Lutetium fine silicate, LFS, is a brand name of the set of Ce-doped solid solutions of rare-earth oxyorthosilicate scintillation crystals, comprising lutetium and crystallizing in the monoclinic system, spatial group C2/c, Z=4. The patented LFS compositions are Ce_xLu_2+2y-x-zA_zSi_1-yO_5+y where A is at least one element selected from the group consisting of Ca, Sc, Y, and rare-earth elements, [1]. For the first time, the results of study of the LFS-5 and LFS-6 compositions crystals were presented in 2004, [2].

The LFS-3, LFS-7, and LFS-8 compositions crystal ingots of the up 10 kg weight, 10 cm diameter and 22 cm length have been grown by the Czochralski method. The LFS crystals are produced industrially by BOET, China. Compositions of the crystals have been studied. Concentrations of the host elements were measured by the ICP-MS method, oxygen concentration was measured by the Leco combustion analysis, concentration of the impurities was measured by the GDMS method. The existence of the two luminescence centres Ce2(7) and Ce1(6) in LSO crystal is a reason for the deterioration of luminescent properties of Ce-doped LSO. The emission from Ce1 centres should be controlled as much as possible in order to improve crystal performance. It may be achieved by means of: (a) an increasing the average distance of the two centres or (b) a decrease of the Ce1 centres concentration.

Results of the study of optical and luminescence characteristics of the LFS-3, LFS-7 and LFS-8 heavy scintillation crystals are presented. Emission spectra of LFS crystals were measured using a setup with luminescence excitation by X-ray photons with energy of 30 KeV. To determine the light yield scintillators, the total-absorption spectra of γ-rays from radioactive sources, i.e., so-called, photopeaks were used. The luminescence time of LFS scintillators was studied with a special installation using the delayed coincidence method consisting in measurements of the distribution of time intervals between scintillator excitation and photoelectron formation on the photomultiplier photocathode with the aid of a time-to-digital converter (TDC).

Using the average quantum efficiency of the R4125Q photomultiplier photocathode in the region of the LFS-3 emission spectrum to convert the number of photoelectrons per MeV to the photon yield, the LFS-3 light yield of 38000 photon/MeV was achieved. The reduction of the energy transfer between two Ce³⁺ centres improves the energy resolution to 7% and slows down the decay time to 35 ns due to the distortion of the crystal  lattice in the Y-containing LFS-3 crystals of the 7.35 g/cm³ density. The LFS-7 crystals of the 7.43 g/cm³  density, exhibit high light yield, up to 36000 ph/MeV, good energy resolution, about 6 - 7.5%, a high uniformity  and reproducibility of scintillation properties within the boule and from boule to boule. An insertion of Ca²⁺-ions in LFS compositions and a proper non-stoichiometry compositional shift shortens decay time to 30 - 32 ns for the LFS-7 composition crystal, and results in superior short decay time of 14 ns - 19 ns for the LFS-8 composition crystal. Density of LFS-8 crystal is 7.4 g/cm³, a light yield is up to 32000 ph/MeV.

Currently there is strong demand for new radiation hard scintillating materials for electromagnetic calorimeters of the HEP experiments. The stability of LFS-3 crystals to radiation damages caused by charged hadrons and γ-rays was studied. The samples were irradiated by the ⁶⁰Co radioactivity source (the maximum power is ~4krad/min). LFS-3 crystals were sequentially irradiated with three doses: 5, 23, and 68 Mrad. The optical transmission spectra were measured before and immediately after irradiation using a Kruess Optronic VIS 6500 spectrophotometer. An analysis of the transmission spectra shows that the optical transmittance in the LFS-3 emission region decreases by 2.5% for a dose of 68 Mrad. For the LFS-3 samples cut out from the upper, middle, and lower parts of the initial crystal, the dose of 23 Mrad had no appreciable effect on optical transmission.

Radiation damages of LFS-3 crystals during their irradiation with hadrons were studied in the proton beam of the synchrotron of the Institute of Theoretical and Experimental Physics (ITEP). LFS crystals were packed into a 3×2 matrix for simultaneous irradiation of six samples in the 50-mm-diameter proton beam. The LFS crystals were irradiated with 155-MeV protons. Due to the high level of induced radioactivity, the first measurements of the optical transmittance of crystal samples irradiated with protons were performed in 30 days after irradiation only. There are no damages in LFS-3 crystals for a fluence of 4.4×10¹² particle/cm² [3].

References

[1] A.I. Zagumennyi, Yu.D. Zavartsev, S.A. Kutovoi, Patent US 7132060, 2004
[2] Th.K. Lewellen, M.L. Janes, R.S. Miyaoka, F.A. Zerrouk, DOI: 10.1109/NSSMIC.2004.1466296, Nuclear Science Symposium Conference Record, 2004 IEEE Vol.5 (2004) 2915-2918 “Initial evaluation of the scintillator LFS for positron emission tomograph applications”
[3] Yu.D. Zavartsev, M.V. Zavertyaev, A.I. Zagumennyi, A.F. Zerrouk, V.A. Kozlov, and S.A. Kutovoi, "New radiation resistant scintillator LFS-3 for electromagnetic calorimeters" Bulletin Lebedev Phys. Institute vol.40 (2013) 34-38 DOI:10.3103/S1068335613020024

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1. Structural peculiarities of undoped and Ce-doped solid solutions of rare-earth oxyorthosilicate crystals