The role of oxygen in the photostimulation luminescence process of europium doped potassium chloride.

A recent suggestion that europium doped potassium chloride (KCl:Eu2+) has the potential to significantly advance the state-of-the-art in radiation therapy dosimetry has generated a renewed interest in a classic storage phosphor material. The purposes of this work are to investigate the role of oxygen in the photostimulation luminescence (PSL) process and to determine if both increased PSL yield and improved temporal stability could be realized in KCl:Eu2+ by incorporating oxygen in the material fabrication process. Regardless of synthesis atmosphere, air or pure nitrogen, PSL amplitude shows a maximum at 1.0 mol % Eu. Depending on europium concentration, dosimeters fabricated in air exhibit stronger PSL by a factor of 2 to 4 compared to those made in N2. There is no change in PSL stimulation spectrum while noticeable shifts in both photoluminescence and PSL emission spectra are observed for air versus nitrogen. Almost all charge-storage centers are spatially correlated, suggesting oxygen's stabilization role in the PSL process. However, oxygen alone does not improve material's temporal stability in the first few hours post irradiation at room temperature, probably because a significant portion of radiation-induced holes are stored in the Vk centers which are mobile.

Two-dimensional high spatial-resolution dosimeter using europium doped potassium chloride: a feasibility study.

Recent research has shown that KCl:Eu²âº has great potential for use in megavoltage radiation therapy dosimetry because this material exhibits excellent storage performance and is reusable due to strong radiation hardness. This work reports the authors' attempts to fabricate 2D KCl:Eu²âº storage phosphor films (SPFs) using both a physical vapor deposition (PVD) method and a tape casting method. X-ray diffraction analysis showed that a 10 µm thick PVD sample was composed of highly crystalline KCl. No additional phases were observed, suggesting that the europium activator had been completely incorporated into the KCl matrix. Photostimulated luminescence and photoluminescence spectra suggested that F (Cl(-)) centers were the electron storage centers post x-ray irradiation and that Eu²âº cations acted as luminescence centers in the photostimulation process. The 150 µm thick casted KCl:Eu²âº SPF showed sub-millimeter spatial-resolution. Monte Carlo simulations further demonstrated that the admixture of 20% KCl:Eu²âº and 80% low Z polymer binder exhibited almost no energy-dependence in a 6 MV beam. KCl:Eu²âº pellet samples showed a large dynamic range from 0.01 cGy to 60 Gy dose-to-water, and saturated at approximately 500 Gy as a result of KCl's intrinsic high radiation hardness. Taken together, this work provides strong evidence that KCl:Eu²âº-based SPF with associated readout apparatus could result in a novel electronic film system that has all the desirable features associated with classic radiographic film and, importantly, water equivalence and the capability of permanent identification of each detector.

Radiation hardness of the storage phosphor europium doped potassium chloride for radiation therapy dosimetry.

PURPOSE: An important property of a reusable dosimeter is its radiation hardness, that is, its ability to retain its dosimetric merits after irradiation. The radiation hardness of europium doped potassium chloride (KC1:Eu2+), a storage phosphor material recently proposed for radiation therapy dosimetry, is examined in this study. METHODS: Pellet-style KCl:Eu2+ dosimeters, 6 mm in diameter, and 1 mm thick, were fabricated in-house for this study. The pellets were exposed by a 6 MV photon beam or in a high dose rate 137Cs irradiator. Macroscopic properties, such as radiation sensitivity, dose response linearity, and signal stability, were studied with a laboratory photostimulated luminescence (PSL) readout system. Since phosphor performance is related to the state of the storage centers and the activator, Eu2+, in the host lattice, spectroscopic and temporal measurements were carried out in order to explore radiation-induced changes at the microscopic level. RESULTS: KCl:Eu2+ dosimeters retained approximately 90% of their initial signal strength after a 5000 Gy dose history. Dose response was initially supralinear over the dose range of 100-700 cGy but became linear after 60 Gy. Linearity did not change significantly in the 0-5000 Gy dose history spanned in this study. Annealing high dose history chips resulted in a return of supralinearity and a recovery of sensitivity. There were no significant changes in the PSL stimulation spectra, PSL emission spectra, photoluminescence spectra, or luminescence lifetime, indicating that the PSL signal process remains intact after irradiation but at a reduced efficiency due to reparable radiation-induced perturbations in the crystal lattice. CONCLUSIONS: Systematic studies of KCl:Eu2+ material are important for understanding how the material can be optimized for radiation therapy dosimetry purposes. The data presented here indicate that KCl:Eu2+ exhibits strong radiation hardness and lends support for further investigations of this novel material.

Theoretical and empirical investigations of KCl : Eu2+ for nearly water-equivalent radiotherapy dosimetry.

PURPOSE: The low effective atomic number, reusability, and other computed radiography-related advantages make europium doped potassium chloride (KCl : Eu2+) a promising dosimetry material. The purpose of this study is to model KCl : Eu2+ point dosimeters with a Monte Carlo (MC) method and, using this model, to investigate the dose responses of two-dimensional (2D) KCl : Eu2+ storage phosphor films (SPFs). METHODS: KCl : Eu2+ point dosimeters were irradiated using a 6 MV beam at four depths (5-20 cm) for each of five square field sizes (5 x 5-25 x 25 cm2). The dose measured by KCl : Eu2+ was compared to that measured by an ionization chamber to obtain the magnitude of energy dependent dose measurement artifact. The measurements were simulated using DOSXYZnrc with phase space files generated by BEAMnrcMP. Simulations were also performed for KCl : Eu2+ films with thicknesses ranging from 1 microm to 1 mm. The work function of the prototype KCl : Eu2+ material was determined by comparing the sensitivity of a 150 microm thick KCl : Eu2+ film to a commercial BaFBr0.85 I0.15 : Eu(2+)-based SPF with a known work function. The work function was then used to estimate the sensitivity of a 1 microm thick KCl : Eu2+ film. RESULTS: The simulated dose responses of prototype KCl : Eu2+ point dosimeters agree well with measurement data acquired by irradiating the dosimeters in the 6 MV beam with varying field size and depth. Furthermore, simulations with films demonstrate that an ultrathin KCl : Eu2+ film with thickness of the order of 1 microm would have nearly water-equivalent dose response. The simulation results can be understood using classic cavity theories. Finally, preliminary experiments and theoretical calculations show that ultrathin KCl : Eu2+ film could provide excellent signal in a 1 cGy dose-to-water irradiation. CONCLUSIONS: In conclusion, the authors demonstrate that KCl : Eu(2+)-based dosimeters can be accurately modeled by a MC method and that 2D KCl : Eu2+ films of the order of 1 microm thick would have minimal energy dependence. The data support the future research and development of a KCl : Eu2+ storage phosphor-based system for quantitative, high-resolution multidimensional radiation therapy dosimetry.

Quantitative megavoltage radiation therapy dosimetry using the storage phosphor KCl: Eu2+.

This work, for the first time, reports the use of europium doped potassium chloride (KCl:Eu2+) storage phosphor for quantitative megavoltage radiation therapy dosimetry. In principle, KCl:Eu2+ functions using the same photostimulatated luminescence (PSL) mechanism as commercially available BaFBr0.85I0.15:Eu2+ material that is used for computed radiography (CR) but features a significantly smaller effective atomic number--18 versus 49--making it a potentially useful material for nearly tissue-equivalent radiation dosimetry. Cylindrical KCl:Eu2+ dosimeters, 7 mm in diameter and 1 mm thick, were fabricated in-house. Dosimetric properties, including radiation hardness, response linearity, signal fading, dose rate sensitivity, and energy dependence, were studied with a laboratory optical reader after irradiation by a linear accelerator. The overall experimental uncertainty was estimated to be within +/-2.5%. The findings were (1) KCl:Eu2+ showed satisfactory radiation hardness. There was no significant change in the stimulation spectra after irradiation up to 200 Gy when compared to a fresh dosimeter, indicating that this material could be reused at least 100 times if 2 Gy per use was assumed, e.g., for patient-specific IMRT QA. (2) KCl:Eu2+ exhibited supralinear response to dose after irradiation from 0 to 800 cGy. (3) After x ray irradiation, the PSL signal faded with time and eventually reached a fading rate of about 0.1 % /h after 12 h. (4) The sensitivity of the dosimeter was independent of the dose rate ranging from 15 to 1000 cGy/min. (5) The sensitivity showed no beam energy dependence for either open x ray or megavoltage electron fields. (6) Over-response to low-energy scattered photons was comparable to radiographic film, e.g., Kodak EDR2 film. By sandwiching dosimeters between low-energy photon filters (0.3 mm thick lead foils) during irradiation, the over-response was reduced. The authors have demonstrated that KCl:Eu2+ dosimeters have many desirable dosimetric characteristics that make the material conducive to radiation therapy dosimetry. In the future, a large-area KCl:Eu2+-based CR plate with a thickness of the order of a few microns, created using modern thin film techniques, could provide a reusable, quantitative, high-resolution two-dimensional dosimeter with minimal energy dependence.