• Proceedings of the Korean Society of Medical Physics Conference
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• 2003.09a
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• pp.64-64
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• 2003

Objectives: Patient dose verification is clinically the most important parts in the treatment delivery of radiation therapy. The three dimensional(3D) reconstruction of dose distribution delivered to target volume helps to verify patient dose and determine the physical characteristics of beams used in intensity modulated radiation therapy(IMRT). We present Beam Intensity Scanner(BInS) system for the pre treatment dosimetric verification of two dimensional photon intensity. The BInS is a radiation detector with a custom made software for relative dose conversion of fluorescence signals from scintillator. Methods: This scintillator is fabricated by phosphor Gadolinium Oxysulphide and is used to produce fluorescence from the irradiation of 6MV photons on a Varian Clinac 21EX. The digitized fluoroscopic signals obtained by digital video camera will be processed by our custom made software to reproduce 3D relative dose distribution. For the intensity modulated beam(IMB), the BInS calculates absorbed dose in absolute beam fluence, which are used for the patient dose distribution. Results: Using BInS, we performed various measurements related to IMRT and found the followings: (1) The 3D dose profiles of the IMBs measured by the BInS demonstrate good agreement with radiographic film, pin type ionization chamber and Monte Carlo simulation. (2) The delivered beam intensity is altered by the mechanical and dosimetric properties of the collimating of dynamic and/or static MLC system. This is mostly due to leaf transmission, leaf penumbra, scattered photons from the round edges of leaves, and geometry of leaf. (3) The delivered dose depends on the operational detail of how to make multileaf opening. Conclusions: These phenomena result in a fluence distribution that can be substantially different from the initial and calculative intensity modulation and therefore, should be taken into account by the treatment planing for accurate dose calculations delivered to the target volume in IMRT.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.9 s.240
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• pp.1022-1031
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• 2005

Temporal behavior of the laser induced incandescence (LII) signal is often used for soot particle sizing, which is possible because the cooling behavior of a laser heated particle is dependent on the particle size. In present study, LII signals of soot particles are modeled using two non-linear coupled differential equations deduced from the energy- and mass-balance of the process. The objective of this study is to obtain an appropriate calibration curve for determining primary particle size by comparing the gated signal ratio and double-exponential curve fitting methods. Not only the effects of laser fluence and gas temperature on the cooling behavior but also heat transfer mechanisms of heated soot particle have been investigated. The second-order exponential curve fitting showed better agreements with the LII signals than the gated signal ratio method which was based on the lust-order exponential curve fit. And the temporal decay rate of the LII signal and primary particle size showed nearly linear relationship, which was little dependent on the laser fluence. And it also could be reconfirmed that vaporization was dominant process of heat loss during first loons after laser pulse, then heat conduction played most important role while thermal radiation had little influence all the time.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2002.09a
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• pp.255-259
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• 2002

To reduce the uncertainty in the calibration of radiation beams, absorbed dose to water for high energy electrons is recommended as the standards and reference absorbed dose by AAPM Report no.51 and IAEA Technical Reports no.398. In these recommendations, water is, defined as the reference medium, however, the water substitute solid phantoms are discouraged. Nevertheless, when accurate chamber positioning in water is not possible, or when no waterproof chamber is available, their use is permitted at beam qualities R$\_$50/ < 4 g/cm$^2$ (E$\_$0/ < 10 MeV). For the electron dosimetry using solid phantom, a depth-scaling factor is used for the conversion of depth in solid phantoms to depth in water, and a fluence-scaling factor is used for the conversion of ionization chamber reading in plastic phantom to reading in water. In this work, the properties, especially depth-scaling factors c$\_$p1/ and fluence-scaling factors h$\_$pl/ of several commercially available water substitute solid phantoms were determined, and the electron dosimetry using these scaling method was evaluated. As a result, it is obviously that dose-distribution in solid phantom can be converted to appropriate dose-distribution in water by means of IAEA depth-scaling.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2002.09a
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• pp.68-73
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• 2002

In standard teletherapy, a treatment plan is generated with the aid of a treatment planning system, but it is common to perform an independent monitor unit verification calculation (MUVC). In exact analogy, we propose and demonstrate that a simple and accurate MUVC in Intensity Modulated Radiotherapy (IMRT) is possible. We introduce a concept of Modified Clarkson Integration (MCI). In MCI, we exploit the rotational symmetry of scattering to simplify the dose calculation. For dose calculation along a central axis (CAX), we first replace the incident IMRT fluence by an azimuthally averaged fluence. Second, the Clarkson Integration is carried over annular sectors instead of over pie sectors. We wrote a computer code, implementing the MCI technique, in order to perform a MUVC for IMRT purposes. We applied the code to IMRT plans generated by CORVUS. The input to the code consists of CORVUS plan data (e.g., DMLC files, jaw settings, MU for each IMRT field, depth to isocenter for each IMRT field), and the output is dose contribution by individual IMRT field to the isocenter. The code uses measured beam data for Sc, Sp, TPR, (D/Mu)$\_$ref/ and includes effects from MLC transmission, and radiation field offset. On a 266 MHZ desktop computer, the code takes less than 15 sec to calculate a dose. The doses calculated with MCI algorithm agreed within +/- 3% with the doses calculated by CORVUS, which uses a 1cm x 1cm pencil beam in dose calculation. In the present version of MCI, skin contour variations and inhomogeneities were neglected.

• Nuclear Engineering and Technology
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• v.20 no.1
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• pp.19-26
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• 1988

The low leakage leading pattern has features as the placement of some fresh fuel assemblies in the core interior to reduce the neutron fluence on the pressure vessel and to enhance the neutron economics. But as fresh fuel assemblies are loaded in the core interior, the local power tends to exceed safety limit due to the high reactivity of the fresh assemblies. Therefore, a large number of burnable poisons must be utilized in a low leakage scheme to suppress the high assembly power as well as the excess reactivity. In this study the effects of burnable poisons are treated as a perturbation on the power distribution, and the 'Power Sensitivity Coefficient' concept is adopted. An application study is performed for cycle 1 of the Korea Nuclear Unit-7 (KNU-7) to justify the usefulness of the reverse depletion method coupled with the above concept. To obtain the optimal burnable poision distribution at the given burnup step, the linear programming technique is adopted. The result shows maximum 4.5% error in the amount of burnable poisons between the calculated and the reference values. It is concluded that the design methodology which consists of the reverse depletion, the power sensitivity coefficient concept, and the linear programming technique can be used to find the optimal turnable poison distribution.

• Proceedings of the Korean Vacuum Society Conference
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• 1999.07a
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• pp.80-80
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• 1999

플라즈마 디스플레이 패널(PDP)의 공정을 간단히 하기 위하여 포토레지스트, ITO, 격벽재료를 Ar+ laser(λ-514 nm, CW)와 Nd:YAG laser(λ=532, 266nm, pulse)로 직접 패터닝 하였다. 레이저에 의한 포토레지스트의 패턴결과, 아르곤 이온 레이저의 포토레지스트 가공의 반응 메카니즘은 레이저 빔의 열에 의한 시료 표면의 국부적인 온도상승에 의한 용융작용이며, 그 결과 식각 후 형성된 패턴의 단면 모양도 레이저빔의 profile과 같은 가우시안 형태를 나타낸다. Nd:YAG 레이저의 4고조파(532nm)를 이용한 경우 200$\mu\textrm{m}$/sce의 주사속도에서 포토레지스트를 패턴하기 위한 임계에너지(threshold energy fluence) 값은 25J/cm2이며, 약 40J/cm2의 에너지 밀도에서 하부기판의 손상이 발생하기 시작하였다. 글미 1은 Nd:YAG 레이저 4고조파를 이용하여 포토레지스트를 식각한 경우 SEM 표면사진(위)과 단차특정기에 의한 단면형상(아래)이다. ITO 막의 레이저에 의한 직접 패턴 결과, ITO 막은 레이저 펄스에 의한 급속 가열 및 증발에 의한 메커니즘으로 식각이 이루어지며, 레이저 파장에 따른 광흡수 정도의 차이에 의해 2고조파 (532nm)에서 ITO 막의 가공 품질이 4고조파(266nm)에 비해 우수하며 패턴의 폭도 출력에 따라 제어가 용이하였다. 그림 2는 Nd:YAG 레이저 2고조파를 이용하여 ITO를 식각한 경우 SEM표면 사진(위)과 단차측정기에 의한 단면형상(아래)이다. 격벽 재료의 레이저에 의한 직접 패턴 결과, Ar+ 레이저(514nm)는 출력 밀도 32NW/cm2에서 격벽을 유리 기판의 경계면까지 식각하였다. Nd:YAG 레이저(532nm)는 laser fluence가 6.5mJ/cm2에서 격벽을 식각하기 시작하였으며, 19.5J/cm2에서 유리기판의 rudraus(격벽 두께 130$\mu\textrm{m}$)까지 식각하였다.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.34.2-34.2
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• 2010

Cosmic rays registered by Neutron Monitors on the surface of the Earth are believed to be coming from outer space, and sometimes also from the exotic objects of the Sun. Ground level enhancement (GLE) is the sudden, sharp and short-lived increase in cosmic rays originated from the Sun. Since GLE is the signature in solar cosmic ray intensity, different solar factors erupted from the Sun can be responsible for causing it. In this context, an attempt has been made to determine quantitative relationships of GLEs > 5% with simultaneous solar, interplanetary and geophysical factors from 1997 through 2006 thereby searching the perpetrators which seem to be causing them. The study has revealed that solar flares are stronger ($0.71{\times}10-4$ w/m2) during GLE peaks than the solar flares ($1.10{\times}10-5$ w/m2) during GLE non-peaks and backgrounds. On the average, the solar wind plasma velocity and interplanetary magnetic field are found stronger during the GLE peaks than the GLE non-peaks and backgrounds indicating that the solar flares, in conjunction with interplanetary shocks, sometimes may cause GLE peaks. Direct proportionality of GLE peaks to simultaneous solar energetic particle (SEP) fluxes imply that the GLE peaks may often be caused by SEP fluxes. Although the high intensity of SEP fluxes are also seen extended few minutes even after GLE peaks, the mean (373.62 MeV) of the GLE associated SEP fluxes is much stronger than the mean (10.35 MeV) of the non-GLE associated SEP fluxes. Evidences are also supported by corresponding SEP fluences that the the mean fluence (${\sim}5.32{\times}107/cm2$) across GLE event was more intense than the mean fluence (${\sim}2.53{\times}106/cm2$) of SEP fluxes across non-GLE event.

• Journal of the Microelectronics and Packaging Society
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• v.14 no.3
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• pp.29-36
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• 2007

The main objectives of this study are to investigate the micro-scale energy transfer mechanism for silicon wafer and to find an efficient way for fabrication of silicon wafer through-hole by using the femtosecond pulse laser ablation. In addition, the electron-phonon interactions during laser irradiation are discussed and the carrier number density and temperatures are estimated. In particular, the present study observes the shapes of silicon wafer through-hole with $100\;{\mu}m$ diameter and it also measures the heat-affected area and the ablation depths fur different laser fluences by using the optic microscope and the three-dimensional profile measurement technique. First, from numerical investigation, it is found that the nonequilibrium state exists between electrons and phonons during laser irradiation. From experimental results, it should be noted that the heat-affected area increases with laser fluence, and the optimal conditions for through-hole formation with minimum heat affected zone are finally obtained.

• Transactions of Materials Processing
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• v.8 no.3
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• pp.241-241
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• 1999

With the aim of assessing the degradation of Zr-2.5Nb pressure tubes operating in the Wolsong unit-1 nuclear power plant, characterization tests are being conducted on irradiated Zr-2.5Nb tubes removed after 10-year operation. The examined tube had been exposed to temperatures ranging from 264 to 306℃ and a neutron fluence of 8.9×$10^{21}$ n/cm²(E>1 MeV) at the maximum. Tensile tests were carried out at temperatures ranging from RT to 300℃. The density of a-type and c-type dislocations was examined on the irradiated Zr-2.5Nb tube using a transmission electron microscope. Neutron irradiation up to 8.9×$10^{21}$ n/cm²(E>1 MeV) yielded an increase in a-type dislocation density of the Zr-2.5Nb pressure tube to 7.5×$10^{14} m^{-2}$, which was highest at the inlet of the tube exposed to the low temperature of 275℃. In contrast, the c-component dislocation density did not change with irradiation, keeping an initial dislocation density of 0.8×$10^{14} m^{-2}$ over the whole length of the tube. As expected, the neutron irradiation increased mechanical strength by about 17-26% in the transverse direction and by 34-39% in the longitudinal direction compared to that of the unirradiated tube at 300℃. The change in the mechanical properties with irradiation is discussed in association with the microstructural change as a function of temperature and neutron fluence.

• Progress in Superconductivity and Cryogenics
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• v.16 no.1
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• pp.9-13
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• 2014

Effects of neutron irradiation on the superconducting properties of the undoped $MgB_2$ and the carbon(C)-doped $MgB_2$ bulk superconductors, prepared by an in situ reaction process using Mg and B powder, were investigated. The prepared $MgB_2$ samples were neutron-irradiated at the neutron fluence of $10^{16}-10^{18}n/cm^2$ in a Hanaro nuclear reactor of KAERI involving both fast and thermal neutron. The magnetic moment-temperature (M-T) and magnetization-magnetic field (M-H) curves before/after irradiation were obtained using magnetic property measurement system (MPMS). The superconducting critical temperature ($T_c$) and transition width were estimated from the M-T curves and critical current density ($J_c$) was estimated from the M-H curves using a Bean's critical model. The $T_cs$ of the undoped $MgB_2$ and C-doped $MgB_2$ before irradiation were 36.9-37.0 K and 36.6-36.8 K, respectively. The $T_cs$ decreased to 33.2 K and 31.6 K, respectively after irradiation at neutron fluence of $7.16{\times}10^{17}n/cm^2$, and decreased to 22.6 K and 24.0 K, respectively, at $3.13{\times}10^{18}n/cm^2$. The $J_c$ cross-over was observed at the high magnetic field of 5.2 T for the undoped $MgB_2$ irradiated at $7.16{\times}10^{17}n/cm^2$. The $T_c$ and $J_c$ variation after the neutron irradiation at various neutron fluences were explained in terms of the defect formation in the superconducting matrix by neutron irradiation.