Ionizing-radiation-induced storage-luminescence for dosimetric applications

Abstract

Storage phosphors have been used for luminescent-type dosimeters to measure radiation dose. Common storage-luminescence phenomena are optically-stimulated luminescence (OSL), thermally-stimulated luminescence (TSL) and radio-photoluminescence (RPL). In this paper, we review the basic mechanisms of OSL, TSL and RPL, and storage phosphor materials for practical dosimetry applications are introduced. Following these reviews, recent R&D of OSL, TSL and RPL materials is summarized.

Introduction

Storage phosphors have a function to record information of radiation dose and distribution, and they have been used for radiation detectors. Radiation detectors which record accumulated radiation dose over a certain period of time are often called dosimeters, which consist of a storage phosphor as a main sensing element. When the energy of ionizing radiations is absorbed by the storage phosphor, a large number of carriers are generated in the matrix, where the number of carriers is proportional to the absorbed radiation energy. Then, these carriers are captured and stably stored at localized trapping centers. These trapped carriers can be stimulated to be freed and then recombine at luminescence center to emit light (from UV to NIR). The stimulation can be in a form of light (often visible or near-infrared) and heat, and the resultant luminescence is referred as optically-stimulated luminescence (OSL) [1] and thermally-stimulated luminescence (TSL) [2]. In some special cases, localized trapping centers with trapped electrons or holes act as a luminescence center, and such phenomenon of generating a new luminescence center is called radio-photoluminescence (RPL) [3]. In many cases, a change of valence state (e.g., 2Ag⁺ → Ag° + Ag²⁺) of impurity ion results in RPL, and in other cases, defect-type luminescence center (e.g., F-center) is generated as RPL. The emission intensity of OSL, TSL and RPL are proportional to the incident radiation dose, so we can record and indirectly measure the radiation dose based on the emission intensity. Fig. 1 represents the emission mechanisms of dosimeter materials.

Storage phosphors for dosimeters have a close relationship with some other phosphors. One common example is persistent luminescence, and we understand the persistent luminescence as TSL stimulated at room temperature. The most common persistent luminescent phosphor is Eu and Dy co-doped SrAl₂O₄ [4], and it shows TSL glow peak around room temperature when it is exposed to UV light or ionizing radiation. When we irradiate phosphors with ionizing radiation, they show a prompt luminescence and such emission is called scintillation. In scintillators, as for storage phosphors, many carriers are generated by ionizing radiation, and most part of these carriers are transported to localized luminescence centers. From the viewpoint of luminescent materials used in radiation detectors, the difference of storage luminescence and scintillation can be understood as a difference of branching ratio of carrier transportation to trapping and luminescent centers. Recently, an inversely-proportional relationship of storage-luminescence (OSL and TSL) and scintillation was observed in some common phosphors [5], [6]. In scintillators, the light yield (LY_sc) can be explained by the Robbins model [7], and it is expressed as LY_sc ∝ S × Q where S is the energy transportation efficiency from the host to luminescence center (or efficiency of carriers directly transported to luminescence center) and Q equals to PL quantum yield (QY). Although the quantitative evaluation has not been established yet, the luminescence yield of storage-luminescence (LY_st) can analogically be expressed as LY_st ∝ S’ × P × Q where S’ means the energy transportation efficiency to trapping centers, and P is de-trapping rate from the trap levels to conduction band. The detailed discussion based on TSL can be found elsewhere [8], and at least we can conclude that PL QY is essentially important for radiation induced luminescent materials.

Among these storage luminescence phenomena, TSL is sometimes used to evaluate some trapping parameters of secondary electrons [9]. The most common method is the initial rise method which can determine the trap depth (energy) and the frequency factor [10]. In this method, the assumption is that the rate of change of trapped carrier population is negligible during the initial rise part of a TL curve (early rising range of temperatures). In addition to the initial rise method, some analysis models have been proposed, and some people use the peak shape method [11], [12], [13] of which differences are some physical assumptions (approximation formula). Following these pioneering works, some modifications and progresses have been done to construct new analytical models (e.g., [14], [15], [16], [17], [18]). By using these analytical ways including the initial rise method, we can easily determine the trap parameters only by fitting over the glow curve, and the advantage of these methods is that only one experiment is required. On the other hand, some people use the various heating rate method to determine the trap parameters. In this method, measurements of the glow curves are done at several times with different heating rate (q K/s), and then, the glow peak temperature (T_m) in each measurement changes. When we plot ln(T_m²/q) and 1/T_m, we can determine the activation energy and the frequency factor [19]. Although this method requires experiments on several times, some groups actually determine the trap parameters by using this method [20], [21], [22], [23], [24], [25], [26]. To our knowledge, most groups use one of these three methods (initial rise, peak shape, and heating rate), and some other methods were proposed previously (e.g., Balarin-Zetzsche method [19], [27], [28]). Up to now, many analysis methods have been proposed to determine the trap parameters, and we cannot comment which method is the best. For the physical interpretation, if the frequency factor is enough large, we can consider the de-trapping process will be due to a typical thermal activation, and in the opposite case, we can interpret the de-trapping will be ascribed to the tunneling effect or something. In the viewpoint of the activation energy, the empirical model to determine the energy gap of energy levels of dopant and conduction band is proposed by Dorenbos [29], [30], and the predicted trap depth (emission wavelength in some cases) shows a good agreement with TSL-based analysis (e.g., [31], [32], [33], [34], [35], [36], [37], [38]). Although the model is useful to predict the glow peak temperature or emission wavelength, it should be noted that the model does not predict the emission intensity.

Recently, we have been interested in a relationship between scintillation and radiation-induced storage luminescence. As mentioned above, the difference of scintillation and storage luminescence can be explained by the difference of branching ratio of radiation-generated charges. In scintillator materials, the majority of the charges is transferred to luminescence center while storage phosphors capture most charges at trapping center. When we consider the energy conservation, scintillation and storage luminescence are complementarily related. In fact, we have experimentally confirmed such relationship in some materials including ceramics, crystals and glasses [5], [6]. Now, we have been strategically developing new luminescent materials for radiation detectors based on such relationship. Although this relationship is not a deductive phenomenon but inductive, it is very useful to consider new materials. Simply speaking, materials with low scintillation intensity can be considered as effective dosimeter materials.

Fig. 2 illustrates a typical dosimeter. Although the design of dosimeters is different by each dosimeter company (rectangular, circle, ring, etc.), typically several pieces of phosphor plates are included. In the case of dosimeters for high energy photons such as X- and γ-rays, these luminescent materials are coupled with some metal filters such as Al, Cu, etc. The role of these filters is to correct the energy dependence of detector since the absorption probability of each metal depends on the density, thickness and the effective atomic number (Z_eff). By using such a combination of luminescent materials and filters, dosimeters have a specific energy response for target radiations.

The properties of luminescent-type dosimeters to be considered include high sensitivity and adequate Z_eff against target ionizing radiation, adequate energy response for the target energy, low fading, chemical stability and production cost. The higher sensitivity, the stronger emission intensity. When the radiation dose is small, high sensitivity is required, and it is especially the case for personal dose monitoring applications. In the case of protection dosimetry, the Z_eff is required to be close to that of human soft tissue (Z_eff ~ 7 [39]) for accurate correction of energy dependence, so storage phosphors of such applications is made of light chemical elements. On the other hand, heavy materials are preferred for imaging plates which are commonly used in dentistry. All the dosimeters have upper and lower detection limits of sensitivity, and we must select dosimeters with adequate dynamic range. The personal dose monitoring applications require the lower detection limit on the order of μGy, and some applications such as radiation therapy requires dosimeters having upper detection limit of 1000 Gy. Fading is defined as a decrease of emission intensity as a function of storage time. As mentioned above, carriers are stored at trapping centers for a certain period of time. Some of these trapped carriers may gradually escape from the trapping centers by unintentional stimulations such as room light, room temperature, mechanical stress etc. A low fading property (in other words high memory retention ability) is an important character for accurate dose measurement. Chemical stability and low production cost are also important characteristics as other phosphor applications.