Modification of NaI crystal scintillation properties by Eu-doping N. Shiran * , A. Gektin, Y. Boyarintseva, S. Vasyukov, A. Boyarintsev, V. Pedash, S. Tkachenko, O. Zelenskaya, D. Zosim Institute for Scintillation Materials, NAS of Ukraine, 60, Lenin Ave., Kharkov, Ukraine article info Article history: Received 2 November 2009 Received in revised form 29 March 2010 Accepted 22 April 2010 Available online 20 May 2010 Keywords: NaI:Eu crystal Scintillator Eu 2+ luminescence Decay Heat treatment abstract High efficiency and reasonably fast decay time of NaI:Eu crystal point out the possibility of crystal appli- cation as scintillator. Modification of crystal properties by varying the activator concentration and heat treatment conditions enabled the identification the solid solution instability. Structure of activator cen- ters varies considerably, resulting in a change of the luminescence intensity and spectral characteristics. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Recently the activation of alkali-earth halides by europium has allowed to produce CaI 2 :Eu, SrI 2 :Eu and BaI 2 :Eu scintillators with uniquely high spectrometric parameters [1]. This fact makes us look for new ways to modify alkali-halide crystal properties. In the nineties of the past century it was demonstrated that ow- ing to cations of the matrix and activator being non-isomorphic the absorption and luminescent parameters of the alkali-halides doped with europium varied depending on heat treatment applied to crystals [2,3]. The scintillation properties of those kinds of compounds were not investigated up to date. The first results obtained in our studies on optical and scintillation parameters of NaI:Eu crystals were pre- sented lately [4]. Preliminary measurements indicated that the high efficiency and reasonably fast decay times of NaI:Eu made for the feasibility of this crystal to be used as scintillators. This pa- per presents results of a subsequent research on properties modi- fication by changing activator concentration and heat treatment of NaI:Eu crystals. 2. Experimental NaI:Eu single crystals were pulled up by the Czochralski method in reactive atmosphere to prevent oxygen contamination. The con- centration of Eu 2+ varied from 10 5 to 0.5 mol% in crystals and was determined by chemical and absorption methods. The absorption spectra were measured by means of SPECORD 40 spectrophotom- eter. Spectral and kinetic characteristics of photoemission were studied using FLS920 combined steady state and fluorescence life- time spectrometer manufactured by Edinb. Instr. Ltd. A Xe900 steady state xenon lamp was used in the continuous mode for UV spectroscopy. Kinetic measurements were made using an nF900 nanosecond flash lamp. Radio luminescence spectra were fixed using isotope c 241 Am (60 keV, 6.66  10 10 Bk). Light yield and energy resolution were checked by a pulse method described in [5]. Scintillation parameters were tested for 662 keV c 137 Cs source with the shaping time 12 ls by a R1307 Hamamatsu PMT connected to a proper amplifier and MCA; and the error of data determination was up to 5%. The size and shape of samples were chosen to be the same (hermetically sealed detectors 14  9 mm) to provide better result correlation. 3. Results and discussion 3.1. Optical absorption and photoluminescence of as-grown crystals The absorption spectrum of NaI:Eu crystal consists of two broad bands in 250–290 and 330–390 nm ranges and corresponds to 8 S 7/2 ? 4f 6 5d transitions in Eu 2+ ion with a crystalline field splitting 10 Dq  10,752 cm 1 (Fig. 1). The excitation of a slightly doped crystal (<0.1% Eu) in those bands stimulates a narrow (0.17 eV) luminescence band at 439 nm. The emission is caused by the spin-allowed transition from the excited states of 4f 6 5d configura- tion to the ground state 4f 7 ( 8 S 7/2 ) of Eu 2+ ion (Fig. 2). The overlap- 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.04.014 * Corresponding author. Tel.: +380 573410367; fax: +380 573404474. E-mail address: shiran@isc.kharkov.com (N. Shiran). Optical Materials 32 (2010) 1345–1348 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat