Characterization and Modeling of Relative Luminescence Efficiency of Optically Stimulated Luminescence Detectors Exposed to Heavy Charged Particles

This work investigates the optically stimulated luminescence (OSL) response of carbon-doped aluminum oxide Al 2 O 3 :C detectors exposed to heavy charged particles (HCPs) with energies relevant to radiation protection in space, and cancer therapy. This investigation includes ground-based experiments in accelerators and theoretical studies of the detector's response. These theoretical studies are based on the track structure model (TSM) and require information of the spatial pattern of energy deposition around the HCP path - the radial dose distribution (RDD). Thus, RDDs were obtained using six analytical models, and Monte Carlo (MC) simulations with the code GEANT4. In addition, we propose a modified analytical model to improve the agreement between calculated and experimental efficiency values.Findings and Conclusions: Dose response experiments showed that beta rays and H 1000 MeV radiations produced similar responses in the detectors and we concluded that the H 1000 MeV and beta radiations deposit energy similarly. We observed a common trend of decreasing the relative luminescence efficiency as increasing the unrestricted linear energy transfer in water (LET in water) for all the detectors. For Luxel TM detectors the relative luminescence efficiency was close to unit for particles with LET in water lower than 3 keV/micrometer. TSM using the RDD from Chatterjee and Schaefer, Butts and Katz, Waligorski et al. , Fageeha et al. , Kiefer and Straaten, and Geiss et al. models failed to predict the relative luminescence efficiency values. We proposed a modified version of the RDD from Butts and Katz model, which agreed within 20 % with relative luminescence efficiency experimental data. This was the first time that such agreement was achieved for a wide range of HCPs of different energies. MC simulations with GEANT4 agreed within 35 % with relative luminescence efficiency experimental data. Finally, we suggested a correction method, based on the calculation of the relative luminescence efficiency using the TSM presented in this dissertation, to measure more reliable absorbed doses when using luminescence detectors in complex radiation fields.Department of Physic

Enhancing the Design and Consistency of an α-particle Irradiator for In Vitro Experiments

https://openworks.mdanderson.org/sumexp23/1033/thumbnail.jp

Combination of ATR Inhibition and X-Ray Irradiation Radiosensitizes Cancer Cells

https://openworks.mdanderson.org/sumexp21/1250/thumbnail.jp

Enhancing Anti-Tumor Immunity through High-LET Radiation and Immunotherapy

https://openworks.mdanderson.org/sumexp22/1040/thumbnail.jp

Is proton radiation more effective than photon radiation at inducing senescence?

https://openworks.mdanderson.org/sumexp21/1158/thumbnail.jp

Adjustment of the lateral and longitudinal size of scanned proton beam spots using a pre-absorber to optimize penumbrae and delivery efficiency

In scanned-beam proton therapy, the beam spot properties, such as the lateral and longitudinal size and the minimum achievable range, are influenced by beam optics, scattering media and drift spaces in the treatment unit. Currently available spot scanning systems offer fewoptions for adjusting these properties. We investigated a method for adjusting the lateral and longitudinal spot size that utilizes downstream plastic pre-absorbers located near a water phantom. The spot size adjustment was characterized usingMonte Carlo simulations of a modified commercial scanned-beam treatment head. Our results revealed that the pre-absorbers can be used to reduce the lateral full width at half maximum (FWHM) of dose spots inwater by up to 14 mm, and to increase the longitudinal extent from about 1 mm to 5 mm at residual ranges of 4 cm and less. A large factor in manipulating the lateral spot sizes is the drift space between the preabsorber and the water phantom. Increasing the drift space from 0 cm to 15 cm leads to an increase in the lateral FWHM from 2.15 cm to 2.87 cm, at a waterequivalent depth of 1 cm. These findings suggest that this spot adjustment method may improve the quality of spot-scanned proton treatments. © 2010 Institute of Physics and Engineering in Medicine

SU‐E‐T‐119: A Phenomenological Model of the Al2O3:C Optically Stimulated Luminescence Detector (OSLD) Fading

Purposes: To explain the causes of fading in the Al2O3:C optically stimulated luminescence detectors (OSLDs). Methods: A phenomenological band diagram model with three electron traps and two recombination centers was used to represent the entities that are related to the OSL of Al2O3 :C OSLDs. The electron traps consist of a shallow trap, the main dosimetric trap and a deep trap. The shallow trap is unstable at room temperature (activation energy of 1.03 eV). Both recombination centers are radiative, however, the OSL signal is due only to one of the recombination centers. This model was based on experimental data of the thermoluminescence, optical absorption and OSL of Al2O3:C. A system of differential equations representing the transport and storage of charge carriers during irradiation, relaxation and stimulation phases was solved numerically. The system of equations was solved at room temperature conditions (T = 295 K). Results: The simulated shape of the OSL decay curve was in good qualitative agreement with experimental data. The simulated dose response of the OSL signal was linear in the interval of doses investigated (up to 10 Gy). The simulated fading of the OSL signal occurred in the first 10 min elapsed since irradiation and then became stable. Conclusions: The fading behavior of the OSL signal of Al2O3:C OSLDs can be explained by the existence of a shallow trap that is unstable at room temperature

SU‐E‐T‐67: The Role of Different Luminescence Centers on the Dose Response of Al2O3:C OSLDs: A Systematic Investigation Using Continuous Wave and Pulsed OSL Readouts

Purpose: To characterize the dose response of optically stimulated luminescence (OSL) from Al2O3:C detectors (OSLDs) using continuous wave (CWOSL) and pulsed (POSL) stimulations and different detection windows. Methods: Al2O3:C OSLDs were irradiated for a dose range from 0.1 to 200 Gy using a Co‐60 source. Readouts of OSLDs were performed with a custom‐made OSL readout system using CWOSL and POSLstimulations. Two separate optical filter sets (Hoya U‐340 and Kopp 5113 optical filters) were employed to filter out the 530 nm stimulation light from the OSL signal and selectively separate the blue and ultraviolet OSLemissions of Al2O3:C OSLDs. The OSL decay curves were then analyzed using a range of intervals of integration to define the OSL signal, i.e., initial intensity or total area under the OSL decay curves. The results from CWOSL and POSL with different filter sets were compared to determine the contributions of the different recombination centers to the net OSL signal of Al2O3:C OSLDs. Results: The blue emission had a larger linearity range (up to ∼2 Gy) than the ultraviolet emission (up to ∼1 Gy). At 10 Gy, the dose response of the blue emission deviated from linearity by about 13% while the ultraviolet emission deviated by about 60%. Conclusions: The dose response of Al2O3:C OSLDs depended on the detection window used to acquire the OSL signal. The ultraviolet emission had much higher supralinearity than the blue emission in the dose response of Al2O3:C OSLDs. A higher range of linearity can be achieved by selective detection of the blue emission, which can be performed using optical filters or POSL. The results of this work may assist in the development of: i) new commercial OSL readers; and ii) dosimetry protocols on the use of OSLDs in clinical settings. “Natural Sciences and Engineering Research Council of Canada and Ontario Graduate Scholarship Program, Ontario Ministry of Training, Colleges and Universities”

SU‐E‐T‐87: The Effect of Bleaching Wavelengths on the Regeneration of the Optically Stimulated Luminescence Signal of NanoDot Dosimeters Pre‐Exposed to High‐Doses

Purpose: To investigate the effect of bleaching wavelengths on the regeneration of optically stimulated luminescence (OSL) signals in Al2O3:C nanoDot dosimeters pre‐exposed to high doses. Regeneration is the increase in the OSL signal during storage of a bleached nanoDot that was previously pre‐exposed to a high dose. This phenomenon affects the accuracy of a calibration protocol proposed by Jursinic 2010 (Med. Phys. 37:102) in which pre‐exposure of nanoDots to a high‐dose was used to minimize changes in the sensitivity of the detector as a function of accumulated dose. Methods: Al2O3:C OSLDs of the type nanoDot were used throughout this study. Readout was performed using the microStar reader. Bleaching of the OSLDs was performed with four 26 W fluorescent light bulbs in two modes: (i) directly under the lamps; and (ii) with the aid of a long‐pass optical filter placed over the nanoDots, partially blocking wavelengths below 495 nm. Eighteen nanoDots were pre‐exposed to 1 kGy dose. Then the pre‐exposed nanoDots were bleached in two sets of 9 to very low residual OSL signals using bleaching modes (i) and (ii) for 12 h and 45 h, respectively. The nanoDots were then stored in dark and readout after various time intervals to monitor the regeneration of the OSL signal. Results: We fitted the regeneration of the OSL signal using a saturation function and obtained rise‐time values of 563 h and 630 h, for bleaching modes (i) and (ii), respectively. At the saturation level, the equivalent doses were about 1.18 Gy and 0.38 Gy for modes (i) and (ii), respectively. Conclusions: The regeneration rates of nanoDot OSLDs pre‐exposed to high doses depend on the bleaching light wavelength used to reset the detectors. A bleaching source that has a low component of wavelengths below 495 nm can minimize the regeneration of the OSL signal. Natural Sciences and Engineering Research Council of Canada

SU‐E‐T‐106: Experimental Characterization of Al2O3:C Optically Stimulated Luminescence Detector (OSLD) Exposed to 6 MV X‐Ray Beams

Purposes: To characterize the response of Al2O3:C optically stimulated luminescence detectors (OSLDs) to 6 MV x‐ray beams and to determine the optimal bleaching method that would allow the re‐use of the OSLDs. Methods: OSLDs were exposed to a 6 MV x‐ray linac beam in the dose range from 50 mGy–10 Gy. Readouts were performed with a commercial OSLD reader (microStarTM, Landauer Inc.). For 50 mGy and 100 mGy doses, OSLDs were readout using the reader's low‐dose mode, which provides higher stimulation power compared to the high‐dose mode. All other readouts were performed using the high‐dose mode. The OSLD response to dose, bleaching time and repeated readouts (depletion) were determined. Bleaching of the OSL signal was performed using a 250 W halogen lamp with two Methods: (a) direct exposure to light; and (b) exposure using a long‐pass optical filter to block wavelengths shorter than 495 nm. Results: The OSLD dose‐response was linear for the investigated dose range. After 100 readouts, the OSL signal was depleted by (24.5 ± 0.7) % and (3.16 ± 0.07) % with a depletion rate of (0.251 ± 0.002) and (0.023 ± 0.002) %/readout for low‐ and high‐dose modes, respectively. After a 5 min bleaching time, (85.1 ± 1.4) and (70.5 ± 2.0) % reductions in the OSL signal for all doses was attained using methods (a) and (b), respectively. After a 100 min bleaching time, (99.5 ± 0.2) % reduction was attained. Conclusions: We observed linearity of the OSLD's dose response for the investigated dose range. A 100 min bleaching time was sufficient to bleach 99.5 % of the OSL signal for both bleaching methods. The depletion rate using the low‐dose mode is 11 times higher than using the high‐dose mode. for the high‐dose mode of the reader, the depletion rate is independent on dose