Fast d–f emission in Ce 3+ , Pr 3+ and Nd 3+ activated RbCl Aleksander Zych ⇑ , Celso de Mello Donegá, Andries Meijerink CMI, Debye Institute for NanoMaterials Science, Utrecht University, 3508TA Utrecht, The Netherlands article info Article history: Received 27 October 2010 Received in revised form 25 November 2010 Accepted 26 November 2010 Available online 28 December 2010 Keywords: Ce 3+ Pr 3+ Nd 3+ Scintillators RbCl abstract In the search for new scintillator materials, Ce 3+ doped chlorides are a promising class of materials, com- bining a high efficiency and fast response time. Even shorter response times may be achieved by replac- ing Ce 3+ by Pr 3+ or Nd 3+ as the lifetime of the d–f emission is substantially shorter for these ions. Here we report on the luminescence properties of Ce 3+ , Pr 3+ and Nd 3+ in RbCl and investigate the potential as a scintillator material. Under UV excitation Ce 3+ shows d–f emission between 325 and 425 nm. The emis- sion originates from multiple (differently charge compensated) Ce 3+ sites. The luminescence lifetime var- ies with wavelength and is 40 ns for the longer wavelength emission. For RbCl:Pr 3+ three d–f emission band are observed between 250 and 350 nm which can be assigned to transitions from the lowest energy fd state to different 3 H J (J = 4–6) states within the 4f 2 configuration of Pr 3+ . The decay time is 17 ns. For the Nd 3+ activated sample a weak emission band around 220 nm is observed only at 8 K which may be due to d–f emission. The very short lifetime (4 ns) is faster than the radiative lifetime, indicating that the d–f emission is quenched by relaxation to lower lying 4f 3 states or by the process of photoionization. Under VUV excitation at wavelengths below 175 nm (the bandgap of RbCl) the d–f emission is very weak for Ce 3+ , Pr 3+ and Nd 3+ doped RbCl and the emission spectra are dominated by defect related emission. This indicates that energy transfer from the host lattice to the fd states is inefficient which prevents appli- cation as a scintillator material. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Lanthanide (Ln) doped inorganic hosts have been intensively investigated in the past as possible scintillation materials and are presently used in multiple applications, such as medical imaging (positron emission tomography (PET), computed tomography (CT) or single photon emission tomography (SPECT)) and radiation detection for homeland security or high energy physics [1–3]. Many of the presently used scintillators are doped with Ce 3+ , which exhibits efficient and relatively fast d–f emission. The typical radi- ative lifetime of Ce 3+ emission is 30–70 ns, depending on the emis- sion wavelength. The highest efficiencies have been reported for Ce 3+ doped composition LnX 3 (X = Cl, Br, I; Ln = La, Lu) [4–7]. The exceptionally high scintillation efficiencies (up to 100,000 pho- tons/MeV) and relatively short emission lifetimes (20–50 ns) result in an excellent energy resolution for these scintillator materials [4–10]. Also for other simple ionic halides d–f emission from Ln 3+ has been investigated, viz. in NaCl [11], SrCl 2 and BaCl 2 [12–15]. It has been shown that incorporation of trivalent lanthanide ions is possible in these materials and fast d–f emission has been observed not only for cerium, but also for praseodymium and neo- dymium ions in these compounds. The radiative lifetime decreases as the emission shifts to higher energies [1,16,17]. For the d–f emission from Ce 3+ and Eu 2+ the radiative decay rate has been demonstrated to be inversely propor- tional to k 3 [16,17]. The higher energy of the d–f emission from Pr 3+ and Nd 3+ is responsible for the shorter decay times for d–f emission from these ions compared to Ce 3+ . The higher energy position of the fd states might impose certain problems in case of lumines- cence originating from these states due to the proximity of the lowest fd level to the conduction band of the material, which might in turn lead to luminescence quenching via a process of photoion- ization. On the other hand, the scintillation light yield becomes lower in large bandgap materials, due to the smaller number of electron–hole (e–h) pairs that can be produced upon an excitation with high energy photons. This leads to an optimization of the combination of host lattice and lanthanide ion which will also de- pend on which property is more desired (response time or light yield) is more important for a specific application. The relative positions of the fd levels of lanthanides are de- scribed by the Dorenbos relation [18]. For the Pr 3+ the fd levels are located at about 12,240 cm 1 higher energies than for Ce 3+ lev- els in the same host lattice [18]. As a result, the lifetime of the d–f emission is typically two times shorter for Pr 3+ in comparison to Ce 3+ [19,20]. In case of neodymium the lowest fd level is about 22,720 cm 1 higher than the cerium fd levels [18] and the decay of the d–f emission is even faster. Short emission lifetimes are 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.11.030 ⇑ Corresponding author. Tel.: +31 30 2532203; fax: +31 30 2532403. E-mail address: a.k.zych@uu.nl (A. Zych). Optical Materials 33 (2011) 347–354 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat