Basic study of Europium doped LiCaAlF 6 scintillator and its capability for thermal neutron imaging application Takayuki Yanagida a,⇑ , Noriaki Kawaguchi b,d , Yutaka Fujimoto d , Kentaro Fukuda b , Yuui Yokota d , Atsushi Yamazaki c , Kenichi Watanabe c , Jan Pejchal d,e , Akira Uritani c , Tetsuo Iguchi c , Akira Yoshikawa a,d a New Industry Creation Hatchery Center (NICHe), Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan b Tokuyama Corporation Shibuya 3-chome, Shibuya-ku, Tokyo 150-8383, Japan c Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan d Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan e Institute of Physics AS CR, Cukrovarnicka 10, 16200 Prague 6, Czech Republic article info Article history: Received 24 December 2010 Received in revised form 14 February 2011 Accepted 15 February 2011 Available online 4 March 2011 Keywords: LiCaAlF 6 Single crystal Scintillation detector Eu 2+ Neutron Scintillator abstract Eu 2+ 0.1, 0.5, 1, and 2 mol% doped LiCaAlF 6 single crystalline scintillators were grown by the micro-pull- ing down (l-PD) method. Eu 2+ 2 mol% doped LiCaAlF 6 was also prepared using the Czochralski method. In the transmittance spectra, 4f–5d absorption lines appeared around 200–220 and 290–350 nm. An intense emission at 375 nm due to Eu 2+ 5d–4f transition was observed under 241 Am a-ray excitation. When 252 Cf excited pulse height spectra were measured, Eu 2% doped one showed the highest light yield of 29,000 ph/n with 1.15 ls decay time. Using the 2 inchu Czochralski grown one coupled with the position sensitive photomultiplier tube covered by Cd mask with various size (1, 2, 3, and 5 mm) pin holes, ther- mal neutron imaging was examined. As a result, the spatial resolution turned out to be better than 1 mm. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Inorganic scintillators have been playing a major role in many fields of radiation detection, including medical imaging, security, astrophysics, and searching resources, for example oil. In these applications, scintillators for thermal neutron detection have re- cently attracted much attention due to diminishing resources of 3 He gas. Up to now, most of the thermal neutron detectors are the 3 He gas counters with high thermal neutron cross section and low background c-ray sensitivity. However, the recent de- mand for 3 He highly exceeds its supply because a use of neutron detectors for security and oil logging applications increased. This huge discrepancy between the demand and supply motivates us to develop novel thermal neutron scintillators which would re- place the contemporary 3 He based systems. Concerning the material design, the lattice of a scintillator for thermal neutron detection should consist of light elements to re- duce the sensitivity for c-ray background. This requirement is a contrary to that for X and c-ray scintillators. Similarly as for the X and c-ray scintillators, high light yield, emission wavelength suitable for the conventional photomultiplier tubes (PMTs), no hygroscopiciy, fast decay time, and high transmittance are re- quired. To satisfy these requirements, we selected Eu doped LiCa- AlF 6 as a candidate. For LiCaAlF 6 , low effective atomic number of 14 and low density of 2.98 g/cm 3 are very attractive properties be- cause the low sensitivity for background c-rays can be expected. Ce 3+ -doped LiCaAlF 6 was originally developed and studied for laser application [1]. As a solid state ultra violet laser material, Ce:LiCa- AlF 6 showed good performance [2]. Up to now, including our works, the energy migration processes and scintillation efficiency under neutron excitation were precisely studied [3–6] in Ce:LiCa- AlF 6 . However, scintillation properties of Eu:LiCaAlF 6 has been rarely reported. It was first reported that Eu 0.5 mol% doped LiCa- AlF 6 exhibited 10,000 ph/MeV under low energy c-ray irradiation [7–8]. Recently, we found that Eu:LiCaAlF 6 has good scintillation properties under neutron irradiation [9]. In order to continue the systematic study of Eu:LiCaAlF 6 , in the present work, we grew Eu 0.1, 0.5, 1 and 2 mol% (nominal concen- trations) doped LiCaAlF 6 crystals by the micro-pulling down (l-PD) method [10] to study the relation between the Eu concentration and the scintillation response. Using these l-PD grown crystals, the optimum Eu concentration of 2 mol% was determined in optical (transmittance and radio luminescence) and scintillation 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.02.016 ⇑ Corresponding author. Present address: New Industry Creation Hatchery Center (NICHe), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-0812, Japan. Tel.: +81 22 217 5822; fax: +81 22 217 5102. E-mail address: t_yanagi@tagen.tohoku.ac.jp (T. Yanagida). Optical Materials 33 (2011) 1243–1247 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat