Elsevier

Radiation Measurements

Volume 71, December 2014, Pages 158-161
Radiation Measurements

Study on fast luminescence component induced by gamma-rays in Ce doped LiCaAlF6 scintillators

https://doi.org/10.1016/j.radmeas.2014.02.019Get rights and content

Highlights

  • The fast luminescence in Ce:LiCaAlF6 scintillator is generated in the host matrix.

  • The origin of the fast luminescence is determined as the Cherenkov radiation.

  • The slow rise time also plays an important role to perform PSD.

Abstract

We discuss the origin of the fast luminescence component induced by fast electrons generated in gamma-ray interactions in Ce doped LiCaAlF6 scintillators. Although the slow luminescence component induced by Ce3+ emissions depends on the Ce concentration in the LiCaAlF6 scintillator, the fast component is independent. The fast component is suggested to be generated in the host matrix of the LiCaAlF6 crystal. From quantitative considerations based on Frank–Tamm equation, which shows the light yield of the Cherenkov radiation, the Cherenkov radiation was determined as the origin of the fast component. We, additionally, found that the slow rise time of main Ce3+ emissions in the Ce:LiCaAlF6 scintillator plays an important role to perform the pulse shape discrimination.

Introduction

Neutron detection techniques play an important role in various fields. For thermal neutron detection, a He-3 proportional counter has been the gold standard detector. Within the last few years, the He-3 gas shortage has been a severe problem for neutron detection applications. Therefore, alternatives to the conventional He-3 neutron detectors have been required. Inorganic scintillators containing lithium or boron have attracted much attention for thermal neutron detection. As one of the promising candidates, Ce doped LiCaAlF6 scintillators have been developed (Yoshikawa et al., 2009, Kawaguchi et al., 2011, Iwanowska et al., 2011). The Ce:LiCaAlF6 scintillator has excellent properties for neutron detection such as significant light yield, fast response, high transparency, light composition and no hygroscopicity. Especially, since Ce:LiCaAlF6 scintillators have a high alpha to beta ratio, which is defined as the ratio of light outputs per unit energy for alpha particles and electrons, they can be promising candidates from viewpoint of the gamma-ray rejection. In addition, Ce:LiCaAlF6 scintillators were reported to distinguish neutron and gamma-ray events by the pulse shape discrimination technique (Yamazaki et al., 2011). For only gamma-ray events, a scintillation pulse shape has a quite fast component with the decay time less than a few nanoseconds. On the other hand, for neutron events, it has only a relatively slow component with the decay time of a few 10 ns, corresponding to the emission of Ce3+ ions. This fast component can be applied to discriminate neutron and gamma-ray events. Although the origin of this fast component was discussed from the viewpoint of optical physics, the origin has not been identified (Koshimizu et al., 2013). In this paper, we discuss the fast luminescence component induced by gamma-rays in Ce:LiCaAlF6 scintillators through the experiments using the digital signal processing technique, which can quantitatively separate a scintillation signal pulse into the fast and slow luminescence components.

Section snippets

Experimental procedures

In order to perform the pulse shape discrimination, we apply the digital signal processing technique. In the basic experiments, the following experimental setup was used to acquire signal data. The Ce:LiCaAlF6 scintillator was optically connected to a photomultiplier tube (PMT, Hamamatsu, R7600U-200). The output signal from the PMT was directly digitized with the fast digitizer (Agilent, U1071A, 8 bit, 1 GHz, 2 GS/s). The digitized signal pulse shapes were processed off-line with PC.

Fig. 1

Ce content dependence

In order to discuss the origin of the fast luminescence, the Ce doping concentration dependence of the luminescence property was measured. LiCaAlF6 scintillators with Ce concentrations of 2%, 3% and 4% in raw material composition were prepared. We irradiated Co-60 gamma rays into the specimens to measure the relationship between the fast and slow components. For each LiCaAlF6 scintillator sample, the digital signal processing was applied and the fast and slow components were extracted

Conclusion

We discuss the origin of the fast luminescence component induced by fast electrons generated in gamma-ray interactions. Although the slow luminescence component induced by Ce3+ emissions depends on the Ce concentration in the LiCaAlF6 scintillator, the fast component is independent of the Ce concentration. The fast component is suggested to be generated in the host matrix of the LiCaAlF6 crystal. From quantitative considerations based on Frank–Tamm equation, which shows the light yield of the

Acknowledgement

This study is the result of “Development of an alternative to He-3 neutron detectors for homeland security and nuclear material safeguards” carried out under the Adaptable and Seamless Technology transfer Program through target drive R&D by Japan Science and Technology Agency.

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