X-ray luminescence of cerium-doped polv crystalline zinc sulfide obtained by self-propagating high-temperature synthesis

Abstract

Many studies have been published recently [I-3] on a series of cerium-activated crystals, with the aim of establishing the feasibility of their use as scintillation materials. In the work reported here, we investigated the x-ray luminescence of polycrystalline ZnS:Ce obtained by self-propagating high-temperature synthesis (SPHTS), following a scheme that was described in [4]. When AIIB v! crystals are doped with cerium, the trivalent ion Ce 3+ replaces the A 2+ cation in the lattice and becomes a donor; therefore, the principle of charge compensation requires the parallel introduction of a coactivator playing the role of an acceptor impurity. As shown in [5], halogen ions facilitate the formation of cationic vacancies and hence promote the insertion of Ce 3+ into the lattice. In this connection, cerium doping has been accomplished by introducing powdered CeF 3 into an original charge consisting of Zn and S powders. It is also known that Ce centers are capable of retaining oxygen [3, 6]. Studies have been made of crystalline specimens of ZnS:Ce from a region of columnar or equiaxial crystals [7] with various concentrations of cerium (C = 3.0, 0.3, and 0.03 mole %). Steady-state x-ray luminescence (XRL) spectra of the specimens were excited by means of a URS-51 unit (Mo anticathode, U = 10 kV, I = 12 mA). For the investigation of the time characteristics, we used pulsed x-ray excitation with a pulse duration Ar ~ 1 nsec and a recurrence frequency f = 100 kHz (U = 35 kV, I = 100 #A). The luminescence spectra in the interval 250-800 nm were analyzed by means of a unit based on an MDR-2 monochromator; the spectra were registered by means of an FI~U-100 photomultiplier. Decay curves were obtained by using a method of counting individual quanta, and the kinetics of luminescence decay were determined by solving the convolution equation of the post-emission and excitation pulse in an ES-1842 personal computer. The spectra of the test specimens with steady excitation (T = 77 K) are shown in Fig. la. The structure of the steady-state XRL spectra depends on the Ce concentration; with C = 0.03 or 0.3 mole %, the cerium favors an increase in the concentration of the centers responsible for emission in the green region of the spectrum and a decrease in the ~:oncentration of centers responsible for emission in the 600-nm region; with a Ce concentration of 3.0 mole %, an additional band is manifested at hma x = 405 nm. The spectral and kinetic parameters of the synthesized crystals were examined over a broad range of time, from 10 -9 to 10 -6 sec. The XRL pulse decay curve for the ZnS--Ce specimens is complex; two time intervals can be distinguished on the curve, one with decay times amounting to several nanoseconds (the initial section of the decay of intensity is well approximated by an exponential with r = 12.5 nsec) and the other with decay times measured in microseconds. The spectral composition of the XRL differs substantially between these two time intervals. The XRL spectra of the ZnS--Ce crystals in the different stages of damping of the luminescence pulse are presented in Fig. lb (for the case t I = 0 nsec, corresponding to the registration of an XRL spectrum with a time window of 2 nsec at the instant of reaching the maximum on the decay curve) and in Fig. 2 (for the case b = 5 #sec, corresponding to the XRL spectrum measured 5 #sec after excitation). Also shown for comparison are XRL spectra of undoped ZnS, taken under analogous conditions.