heavy-ion irradiation Color center Thermal annealing Scintillation screens abstract Polycrystalline alumina samples (a-Al 2 O 3 , purity: 99.8%) were irradiated by 63 Cu heavy ions (E = 0.5 MeV/u) at various fluences. After irradiation, absorption measurements were performed within the wavelength range from 200 to 1000 nm to evaluate color center evolution. Thermal annealing behavior of the created defects was investigated with respect to annealing temperature and duration. Complex color center formation processes depending on particle fluence and temperature could be observed. Calculated activation energies necessary for F- and F + -center migration are 0.3 eV for temperatures ranging from RT to 673 K. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Scintillating screens are used at accelerator facilities for ion beam diagnostics up to very high ion fluxes. Applications range from beam alignment to beam profile measurements or detectors for pepper-pot emittance systems [1]. However, during irradiation of the material formation of color centers occurs. The increasing radiation damage and destruction of active emission centers lead to a significant degradation of scintillation yield, which is one of the main problems using the screens as an appropriate tool for beam imaging [2]. Radiation damage and change in optical proper- ties were studied under various irradiation conditions for a large variety of scintillating materials [3–6]. Due to its good radiation hardness, alumina is an interesting material for scintillation applications [7]. Another advantage of using alumina is that the understanding of its luminescence and degradation behavior is at an advanced stage [8]. Ionizing radiation causes free electrons and holes in Al 2 O 3 , resulting in trapping at the sites of lattice defects or impurities. Under irradiation, the primary defects produced in a-Al 2 O 3 are oxygen vacancies (F and F + color centres with two or one trapped electron) [9,10]. At high irradiation doses the concentration of single defects becomes large and neighboring defects can interact and also form more complex F-center aggregates (e.g. F 2 ,F þ 2 or F 2þ 2 ) [11,12]. Besides the dominant formation of these oxygen monovacanies and vacancy clusters, also Al þ i interstitial ions can be formed [8,13]. However, thermal annealing leads to a change of the optical absorption of the material and a decrease in color center concentration [14,15]. At elevated temperatures, created point defects may migrate in the solid. The diffusion of defects can be expressed as thermal activated jumps to another lattice sites. For a current defect concentration N F which changes with temperature T and time t, first order annealing kinetics are assumed. N F N 0 ¼ expðmtÞ; ð1Þ where N 0 is the defect concentration after ion irradiation. The temperature dependent diffusion frequency m is given by Eq. (2). m ¼ m 0  exp  E mig k B T   ; ð2Þ where m 0 is a frequency factor, E mig the activation energy necessary for migration of vacancies and k B is the Boltzmann constant. Insertion of Eq. (2) into Eq. (1) yields the following expression for defect migration. http://dx.doi.org/10.1016/j.nimb.2015.08.024 0168-583X/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstraße 1, 64291 Darmstadt, Germany. E-mail address: s.lederer@gsi.de (S. Lederer). Nuclear Instruments and Methods in Physics Research B 365 (2015) 548–552 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb