Transverse impendance of LHC collimators
E. MétralG. ArduiniR. AßmannAndrea BoccardiThomas BohlF. CaspersM. GąsiorR. JonesKrzysztof KasińskiT. KroyerStefano RedaelliG. Robert-DemolaizeG. RumoloRalph SteinhagenT. WeilerFrank ZimmermannChiara BraccoBenoît SalvantF. Roncarolo
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The transverse impedance in the LHC is expected to be dominated by the numerous collimators, most of which are made of Fibre-Reinforced-Carbon to withstand the impacts of high intensity proton beams in case of failures, and which will be moved very close to the beam, with full gaps of few millimetres, in order to protect surrounding super-conducting equipments. We present an estimate of the transverse resistive-wall impedance of the LHC collimators, the total impedance in the LHC at injection and top energy, the induced coupled-bunch growth rates and tune shifts, and finally the result of the comparison of the theoretical predictions with measurements performed in 2004 and 2006 on a prototype collimator installed in the SPS.Keywords:
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shift as a function of bunch intensity and deriving the total accelerator machine transverse impedance. For RHIC, we could obtain first promising results of impedance localization measurements as well.
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Transverse impedances can be derived from transmission coefficients measured along one of the two coaxial wires driven by a 180 hybrid. Our results show that the newly designed cavities behave better than the old prototype cavity with larger transverse impedance,and between the newly designed cavities,dielectric cavity with a coxial like accelerating gap geometry should be the first choice because the transverse impedance of the lower mode is oppressed efficiently.
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Collimator angle is usually rotated when planning volumetric modulated arc therapy (VMAT) due to the leakage of radiation between multi-leaf collimator (MLC) leaves. We studied the effect of the collimator angles on the results of dosimetric verification of the VMAT plans for head and neck patients. We studied VMAT plans for 10 head and neck patients. We made 2 sets of VMAT plans for each patient. Each set was composed of 10 plans with collimator angles of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45 degrees. Plans in the first set were optimized individually and plans in the second set shared the 30 degree collimator angle optimization. Two sets of plans were verified using the 2-dimensional ion chamber array MatriXX (IBA Dosimetry, Germany). The comparison between the calculation and measurements were made by the $\gamma$-index analysis. The $\gamma$-index (2\%/2 mm) and (3\%/3 mm) passing rates had negative correlations with the collimator angle. Maximum difference between $\gamma$-index (3\%/3 mm) passing rates of different collimator angles for each patient ranged from 1.46\% to 5.60\% with an average of 3.67\%. There were significant differences (maximum 5.6\%) in the passing rates of different collimator angles. The results suggested that the accuracy of the delivered dose depends on the collimator angle. These findings are informative when choosing a collimator angle in VMAT plans.
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An experimental arrangement which permitted the parallel multihole collimator of a gamma camera to be set in rotary motion during image formation was fabricated. Various test sources of radioactive 99mTc were imaged with the collimator. With the collimator stationary an image of holes and septa of the collimator was projected into the picture of test sources as imaged by the camera. With the collimator in motion the image of the collimator was eliminated from pictures of test sources. Possible practical use of a moving collimator to eliminate collimator artifacts from gamma-camera images is considered.
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Collimator angle is usually rotated when planning volumetric modulated arc therapy (VMAT) due to the leakage of radiation between multi-leaf collimator (MLC) leaves. We studied the effect of the collimator angles on the results of dosimetric verification of the VMAT plans for head and neck patients. We studied VMAT plans for 10 head and neck patients. We made 2 sets of VMAT plans for each patient. Each set was composed of 10 plans with collimator angles of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45 degrees. Plans in the first set were optimized individually and plans in the second set shared the 30 degree collimator angle optimization. Two sets of plans were verified using the 2-dimensional ion chamber array MatriXX (IBA Dosimetry, Germany). The comparison between the calculation and measurements were made by the $\gamma$-index analysis. The $\gamma$-index (2\%/2 mm) and (3\%/3 mm) passing rates had negative correlations with the collimator angle. Maximum difference between $\gamma$-index (3\%/3 mm) passing rates of different collimator angles for each patient ranged from 1.46\% to 5.60\% with an average of 3.67\%. There were significant differences (maximum 5.6\%) in the passing rates of different collimator angles. The results suggested that the accuracy of the delivered dose depends on the collimator angle. These findings are informative when choosing a collimator angle in VMAT plans.
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The nuclear accident that occurred in Fukushima, Japan, in 2011, resulted in considerable radiation leaks and mass exposure owing to an insufficient initial response to the disaster. To prevent similar disasters in the future, their cause should be identified, analyzed, and addressed using radiation-monitoring systems. The core component of a radiation-monitoring system is the gamma camera. It consists mainly of a gamma detector and a collimator. We proposed a diverging collimator that incorporates a novel concept of a curved surface such that the collimator hole length is constant for all the incident hole angles. Therefore, the sensitivity is likely to be more uniform across the field of view (FOV) compared with the case of the conventional flat collimator. Through a simulation study, we determined the collimator parameters and compared the performance of the curved collimator with that of the conventional flat collimator. The results revealed the feasibility of achieving higher uniform sensitivity, up to 10.5% at the edge of the FOV by using the proposed curved collimator. The average peak-to-background ratio (PBR) of the curved collimator was determined to be higher by 10.8% across the entire FOV. This indicates that it produced a more distinct signal than the flat collimator. In our future research, we intend to fabricate a curved collimator and evaluate its performance experimentally.
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