INTRODUCTION

Today, there are several dozen different types of inorganic scintillation crystals that can be successfully used in a number of nuclear physics experiments. It should be noted that ions with the nS2 configuration of the electron shell (Tl+, Sn2+, Bi3+, Ce3+, etc.) act as luminescence centers in inorganic scintillation crystals. Many scintillation characteristics of the crystal depend on the type of activator and its amount in the crystal [1].

The development of technologies for the manufacture of laser materials has led to the synthesis of a number of crystals with good scintillation properties, which may well be used as a detector of ionizing radiation in many branches of nuclear physics [1–3].

Some of these crystals are crystals based on complex oxides of group 3 elements; they have high mechanical strength, thermal shock resistance, and corrosive resistance. Scintillation crystals Y3Al5O12:Ce and YAlO3:Ce are well known and are actively used in nuclear physics experiments (scintillation properties of YAlO3:Ce and NaI(Tl) are given in Table 1 for comparison) [3–6]. Other types of crystals of complex oxides of group 3 elements have not been studied.

Table 1.   Scintillation properties of crystals YAlO3:Ce and NaI(Tl)
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We studied scintillation properties of crystals based on complex oxides of group 3 elements such as Y3Al5O12:Ce, Y3Al5O12:Sc, Y3Al5O12:Re, Y3Al5O12:Sc:La, YAlO3:Ce, YAlO3:Ce:Yb, YAlO3:Ce:Sc, YAlO3:Eu, YAlO3: Re, YAlO3:Ti, and YAlO3:Ce:Ti. These single crystals were grown by the Czochralski method.

EXPERIMENTAL

To study the relative light yield of scintillators based on complex oxides of group 3 elements, depending on the type and content of the activator, we used a standard spectrometric route consisting of an FEU-143 photomultiplier and analyzing equipment [1]. The results of investigations are shown in Fig. 1. The light yield of crystals increases with an increase in the concentration of the activator in the initial charge.

Fig. 1.
figure 1

Dependence of the light yield of crystals YAlO3:Ce (1), Y3Al5O12:Re (2), and Y3Al5O12:Ce (3) (relative to NaI(Tl) crystal) on the activator concentration.

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In our experiments, we measured the value of the α/β ratio for different scintillation crystals. The dependence of the α/β ratio on the α-particle energy for a number of scintillators is shown in Fig. 2. For scintillation crystals based on complex oxides of group 3 elements, the α/β ratio is 0.20 ± 0.09, which indicates a strong dependence of the light yield on the ionization density in the crystal. The strong dependence of the light yield on the ionization density in the crystal leads to the effect of detector nonlinearity [1, 2].

Fig. 2.
figure 2

Dependences of the α/β ratio on the α-particle energy for a number of scintillators (for CsI(Tl) crystals with different concentrations of an activator).

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We have experimentally established the value of the α/β ratio increases with an increase in the concentration of the activator in scintillation crystals based on complex oxides of group 3 elements.

The temporal characteristics of the scintillators are studied by the single-photon method and using oscillographic testing of the scintillation signal.

Quite a lot of works are devoted to the study of temporal characteristics [1–5]. Such crystal parameters as the rise time and luminescence time of the scintillation pulse, as well as the presence of background, mainly determine the temporal characteristics of scintillation detectors. These studies are important both for the development of the detecting part and for understanding the scintillation process [1–4].

The facility shown in Fig. 3 was used to study the temporal characteristics of scintillators by the single-photon method. The scintillator crystal under study was located in an opaque casing 1 between two FEU-143 photomultipliers operating in the single-photon mode. The scintillation in the crystal was caused by radioactive sources (γ-source or α-source depending on the experiments).

Fig. 3.
figure 3

Functional diagram of the facility for studying the waveform of a scintillation pulse in crystals. (1) Opaque casing, (2) test sample, (3) collimated radiation source, (4) diaphragm, and (5) filter wheel; ВС-22 is a power supply; F is a pulse former; CC is a coincidence circuit; PC is a pulse counter; Т → А is a time amplitude converter; AA is an amplitude analyzer.

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The photons from the scintillation hit the START photomultiplier, and there was a measuring signal of the time interval between the start of scintillation and the photon that passed through the diaphragm and was recorded by the STOP photomultiplier. The diaphragm was adjusted in such a way that only one photon from scintillation passed through its hole. Thus, we measured the temporal characteristics of the scintillators. The temporal resolution of the facility measured using Cherenkov radiation was 1 ns, which corresponded to a rise time of 0.5 ns. Measurement of the pulse waveform of standard scintillators (stilbene, anthracene, and tolan) is in good agreement with the published data. The results of the studies are shown in Table 2.

Table 2.   Scintillation characteristics of crystals based on complex oxides of group 3 elements
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The studies showed that the rise time of the scintillation pulse upon alpha excitation and beta excitation was practically independent of the activator concentration and was 4.5 ± 1 ns for crystals Y3Al5O12:Ce, Y3Al5O12:Sc, Y3Al5O12:Re, and Y3Al5O12:Sc:La and 2.7  ± 1 ns for crystals YAlO3:Ce, YAlO3:Ce:Yb, YAlO3:Ce:Sc, YAlO3:Eu, YAlO3:Re, YAlO3:Ti, YAlO3:Ce:Ti.

On the basis of the model from [1, 7], the scintillation emission in this type of crystals is due to radiative recombination of nonequilibrium charge carriers at activator centers [1, 3–6]. The most probable way of the appearance of activator luminescence in crystals of this type, for example, Y3Al5O12:Ce and YAlO3:Ce, is as follows. Holes generated by ionizing radiation in a time less than or on the order of the rise time of the light flash, i.e., ≤1 ns, localize on the activator ions Ce3+ with the formation of ions Ce4+ and then radiatively recombine with electrons. In this case, the time to localization of an electron near the ion Ce4+ does not exceed a few nanoseconds. It is most likely that the capture of holes and electrons by the activator ion occurs without intermediate localization of charge carriers at any other capture centers. The process of the activator luminescence in these crystals can be schematically depicted as follows:

$$h\nu + {\text{C}}{{{\text{e}}}^{{3 + }}} \to {\text{C}}{{{\text{e}}}^{{4 + }}},$$
$${\text{C}}{{{\text{e}}}^{{4 + }}} + \bar {e} \to ({\text{C}}{{{\text{e}}}^{{3 + }}}){\kern 1pt} {\text{*}} \to {\text{C}}{{{\text{e}}}^{{3 + }}} + h\nu .$$

The characteristic lifetime of the luminescence center in such crystals is several tens of nanoseconds.

It should be noted that a distinctive feature of scintillation crystals based on complex oxides of group 3 elements is a relatively weak dependence of the energy conversion efficiency on temperature [1, 3–6].

Scintillation crystals YAlO3:Ce can be used to develop α spectrometers with good energy resolution. The high strength of YAlO3:Ce crystals makes it possible to produce thin plates with a thickness of 100 μm and up to 30 mm in diameter for registration of α particles. A spectrometer based on a thin scintillator allows one to obtain an energy resolution of 2.5% for α particles with energy of 7.7 MeV (226Ra).

CONCLUSIONS

Our studies have shown the most promising material for use as a scintillator is a YAlO3:Ce crystal. This crystal differs from other crystals of this type in a sufficiently high light yield and good temporal characteristics. In order to reduce luminescence time of the crystal, we used other types of activator. However, the energy conversion efficiency decreased (Table 2).

It has been shown that YAlO3:Ce scintillators can be used to develop α spectrometers with good energy resolution.

The combination of properties of scintillation crystals based on complex oxides of group 3 elements makes them promising for use in hot plasma pulsed X-ray spectrometers [2].

It should be noted that our studies of crystals of this type allowed us to conclude the crystals can be used as thermoluminescent detectors and track detectors, which is very important for developing a combined detector using hot plasma in X-ray diagnostics.