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Evolution of strain and mechanical properties upon annealing in He-implanted 6H-SiC
No full textThe effects of annealing temperature on strain and mechanical property changes of 6H-SiC implanted with helium ions at 600 K to doses of 3 x 10(15) cm(-2), 1 x 10(16) cm(-2) and 3 x 10(16) cm(-2) and at an ion energy of 100 keV were investigated by using high-resolution X-ray diffraction (XRD), nano-indentation and transmission electron microscopy (TEM). Strain increases with increasing displacements per atom (dpa). Strain relaxation in terms Of changes in Delta d/d exhibited a linear decrease with increasing annealing temperature ranging from 873 K to 1473 K for 30 min in vacuum. The relaxation activation energies of the strains were estimated by Arrhenius law to be in the range of 0.4-0.7 eV. Irradiation-induced hardening was observed via nano-indentation measurements as a function of annealing. The hardness of the highly damaged layer decreased monotonically with increasing annealing temperature for the samples implanted with He ions to doses of 3 x 10(15) cm(-2) and 1 x 10(16) cm(-2), and where no helium bubbles were formed in the damaged layer. The hardness of the damaged layer initially decreased and then increased with increasing annealing temperature from 600 K to 1073 K for the sample implanted He ions to a dose of 3 x 10(16) cm(-2), where numerous helium bubbles were formed in the damaged layer. The TEM results suggest that the growth of helium bubbles emits interstitials upon annealing. These interstitials agglomerate into stacking faults and dislocation loops, which increase the hardness. (C) 2014 Elsevier B.V. All rights reserved
Respiratory motion management using audio-visual biofeedback for respiratory-gated radiotherapy of synchrotron-based pulsed heavy-ion beam delivery
No full textPurpose: To efficiently deliver respiratory-gated radiation during synchrotron-based pulsed heavy-ion radiotherapy, a novel respiratory guidance method combining a personalized audio-visual biofeedback (BFB) system, breath hold (BH), and synchrotron-based gating was designed to help patients synchronize their respiratory patterns with synchrotron pulses and to overcome typical limitations such as low efficiency, residual motion, and discomfort. Methods: In-house software was developed to acquire body surface marker positions and display BFB, gating signals, and real-time beam profiles on a LED screen. Patients were prompted to perform short BHs or short deep breath holds (SDBH) with the aid of BFB following a personalized standard BH/SDBH (stBH/stSDBH) guiding curve or their own representative BH/SDBH (reBH/reSDBH) guiding curve. A practical simulation was performed for a group of 15 volunteers to evaluate the feasibility and effectiveness of this method. Effective dose rates (EDRs), mean absolute errors between the guiding curves and the measured curves, and mean absolute deviations of the measured curves were obtained within 10%-50% duty cycles (DCs) that were synchronized with the synchrotron's flat-top phase. Results: All maneuvers for an individual volunteer took approximately half an hour, and no one experienced discomfort during the maneuvers. Using the respiratory guidance methods, the magnitude of residual motion was almost ten times less than during nongated irradiation, and increases in the average effective dose rate by factors of 2.39-4.65, 2.39-4.59, 1.73-3.50, and 1.73-3.55 for the stBH, reBH, stSDBH, and reSDBH guiding maneuvers, respectively, were observed in contrast with conventional free breathing-based gated irradiation, depending on the respiratory-gated duty cycle settings. Conclusions: The proposed respiratory guidance method with personalized BFB was confirmed to be feasible in a group of volunteers. Increased effective dose rate and improved overall treatment precision were observed compared to conventional free breathing-based, respiratory-gated irradiation. Because breathing guidance curves could be established based on the respective average respiratory period and amplitude for each patient, it may be easier for patients to cooperate using this technique. (C) 2014 American Association of Physicists in Medicine
Electromagnetic Design and Optimization of Superconducting Spoke021 Cavity for C-ADS
No full text为满足C-ADS 项目建设对超导轮辐腔Spoke021的需求，对Spoke021进行了详细的电磁参数优化。对Spoke021的参数化模型进行参数扫描，针对腔体的关键电磁特征量寻找可能存在的极值，详细分析、解释了优化过程中各个参数发生变化的物理意义。在Spoke021各参数达到最终优化值时，表征腔体性能的两个关键比值分别为：E_p/E_(acc) = 3:14，B_p/E_(acc) = 4:77 mT/(MV/m)。考虑到次级电子倍增(Multipacting，MP)对Spoke021运行中所能达到的性能指标有很重要影响，对腔体的MP进行了建模分析。结果表明，当Spoke021工作在Eacc =10 MV/m情况下，没有发生MP，优化得到的参数可以满足Spoke021工程设计的需要；最后计算了腔体的TTF曲线，表明该腔体具有较宽的速度接受度。<div>
The electromagnetic optimizations of a superconducting Spoke021 cavity is studied in detail for C-ADS project, in this paper. The cavity model has been parameterized, in order to find the possible maxima or minima of the key electro-magnetic parameters, the parameter sweep is performed. The cavity model has been parameterized. For finding the key electromagnetic feature, the geometric parameter sweep is performed. The physical interpretation for the variation of these electromagnetic parameters has been elaborated. The electromagnetic simulation gives the optimum parameters E_p/E_(acc) of 3.14 and B_p/E_(acc) of 4.77 mT/(MV/m). In addition, the Multipacting(MP) has a great effect on the electromagnetic properties of superconducting Spoke021 cavity. So the MP simulations have been done for this cavity, which shows that there is no MP phenomenon at 10 MV/m accelerator gradient. Therefore, the optimized cavity parameters can satisfy the requirements of the C-ADS project. Finally, the TTF(Transit Time Factor) curve of this cavity is calculated and it shows that this cavity has wide velocity acceptance.</div>
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</div>中国科学院战略性先导科技专
Energy deposition by heavy ions: Additivity of kinetic and potential energy contributions in hillock formation on CaF2
No full text<span style="color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; font-size: 13px; line-height: 22px; background-color: rgb(248, 248, 248);">Modification of surface and bulk properties of solids by irradiation with ion beams is a widely used technique with many applications in material science. In this study, we show that nano-hillocks on CaF2 crystal surfaces can be formed by individual impact of medium energy (3 and 5 MeV) highly charged ions (Xe22+ to Xe30+) as well as swift (kinetic energies between 12 and 58 MeV) heavy xenon ions. For very slow highly charged ions the appearance of hillocks is known to be linked to a threshold in potential energy (E-p) while for swift heavy ions a minimum electronic energy loss per unit length (S-e) is necessary. With our results we bridge the gap between these two extreme cases and demonstrate, that with increasing energy deposition via Se the E-p-threshold for hillock production can be lowered substantially. Surprisingly, both mechanisms of energy deposition in the target surface seem to contribute in an additive way, which can be visualized in a phase diagram. We show that the inelastic thermal spike model, originally developed to describe such material modifications for swift heavy ions, can be extended to the case where both kinetic and potential energies are deposited into the surface.</span