Scintillation Properties of Rb 2 Cu(Cl,Br) 3 Crystals

Rb 2 Cu(Cl,Br) 3 crystals were synthesized by the slow cooling method and their photoluminescence (PL) and scintillation properties were evaluated. PL emission peaks at 390 nm due to the recombination of excitons were observed, and the PL quantum yields of Rb 2 CuCl 3 , Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 , and Rb 2 CuBr 3 were 91.2, 95.3, and 96.3%, respectively. PL decay curves were approximated using a single exponential function model, and the obtained decay time constants were 15–66 μs. Under X-ray irradiation, the scintillation emission peaks of the samples were observed at 390 nm, and the decay times were 11–63 μs. The afterglow levels at 20 ms after X-ray irradiation were calculated to be 30–450 ppm. Rb 2 CuCl 3 showed the highest scintillation light yield of 11000 photons/MeV among the samples when calculated from the pulse height spectra of 137 Cs γ-rays (662 keV).


Introduction
(3) They are generally optically coupled with photomultiplier tubes (PMTs) or photodiodes to convert scintillation photons to electric signals. (3,4)There is a wide range of applications of scintillators, for example, medical imaging, (5) security inspection, (6) natural resource exploration, (7) and materials analysis. (8)The following characteristics are demanded in scintillators for X-and γ-ray detection: high scintillation light yield (LY), high density, large effective atomic number, fast scintillation lifetime, and good transparency.However, no scintillators suitable for all the abovementioned applications have been reported; thus, it is necessary to select scintillators depending on the required performance of the applications.Owing to their excellent optical properties such as high transparency, single-crystal scintillators have been widely studied (9)(10)(11)(12)(13) and put into practical applications.For example, Bi 4 Ge 3 O 12 (BGO), (14)(15)(16) CdWO 4 (CWO), (17)(18)(19) and Tl:CsI (20)(21)(22) have been conventionally used for nondestructive inspections, as represented by X-ray CT and baggage inspections.

Results and Discussion
Figure 1 shows photographs and XRD patterns of the synthesized samples.Several crystals were precipitated after crystal growth.The obtained samples were colorless and transparent, and the maximum size among the samples was 5-10 mm in length and 1 mm in thickness.The rest of the crystal samples were crushed into powders for XRD measurement.All diffraction peaks of the samples were in good agreement with the reported data of Rb 2 CuCl 3 (39,45) and the reference data of Rb 2 CuBr 3 (JCPDS: 98-015-0295 (36) ).Therefore, the obtained samples had an orthorhombic crystal system with a space group of Pnma.
Figure 2 shows the PL excitation and emission spectra of all the samples.A broad emission peak at 350-550 nm was confirmed under excitation at 250-350 nm.The origin of the emission is the recombination of self-trapped excitons (STEs). (46)When the range of 320-800 nm was monitored under the excitation light of 310 nm, the PL QY values of Rb 2 CuCl 3 , Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 , and Rb 2 CuBr 3 were 91.2, 95.3, and 96.3%, respectively.The QY of Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 slightly increased compared with that of Rb 2 CuCl 3 via the partial anion substitutions of Cl and Br.In addition, the emission wavelength was shifted to a longer wavelength by Cl replacement.
The insets of Fig. 2 show the PL decay curves of the obtained samples monitored at 390 nm under excitation at 315 nm.The observed decay curves were in good agreement with a single exponential function model.The obtained decay time constants of Rb 2 CuCl 3 , Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 , and Rb 2 CuBr 3 were 15, 31, and 66 μs, respectively, which were comparable to those of previous studies. (36,39,47)The decay time constants decreased as the Cl proportion decreased.
Figures 3 and 4 show the X-ray-induced scintillation spectra and scintillation decay curves, respectively.A broad emission peak at 390 nm was observed, and the peak position shifted toward longer wavelength as the Cl ratio increased.The spectral profile was similar to scintillation in a previous study; (39) therefore, they were considered to be derived from the recombination of STE.All the scintillation decay curves were approximated by a single exponential function model except for the instrumental response function (IRF).The obtained PL and scintillation values were not significantly different and were considered to be within the typical errors.
Figure 5 shows the afterglow curves of the samples after 2 ms irradiation with X-rays.The afterglow level (A) was defined as A = (I 20 − I BG )/(I MAX − I BG ).I 20 , I MAX , and I BG are the intensity at 20 ms after X-ray irradiation for 2 ms, the intensity during X-ray irradiation, and the background intensity before X-ray irradiation, respectively.The obtained A values of Rb 2 CuCl 3 , Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 , and Rb 2 CuBr 3 were 30, 36, and 450 ppm, respectively.These values significantly decreased as the Cl ratio increased.In addition, the values of Rb 2 CuCl 3 and Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 were lower than that of Tl:CsI (300 ppm), which has been one of the conventional halide scintillators, (48) and comparable to that of Bi 4 Ge 3 O 12 (10 ppm), (48) a scintillator for X-ray detection under the same measurement condition.
Figure 6 shows the pulse height spectra of 137 Cs γ-rays measured using the samples and CWO as a reference sample, which shows LY of 15800 photons/MeV. (49)All the samples exhibited a photoabsorption peak.The peak channel was determined by Gaussian approximation, and the  calculated LY values of Rb 2 CuCl 3 , Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 , and Rb 2 CuBr 3 were 11000, 4900, and 1500 photons/MeV, respectively, after corrections of the PMT sensitivity.In addition, the DE values at 662 keV for Rb 2 CuCl 3 , Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 , and Rb 2 CuBr 3 were 10.4, 10.8, and 12.1%, respectively.The difference in the estimation of LY was attributed to the different measurement methods used.In a previous study, (39) the LY of Rb 2 CuCl 3 was estimated from the scintillation spectrum, whereas the current work used the pulse height spectrum.Rb 2 CuCl 3 showed the highest LY and the lowest A among the samples.They did not follow the tendency of QY.In general, scintillation and storage-type luminescence properties such as afterglow show an inverse relationship. (50,51)The A values of Cl-containing samples were lower than those of Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 and Rb 2 CuBr 3 , and the trend was consistent with the relationship.

Conclusions
Continuous solid solutions of Rb 2 Cu(Cl 0.5 ,Br 0.5 ) 3 crystals were successfully synthesized by the slow cooling method.In the PL and scintillation spectra, a broad emission peak was observed at 390 nm, and the emission mechanism was attributed to the recombination of STEs.The PL    and scintillation decay curves were approximated using a single exponential function model, and the obtained decay constants (11-66 µs) decreased as the Cl proportion increased.A was improved from 450 ppm (Rb 2 CuBr 3 ) to 36 ppm [Rb 2 Cu(Cl,Br) 3 ] and 30 ppm (Rb 2 CuCl 3 ) by changing the anion part to Cl from Br.The LY values of the samples tended to increase as the Cl ratio in the composition increased; the maximum LY was 11000 photons/MeV calculated for Rb 2 CuCl 3 .As a comprehensive result, the unsubstituted sample Rb 2 CuCl 3 showed the best scintillation properties among the samples.The A and LY of Rb 2 CuCl 3 were comparable to those of BGO, which is one of the representative commercially available scintillators for security and anticompton spectrometers. (52)Therefore, Rb 2 CuCl 3 can be a promising material for the above applications.

Fig. 2 .
Fig. 2. (Color online) PL excitation and emission spectra of Rb 2 Cu(Cl,Br) 3 .Horizontal and vertical axes indicate emission and excitation wavelengths, respectively.Insets show PL decay curves measured under excitation at 315 nm and monitored at 390 nm.