Elsevier

Optical Materials

Volume 35, Issue 11, September 2013, Pages 1987-1992
Optical Materials

Study of rare-earth-doped scintillators

https://doi.org/10.1016/j.optmat.2012.11.002Get rights and content

Abstract

Scintillators are widely used in radiation detection applications in basic science and industry. Scintillators are generally solids and consist of an emission center and a host lattice. Especially in medical applications that detect high-energy photons, the emission centers are rare-earth (RE) ions, mostly Ce3+. However, recently Pr3+ also exhibited fast 5d–4f emission and became an alternative to Ce3+. In X-ray computed tomography, a scintillation decay time on the order of microseconds is acceptable, and other rare-earth ions such as Nd3+, Eu2+, Ho3+, Er3+, and Tm3+ are candidates for the emission center. The characteristics of these inorganic materials and their advantages and disadvantages for scintillators will be discussed.

Highlights

► The present paper reviews recent progress of rare earth doped scintillators. ► This work covers from VUV to NIR emitting scintillators. ► Novel X-ray, gamma-ray, and neutron scintillators are described.

Introduction

Scintillators are phosphors that convert a single photon of high-energy ionizing radiation to hundreds of visible–ultraviolet photons. Scintillation detectors, which usually consist of a scintillator and photodetector [photomultiplier tube (PMT) or photodiode (PD)], have played an important role in radiation detection applications, for example, in medicine [1], security [2], well-logging [3], and high-energy physics [4], [5]. The main difference between scintillation and photoluminescence is whether or not the host material is excited. In scintillation, the host material is first excited by high-energy radiation; then the absorbed energy in the host migrates to the emission center, and finally scintillation photons are emitted via a relaxation process in the emission center. Emission centers have generally been rare-earth (RE) ions, as in other phosphors.

To date, the emission center of most scintillators is Ce3+, which exhibits bright luminescence with a decay time of several tens of nanoseconds owing to the spin- and parity-allowed 5d–4f transition. Among scintillator applications, positron emission tomography (PET) is a common application where Ce3+-doped scintillators play an important role because PET requires fast scintillators owing to its high counting rate. However, other RE dopants should also be considered as the emission center because Pr3+ [6], Nd3+, Ho3+, Tm3+, and Er3+ [7], [8] can exhibit rapid 5d–4f-transition-based emission at vacuum ultraviolet (VUV) or near ultraviolet wavelengths. In addition, charge transfer luminescence caused by Yb3+ is also attractive because of its subnanosecond decay time at room temperature [9].

In addition to rapid 5d–4f luminescence, 4f–4f emission with microsecond decay or the 5d–4f transition from Eu2+ can be used for some applications. In security systems at airports or ports, CdWO4 (CWO) single crystalline scintillators coupled with a Si PIN PD are used; their main decay time is ∼12 μs [10]. In X-ray computed tomography (CT), the detector is essentially the same as in the above security instruments; scintillators coupled with PDs are used, and Gd2O2S (GOS) doped with Ce and Pr is widely applied [11]. The emission from GOS is caused by 4f–4f emission from Pr3+, and a Ce3+ co-dopant is added to suppress the afterglow. Unlike PET, where each radiation signal is processed event-by-event (pulse counting type), these systems (integrated-type detectors) read out scintillation photons as a current with a few milliseconds of integration time, so the scintillators are not necessarily fast. In such cases, Pr3+, Nd3+, Ho3+, Tm3+, Er3+, and Eu2+ become attractive dopants because they have many emission lines due to 4f–4f transitions from visible to near infrared (NIR) wavelengths with a decay time of a few microseconds. In this paper, we will discuss traditional and novel inorganic scintillators doped with RE ions.

Section snippets

RE-doped scintillators for γ-ray detection

In γ-ray detectors, bulk inorganic scintillators are essential because the stopping power for γ-rays is basically proportional to the volume and ρZeff4 value of the scintillator, where ρ and Zeff are the density (g/cm3) and effective atomic number, respectively. The interaction processes between γ-rays and scintillators are photoabsorption, Compton scattering, and pair creation, and most γ-ray detectors use photoabsorption events. When γ-rays are absorbed by a scintillator, the absorbed energy

RE-doped scintillators for X-ray detection

The requirements for scintillators for X-ray detection generally differ from those for γ-ray detection because most detectors are the integrated type, as mentioned above. Unlike the case in pulse counting detectors, both the light yield and stopping power affect the signal output, and a microsecond-order decay time is acceptable. In the current stage, CWO crystals [10] and Ce/Pr:GOS ceramics [11] are used in security and X-ray CT applications, respectively. In some applications, Tl:CsI [12] is

RE-doped scintillators for neutron detection

In addition to high-energy photon detection, scintillators are also applied to particle detection. Organic scintillators have played a major role in the detection of charged particles (e.g., α-rays, β-rays), so we do not treat charged particle detection in this work. RE-doped inorganic scintillators attract much attention for neutron detection because of a worldwide 3He supply crisis. To date, most thermal neutron detectors are 3He gas counters, which have a high thermal neutron cross section

Conclusion

We discussed several of the main trends in RE-doped inorganic scintillator development for high-energy photon and neutron detectors. In addition to the traditional scintillators used in γ-ray (Tl:NaI, BGO, Ce:LYSO), X-ray (CWO, GOS, Tl:CsI) and neutron (Li glass, Eu:LiI) detection, some novel scintillators are under investigation. Ce- or Pr-doped garnet scintillators show good scintillation responses, and VUV scintillators for use in gas-PMT-based radiation detectors are studied intensively.

Acknowledgements

This work was mainly supported by JST Sentan and partially by a Grant in Aid for Young Scientists (A)-23686135, and Challenging Exploratory Research-23656584 from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT). Partial assistance from the Yazaki Memorial Foundation for Science and Technology, Sumitomo Foundation, and Iketani Science and Technology Foundation are also gratefully acknowledged. The part of this work was performed under the

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