Local Shielding Research Articles

Measurement of Gamma and Neutron Dose from the Operation of the ANKA Storage Ring

ANKA is a ramped 2.5-GeV electron synchrotron light source, which can be operated at all energies from 500 MeV to 2.5 GeV. An online area monitoring network makes it possible to measure the radiation from the decaying beam at eight stations every minute. It consists of two stationary and six mobile stations with a gamma and a neutron detector, which are connected with a central computer by a bus system. Measuring devices and software were developed or customized for ANKA.Special machine shifts were dedicated to the systematic investigation of beam lifetime and ambient equivalent dose, which depend on the beam energy, beam current, and operating conditions of the storage ring.The highest dose rates are measured in the forward direction of insertion devices. The beam is lost at the device with the smallest aperture. Local shielding of the storage ring and operating conditions can be improved with time-resolved dose rate measurements at locations with high dose levels. Low beta optics improves the lifetime of electron beams circulating in vacuum chambers with small aperture. This helps to reduce the dose rate. Furthermore, this has the advantage of reducing the dose rate without the drawback of extra costs for additional shielding.

Estimate of the space station shielding thickness at a USLab site using ALTEA measurements and fragmentation cross sections

In this work we presented a method to estimate a local shielding thickness of the International Space Station (USLab), expressed as equivalent of aluminum. The calculation took advantage of total and partial charge-changing cross sections, both measured experimentally and computed theoretically, to obtain the total survival fractions of ions with charge 5 ⩽ Z ⩽ 26 when transversing the space craft hull. The combination of these values with the measured relative abundances outside the space station provided the estimates of the internal relative abundances at several thicknesses. The results have been compared with data measured by ALTEA experiment in the USLab to find the thickness that minimizes the discrepancy between the calculated data and the measurements: the result yielded to an estimate of the shielding thickness at ALTEA site of about 5 cm of aluminum equivalent.

Design of a beam dump for the IFMIF-EVEDA accelerator

The IFMIF-EVEDA accelerator will be a 9 MeV, 125 mA cw deuteron accelerator prototype for verifying the validity of the accelerator design for IFMIF. A beam stop will be used for the RFQ and DTL commissioning as well as for the EVEDA accelerator tests. Therefore, this component must be designed to stop 5 MeV and 9 MeV deuteron beams with a maximum power of 1.13 MW. The first step of the design is the beam-facing material selection. The criteria used for this selection are low neutron production, low activation and good thermomechanical behavior. In this paper, the mechanical analysis and radioprotection calculations that have led to the choice of the main beam dump parameters will be described. The present design is based on a conical beam stop (2.5 m length, 30 cm diameter, and 3.5 mm thickness) made of copper plus a cylindrical 0.5 m long beam scraper. The cooling system is based on an axial high velocity flow of water. This design is compliant with the mechanical design rules during full power stationary operation of the accelerator. The radioprotection calculations performed demonstrate that, with an adequate local shielding, doses during beam on/off phases are below the limits.

Neutron production from beam-modifying devices in a modern double scattering proton therapy beam delivery system

In this work the neutron production in a passive beam delivery system was investigated. Secondary particles including neutrons are created as the proton beam interacts with beam shaping devices in the treatment head. Stray neutron exposure to the whole body may increase the risk that the patient develops a radiogenic cancer years or decades after radiotherapy. We simulated a passive proton beam delivery system with double scattering technology to determine the neutron production and energy distribution at 200 MeV proton energy. Specifically, we studied the neutron absorbed dose per therapeutic absorbed dose, the neutron absorbed dose per source particle and the neutron energy spectrum at various locations around the nozzle. We also investigated the neutron production along the nozzle's central axis. The absorbed doses and neutron spectra were simulated with the MCNPX Monte Carlo code. The simulations revealed that the range modulation wheel (RMW) is the most intense neutron source of any of the beam spreading devices within the nozzle. This finding suggests that it may be helpful to refine the design of the RMW assembly, e.g., by adding local shielding, to suppress neutron-induced damage to components in the nozzle and to reduce the shielding thickness of the treatment vault. The simulations also revealed that the neutron dose to the patient is predominated by neutrons produced in the field defining collimator assembly, located just upstream of the patient.

Development of a local electromagnetic shielding for an extremity magnetic resonance imaging system

A local radio frequency (rf) shielding consisting of a Cu plate and an LC balun circuit has been developed for a compact magnetic resonance imaging (MRI) system with a 0.3 T permanent magnet. Performance of the local rf shielding was evaluated using an artificial external noise source irradiating a human subject whose hand was inserted into the rf coil of the MRI system. Power spectra of the rf signal detected through the rf coil demonstrated that the local rf shield achieved 30.1 dB external noise suppression. With the local rf shielding, a MRI of the subject's hand was performed using a three-dimensional gradient-echo sequence. Anatomical structures of the subject's hand were clearly visualized. It was concluded that the local rf shielding could be used for the compact MRI system instead of a rf shielded room.

Growth of a short fatigue crack – A long term simulation using a dislocation technique

In this study, the growth of a short edge crack during more than 14 000 cycles of fatigue loading is investigated in detail. An edge crack, in a semi-infinite body with no pre-existing obstacles present, is modelled in a boundary element approach by a distribution of dislocation dipoles. The fatigue cycles are fully reversed ( R = - 1 ) , and the load range is well below the threshold for long fatigue cracks. The developing local plasticity consists of discrete edge dislocations that are emitted from the crack tip. The movements of discrete dislocations are restricted to slip along preferred slip planes. The present model is restricted to a 2D plane strain problem with a through-thickness crack, assuming no 3D irregularities. A remote load is applied perpendicular to the crack extension line, and the material parameters are those of a BCC crystal structure. The competition between influence of the global loading on and local shielding of the crack tip governs the crack growth. The growth rate increases in discrete steps with short periods of retardation, from approximately the size of Burgers vector, b, up to 25 b per cycle as the length of the crack is tripled. The plastic zone changes from having an elongated, slender form to include a low angle grain boundary, which, eventually, divides into two parts. The crack growth is found to change from constant acceleration to constant growth rate as the event of the low-angle grain boundary split is approached. The results are compared to long crack characteristics, for which linear elastic fracture mechanics and Paris law can be used to predict fatigue crack growth. The exponent in Paris law varies between 1 and 0 in the present study, i.e. smaller than typical values for ductile BCC materials. The ratio between static and cyclic plastic zone sizes is found to increase during crack growth, and the angle of the general plastic zone direction increases, showing a tendency towards long crack values. The characteristics of the simulated crack growth, found in the present study, are typical for below-threshold growth, with slow acceleration, constant growth rate, and, eventually, either arrest or transition to long crack growth behaviour, as reported in the literature.

Compact linear electron accelerator for radiation technologies

A prototype of a commercial compact continuous-beam linear electron accelerator has been designed and constructed. With its energy of 600 keV and a beam with 30 kW maximum power, the accelerator can be used in various radiation technologies. Its compact size, easy operation, reliability, wide adjustment range for the main parameters, and possible local shielding allow it to be integrated into the majority of radiation technology processes.

Nanoscale and effective mechanical behavior and fracture of silica nanocomposites

An experimental investigation of the composite and local mechanical and fracture behavior of an EPON based epoxy with 12 nm (primary) and 100 nm (secondary) fumed silica particles was carried out. The secondary particles promoted matrix stiffening at small weight fractions, which decreased at larger weight fractions and converged to that of the primary nanoparticles. The elastic modulus of the composites with 12-nm silica particles increased by as much as 30% for 15 wt.% silica loading. Atomic Force Microscopy coupled with Digital Image correlation (AFM/DIC) full-field strain measurements showed matrix strain localization in the vicinity of 100-nm fillers, which controlled the overall composite stiffness. In composites with 5 wt.% secondary particles, neighboring particles located at small proximities to each other behaved as single large particles or resulted in local matrix strain shielding. The tensile strength of all composites was independent of particle size and weight fraction, which was attributed to strong particle bonding and failure initiation in the matrix. The critical mode I stress intensity factor of 12-nm silica composites increased with silica weight fraction, by as much as 35% for 15 wt.% silica. SEM fractographs showed enhanced matrix yielding for 15 wt.% silica compared to significant roughening of the fracture plane and void formation at smaller weight fractions.

Commissioning of the Superconducting Ring Cyclotron for the RIKEN RI Beam Factory

Since 1997, RIKEN Nishina Center has been constructing the Radioactive Isotope Beam Factory (RIBF) and succeeded in beam commissioning of its accelerator complex at the end of 2006. The world's first superconducting ring cyclotron (SRC) is the final booster in the RIBF accelerator complex which is able to accelerate all-element heavy ions to a speed of about 70% of the velocity of light. Assembling of the superconducting sector magnets was completed in August 2005. The superconducting coils were successfully cooled down and excited for tests on many operational aspects: magnetic force, coil protection and quality of magnetic field, showing that they work as designed. After a series of tests the other components were installed and tested under stray fields from the sector magnets. Local magnetic shields were put to the components which could not work under the stray fields. After setups of beam vacuum and radio frequency, beam commissioning started. The first beam was extracted at the end of 2006 and the first uranium beam was extracted in March 2007.

SU-GG-T-360: Monte Carlo-Based Pre-Installation Proton Therapy Shielding Verification for a Single-Room Proton Therapy Facility

Purpose: To validate analytic shielding and maze calculations prior to construction of a doorless in-room accelerator proton therapy facility. Method and Materials: The MCNPX Monte Carlo program (version 2.6c) was used to model the treatment facility. LA 150 cross-sections were used for all materials available. Maze walls, floors, and shielding walls, including all passage ways were included in the room geometry. Treatment losses were modeled with an Fe target placed at isocenter. The cyclotron was geometrically modeled with anticipated proton loss points. The cyclotron was modeled both with and without localized compound shielding. Simulations were performed for gantry angles of 0°, 90°, and 180° for the target source and 0°, 90°, and 135° for the cyclotron source. The 135° beam was simulated since it places the accelerator in a direct line-of-sight of an personnel access area in the lower level shielding wall. Neutron dose equivalent and photon dose values were tallied using mesh tallies throughout the treatment room, maze, and outside of the shielding enclosure and compared with design values. Neutron dose equivalent estimates used the ICRP-74 neutron flux-to-dose rate conversion factors. Results: MCNPX simulations confirmed the analytical shielding estimates and indicated areas of under and over-shielding in the facility plans. Beyond the shielding walls, photon dose equivalent values were less than 10% of the neutron values. Neutrons streaming from the accelerator through a lower maze shielding void contribute to the personnel dose to in the treatment control area. Use of 1″ of simple compound shielding reduces this contribution by a factor of 2.25. Conclusion: MCNPX is a valuable tool for pre-installation verification of shielding design and indicate that doorless proton therapy vaults are feasible.

SU‐GG‐T‐522: Reducing Stray Radiation Dose for a Pediatric Patient Receiving Craniospinal Irradiation with Protons

Purpose: Proton therapy is an emerging treatment modality for pediatric cancer patients because of its advantageous depth‐dose relationship. Although proton dose distributions conform to the target volume, stray neutrons deliver low doses to healthy tissues throughout the body. These exposures may increase the risk of radiation carcinogenesis, particularly for pediatric patients. The aim of this study was to quantify stray radiation dose for a pediatric patient receiving proton craniospinal irradiation and to evaluate methods of reducing stray radiation dose to the patient. Method and Materials: Equivalent dose to each organ and effective dose from stray radiation was estimated for a 30.6 Gy craniospinal treatment using Monte Carlo simulations of a passively‐scattered proton nozzle and a human phantom. The treatment plan and phantom were based on CT images of a 10‐year‐old boy treated for medulloblastoma. Comparisons were made between the existing nozzle, an idealized nozzle with no stray radiation, and the existing nozzle with modest modifications to suppress stray radiation. The existing nozzle was modified by increasing the distance from the patient to the primary external neutron source and enhancing the local shielding. Results: Effective dose from stray radiation was 396 mSv and 74 mSv for the existing and idealized nozzles, respectively, while modest modifications to the nozzle reduced effective dose emanating from the nozzle by 43%. Conclusion: These results add to the body of evidence supporting the suitability of passively‐scattered proton beams for the treatment of pediatric cancer and confirm that the effective dose from stray radiation was not excessive and can be substantially reduced by modest enhancements to the nozzle. Conflict of Interest (only if applicable): We would like to disclose that a similar presentation will be given to a largely different audience at the International Conference on Radiation Shielding in April 2008.

超伝導リングサイクロトロン（ＳＲＣ）の概要

The RIKEN Nishina Center began construction of the Radioactive Isotope Beam Factory (RIBF) in 1997, and succeeded in commissioning the beam of its accelerator complex at the end of 2006. The world’s first superconducting ring cyclotron (SRC) is the final booster in the RIBF accelerator complex. It is capable of accelerating all-element heavy ions to a speed of about 70% of the speed of light. Assembly of the superconducting sector magnets was completed in August 2005. The superconducting coils were successfully cooled down and excited for tests on many operational aspects: magnetic force, coil protection and quality of magnetic field, showing that they work as designed. After a series of tests the other components were installed and tested under stray fields from the sector magnets. Local magnetic shields were supplied for the components that could not work under the stray fields. After setting the beam vacuum and radio frequency, beam commissioning started. The first beam was extracted at the end of 2006, and the first uranium beam was extracted in March 2007.

Hydrogen evolution on the polyethylene-modified Ni–Mo composite layers

The Ni–Mo + PE Ni layers were prepared by electrodeposition from a bath containing a suspension of polyethylene subjected to preliminary nickel plating (PE Ni). Surface morphology was examined using scanning electron microscope. The potentiodynamic and electrochemical impedance spectroscopy methods were used for characterisation of electrochemical activity during hydrogen evolution. Based on the obtained results parameters of hydrogen evolution process have been determined. It was found that the process proceeds better on the layers containing the embedded PE Ni than on the Ni–Mo layers. Kinetic parameters of hydrogen evolution are better for the Ni–Mo + PE layers than for comparable Ni–Mo alloy. It was concluded that embedding the polymer into the layer results in local shielding of the electrode surface and directing the electrochemical process to the free areas thus improving kinetics of hydrogen evolution process.

SU-FF-T-305: Monte Carlo Investigation of Local Shielding to Reduce Stray Radiation Doses to Patients Receiving Proton Therapy

Purpose: The purpose of this study was to quantify the effectiveness of local shielding in reducing the stray radiation exposure to a patient receiving curative proton therapy for prostate cancer by varying the thickness and material of the shielding in the treatment head. Method and Materials: Effective dose,E, was predicted using Monte Carlo simulations of a 76‐Gy prostate treatment with protons. A passive‐scattering treatment nozzle was modeled together with a detailed anthropomorphic phantom. Protons,neutrons, and photons were tracked throughout the geometry. Tallies of energy deposition and fluence were made for these particles in selected organs. Eleven different compositions of shielding material were tested, and the thickness of additional shielding was varied from 0 to 35 cm. A comparison was made between using brass and tungsten alloy for the field‐defining collimator.Results: E from stray radiation for the unmodified nozzle was 583 mSv for the prostate treatment. Additional shielding of the treatment head reduced E by up to 66 mSv. Using tungsten alloy instead of brass for the collimator material reduced E by 50 mSv. Implementing both techniques together reduced E by 87 mSv. Conclusion: This study suggests that a significant reduction in effective dose from stray radiation may be reasonably achievable for patients receiving proton therapy. By adding local shielding to the treatment head and exchanging the brasscollimator with a tungsten alloy replacement, the effective dose to proton therapy patients from stray radiation was reduced by 15%.

Homogeneity of supercurrent flow in coated conductors

The development of coated conductors is now entering a stage where they become available in lengths of several meters. A technique capable of analysing the local superconducting properties of long samples within a relatively short time is therefore highly desirable. In this paper, the application of a scanning technique, initially developed for the characterisation of bulk materials [M. Eisterer, S. Haindl, T. Wojcik, H.W. Weber, Supercond. Sci. Technol. 16 (2003) 1282] to coated conductors is presented. The principle consists of spatially mapping the magnetic field ( B z ) perpendicular to the top sample surface, generated by local shielding currents. The supercurrents are induced by a permanent magnet placed above the scanning Hall probe. This technique was successfully applied to a sample with a length of 10 cm, providing information on the tape homogeneity. Local disturbances of the supercurrent flow and spatial variations of the critical current density are detected in this way on the scale of ⩽1 mm.

Radiation shielding considerations against gas bremsstrahlung for the transverse wall at the Canadian Light Source

A radiation shielding study against primary and secondary gas bremsstrahlung is carried out for the existing transverse wall at the Canadian Light Source (CLS). A Monte Carlo method for mapping the radiation dose behind the transverse wall is developed. Two shielding methods are studied: (1) local shielding and (2) a lead wall, to bring the radiation level below the regulation limit. Also discussed are the lead beam stops used for an insertion device (ID) and a bending magnet (BM) beamline since there exists a considerable difference in length of the straight section between the ID and BM beamlines.

Review of research into wheel/rail rolling noise reduction

Rolling noise is the main source of noise from railway operations at conventional speeds, and even at 300 km/h, it is at least as important as aerodynamic noise. In the last decade, several large research projects have been carried out across Europe with the aim of reducing wheel/rail rolling noise at source. Much of the research has concentrated on freight traffic as this has been perceived as the major source of nuisance because of its relatively high noise levels and the operational necessity of running at night. The solutions considered have included means of reducing wheel and rafl surface roughness, improved wheel and track designs for low noise generation, and local shielding applied to the vehicle and track. These have demonstrated a number of successful concepts, including wheel shape optimization, wheel damping, rail damping, bogie shrouds, low barriers, and modified tread braking. This article summarizes the main results obtained and discusses the prospects for implementation of the technology in practice.

A SQUID NDE system for the investigation of pinch welds

HTS SQUID NDE systems attempt to achieve both high spatial resolution and high signal sensitivity, though often the more important parameter is determined by the sample of interest. SQUID microscopes offer micron-sized resolution by either bringing the sample into very close contact with a small SQUID sensor or by using a ‘‘flux guide’’ that is located in the tail of the cryostat. Conventional SQUID NDE systems typically have a larger sensor cooled by a liquid cryogen (or possibly a cryocooler) and place more import on signal resolution. Our system is a hybrid of the above ideas, developed specifically to permit the characterisation of small, irregular-shaped stainless steel samples containing pinch welds. We have investigated using a ferromagnetic flux guide with our system, but for convenience have simply attached it to the tail of the cryostat 4 mm below a washer SQUID magnetometer. To increase the signal resolution we have chosen to employ local electromagnetic shielding around the measurement system. We detail the characteristics of the system and present experimental NDE data as individual time series traces of the SQUID output for a set of samples that were fabricated using a broad range of weld parameters.

Simulation of the current dynamics in a superconductor induced by a small permanentmagnet: application to the magnetoscan technique

The magnetoscan is a rather new technique for investigating local inhomogeneities of large bulkhigh-Tc superconductors. Local (shielding) currents, which are induced by a small permanentmagnet slightly above the sample surface, generate a magnetic field that is recorded by aHall probe. We report on a theoretical analysis of such a three-dimensional system usingnumerical simulations to calculate the current dynamics during a magnetoscan.The magnetoscan signal is analysed for different experimental setups and different criticalcurrent densities of the superconductor. We show that the shape of small defects(∼1 mm in size) is almost exactly displayed, even if the radius of the permanent magnet is muchlarger than the defect size. The critical current density of larger inhomogeneities can bedirectly determined at the sample surface, if all details of the experimental setup areknown. Different aspects of how to improve the experimental resolution are discussed.

The effect of silicon content on long crack fatigue behaviour of aluminium–silicon piston alloys at elevated temperature

The microstructure of aluminium piston alloys comprises primary and eutectic silicon together with numerous intermetallics. Previous research has shown that primary silicon strongly influences both fatigue crack initiation and subsequent propagation behaviour, however, the detailed effects of varying silicon volume fraction and morphology have not been fully addressed. Therefore, the fatigue properties of a number of candidate piston alloys with varying volume fractions of silicon have been studied. Long crack fatigue tests have been performed at room and elevated temperature typical of the gudgeon pin boss (200 °C) using a test frequency of 15 Hz (a typical engine frequency at engine idle condition). Microstructural characterisation using image analysis approaches combined with optical profilometry has been used to assess the fracture surfaces of test samples. The role of primary Si in enhancing crack growth rates at high Δ K levels, whilst affording improvements in crack growth rates at lower Δ K levels due to local crack deflections and shielding, has been confirmed. In the absence of primary Si (lower Si content alloys) the low Δ K level crack growth behaviour is dominated by matrix properties (intra-dendritic crack growth pre-dominates) whilst the high Δ K level crack growth behaviour is inter-dendritic and occurs along the weak path of the eutectic Si and/or intermetallic network.