Effect of annealing and impurity concentration on the TL characteristics of nanocrystalline Mn-doped CaF 2 P.D. Sahare a, * , Manveer Singh a , Pratik Kumar b a Department of Physics and Astrophysics, University of Delhi, Delhi 110 007, India b Medical Physics Unit, BRAIRCH, AIIMS, Ansari Nagar, New Delhi 110029, India highlights  Nanocrystalline material CaF 2 :Mn is prepared by simple coprecipitation method.  The material is studied by XRD, TEM, ESR, TL and PL techniques.  High impurity concentrations give rise to clusters causing material instability.  Changes in ESR and PL and glow curve structures are studied and explained.  Better characteristics than the bulk make the nanophosphor useful for dosimetry. article info Article history: Received 18 October 2013 Received in revised form 14 April 2015 Accepted 3 July 2015 Available online 6 July 2015 Keywords: CaF 2 :Mn nanoparticles Thermoluminescence (TL) Concentration effects EPR PL abstract Nanocrystalline samples of Mn-doped CaF 2 were synthesized by chemical coprecipitation method. The impurity concentration was varied in the range of 0.5e4.0 mol%. The structure of the synthesized ma- terial was confirmed using powder XRD analysis. TEM images of the nanoparticles show their size occurring mostly in the range of 35e40 nm, with clusters of some impurity phases formed on annealing of the material at higher temperatures. Detailed studies on TL showed that the structures of glow curves depend on Mn concentrations and annealing temperatures. Optimization of the concentration and annealing temperature showed that the sample (doped with 3.0 mol% and annealed at 673 K) has almost a single dosimetric glow peak appearing at around 492 K. EPR and PL spectra were further studied to understand the reasons for changes in the glow curve structures. All detailed studies on TL, PL and EPR showed that the changes in glow curve structures are caused not only by the stress connected with the difference in ionic radii of host Ca 2þ and the guest impurity Mn 3þ /Mn 2þ , but are also governed by other reasons, like diffusion of atmospheric oxygen and formation of impurity aggregates, such as, MnO 2 , Mn 3 O 4 , etc. This is true not only for nanocrystalline CaF 2 :Mn but could also be so for the bulk CaF 2 :Mn (TLD-400) and would thus help in understanding complex glow curve structure, high fading and the loss of reusability on annealing beyond 673 K. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction TLD phosphor CaF 2 :Mn (TLD-400, microcrystalline powder or hot pressed chips) shows a high sensitivity and linear response over a wide range of radiation doses (0.5 mGye1.0 kGy) (Thermscientific, TLD Materials, Features and Technical Specifications, 1981; Fehl et al., 1994). It is widely used for radiation dosimetry for last four decades. However, there are some drawbacks, such as, complicated glow curve structure, low stability and fast fading, loss of reusability, etc. (Danilkin et al., 2008). One of the main disadvantages of CaF 2 :Mn is its low stability, on annealing and during repeated TL readouts, introducing inaccuracies in dose measurements. It is caused by a strong dependence of TL glow curve structures on the concentration of the impurity (Mn) and its related redox reactions during annealing. It is believed that low stability of the phosphor is caused by the oxidation of Mn 2þ to higher charge states at higher temperatures (Danilkin et al., 2008; Planque, 1984). However, a well defined and simple high- temperature TL peak is observed when concentration of Mn ions * Corresponding author. E-mail addresses: pdsahare@yahoo.co.in, pdsahare@physics.du.ac.in (P.D. Sahare). Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas http://dx.doi.org/10.1016/j.radmeas.2015.07.003 1350-4487/© 2015 Elsevier Ltd. All rights reserved. Radiation Measurements 80 (2015) 29e37