IEEE Proof IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 3, JUNE 2010 1 Eu Doped and Eu, Tl Co-Doped NaI Scintillators Natalia V. Shiran, Alexander V. Gektin, Yanina Boyarintseva, Sergey Vasyukov, Andrej Boyarintsev, Vyacheslav Pedash, Sergej Tkachenko, Olga Zelenskaya, N. Kosinov, O. Kisil, and L. Philippovich Abstract—It is shown that Eu co-doping of NaI:Tl allows to modify scintillation properties and reach a better performance comparing to NaI:Tl. The overlapping of absorption, excitation and emission bands typical for and ions might be the reason for energy exchange between these luminescence centers in NaI:Tl, Eu. Enhanced efficiency of emission and delayed rise times in NaI:Tl, Eu confirm this assumption. Eu co-doping al- lows to utilize an additional excitation channels in “conventional” NaI:Tl scintillator. Index Terms—Energy transfer, integration time, luminescence, NaI-Tl, Eu, scintillation. I. INTRODUCTION L AST trends in radiation detection concentrate increas- ingly more on selective spectroscopy devices develop- ments. Large portion of such elaborations is connected with low activity counting that allows using “slow scintillators”, i.e. materials with decay time in the microsecond range. It relates to security systems (Advanced Spectroscopy Portals) first of all. Such driving forces renewed the study and search of -doped halide scintillators that posses spin-allowed ra- diative transitions. The high efficiency luminescence center re- veals bright blue emission in the range of 410–470 nm with decay time in Eu-doped halides. The list of the scintil- lator discovery includes Eu doped alkali-earth halides [1], [2], , , [3]. , , crystals demonstrate excellent light yield, 90 000 ph/MeV, and ability to reach energy resolutions (FWHM) better than 3% at 662 keV [4]. Sixty years ago NaI:Tl invention started the new era of scin- tillation physics, but even today this crystal is still the dominant option. At the same time theoretical estimations [5] show that this scintillator performance is far from the top limits both for light yield and energy resolution. It means we do not use all the resources for NaI:Tl scintillator improvement. If activator (Tl) concentration is optimal we can try to achieve higher scintilla- tion efficiency by co-doping and using more excitations. The aim of the present study was to ascertain the resource for alkali halide scintillator improvement by Eu co-doping. The properties of the well known NaI:Tl scintillators after Eu co- Manuscript received June 30, 2009; revised September 07, 2009 and October 27, 2009; accepted November 13, 2009. Date of current version June 16, 2010. N. V. Shiran, A. V. Gektin, Y. Boyarintseva, S. Vasyukov, A. Boyarintsev, V. Pedash, S. Tkachenko, O. Zelenskaya, and N. Kosinov are with the Institute for Scintillation Materials NAS of Ukraine, 61001 Kharkov, Ukraine (e-mail: shiran@isc.kharkov.com). O. Kisil and L. Philippovich are with the Institute for Single Crystals NAS of Ukraine, 61001 Kharkov, Ukraine. Digital Object Identifier 10.1109/TNS.2010.2048578 doping were studied. A special attention was given to the energy transfer and efficiency of doubly (Eu and Tl) doped NaI crystal. II. EXPERIMENTAL Industrial technologies of NaI:Tl crystals growth with op- timal Tl concentration and scintillator parameters are well known [6]. Such crystal was used as the reference sample to compare with modified scintillator parameters. 1 inch dia. NaI based single crystals were pulled up by Czochralski method in reactive atmosphere to prevent oxygen contamination. Concentrations of and were varied from to in crystals and determined by chemical and absorption methods. The absorption spectra were measured by means of a SPECORD 40 spectrophotometer. Spectral and kinetic characteristics of luminescence were studied using the FLS920 combined steady state and fluorescence lifetime spectrometer Edinburgh Instr. Ltd. Xe900 steady state xenon lamp was used in the continuous mode for UV spectroscopy. Kinetic measurements were done using nF900 nanosecond flashlamp. All scintillation data were checked at hermetically sealed detectors. Light yield (LY) and energy resolution (R) were tested with a R1307 Hamamatsu PMT connected to a proper amplifier and MCA by method described in [7]. Shaping time was varied from 1 to 8 . Instrumental error of LY and R determinations was not exceeding 5%. III. RESULTS AND DISCUSSION Absorption spectrum of NaI:Eu crystal consists of two broad bands in 250–290 and 330–390 nm ranges with a crystalline field splitting (Fig. 1). Excitation in these bands stimulates a narrow (0.16 eV) luminescence band at 440 nm caused by the spin-allowed transition from the excited states of configuration to the ground state of . Decay time of photoluminescence is close to 970 40 ns. Intense emission under UV and gamma irradiation was re- vealed even though very small concentration of Eu . It may be explained by the high quantum efficiency of lu- minescence center. The overlapping of absorption and emission bands is the reason for the emission reabsorption of NaI:Eu. Ab- sorption and photoluminescence characteristics of NaI:Eu and NaI:Tl crystals are summarized in Table I. Gamma-luminescence of single and double (Tl, Eu) activated crystals were studied under identical steady-state conditions (Fig. 2). Spectrum of NaI:Tl shows a typical wide emission band (0.63 eV) with maximum at 420 nm, whereas band of NaI:Eu is narrow and shifted to 450 nm. It has to be noted that crystal with high Tl content and trace of Eu reveals a luminescence spectrum typical for 0018-9499/$26.00 © 2010 IEEE