Direct measurement of the image displacement instability in a linear induction accelerator
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43 PARTICLE ACCELERATORS↗
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The Ion-Hose Instability in a High-Current Multipulse Linear Induction Accelerator
Not Available
43 PARTICLE ACCELERATORS↗
Hamiltonian analysis for linearly acceleration-dependent Lagrangians
We study the constrained Ostrogradski-Hamilton framework for the equations of motion provided by mechanical systems described by second-order derivative actions with a linear dependence in the accelerations. We stress out the peculiar features provided by the surface terms arising for this type of theories and we discuss some important properties for this kind of actions in order to pave the way for the construction of a well defined quantum counterpart by means of canonical methods. In particular, we analyse in detail the constraint structure for these theories and its relation to the inherent conserved quantities where the associated energies together with a Noether charge may be identified. The constraint structure is fully analyzed without the introduction of auxiliary variables, as proposed in recent works involving higher order Lagrangians. Finally, we also provide some examples where our approach is explicitly applied and emphasize the way in which our original arrangement results in propitious for the Hamiltonian formulation of covariant field theories.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
SU-C-BRE-06: Radiobiological Advantage of Very Rapid Irradiation
Purpose: To characterize the effect of very rapid dose delivery as compared to conventional therapeutic irradiation times on clonogenic cell survival. Methods: We used a Varian Trilogy linear accelerator to deliver doses up to 10 Gy using a 6 MV SRS photon beam. We irradiated four cancer cell lines in times ranging from 30 sec to 30 min. We also used a Varian TrueBeam linear accelerator to deliver 9 MeV electrons at 10 Gy in 10 s to 30 min to determine the effect of irradiation time on cell survival. We then evaluated the effect of using 60 and 120 MeV electrons on cell survival using the Next Linear Collider Test Accelerator (NLCTA) beam line at the SLAC National Accelerator Laboratory. During irradiation, adherent cells were maintained at 37oC with 20%O2/5%CO2. Clonogenic assays were completed following irradiation to determine changes in cell survival due to dose delivery time and beam quality, and the survival data were fitted with the linear-quadratic model. Results: Cell lines varied in radiosensitivity, ranging from two to four logs of cell kill at 10 Gy for both conventional and very rapid irradiation. Delivering radiation in shorter times decreased survival in all cell lines. Log differences in cell kill ranged from 0.2 to 0.7 at 10 Gy for the short compared to the long irradiation time. Cell kill differences between short and long irradiations were more pronounced as doses increased for all cell lines. Conclusion: Our findings suggest that shortening delivery of therapeutic radiation doses to less than 1 minute may improve tumor cell kill. This study demonstrates the potential advantage of technologies under development to deliver stereotactic ablative radiation doses very rapidly. Bill Loo and Peter Maxim have received Honoraria from Varian and Research Support from Varian and RaySearch.
60 APPLIED LIFE SCIENCES↗
Cost-efficiency enhancement of X- and Ku-band split waveguides for industrial accelerators
Compact microwave electron linear accelerators must operate at high frequencies in order to maintain tight SWaP (size, weight, and power) requirements, set by modern applications. Currently, the frequencies are practically limited to X- and Ku-bands because of the availability of MW-class magnetrons, required to achieve beam energies of several MeV, and tight fabrication tolerances. The latter is associated with micron-level accuracy and further waveguide frequency tuning process, both increasing its costs dramatically. By implying a novel split structure fabrication approach, it is possible to relax these tolerances to the values of 10–20 μm and eliminate the tuning step, greatly reducing their total costs. Finally, this paper presents the results of microwave measurements of split X- and Ku-band accelerating structures of different types and discusses the benefits of our fabrication approach.
43 PARTICLE ACCELERATORS↗
Technology development for modern SRF accelerators
Superconducting RF (SRF) linear accelerators are widely used for scientific application. Challenges, status, breakthroughs and progress in SRF technology for accelerator application are discussed.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
A novel electron gun for inline MRI-linac configurations
Purpose: This work introduces a new electron gun geometry capable of robust functioning in the presence of a high strength external magnetic field for axisymmetric magnetic resonance imaging (MRI)-linac configurations. This allows an inline MRI-linac to operate without the need to isolate the linear accelerator (linac) using a magnetic shield. This MRI-linac integration approach not only leaves the magnet homogeneity unchanged but also provides the linac flexibility to move along the magnet axis of symmetry if the source to target distance needs to be adjusted. Methods: Simple electron gun geometry modifications of a Varian 600C electron gun are considered and solved in the presence of an external magnetic field in order to determine a set of design principles for the new geometry. Based on these results, a new gun geometry is proposed and optimized in the fringe field of a 0.5 T open bore MRI magnet (GE Signa SP). A computer model for the 6 MeV Varian 600C linac is used to determine the capture efficiency of the new electron gun-linac system in the presence of the fringe field of the same MRI scanner. The behavior of the new electron gun plus the linac system is also studied in the fringe fields of two other magnets, a 1.0 T prototype open bore magnet and a 1.5 T GE Conquest scanner. Results: Simple geometrical modifications of the original electron gun geometry do not provide feasible solutions. However, these tests show that a smaller transverse cathode diameter with a flat surface and a slightly larger anode diameter could alleviate the current loss due to beam interactions with the anode in the presence of magnetic fields. Based on these findings, an initial geometry resembling a parallel plate capacitor with a hole in the anode is proposed. The optimization procedure finds a cathode-anode distance of 5 mm, a focusing electrode angle of 5°, and an anode drift tube length of 17.1 mm. Also, the linac can be displaced with ±15 cm along the axis of the 0.5 T magnet without capture efficiency reduction below the experimental value in zero field. In this range of linac displacements, the electron beam generated by the new gun geometry is more effectively injected into the linac in the presence of an external magnetic field, resulting in approximately 20% increase of the target current compared to the original gun geometry behavior at zero field. The new gun geometry can generate and accelerate electron beams in external magnetic fields without current loss for fields higher than 0.11 T. The new electron-gun geometry is robust enough to function in the fringe fields of the other two magnets with a target current loss of no more than 16% with respect to the current obtained with no external magnetic fields. Conclusions: In this work, a specially designed electron gun was presented which can operate in the presence of axisymmetric strong magnetic fringe fields of MRI magnets. Computer simulations show that the electron gun can produce high quality beams which can be injected into a straight through linac such as Varian 600C and accelerated with more efficiency in the presence of the external magnetic fields. Also, the new configuration allows linac displacements along the magnet axis in a range equal to the diameter of the imaging spherical volume of the magnet under consideration. The new electron gun-linac system can function in the fringe field of a MRI magnet if the field strength at the cathode position is higher than 0.11 T. The capture efficiency of the linac depends on the magnetic field strength and the field gradient. The higher the gradient the better the capture efficiency. The capture efficiency does not degrade more than 16%.
43 PARTICLE ACCELERATORS↗
Radio to Gamma-Ray Emission from Shell-Type Supernova Remnants: Predictions from Non-Linear Shock Acceleration Models
Supernova remnants (SNRs) are widely believed to be the principal source of galactic cosmic rays, produced by diffusive shock acceleration in the environs of the remnant's expanding blast wave. Such energetic particles can produce gamma-rays and lower energy photons via interactions with the ambient plasma. The recently reported observation of TeV gamma-rays from SN1006 by the CANGAROO Collaboration, combined with the fact that several unidentified EGRET sources have been associated with known radio/optical/X-ray-emitting remnants, provides powerful motivation for studying gamma-ray emission from SNRs. In this paper, we present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like SNRs. These non-linearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays which deviate from pure power-laws. Such deviations are crucial to acceleration efficiency considerations and impact photon intensities and spectral shapes at all energies, producing GeV/TeV intensity ratios that are quite different from test particle predictions.
Baring, Matthew G.↗
SU-G-JeP4-10: Measurement of Prostate Motion Trajectories During Prostate SBRT Boost Treatments with a Rectafix
Purpose: To determine prostate motion during SBRT boost treatments with a Rectafix rectal sparing device in place using kV imaging during treatment. Methods: Patients each had three gold fiducial markers inserted into the prostate and received two VMAT boost fractions of 9.5–10 Gy under the PROMETHEUS clinical trial protocol with a Rectafix rectal retractor in place. Two-dimensional kilovoltage images of fiducial markers were acquired continuously during delivery. Three patients were treated on a Varian Clinac iX linear accelerator (6X, 600 MU/min), where kV images were acquired at 5 Hz during treatment. Seven patients were treated on a Varian Truebeam linear accelerator (10XFFF, 2400 MU/min) where kV images were acquired every 3 seconds. Images were processed off-line using the Kilovoltage Intrafraction Monitoring (KIM) software after treatment. KIM determines prostate position in three dimensions from 2D kV projections using a probability density model and a pre-treatment kV arc. The 3D displacement of the prostate was quantified as a function of time throughout each fraction. Results: From all fractions analyzed, it was found that the prostate had moved less than 1 mm in any direction from its initial position 84.6% of the time. The prostate was between 1 and 2 mm from its initial position 14.2% of the time, between 2 and 3 mm of its initial position 0.8% of the time and was greater than 3 mm from its initial position only 0.4% of the time. Conclusion: The amount of prostate motion observed during prostate SBRT boost treatments with a Rectafix device in place was minimal and lower than that observed in non-Rectafix studies. The Rectafix device reduces rectal dose as well as immobilizing the prostate. Kimberley Legge is the recipient of an Australian Postgraduate Award.
60 APPLIED LIFE SCIENCES↗
SU-F-T-84: Measurement and Monte-Carlo Simulation of Electron Phase Spaces Using a Wide Angle Magnetic Electron Spectrometer
Purpose: To measure and simulate well-defined electron spectra using a linear accelerator and a permanent-magnetic wide-angle spectrometer to test the performance of a novel reconstruction algorithm for retrieval of unknown electron-sources, in view of application to diagnostics of laser-driven particle acceleration. Methods: Six electron energies (6, 9, 12, 15, 18 and 21 MeV, 40cm × 40cm field-size) delivered by a Siemens Oncor linear accelerator were recorded using a permanent-magnetic wide-angle electron spectrometer (150mT) with a one dimensional slit (0.2mm × 5cm). Two dimensional maps representing beam-energy and entrance-position along the slit were measured using different scintillating screens, read by an online CMOS detector of high resolution (0.048mm × 0.048mm pixels) and large field of view (5cm × 10cm). Measured energy-slit position maps were compared to forward FLUKA simulations of electron transport through the spectrometer, starting from IAEA phase-spaces of the accelerator. The latter ones were validated against measured depth-dose and lateral profiles in water. Agreement of forward simulation and measurement was quantified in terms of position and shape of the signal distribution on the detector. Results: Measured depth-dose distributions and lateral profiles in the water phantom showed good agreement with forward simulations of IAEA phase-spaces, thus supporting usage of this simulation source in the study. Measured energy-slit position maps and those obtained by forward Monte-Carlo simulations showed satisfactory agreement in shape and position. Conclusion: Well-defined electron beams of known energy and shape will provide an ideal scenario to study the performance of a novel reconstruction algorithm using measured and simulated signal. Future work will increase the stability and convergence of the reconstruction-algorithm for unknown electron sources, towards final application to the electrons which drive the interaction of TW-class laser pulses with nanometer thin target foils to accelerate protons and ions to multi-MeV kinetic energy. Cluster of Excellence of the German Research Foundation (DFG) “Munich-Centre for Advanced Photonics”.
07 ISOTOPE AND RADIATION SOURCES↗
Initial performance of the magnet system in the splitter/combiner section of the Cornell-Brookhaven Energy-Recovery Linac Test Accelerator
The Cornell-Brookhaven Energy-recovery Linac Test Ac- celerator is a four-pass, 150-MeV electron accelerator with a six-cell 1.3 GHz superconducting-RF linear accelerator and a fixed-field alternating-gradient (FFAG) return loop made up of Halbach-style quadrupole magnets. The optics matching between the linear accelerator and the return loop is achieved with a conventional magnet system comprised of 50 dipole magnets and 64 quadrupole magnets in four beamlines at each end of the linac. The 42-, 78-, 114- and 150-MeV electron beams are separated into independent vacuum chambers in order to allow for the path-length ad- justment required by energy recovery. We report on the first beam tests of the initial installation of the splitter/combiner section at the exit of the linac. The vacuum system of the 42-MeV S1 line was installed during the first week of April. Nine dipole and four quadrupole magnets were installed and surveyed into position the following week, and the water cooling system was commissioned. A 6-MeV beam passed through the line on April 11 with no need for adjusting pre-set magnet excitation currents. One week later, time- of-flight measurements were used to calibrate and phase the individual superconducting RF cavities. The S1 magnet settings were then scaled up to achieve 5-cavity, 42-MeV operation through the first nine FFAG permanent-magnet quadrupoles. This initial Fractional Arc Test will conclude on May 18, when the installation of the remaining seven split- ter/combiner lines and the return loop will begin. CBETA operations are scheduled to begin in early 2019.
43 PARTICLE ACCELERATORS↗
Plastic scintillator block as photon beam monitor for EGRET calibration
The EGRET (Energetic Gamma Ray Experiment Telescope) detector has been calibrated at SLAC (Stanford Linear Accelerator) and, to a lesser degree, at the MIT Bates Linear Accelerator Center. To monitor the photon beams for the calibration, a plastic scintillator block, 5 cm x 5 cm in cross section, 15 cm in length, and viewed by a single photomultiplier tube, was used for the entire beam energy range of 15 MeV to 10 GeV. The design operation, and method of analysis of the beam intensity are presented. A mathematical framework has been developed to treat the general case of a beam with multiphoton beam pulses and with a background component. A procedure to deal with the fluctuations of the beam intensity over a data-taking period was also developed. The photon beam monitor is physically sturdy, electronically steady, simple to construct, and easy to operate. Its major merits lie in its sheer simplicity of construction and operation and in the wide energy range it can cover.
Lin, Y. C.↗
SU-E-T-589: A Comparison of Field Size Dependence of Electron Depth Dose From Different Linacs
Purpose: For accurate dose calculation in electron beam therapy, it is important to know the percentage depth dose (PDD) for each beam. This can vary depending on the machine make and model and the field size. Three different linear accelerators were compared in this study. Methods: PDD data was collected for different output beam energies and different field sizes for three different linear accelerators (Siemens Primus, Varian 2300ix, Varian Truebeam). Data was compared for the same energy with the same field size to see if the PDD differed among manufacturers. Furthermore, PDD was compared for different field sizes of the same machine at the same energy. Results: For the same beam energy and the same field size, the PDD curves were comparable for the three linacs with variations within 13%. PDD curves for different field sizes and beam energies were compared to verify this result. At higher beam energies, the disagreement between PDD curves is more pronounced between different field sizes for all three of the linacs compared. Conclusions: For the same energy and field size, the variation between different machines was within 13%. For the same manufacturer (Varian Clinac 2300ix and Truebeam), the agreement is within 3% with a standard deviation of less than 2.5%. PDD curves for different field sizes for the same energy were also investigated for the three linacs.
60 APPLIED LIFE SCIENCES↗
Computational Study of 15 MeV Linac Radiation Doses at the UNLV Accelerator Facility
Linear electron accelerators (linacs) are used to generate Bremsstrahlung photons through the interaction of an electron beam with a heavy metal target. These Bremsstrahlung photons with endpoint energy equal to that of the impinging electron beam are typically collimated into a desired beam shape (fan or cone beams). At electron energies larger than 10 MeV, the interaction of the newly produced photons with the linac component materials leads to the production of neutrons. These neutrons are often undesired and pollute the collimated photon beam while increasing the dose rates at the facility due to the linac operation. MCNPX 2.7.0 code was used to investigate the production of neutrons due to the normal operation of a 15 MeV linac. Dose rates within the UNLV accelerator facility were calculated for both photons and neutrons. Furthermore, the irradiation of several materials using the K15 linac was modeled; the resulting neutron fluxes and dose rates were determined. (authors)
43 PARTICLE ACCELERATORS↗
Influence of Exposure to Multiple Brazing Cycles on the Integrity of OFE Copper Brazed Joints
A 10 MeV, 5 kW S-band travelling wave electron linear accelerator has been developed at Raja Ramanna Centre for Advanced Technology for industrial applications. The electron accelerating structure is made of oxygen-free electronic (OFE) copper and comprises of fifty-six brazed joints and is fabricated in multiple stages, as per the process and testing requirements. The accelerating structure is often required to be repaired through an additional brazing cycle with the same filler. The present study evaluates possible microstructural and mechanical property degradation of OFE copper brazed joints, as a result of its exposure to multiple brazing cycles. The study was performed on an OFE copper cavity stack brazed assembly, made through a four-stage brazing using BVAg-8 as braze filler. The resultant brazed assembly did not exhibit any leak at a sensitivity of 1 × 10{sup −10} mbar.L/s. The results of the study demonstrate that OFE copper brazements can be safely subjected to an additional brazing cycle without causing significant degradation in its microstructure and strength. However, brazed joint’s exposure to any additional brazing cycles results in the formation of voids and cracks due to intergranular penetration of silver, with associated drop in tensile strength from about 210 MPa to about 150-180 MPa. Based on the results of study, it is recommended to qualify the accelerating structure, fabricated through multiple brazing cycles, by a suitable non-destructive technique. The results of the study are important for design, fabrication and maintenance of electron linear accelerator components.
36 MATERIALS SCIENCE↗
The development and verification of a highly accurate collision prediction model for automated noncoplanar plan delivery
Purpose: Significant dosimetric benefits had been previously demonstrated in highly noncoplanar treatment plans. In this study, the authors developed and verified an individualized collision model for the purpose of delivering highly noncoplanar radiotherapy and tested the feasibility of total delivery automation with Varian TrueBeam developer mode. Methods: A hand-held 3D scanner was used to capture the surfaces of an anthropomorphic phantom and a human subject, which were positioned with a computer-aided design model of a TrueBeam machine to create a detailed virtual geometrical collision model. The collision model included gantry, collimator, and couch motion degrees of freedom. The accuracy of the 3D scanner was validated by scanning a rigid cubical phantom with known dimensions. The collision model was then validated by generating 300 linear accelerator orientations corresponding to 300 gantry-to-couch and gantry-to-phantom distances, and comparing the corresponding distance measurements to their corresponding models. The linear accelerator orientations reflected uniformly sampled noncoplanar beam angles to the head, lung, and prostate. The distance discrepancies between measurements on the physical and virtual systems were used to estimate treatment-site-specific safety buffer distances with 0.1%, 0.01%, and 0.001% probability of collision between the gantry and couch or phantom. Plans containing 20 noncoplanar beams to the brain, lung, and prostate optimized via an in-house noncoplanar radiotherapy platform were converted into XML script for automated delivery and the entire delivery was recorded and timed to demonstrate the feasibility of automated delivery. Results: The 3D scanner measured the dimension of the 14 cm cubic phantom within 0.5 mm. The maximal absolute discrepancy between machine and model measurements for gantry-to-couch and gantry-to-phantom was 0.95 and 2.97 cm, respectively. The reduced accuracy of gantry-to-phantom measurements was attributed to phantom setup errors due to the slightly deformable and flexible phantom extremities. The estimated site-specific safety buffer distance with 0.001% probability of collision for (gantry-to-couch, gantry-to-phantom) was (1.23 cm, 3.35 cm), (1.01 cm, 3.99 cm), and (2.19 cm, 5.73 cm) for treatment to the head, lung, and prostate, respectively. Automated delivery to all three treatment sites was completed in 15 min and collision free using a digital Linac. Conclusions: An individualized collision prediction model for the purpose of noncoplanar beam delivery was developed and verified. With the model, the study has demonstrated the feasibility of predicting deliverable beams for an individual patient and then guiding fully automated noncoplanar treatment delivery. This work motivates development of clinical workflows and quality assurance procedures to allow more extensive use and automation of noncoplanar beam geometries.
60 APPLIED LIFE SCIENCES↗
Small field in-air output factors: The role of miniphantom design and dosimeter type
Purpose: The commissioning of treatment planning systems and beam modeling requires measured input parameters. The measurement of relative output in-air, S{sub c} is particularly difficult for small fields. The purpose of this study was to investigate the influence of miniphantom design and detector selection on measured S{sub c} values for small fields and to validate the measurements against Monte Carlo simulations. Methods: Measurements were performed using brass caps (with sidewalls) or tops (no sidewalls) of varying heights and widths. The performance of two unshielded diodes (60012 and SFD), EBT2 radiochromic film, and a fiber optic dosimeter (FOD) were compared for fields defined by MLCs (5–100 mm) and SRS cones (4–30 mm) on a Varian Novalis linear accelerator. Monte Carlo simulations were performed to theoretically predict S{sub c} as measured by the FOD. Results: For all detectors, S{sub c} agreed to within 1% for fields larger than 10 mm and to within 2.3% for smaller fields. Monte Carlo simulation matched the FOD measurements for all size of cone defined fields to within 0.5%. Conclusions: Miniphantom design is the most important variable for reproducible and accurate measurements of the in-air output ratio, S{sub c}, in small photon fields (less than 30 mm). Sidewalls are not required for fields ≤ 30 mm and tops are therefore preferred over the larger caps. Unlike output measurements in water, S{sub cp,} the selection of detector type for S{sub c} is not critical, provided the active dosimeter volume is small relative to the field size.
61 RADIATION PROTECTION AND DOSIMETRY↗
Dosimetric characterization and output verification for conical brachytherapy surface applicators. Part II. High dose rate {sup 192}Ir sources
Purpose: Historically, treatment of malignant surface lesions has been achieved with linear accelerator based electron beams or superficial x-ray beams. Recent developments in the field of brachytherapy now allow for the treatment of surface lesions with specialized conical applicators placed directly on the lesion. Applicators are available for use with high dose rate (HDR){sup 192}Ir sources, as well as electronic brachytherapy sources. Part I of this paper discussed the applicators used with electronic brachytherapy sources. Part II will discuss those used with HDR {sup 192}Ir sources. Although the use of these applicators has gained in popularity, the dosimetric characteristics have not been independently verified. Additionally, there is no recognized method of output verification for quality assurance procedures with applicators like these. Methods: This work aims to create a cohesive method of output verification that can be used to determine the dose at the treatment surface as part of a quality assurance/commissioning process for surface applicators used with HDR electronic brachytherapy sources (Part I) and{sup 192}Ir sources (Part II). Air-kerma rate measurements for the {sup 192}Ir sources were completed with several models of small-volume ionization chambers to obtain an air-kerma rate at the treatment surface for each applicator. Correction factors were calculated using MCNP5 and EGSnrc Monte Carlo codes in order to determine an applicator-specific absorbed dose to water at the treatment surface from the measured air-kerma rate. Additionally, relative dose measurements of the surface dose distributions and characteristic depth dose curves were completed in-phantom. Results: Theoretical dose distributions and depth dose curves were generated for each applicator and agreed well with the measured values. A method of output verification was created that allows users to determine the applicator-specific dose to water at the treatment surface based on a measured air-kerma rate. Conclusions: The novel output verification methods described in this work will reduce uncertainties in dose delivery for treatments with these kinds of surface applicators, ultimately improving patient care.
62 RADIOLOGY AND NUCLEAR MEDICINE↗