Chul-Young Yi

Monte Carlo calculation of the ionization chamber response to 60Co beam using PENELOPE

The calculation of the ionization chamber response is one of key factors to develop a primary standard of air kerma. Using Monte Carlo code PENELOPE, we simulated the cavity response of the plane parallel ionization chamber to the monoenergetic 60Co beam incident normally on a flat surface of the chamber. Two simulation techniques, namely, the uniform interaction technique and the reentrance technique, were introduced. The effect of the input parameters such as C1 (average angular deflection in a single step between hard elastic events), C2 (maximum average fractional energy loss in a step), S(max) (maximum step length) and W(cc) (the lower energy of secondary electrons created as a result of a hard collision) on the simulated cavity dose was evaluated. We found that the simulated cavity response of the graphite and solid air chambers obtained by PENELOPE to the monoenergetic 60Co beam could be consistent with the value expected from the cross-sections of PENELOPE to within 0.2% (one standard deviation) when W(cc) and S(max) were selected carefully.

Comparison of the standards for air kerma of the NIST and the BIPM for 60Co gamma radiation

An indirect comparison of the standards for air kerma of the Dutch Metrology Institute (VSL), The Netherlands, and of the Bureau International des Poids et Mesures (BIPM) was carried out in the 60Co radiation beam of the BIPM in September 2005. The comparison result, based on the calibration coefficients for three transfer standards and expressed as a ratio of the VSL and the BIPM standards for air kerma, is 0.9985 with a combined standard uncertainty of 2.2 ? 10?3. The latest result agrees with the result of the previous comparison in 60Co ? rays, made in 1996 and updated for recent changes made to the standards to give a value of 0.9982(37). The degrees of equivalence between the VSL and the other participants in the key comparison BIPM.RI(I)-K1 have been calculated and the results are presented in the form of a matrix. A graphical presentation is also given. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCRI Section I, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Monte Carlo calculation of response functions to gamma-ray point sources for a spherical NaI(Tl) detector.

Response functions of a 7.62 cm-diameter spherical NaI(Tl) detector to gamma-ray point sources in the energy range up to 1.5 MeV were calculated by means of the Monte Carlo method using PENELOPE-2006 (Salvat et al., 2006). The detector materials and dimensions were modeled realistically. The calculated response functions agreed well with the experimental spectra.

Temperature dependence of cavity ionization chamber response

The temperature dependence of the cavity ion chamber response was measured at room temperature in the range 17 °C to 27 °C. By analysing the variation of the ionization current with temperature produced in the cavity chamber, the temperature coefficient of the cavity chamber response was evaluated. The values were 4.1 × 10−4 °C−1, 4.3 × 10−4 °C−1 and 2 × 10−5 °C−1 for chambers made of C552 air equivalent plastic, polyoxymethylene and graphite, respectively.

X-Ray Acoustic-Based Dosimetry Using a Focused Ultrasound Transducer and a Medical Linear Accelerator

High-energy ionizing radiation therapy is a highly effective method for destroying cancer cells. Monitoring the dose distribution during radiation therapy is extremely important to deliver an optimal X-ray dose to diseased areas and minimize radiation exposure to adjacent healthy tissues. Here, we present an X-ray acoustic (XA) dosimetry system that successfully combines a spherically focused ultrasound (US) transducer with a medical linear accelerator. The system can be potentially utilized in clinical radiation therapy as an intratherapy dosimetry tool. The measured XA signal showed good correlation with the water-absorbed dose measured with conventional dosimetry. We acquired the absorbed X-ray dose distribution in a lead sample by mechanically scanning the focused US transducer. The lateral spatial resolution of the XA signal was 2.1 ± 0.5 mm, and signal-to-noise ratio of the signal was maintained with 100-mm penetration depth, whereas that of a laser-driven photoacoustic signal was exponentially decreased.

Comparison of the standards for air kerma of the KRISS and the BIPM for 137Cs gamma radiation

A direct comparison of the standards for air kerma of the Korea Research Institute of Standards and Science (KRISS), Korea, and of the Bureau International des Poids et Mesures (BIPM) was carried out in the 137Cs radiation beam of the BIPM in August 2010. The result, expressed as a ratio of the KRISS and the BIPM standards for air kerma, is 0.9986 with a combined standard uncertainty of 2.2 ? 10?3. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCRI Section I, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

SU‐E‐T‐31: The Evaluation of Dosimetric Characteristics of OSLDs Based On Output Correction Factor in Low Energy

Purpose: The output of General X‐ray unit (low energy) has variation against 60Co unit. So it is necessary to minimize output variation when evaluating dosimetric characteristics of optical stimulated luminance dosimeters (OSLDs). Considering output variation of X‐ray unit, we evaluated dosimetric characteristics of OSLDs in low energyMethods: The OSLDs that used were nanoDotTM Dosimeter (Landauer Inc, Glenwood, USA) and this dosimeter had never been irradiated. Through single irradiate them (test dose was 7mGy), we certified batch homogeneity and sampled dosimeters with variation of radiation sensitivity within ± 1.5% among them. Using to these dosimeters certified reproducibility considering output variation every time when irradiated OSLDs. Through this process, we certified element correction factor (ECF) and coefficient of variation (COV) about each OSLD. Based on these OSLDs, we studied linearity, energy dependence and angular dependenceResults: The batch homogeneity was 1.21% of the coefficient of variation (after sampling). The average value of COV about reproducibility of OSLDs was reduced from 1.3% to 0.96% after applying to output correction factor. The linearity was that the correlation of between dose and count was fitted by linear function (R2 =0.997). The energy dependence study showed a range of ion chamber‐to OSLD rations from 0.23 (24.7 keV) to 0.27 (34.5 keV). According to energy, the range of angular dependence was from 0.1% to 8.4% variation when each degree was normalized by zero degreeConclusion: Considering output correction factor, reduced uncertainty occurred in general x‐ray unit. We acquired information that was ECF, COV of OSLDs in low energy. Using to these OLDs, It is feasible to measure patient dose of diagnostic radiography and Cone beam computed tomography for radiotherapy

SU‐E‐T‐267: Development of the Compact Graphite Calorimetry System for the High Energy Photon Beam

Purpose: Graphite calorimeter systems are used for the absolute photon dosimetry. But many electronics are demanded in order to measure the tiny temperature changes. Minimizing the control system is needed to make a portable graphite calorimeter. Methods: A Domen-type graphite calorimetry system is constructing to measure the absorbed dose of the high energy photon beam. The graphite calorimeter divided into three parts, Core, Jacket, and Shield. In order to measure the temperature rising of the core due to the radiation accurately, the temperatures of the jacket and the shield should be controlled properly. A commercial temperature controller (Model 350, Lake Shore Cryogenics) was used to minimize the size of control system for making a portable graphite calorimetry system at the cost of the measurement uncertainty. The PID control of the jacket is conducted by the software (LabView) and Model 350 maintain the temperature of shield. Results: Our design value of the heat deposition power in the core is 0.04 mW for the dose rate of 3 Gy/min where the temperature sensitivity of the graphite is 1.4 mK/Gy. While the residuals of the Steinhart-hart equation fitting for the core thermistor were less than 0.1 mK, the temperature resolution of Model 350 is 1 mK. The temperature of the shield was kept within the 5 mK when the room temperature variation was about 0.5 K. Conclusion: The resolution of Model 350 for the temperature measurement and control is not good enough as the control system for the compact graphite calorimetry system. But The performance of Model 350 is good enough to maintain the temperature of the shield constantly. The Model 350 will be replaced by the AC resistance bridge (Model 372, Lake Shore Cryogenics) for the core temperature measurement and the jacket control.

SU-E-T-315: The Change of Optically Stimulated Luminescent Dosimeters (OSLDs) Sensitivity by Accumulated Dose and High Dose

PURPOSE The objective of this study is to evaluate radiation sensitivity of optical stimulated luminance dosimeters (OSLDs) by accumulated dose and high dose. METHODS This study was carried out in Co-60 unit (Theratron 780, AECL, and Canada) and used InLight MicroStar reader (Landauer, Inc., Glenwood, IL) for reading. We annealed for 30 min using optical annealing system which contained fluorescent lamps (Osram lumilux, 24 W, 280 ∼780 nm). To evaluate change of OSLDs sensitivity by repeated irradiation, the dosimeters were repeatedly irradiated with 1 Gy. And whenever a repeated irradiation, we evaluated OSLDs sensitivity. To evaluate OSLDs sensitivity after accumulated dose with 5 Gy, We irradiated dose accumulatively (from 1 Gy to 5 Gy) without annealing. And OSLDs was also irradiated with 15, 20, 30 Gy to certify change of OSLDs sensitivity after high dose irradiation. After annealing them, they were irradiated with 1Gy, repeatedly. RESULTS The OSLDs sensitivity increased up to 3% during irradiating seven times and decreased continuously above 8 times. That dropped by about 0.35 Gy per an irradiation. Finally, after 30 times irradiation, OSLDs sensitivity decreased by about 7%. For accumulated dose from 1 Gy to 5 Gy, OSLDs sensitivity about 1 Gy increased until 4.4% after second times accumulated dose compared with before that. OSLDs sensitivity about 1 Gy decreased by 1.6% in five times irradiation. When OSLDs were irradiated ten times with 1Gy after irradiating high dose (10, 15, 20 Gy), OSLDs sensitivity decreased until 6%, 9%, 12% compared with it before high dose irradiation, respectively. CONCLUSION This study certified OSLDs sensitivity by accumulated dose and high dose. When irradiated with 1Gy, repeatedly, OSLDs sensitivity decreased linearly and the reduction rate of OSLDs sensitivity after high dose irradiation had dependence on irradiated dose.

Inherent uncertainty of air kerma

Air kerma is defined in air, the constituents and the amounts of which are determined experimentally with some uncertainties. The inherent uncertainty of air kerma in a photon beam stemming from the uncertainties of atomic weights and composition data in air was evaluated as a function of photon energy. The uncertainty was 1 part in 104 for photon beams with energies in the range 1 keV to 20 MeV.