Temporal signal stability of KCl:Eu²⁺ storage phosphor dosimeters

Abstract

Purpose: Current KCl:Eu 2+ prototype dosimeters require a wait time of 12 h between irradiation and dosimetric readout. Although irradiating the dosimeters in the evening and reading on the following day works well in the clinical schedule, reducing the wait time to few hours is desirable. The purposes of this work are to determine the origin of the unstable charge-storage centers and to determine if these centers respond to optical or thermal excitation prior to dosimetric readout. Methods: Pellet-style KCl:Eu 2+ dosimeters were fabricated in-house for this study. A 6 MV photon beam was used to irradiate the dosimeters. After x ray irradiation, dosimeters were subjected to external excitation with near-infrared (NIR) light, ultraviolet (UV) light, or thermal treatment. Photostimulated luminescence (PSL) signal's temporal stability was subsequently measured at room temperature over a few hours using a laboratory PSL readout system. The dosimeters were also placed in a cryostat to measure the temperature dependence of the temporal stability down to 10 K. Results: Strong F-band was present in the PSL stimulation spectrum, indicating that F-centers were the electron-storage centers in KCl:Eu 2+ where an electron was stored at a chlorine anion vacancy. Due to deep energy-depth (2.2 eV), F-centers were probably not responsible for the fast fading in the first a few hours post x ray irradiation. In addition, weak NIR bands were present. However, there was no change in PSL stabilization rate with intense NIR excitation, suggesting that the NIR bands played no role in the PSL fading. At temperatures lower than 77 K there was almost no signal fading with time. Noticeable PSL was observed for undoped KCl samples at room temperature, suggesting that Cl 2 − V k centers served as hole-storage centers for both undoped and doped KCl where a hole was trapped by a chlorine molecular ion. V k centers were stable at low temperature and became mobile at room temperature, probably causing the observed PSL fading with time. On the other hand, V k center could be stabilized by Eu 2+ activator or oxygen in the lattice, leading to the stable component in the PSL. A thermal process at elevated temperatures (60• C or higher) was able to significantly accelerate the migration process resulting in a fast stabilization of PSL. However, this could not be accomplished using intense UV excitation. Conclusions: Thermal treatment enables KCl:Eu 2+ prototypes to be ready for readout in 1 h without the need of applying a large time-dependent correction factor. However, this cannot be achieved using optical preexcitation.

The role of activator concentration and precipitate formation on optical and dosimetric properties of KCl:Eu2+ storage phosphor detectors

“…2 show that KCl:Eu 2+ detectors can be available for readout within 24 h because the fading behavior is stable within several hours postirradiation. (Note: In previous work, 22 we have shown that, for 0.05% Eu samples, a 15 min thermal treatment at 70 • C in air eliminates the fast fading component and renders the detector available for readout in less than 1 h.) Further, the results of Fig. 2 have important implications for future manufacturing challenges.…”

“…After 12 h, the PSL intensity continues to decrease; however, the rate of decrease is much slower (referred to as the slow fading component). According to Xiao et al, 22 V k centers (a dihalide ion with a trapped hole) are mobile and can become stabilized by forming a complex with an activator ion. These complexes will remain stable until photon stimulation.…”

Performance of KCl:Eu2+storage phosphor dosimeters for low-dose measurements

“…Previous work has shown that a short thermal treatment can result in a fast stabilization of the PSL signal (Xiao et al, 2013). Therefore, the second readout scheme involved a pre-readout thermal annealing step.…”

Dual‐storage phosphor proton therapy dosimetry: Simultaneous quantification of dose and linear energy transfer

“…Charges remaining trapped after readout can be erased with a bright, broadband light and the material can then be reused. Readout entails optical detection of PSL and can be done quickly, repeatedly, and with temporal stability 29 in ambient laboratory conditions. 30 In addition to being nearly energy independent in proton fields, KCl:Eu 2+ dosimeters also respond linearly to proton dose, exhibit dose-rate independence, and show both high radiation hardness and dynamic range.…”

“…Charges remaining trapped after readout can be erased with a bright, broadband light and the material can then be reused. Readout entails optical detection of PSL and can be done quickly, repeatedly, and with temporal stability 29 in ambient laboratory conditions 30 …”