Collimation System Research Articles

Neutron imaging at the n_TOF facility of CERN

A neutron radiography station has been developed at the n_TOF facility of CERN. The characteristics of the neutron beam line and the imaging setup are presented in this paper together with first experimental results. A comparison of two different collimation systems is discussed and further improvements and upgrades are outlined.

Simulation Research of Multi-Pinhole Collimated L-Shell XFCT Imaging System

Polychromatic source X-ray fluorescence computed tomography (XFCT) is a novel imaging method, which is applied for the diagnosis and treatment studies of early stage cancer. In this paper, we propose a fast multi-pinhole L-shell XFCT imaging system to reduce the scan time and radiation dose, which has potential in detecting lower concentration of contrast agents (gold nanoparticles, GNPs). Two kinds of phantoms - the concentration-phantom 1 and the size-phantom 2 are designed to verify imaging performance for low-concentration and small-size ROI. The scanning processes are simulated by Geant4, and images are reconstructed by Optimized EM-TV algorithm. It is concluded that this imaging system is more sensitive in detecting low concentration GNPs than K-shell imaging system. Simulation experiments show the reconstructed images can achieve the highest CNR both for phantom 1 and phantom 2 with iterating 10 times. The detection limit can reach 0.16% when pinhole radius is 0.08mm.

The comparison of VMAT test results for Clinac 2300C/D and TrueBeam accelerators

Volumetric Modulated Radiotherapy (VMAT) implementation requires additional Quality Assurance (QA) tests to assure stable machine performance especially in terms of dynamic parameters synchronization. The lack of a twin machine for TrueBeam led us to the investigation of backup workflow with Clinacs 2300C/D. These Clinacs were upgraded to make them VMAT-enabled. This study aimed to compare long-term VMAT performance QA on 3 accelerators: TrueBeam (TB) and 2 Clinacs (V4 and V5). All VMAT test plans were provided by Varian. The test set consisted of initial and advanced tests. Initial tests were intended to check the gravity effect on Multileaf Collimator (MLC) and dosimetry system. As the results of these tests were correct and there was visual inspection applied for MLC positioning accuracy analysis, they were not presented in the paper. We focused on 2 advanced tests: dose rate vs gantry speed and dose rate vs MLC speed. The idea of the advanced test was to compare segments irradiated with the same fluence but different dose rate, gantry speed or MLC speed. Test sets were irradiated weekly on average for 12 months. These tests were analysed following Varian procedures and criteria using in-house-developed software. Apart from that we calculated correlation between all segments pairs and performed profile analysis. According to Varian criteria, all tests for TrueBeam were very well within the tolerances. Dose rate vs gantry speed tests for Clinacs were within allowed limits while as many as 28% and 6% of dose rate vs MLC speed tests failed for Clinacs V4 and V5, respectively. The profile analysis revealed tests for which the difference between measured and planned dose was over 3% and still met the criteria. The correlation analysis showed that VMAT plans on TrueBeam were irradiated repeatably because all segments were strongly correlated. There was no correlation between the segment with the highest MLC speed and the other segments on Clinacs in dose rate vs MLC speed test. This segment was irradiated randomly. TrueBeam is more reliable than upgraded Clinacs 2300C/D for VMAT performance. That is why in our centre the one of upgraded Clinac that performed tests better served only as a backup machine for VMAT technique, and the second one was excluded from clinical use for this technique.

Bismuth nanosheets based saturable-absorption passively <i>Q</i>-switching mid-infrared single-crystal fiber laser

As a new two-dimensional material, bismuth nanosheet is an effective modulator for realizing a mid-infrared pulsed laser, which benefits from its suitable band gap, higher carrier mobility and better room temperature stability, as well as its excellent electrical and optical properties. The mid-infrared single-crystal fiber is a preferable gain medium for high-power laser because of its advantages of both crystal and fiber. In this paper, a bismuth nanosheet saturable absorber is successfully prepared by the ultrasonic method and used for the first time in a diode-pumped Er:CaF<sub>2</sub> single-crystal fiber mid-infrared passively <i>Q</i>-switching pulsed laser. A compact concave planar linear resonator is designed to study the <i>Q</i>-switching Er:CaF<sub>2</sub> single-crystal fiber laser with bismuth nanosheets serving as saturable absorbers. The pump source is a fiber-coupled semiconductor laser with a core diameter of 105 μm, a numerical aperture of 0.22, and a central emission wavelength of 976 nm. The pump light is focused onto the front end of the gain medium through a coupled collimating system with a coupling ratio of 1∶2. The gain medium is a 4 at.% Er<sup>3+</sup>:CaF<sub>2</sub> single-crystal fiber grown by the temperature gradient method, and this fiber has two polished but not coated ends, a diameter of 1.9 mm, and a length of 10 mm. To reduce the thermal effect, the single-crystal fiber is tightly wrapped with indium foil and mounted on a copper block with a constant temperature of 12 ℃. The input mirror has a high reflection coating at 2.7–2.95 μm and an antireflection coating at 974 nm, with a curvature radius of 100 mm. A group of partially transmitting plane mirrors are used as output couplers, respectively, with transmittances of 1%, 3%, and 5% at 2.7–2.95 μm. The total length of the resonant cavity is 26 mm. By inserting the bismuth nanosheet into the resonator and carefully adjusting its position and angle, a stable mid-infrared <i>Q</i>-switching laser is obtained. At the absorbed pump power of 1.52 W, a pulsed laser with an average output power of 190 mW is obtained for an output mirror with a transmittance of 3%. The shortest pulse width is 607 ns, the repetition frequency is 58.51 kHz, and the corresponding single pulse energy and peak power are 3.25 μJ and 5.35 W, respectively.

Non-Destructive Examination Development for the JHR Material Testing Reactor

The Jules Horowitz Reactor (JHR) is a European material testing reactor (MTR) under construction at the CEA Cadarache centre. It will be dedicated to material and fuel irradiation tests, as well as to the production of medical isotopes. Gamma and X-Ray benches will be implemented in the reactor pool (RER), the irradiated component storage pool (EPI) and in a shielded hot cell for measuring either the whole underwater test device still containing the experimental sample or just the experimental sample before its extraction in the hot cell. The CEA/Cadarache Nuclear Measurement Laboratory (LMN) has been working in collaboration with VTT (Technical Research Centre in Finland Ltd.) since 2008 under a Finnish in-kind contribution agreement. This agreement focuses on the development of NDE systems implementing gamma-ray spectroscopy and high-energy X-ray imaging of the sample and irradiation device with the highest definition possible (resolution of 100 μm). The CEA-VTT technical specifications led to a European call for tenders launched by VTT. The contract was awarded to the Spanish company IDOM for the design, manufacturing, assembly and commissioning of: - Underwater gamma and X-ray (UGXR) mechanical benches and their associated gamma and X-ray collimation systems for the RER and EPI pools - Hot cell gamma and X-ray (HGXR) bench in the JHR NDE hot cell. The Final Design Reviews (FDR) of the UGXR and HGXR systems were completed in 2016. The design phase has been an iterative process in order to manage interfacing specifications and constraints: - Challenging experimental requirements, mainly to cover the wide diversity of sample shapes, sample activity levels and measurement processes, but also to achieve a level of mechanical accuracy to reach the ambitious geometrical resolution target in X-ray imaging, - Environmental constraints (immersion, radiation, compactness, limited accessibility for maintenance), - Nuclear safety constraints (seism, radiation protection). The whole design process has produced a number of elaborate and innovative mechatronic systems, which is rather unusual in nuclear applications since the resulting solutions have benefited from IDOM’s technological expertise in designing and commissioning large telescopes for the astronomy sector. Once the manufacturing phase and assembly finalised, the site acceptance tests for the UGXR and HGXR mechanical systems will be performed in 2019-2020 in the TOTEM facility at the CEA Cadarache center. The underwater benches will be tested in the CESARINE pool to check their requirements.

Highly versatile laboratory X-ray scattering instrument enabling (nano-)material structure analysis on multiple length scales by covering a scattering vector range of almost five decades.

A compact laboratory X-ray scattering platform that uniquely enables for high-performance ultra-small-angle X-ray scattering (USAXS), small- and wide-angle X-ray scattering (SAXS/WAXS), and total scattering (atomic pair distribution function analysis; PDF) experiments was developed. It covers Bragg spacings from sub-Angstroms to 1.7 μm, thus allowing the analysis of dimensions and complex structures in (nano-)materials on multiple length scales. The accessible scattering vector q-range spans over almost five decades (qmin = 0.0036 nm-1, qmax = 215 nm-1), without any gaps. Whereas SAXS is suitable to characterize materials on a length scale of 1-100 nm, with USAXS, this range can be significantly extended to the micrometer range. On the other end, from WAXS and particularly from PDF measurements, information about the local atomic order and disorder can be obtained. The high performance, exceptional versatility, and ease-of-use of the instrument are enabled by a high-resolution 2-circle goniometer with kinematic mounts, a modular concept based on prealigned, quickly interchangeable X-ray components, and advanced detector technology. For USAXS measurements, a modified Bonse-Hart experimental setup with single crystal collimator and analyzer optics is used. SAXS/WAXS measurements are enabled by focusing optics, an evacuated beam path, and a 2D detector. For total scattering experiments, a high-energy X-ray source is used in combination with a hybrid pixel array detector that is based on a CdTe sensor for the highest counting efficiency. To ensure high resolution and sensitivity in these various applications, special care is taken to suppress any type of background scattering signal. The high resolution that can be achieved with the USAXS collimation system is demonstrated on a set of monodisperse, colloidal silica dispersions and derived colloidal crystals, with particle diameters in the range of hundreds of nanometers up to 1.6 µm. USAXS and SAXS results are shown to be consistent with those obtained by static light scattering (SLS) and dynamic light scattering. It is demonstrated that the obtainable USAXS data bridge the gap in q between SAXS and SLS. The capabilities of the instrument to acquire high-quality total scattering data for PDF analysis are demonstrated on amorphous SiO2 nanoparticles as well as on NaYF4 upconversion nanocrystals. To the best of our knowledge, it is for the first time that we present a single laboratory instrument that enables measurements of high-quality X-ray scattering data within such a wide q-range, by combining four complementary elastic X-ray scattering techniques. The modular design concept of the instrument allows for incremental improvements as well as to add more applications in the future.

Wavefront distortion analysis of a microwave collimator system with complex horn feed

Phase structure distortion of the probe field that occurs when the collimator-type assemblies of irradiators used in measuring the polarization scattering matrices of location objects are used as part of the radar measuring complex was discussed. It is shown that, due to the specific design, the phase centers of the irradiators in the assembly are removed from the collimator focus, resulting in the appearance of longitudinal-transverse defocusing, which is expressed in the slope and curvature of the wavefront of the probe field. For a quantitative analysis of these distortions, a method for calculating the ray path in the «irradiator – collimator mirror – test zone» system is proposed. For the MAK-5 collimator, numerical estimates of the phase distribution and slope of the wavefront of the probe field, as well as variations in bistatic angles corresponding to the use of an irradiator assembly for radar measurements, were obtained. It is shown that, with the displacements of the phase centers of the irradiators in the assembly not exceeding the wavelength, the field formed by them has a quasi-plane character – the phase change in the plane perpendicular to the propagation direction does not exceed 8°. However, the slope of the wavefront of the probe field to the longitudinal axis of the collimator, as well as the fundamentally bistatic nature of the measurements, are sources of inextricable systematic errors in measuring the scattering characteristics of targets when using irradiator assemblies.

Heavy ion accelerator facility front end design and commissioning

A new project, high intensity heavy ion accelerator facility (HIAF), is currently under design and construction in China. The HIAF front end, composed of electron cyclotron resonance (ECR) ion sources, low energy beam transport (LEBT) and radio frequency quadrupole (RFQ), will produce and provide beams of ions with a mass up to uranium at a beam energy of $0.5\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$. The typical beam intensity is designed up to 2 emA for the uranium beam with a charge state of $35+$. This paper presents an overall design of the front end for HIAF and discusses several key issues in the design. By modeling the beam extraction from the ECR source, we got a reliable starting beam condition to perform the design. Transverse coupling of the beam from the source was elaborated. To relieve the coupling we implanted two solenoids after the source. Space charge effect in the charge state selection of the ion source was evaluated. An overall space charge compensation degree of no less than 70% was predicted. A beam dynamics simulation was performed by using the initial particle distribution obtained from the extraction modeling. The simulation resulted in development of a beam collimation system in the LEBT to confine the transverse emittance. The RFQ design will follow the development of LEAF-RFQ at Institute of Modern Physics, which has successfully commissioned with several beams and demonstrated as an excellent design. Recent beam commissioning results of LEAF-RFQ will also be presented in this paper.

Collimation system studies for the FCC-hh

The Future Circular Collider (FCC-hh) is being designed as a 100 km ring that should collide 50 TeV proton beams. At 8.3 GJ, its stored beam energy will be a factor 28 higher than what has been achieved in the Large Hadron Collider, which has the highest stored beam energy among the colliders built so far. This puts unprecedented demands on the control of beam losses and collimation, since even a tiny beam loss risks quenching superconducting magnets. We present in this article the design of the FCC-hh collimation system and study the beam cleaning through simulations of tracking, energy deposition, and thermo-mechanical response. We investigate the collimation performance for design beam loss scenarios and potential bottlenecks are highlighted.

Ultra-High Vacuum characterization of Molybdenum-Carbide Graphite for HL-LHC collimators

In view of the High-Luminosity upgrade of the Large Hadron Collider (LHC) collimation system, a family of novel molybdenum-carbide graphite (MoGr) composites was developed to meet the challenging requirements of HL-LHC beam-halo collimation, in particular the electrical conductivity and thermo-mechanical performances. The Ultra-High Vacuum (UHV) behaviour of this material was extensively characterized to assess its compatibility with the accelerator’s specifications. The results presented in this paper correlate the outgassing behaviour with the microscopic features of MoGr compared to other graphite-based materials. Residual gas analysis (RGA) was exploited to optimize post-production treatments.

First measurements of a magnetically driven fast-ion loss detector on ASDEX Upgrade

A new scintillator-based fast-ion loss detector (FILD) has been deployed ∼45̂ below the midplane of the ASDEX Upgrade tokamak. Port unavailability at this remote location requires an in-situ magnetically driven manipulator to move the diagnostic head horizontally through the scrape-off layer (SOL). The linear displacement is produced by an externally energized coil, whose magnetic dipole tries to align with the toroidal component of the tokamak magnetic field. The insertion is given by force balance between a retaining spring and the energized solenoid, whose current is regulated in real-time, opening the possibility of self-adaptive real-time control of the probe head location based on its temperature. The diagnostic head contains a scintillator screen, Faraday cup, thermocouple and collimator systems. The scintillator image is transferred to a vacuum window using a 3.5 meters quartz image guide. The light acquisition system is composed by a charge coupled device (CCD) camera, for high velocity-space resolution, and by an 8×4 channels avalanche photo diode (APD) camera, for high temporal resolution (up to 2 MHz). First measurements of magnetohydrodynamic (MHD) induced fast-ion losses and radially resolved fast-ion losses are presented.

High Luminosity LHC Optics and Layout HLLHCV1.4

The goal of the High Luminosity Project is the upgrade of the LHC to deliver an integrated luminosity of at least 250 fb-1 per year in each of the two high-luminosity, general-purpose detectors ATLAS and CMS. This article presents the latest layout design and the corresponding optics features, which comprise optimisation of the orbit corrector and crab cavity systems, and new estimates of the performance reach thanks to the new concept of fully remote alignment. In addition, the new optics version incorporates improvements required by beam instrumentation, dump system, and collimation system, as well as low-beta solutions for the LHCb experiment.

Collimation of target induced halo following MAGIX at MESA

The Mainz Energy-recovering Superconducting Accelerator (MESA) will be an electron accelerator allowing operation in energy-recovery linac (ERL) mode. It provides the opportunity to operate scattering experiments at energies of ~ 100 MeV with thin gas-targets. The MESA Internal Gas Target Experiment (MAGIX) aims to operate windowless jet targets and different gases up to Xenon to search for possible dark photon interactions, to precisely measure the magnetic proton radius and astrophysical S-factors. Investigations on the impact of the target on beam dynamics and beam losses are required for machine safety and to examine limits to ERL operation. The goal of this work is to understand target induced halo in the different experimental setups, track halo particles through downstream sections to examine beam losses and include a suitable collimation system and shielding into the accelerator layout to protect the machine from direct and indirect damage through beam losses and radiation. The present status of the investigations is presented.

The development programme of cathodes and electron guns for the Hollow Electron Lenses of the High Luminosity LHC project1

The High Luminosity LHC project (HL-LHC) foresees the construction and installation of important new equipment to increase the performance of the LHC machine. The Hollow Electron Lens (HEL) is a promising system to control the beam halo. It improves the beam collimation system of the HL-LHC and mitigates possible equipment damage in case of failure scenarios from halo losses. The halo can store up to 30 MJ energy. The specifications for this new device are quite demanding. The source, an electron gun with an annular shaped cathode, has to deliver a current up to 5 A. This is five times higher than the current in the existing electron lenses in Fermi and Brookhaven national laboratories. This note describes the programme carried out to design and test high-perveance guns equipped with two types of high-performance scandate cathodes. The size of the final gun is now considerably smaller than the one of the first prototype, allowing a reduction of diameter and cost of the superconducting magnet system used to steer the electron beam. The tests carried out at FNAL, BVERI and BJUT demonstrated that the developed cathodes fulfil the specifications and can supply a 5 A fully Space Charge Limited (SCL) current.

Collimation of heavy-ion beams in the HE-LHC

A design study for a future collider to be built in the LHC tunnel, the High-Energy Large Hadron Collider (HE-LHC), has been launched as part of the Future Circular Collider (FCC) study at CERN. It would provide proton collisions at a centre-of-mass energy of 27 TeV as well as collisions of heavy ions at the equivalent magnetic rigidity. HE-LHC is being designed under the stringent constraint of using the existing tunnel and therefore the resulting lattice and optics differ in layout and phase advance from the LHC. It is necessary to evaluate the performance of the collimation system for ion beams in HE-LHC in addition to proton beams. In the case of ion beams, the fragmentation and electromagnetic dissociation that relativistic heavy ions can undergo in collimators, as well as the unprecedented energy per nucleon of the HE-LHC, requires dedicated simulations. Results from a study of collimation efficiency for the nominal lead ion (208Pb82+) beams performed with the SixTrack-FLUKA coupling framework are presented. These include loss maps with comparison against an estimated quench limit as well as detailed considerations of loss spikes in the superconducting aperture for critical sections of the machine such as the dispersion suppressors.

Variable tilt-angle, parallel-hole collimation system for high-resolution molecular imaging gamma tomosynthesis

PurposeThis study investigates a novel gamma tomosynthesis (GT) method based on a variable tilt-angle, parallel-hole collimator (VAPHC) which, mounting to a conventional gamma, is able to perform high-resolution three-dimensional imaging. MethodsThe VAPHC has the remarkable feature to be modular, consisting of independent collimation elements able to tilt according to variable angles [−45° to +45°]. Spatial resolutions were measured in reconstructed GT images using a point source at different source-to-collimator distances, while sensitivity was evaluated over the range of slant angles using a disk-source. Image contrast (IC) and contrast-to-noise-ratio (CNR) of sub-centimeters tumors were evaluated using a breast phantom containing a background activity and spheres filled with 99mTc to simulate lesions at two depths. Breast phantom GT images were compared with planar and circular-orbit SPECT acquisitions of equal scan-time. ResultsPlanar spatial resolutions range from 9 to 14 mm over a depth range of 6–10 cm; spatial resolution in depth dimension becomes two times greater than those in the other dimensions. The measured sensitivity decreases from 9 cps/μCi to 6 cps/μCi varying the slant angle from 5° to 45°. The measured IC and CNR of GT reconstructed images demonstrated that it was possible to improve the spatial resolution/sensitivity trade-off. ConclusionsThe proposed GT based VAPHC demonstrated the potential for superior spatial resolution and contrast compared to planar and SPECT acquisitions. A conventional gamma camera equipped with the VAPHC could be located at the minimum distance from the patient, thus improving detection, localisation and characterisation of sub-centimetre lesions.

Development of Cold Neutron Radiography Facility (CNRF) based on China Mianyang Research Reactor (CMRR)

To carry on neutron imaging researches (radiography/tomography), we built a Cold Neutron Radiography Facility (CNRF), based on the China Mianyang Research Reactor (CMRR) at Institute of Nuclear and Chemistry (INPC). This CNRF system utilizes cold neutrons from the C1 neutron guide. The facility consists of beam shutter, flight tubes, beam limiter, object handling system, imaging system, beam stop and data acquisition system. The maximum neutron flux at the sample position is 6 × 106 n/cm2/s, approximately. There are seven pinholes in the collimator system, to offer a possible L/D from 200 to 1300. With different imaging lenses, the imaging system could provide four different FOVs. The performance parameters are tested by Siemens Star and ASTM standard devices-beam purity indicator (BPI) and image sensitivity indicator (SI). Our results show the resolution of CNRF is better than 20μm (i.e. 25lp/mm), and its typical radiography image is of Category-I, as defined in ASTM E545.

Influence of dynamic impact on optical properties of optical transmission window of laser measurement system

The light transmission window is an important part of the optical system, and the high overload environment seriously affects the working quality of the laser receiving system. In this paper, the combination of dynamics and optical numerical simulation is used to analyze the optical characteristics of the light-transmissive window after undergoing high overload. Using the dynamic finite element analysis method, a simulated high overload shock was applied to the light transmission window to study the deformation of the light transmission window. The light transmission windows before and after the impact were optically simulated under the same conditions, and the optical characteristics of the light transmission window before and after high overload were compared and analyzed. The results show that with the increase of the impact acceleration, the brightness of the center of the exit spot after the collimation system is collimated and transmitted through the light transmission window is reduced, the spot size is increased, and the energy is diverged.

Trimmer sequencing time minimization during dynamically collimated proton therapy using a colony of cooperating agents

The dynamic collimation system (DCS) can be combined with pencil beam scanning proton therapy to deliver highly conformal treatment plans with unique collimation at each energy layer. This energy layer-specific collimation is accomplished through the synchronized motion of four trimmer blades that intercept the proton beam near the target boundary in the beam’s eye view. However, the corresponding treatment deliveries come at the cost of additional treatment time since the translational speed of the trimmer is slower than the scanning speed of the proton pencil beam. In an attempt to minimize the additional trimmer sequencing time of each field while still maintaining a high degree of conformity, a novel process utilizing ant colony optimization (ACO) methods was created to determine the most efficient route of trimmer sequencing and beamlet scanning patterns for a collective set of collimated proton beamlets. The ACO process was integrated within an in-house treatment planning system optimizer to determine the beam scanning and DCS trimmer sequencing patterns and compared against an analytical approximation of the trimmer sequencing time should a contour-like scanning approach be assumed instead. Due to the stochastic nature of ACO, parameters where determined so that they could ensure good convergence and an efficient optimization of trimmer sequencing that was faster than an analytical contour-like trimmer sequencing. The optimization process was tested using a set of three intracranial treatment plans which were planned using a custom research treatment planning system and were successfully optimized to reduce the additional trimmer sequencing time to approximately 60 s per treatment field while maintaining a high degree of target conformity. Thus, the novel use of ACO techniques within a treatment planning algorithm has been demonstrated to effectively determine collimation sequencing patterns for a DCS in order to minimize the additional treatment time required for trimmer movement during treatment.

Modelling of the NBI contribution to the neutron energy spectra for the ITER Vertical Neutron Camera

The ITER Vertical Neutron Camera (VNC) is a multichannel collimator system designed to measure the neutron emissivity profile and spectra. The energy spectrum measurements of neutrons made by the VNC will contribute to the reconstruction of the ion temperature, fuel ratio, and fast ions' distribution function. In this paper, the results of neutron spectra calculations for the ITER VNC are presented. The developed software allows for the simulation of particle spectra resulting from interaction of suprathermal ions with thermal background plasma. The method relies on direct reaction rate calculation and explicit fusion reaction kinematics modelling. The assessment of the D-T neutron spectra is carried out for D-NBI heated baseline D-T ITER operation scenarios. It is shown that neutron flux resulting from fast (“beam”) D with thermal T ion interaction dominates at En> 16 MeV part of the spectra. Thus, it is possible to assess fuel ratio based on calculated “thermal-thermal” and “beam-thermal” neutron energy spectra. An ability to observe sawtooth-induced mixing via VNC is studied. Lower energy boundary for the observable part of the suprathermal ion distribution is derived.