Degradation of emission parameters in LiF-MeO crystal scintillators

The paper presents the results of the research in spectral and kinetic parameters of the luminescence in phosphors based on LiF-MeO crystals. The potential of these phosphors used as scintillators in the ionizing radiation field is discussed.


Introduction
Scintillating materials based on oxygen compounds may be divided into two types: scintillators of a doped type and natural scintillators. LiF crystal is the scintillator of a doped type. The prospect of widespread using of this material based on the features of doped crystals structure making light output dopant emission comparable to that in natural scintillators [1,2]. Application of oxygen-containing crystal phosphors as radiation detectors in medical diagnosis, customs control, detectors for fundamental science is being discussed in [3-5, 6, 7]. However, the potential of these materials as scintillators in the ionizing radiation field cannot be assessed since no data of the effect of radiationinduced defects in lattice crystals on scintillation characteristics of the material is available. (It is known that LiF lattice is very sensitive to ionizing radiation). Furthermore, information on radiation modification of the oxygen-containing impurity in the composition of the luminescence centers especially important for the understanding of the aging process in doped crystals placed in radiation field.

Object and Method
We used pulse spectroscopy methods to investigate LiF crystals doped with oxides of various metals (Me: W, Ti, Fe, Li). Imperfection of the crystals (initial and induced by ionizing radiation) was estimated through the analysis of the infrared absorption spectra in the range of 4000-1000 cm -1 at 300 K, pulsed photoluminescence (PPL) and pulsed cathodoluminescence (PCL) spectra in the spectral region of 6−1 eV and in the time interval of 10 −8 -10 −1 s after the action of exciting pulses. PPL was excited at 300 K by laser pulses with photon energy of 4.66 eV and 10 ns pulse duration. PCL was excited by single electron pulse (EP) with the following parameters: pulse width of 10 ns, the average electron energy of 250 keV. The magnitude of the electron fluence per pulse was varied in the range of 10 10 − 10 13 cm -2 .
Irradiation of the crystal was carried out by electron pulses with repetition rate of 2 . 10 -2 Hz in the temperature range of 100-300 K.
The temporal and spectral resolutions of the measurement path were equal to 20 ns and 2 nm, respectively. The transparency cutoff of undoped LiF crystal and all doped crystals LiF was found in the UV region near 12 and 6 eV, respectively. The IR spectra of the LiF crystals doped with oxides of W, Ti or Fe contained a band at 3730 cm -1 that belongs to hydroxide, and a number of narrow bands in Under exposure to laser radiation with photon energy of 4.66 eV, in all the as-grown crystals the PPL at 3.1 eV occurred 'figure 1'. It was inertialess relative to the excitation pulse.
The kinetics of the dopant PPL decay at 300 K is described as a sum of two exponential functions:

Luminescence spectra of the preliminary irradiated doped crystals
Exposure to ionizing radiation (flux of 250 keV electrons) of the crystals causes formation of intrinsic primary electron and hole color centers (CCs) F 1 and F i o , where F 1 is an electron localized at anion vacancy (V a ), F i o is a fluorine atom in interstitial state. At K T 250  addition to primary CCs complex CCs are created (F 2 , F 2 + , F 3 + …) under ionizing radiation of crystal. In PPL spectrum in addition to dopant emission bands new bands at 2.33 and 1.85 eV belonging to radiative transitions in the F 3 + and F 2 CCs, respectively, occur. The characteristic decay time of the additional bands coincides with the laser pulse duration. Figure 4 shows the PPL spectrum of the LiF-Li 2 O crystal irradiated by electrons beam at 300 K and measured with time delay of 10 ns (curve 1) and 10 µs (curve 2) with respect to the end of the laser pulse action. (In both spectra, the bands at 3.1 eV belong to short-living and longliving components of the dopant center PPL. F 3 + and F 2 emission bands positions are marked with arrows).  It is evident from figure 5 that emission intensity due to degradation of the dopant emission band decreases in the region of 2.75 eV, i.e., in the region of maximum sensitivity of phosphor to radiation. The effect increases if concentration of F 2 (F 3 + ) CCs increases with increasing absorbed dose by crystals.
To avoid a decrease in the sensitivity of the scintillator due to the reabsorption of the dopant emission by radiation defects it is necessary to decrease the temperature of operation of the scintillator (at ions is short-living and the value of the characteristic decay time at 300 K is 40 ns. The О 2 − ion emission cannot be optically excited with quantum energy of 4.66 eV in LiF-MeO crystals. Figure 6 shows, for example, the PPL and PCL spectra for irradiated LiF crystal doped with iron oxide measured in a nanosecond time interval after the end of the excitation pulse action.
Result presented in figure 6 shows that in one the same irradiated crystal spectra PPL and PCL are different. PPL spectrum consists of dopant band at 3.1 eV and the bands at 2.33.and 1.85 eV belonging to

Effect of irradiation temperature on PCL kinetic parameters
where τ is emission decay time (ns) of three types of short-living centers in excited state (   The nature of the doped crystal emission can be interpreted as follows. 1. The band in the region of 3.1 eV belongs to allowed intracenter transition on free О 2− ions being present in all the studied crystals can be described by equation (3):

Conclusions
In the process operation of phosphorus based on crystals LiF-MeO in the field of ionizing radiation a decrease of light output of the scintillators and a change in the parameters of the scintillation pulse occur with increasing absorbed dose.
Reducing the light output due to the decrease in the intensity of the long-living dopant emission is caused by the reabsorption of the dopant emission by intrinsic radiation-induced defects, which concentration increases when the absorbed dose grows. The external parameters such as the value of the absorbed dose or the temperature of the crystal by irradiation may lead to an invariance of the dopant emission. So, if K T 250  , ionizing radiation virtually does not lead to creation of complex electron CCs, and hence, it does not cause dopant emission reabsorption. In the operation of the scintillator in the field of ionizing radiation at K T 250  post-radiation dopant emission buildup of the PCL occurs. Overlapping emission decay and emission buildup results in changing the shape of the scintillation pulse in PCL and, as a result, the increase in the duration of the scintillation pulse.
The duration of the scintillation pulse can be maintained stable through the introduction of restrictions on the temperature of crystal operating in field of ionizing radiation. At K T 250  no emission buildup process can be observed. The radiation fields with photon energy less than the bandgap are considered to be most favorable for operation of scintillators based on LiF-MeO. This provides the stability of the emission pulse duration and high light output.