Characterization of optically stimulated luminescent dosimeters, OSLDs, for clinical dosimetric measurements
A schematic diagram of the energy levels of a crystalline material that sustains thermoluminescence or optical luminescence. The numbers in the diagram represent the following: 1 is the absorption of radiation and subsequent charge separation; 2 is the migration and trapping of an electron, , after charge separation; 3 is the migration and trapping of an hole, 엯, after charge separation; 4 is a moderately deep electron trap; 5 is a moderately deep hole trap; 6 is a very deep electron trap; 7 is a shallow electron trap; 8 is the ejection of an electron out of a trap by absorption of heat, thermoluminescence, or light, optical luminescence; 9 is the migration of the untrapped electron; 10 is the recombination of an electron and hole at a hole trap; 11 is the emission of light at a luminescence center, ∗∗∗, which received energy from the recombination of the electron and hole.
The depletion of an OSLD optical signal when given sequential readings. The OSLD was exposed to 100 cGy of 6 MV x rays and then kept in the dark for 10 min. After the dark period, the OSLD was read 25 times. The OSLD signals were normalized to value of the first reading. The solid line is a calculation based on Eq. (1) with .
Repeated measurements of an OSLD vs the time after irradiation. These repeated measurements were corrected for the signal depletion by measurement, using Eq. (1) with . OSLDs irradiated with 100 cGy were normalized to the first signal measured. The solid lines are calculations based on Eq. (3) with parameters shown in Table III.
Measurements of an OSLD signal after various times of optical illumination. The OSLDs were irradiated with 100 cGy of 6 MV x rays and were kept in the dark for 30 min before reading. The OSLDs were then illuminated for the indicated times with a 150 W tungsten-halogen lamp, bright room light that was typical of an office environment, and dim room light that was typical of a linear accelerator environment.
Sensitivity of OSLDs vs accumulated dose. Two different OSLDs were exposed to different amounts of accumulated dose. The sensitivity of the OSLD was then tested by bleaching, irradiating with 100 cGy of 6 MV x rays, and then reading the OSLD signal after an 8 min wait. The error bars indicate the measurement uncertainty of observed for these OSLD measurements.
OSLD response vs absorbed dose. Each dose point is a different OSLD randomly chosen from a batch of OSLDs. The points are measured values with error bars indicating the measurement uncertainty of observed for these OSLD measurements. The broken line corresponds to values calculated with Eq. (4). The solid line shows linear dependence on dose based on the response of OSLD data up to 100 cGy.
The radiation sensitivity of an OSLD, a TLD, and a surface diode as a function of the angle of incidence of radiation. The OSLD and TLD were zeroed, irradiated, and read for each incident angle. All signals were normalized to the sensitivity measured at , which was the incident angle of the central axis of the beam when it was perpendicular to the front surface of the detector. The error bars are one standard deviation of measurement error for the OSLD.
The depth dose of a 15 MV x-ray beam measured with a parallel-plate ion chamber, an OSLD in its light-tight case, and a surface diode. The source-to-surface distance for these measurements was kept constant at 100 cm. The depth was adjusted by adding pieces of solid water and a 5 mm piece of Superflab directly over the OSLD and the surface diode. All data are normalized to the maximum values at a depth of 3 cm. The broken line is a linear fit to the parallel-plate ion chamber data for depths 0, 1, 2, and 3 mm. The error bars indicate the measurement error of observed for these OSLD measurements.
OSLD and diode sensitivity for various modes and energies of radiation used in the clinic. The detector was positioned at the depth of maximum dose as described in Table I for the linear accelerator irradiation. The Ir-192 source was positioned 7.1 cm away from the detectors. The delivered dose was 100 cGy for all modes and energies. The detector sensitivities were normalized to their response to 6 MV x rays. The OSLD was bleached between each irradiation. The error bars correspond to one standard deviation of measurement uncertainty.
Dose response of an OSLD and the surface diode as a function of dose-per-pulse of a 6 MV x-ray beam. Exposures were made with a field measured at 100 cm from the radiation source of a linear accelerator. The dose-per-pulse value was varied from 1.00 to 0.0043 by adding attenuators. The dose-per-pulse value of 0.0043 was obtained by irradiating the detector with it positioned under a solid-collimator jaw. The dose-per-pulse value was varied from 1.79 to 0.156 by changing the distance to the detector from 75 to 250 cm. The dose-per-pulse values were normalized to the value at a detector-to-source distance of 100 cm without attenuators. The corresponding dose-per-pulse value was , 53.4 Gy/s, 3208 Gy/min. The error bars correspond to one standard deviation of measurement uncertainty.
Sensitivity to dose as a function of the temperature at the time of irradiation. The detectors were irradiated by 100 cGy of 15 MV x rays. Each temperature point was accomplished by adjusting the water bath and allowing 5 min for thermal equilibrium. The temperature of the detectors was established by measuring the forward-bias resistance of the diode immediately before the irradiation. The dose sensitivity of the detectors is normalized to the values measured at . The solid line is a calculated temperature sensitivity of . The error bars correspond to one standard deviation of measurement uncertainty.
Geometric parameters for various modes and energies of the linear accelerator operation that correspond to 1 cGy/MU calibrated according to TG-51 (Ref. 34). The detector was placed at the depth of maximum dose. The depth of the detector was provided by a combination of a 0.5 cm thick piece of Superflab and various pieces of solid water.
Analysis of the reading depletion for three OSLDs. These OSLDs were irradiated with 100 cGy of 6 MV x rays and were read 25 times sequentially with the low-intensity LED beam. The reading data were analyzed with linear regression, according to Eqs. (1) and (2).
Analysis of the decay of the OSLD signal after irradiation for three OSLDs. These OSLDs were irradiated with 6 MV x rays and were read beginning at 45 s after the end of irradiation. Equation (1) was used to correct for signal depletion with these multiple sequential readings of the OSLDs. The reading data were analyzed by nonlinear regression, according to Eq. (3).
Exposure data and statistical analysis for one OSLD, 34636H, repeatedly exposed to 100 cGy, read, and then optically annealed. These data for OSLD 34636H are the first six data points in Fig. 5. Data are also shown for six different OSLDs randomly chosen from a batch of detectors.
Article metrics loading...
Full text loading...