Temperature dependence of the photostimulated luminescence in KCl:Eu2+

The goal of this work is to understand the physical mechanism behind the signal stabilization process in KCl:Eu2+, a storage phosphor material that has generated renewed interest due to its potential in radiation therapy dosimetry application. The temperature dependency of the photostimulated luminescence (PSL) spectra and intensity vs. time post x ray irradiation was measured. Commercial BaFBr:Eu2+ materials were included in this study for comparison. Unlike BaFBr:Eu2+, broadening of the F(Cl-) stimulation band and red-shift of the peak were observed for KCl:Eu2+ with increasing temperature. For irradiations at temperatures lower than 200 K, PSL intensity of KCl:Eu2+ showed recuperation behavior in the first 2 hrs post-irradiation and stayed almost constant with time thereafter. Moreover, spatially-correlated storage centers increased from 24% for irradiation at 50 K to 31% at 195 K and almost 100% at room temperature. The data suggest that certain types of charge storage-centers were mobile and contribute to the fast fading in PSL.

X ray storage performance of KCl:Eu2+ with high cumulated dose.

The effects of high cumulative radiation dose on the luminescence properties of KCl:Eu2+ are investigated. Pellet samples of KCl:Eu2+ were given doses of up to 200 kGy at the Louisiana State University Synchrotron facility. After synchrotron irradiation, samples were optically bleached and given a clinical dose of 2 Gy from a 6 MV medical linear accelerator. Optical properties were evaluated using photostimulated luminescence (PSL), photoluminescence (PL), and temperature-dependent PSL measurements. For a cumulated dose of up to 5-10 kGy, the PSL emission intensity increased by 15% compared to the PSL signal with no radiation history. For doses higher than 10 kGy, the PSL emission intensity retained at least 70% of the original intensity. Spatial correlation of the charge storage centers increased for doses up to 5 kGy and then decreased for higher cumulative doses. Emission band at 975 nm was attributed to transitions of Eu1+. PL spectra showed an intense peak centered at 420 nm for all cumulative doses. The results of this work show that KCl:Eu2+ storage phosphors are excellent reusable materials for radiation therapy dosimetry.

Determining molecule-carbon surface adsorption energies using molecular mechanics and graphene nanostructures.

Five model surfaces were developed using molecular mechanics with MM2 parameters. A smooth, flat model surface was constructed of three parallel graphene layers where each graphene layer contained 127 interconnected benzene rings. Four rough surfaces were constructed by varying the separation between a pair of graphene nanostructures placed on the topmost layer of graphene. Each nanostructure contained 17 benzene rings arranged in a linear strip. The parallel nanostructures were moved closer together to increase the surface roughness and to enhance the molecule-surface interaction. Experimental adsorption energy values from the temperature variation of second gas-solid virial coefficients values were available for 16 different alkanes, haloalkanes, and ether molecules adsorbed on Carbopack B (Supelco, 100 m(2)/g). For each of the five different surface models, sets of 16 calculated adsorption energies, E(cal)( *), were determined and compared to the available experimental adsorption energies, E( *). The best linear regression correlation between E( *) and E(cal)( *) was found for a 1.20 nm internuclei separation of the surface nanostructures, and for this surface model the calculated gas-solid interaction energies closely matched the experimental values (E( *)=1.018E(cal)( *), r(2)=0.964).

Temporal signal stability of KCl:Eu(2+) storage phosphor dosimeters.

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:Eu(2+) storage phosphor detectors.

PURPOSE: The activator ion (Eu(2+) in KCl:Eu(2+)) plays an important role in the photostimulated luminescence (PSL) mechanism of storage phosphor radiation detectors. In order to design an accurate, effective, and robust detector, it is important to understand how the activator ion concentration affects the structure and, consequently, radiation detection properties of KCl:Eu(2+). METHODS: Potassium chloride pellets were fabricated with various amounts of europium dopant (0.01-5.0 mol.% Eu(2+)). Clinical radiation doses were given with a 6 MV linear accelerator. Radiation doses larger than 100 Gy were given with a (137)Cs irradiator. Dose response curves, radiation hardness, and temporal signal stability were measured using a laboratory PSL readout system. The crystal structure of the material was studied using x ray diffraction and luminescence spectroscopy. RESULTS: The most intense PSL signal was from samples with 1.0 mol.% Eu. However, samples with concentrations higher than 0.05 mol.% Eu exhibited significant degradation in PSL intensity for cumulated doses larger than 3000 Gy. Structural and luminescence spectroscopy showed clear evidence of precipitate phases within the KCl lattice, especially for high activator concentrations. Analysis of PL emission spectra showed that interactions between Eu-Vc dipoles and Eu-Vc trimers could explain trends in PSL sensitivity and radiation hardness observations. CONCLUSIONS: The concentration of the activator ion (Eu(2+)) significantly affects radiation detection properties of the storage phosphor KCl:Eu(2+). An activator concentration between 0.01 and 0.05 mol.% Eu in KCl:Eu(2+) storage phosphor detectors is recommended for linear dose response, good PSL sensitivity, predictable temporal stability, and high reusability for megavoltage radiation detection.

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

Recent research has demonstrated that europium doped potassium chloride (KCl:Eu(2+)) storage phosphor material has the potential to become the physical foundation of a novel and reusable dosimetry system using either film-like devices or devices similar to thermoluminescent dosimeter chips. The purposes of this work are to quantify the performance of KCl:Eu(2+) prototype dosimeters for low-dose measurements and to demonstrate how it can be incorporated into clinical application for in vivo peripheral dose measurements. Pellet-style KCl:Eu(2+) dosimeters, 6 mm in diameter, and 1 mm thick, were fabricated in-house for this study. The dosimeters were read using a laboratory photostimulated luminescence detection system. KCl:Eu(2+) prototype storage phosphor dosimeter was capable of measuring a dose-to-water as low as 0.01 cGy from a 6 MV photon beam with a signal-to-noise ratio greater than 6. A pre-readout thermal annealing procedure enabled the dosimeter to be read within an hour post-irradiation. After receiving large accumulated doses (~10 kGy), the dosimeters retained linear response in the low-dose region with only a 20% loss of sensitivity comparing to a fresh sample (zero Gy history). The energy dependence encountered during low-dose peripheral measurements could be accounted for via a single point outside-field calibration per each beam quality. With further development the KCl:Eu(2+-)-based dosimeter could become a versatile and durable dosimetry tool with large dynamic range (sub-cGy to 100 Gy).