Internal Shielding Research Articles

SU‐FF‐T‐62: Dosimetric Evaluation of An Internal Shielding Used with a HDR Skin Applicator

Purpose: The Valencia HDR skin applicators are accessories of the microSelectron HDR afterloading system (Nucletron) cup shaped to limit the dose to the irradiation area. Applicators sizes are: diameter 2 and 3 cm. The typical prescription depth is 3 mm. This work aims to evaluate the backscatter and electron contamination of an internal shielding used to reduce the dose to the ocular globe when the lesion is located at the eyelid. Monte Carlo (MC) and experimental methods have been used. Method and Materials: The geometry has been: the Valencia applicator (2 cm diameter) on the surface of a solid polystyrene slab phantom; inside, a 2 mm lead slab located at 3 mm depth. MC simulation characteristics: GEANT4 code (Version 9.0); Scored absorbed dose; Cell size: 0.1 mm in the depth axis; Photon histories: 3×108. Radichromic EBT film dosimetry setup and processing: Films depths: 1,5, 3, 5 and 7 mm, inside the phantom; Irradiations, with and without the lead shielding slab; Flatbed scanner; Matlab software to obtain the red channel and to filter noise; Verisoft (PTW) software utilities to calibrate the optical density against dose and to analyse the results. Results: MC simulation gives an overdose close the lead (backscatter component) of about 190% (range: 0.5 mm). The electron contamination under the lead is about 130% (range: 1 mm). These results agree with measurements with radichromic film within experimental uncertainties. The dose reduction behind the shield is about 60%. Conclusion: The use of an internal 2 mm lead shielding, in eyelid skin treatments with the Valencia applicators has been evaluated. The reduction in dose to the ocular globe with a 2 mm lead shielding is about 60%. A minimum bolus of 0.5 mm and 1 mm thickness must be added above and below the shielding, respectively to remove the dose contamination.

SU-FF-T-19: Accelerated Partial Breast Irradiation With Shielded MammoSite-Type Applicator

Purpose: We propose a method to improve delivery of APBI and therefore improve patient access to BCT. Method and Materials: BCT in conjunction with accelerated partial breast irradiation (APBI), using for instance the Hologic MammoSite applicator, is becoming popular due to the good dose distribution, and relatively short five day treatment procedure. Brachytherapy‐based APBI procedures typically work by implanting a double lumen balloon catheter in the lumpectomy cavity of breast following surgery. The balloon is inflated with an iodine‐based material mixed with saline. If the skin to balloon distance is less than 7mm, the procedure may not be recommended due to adverse skin reactions leading to poor cosmesis. This problem could be avoided by partially shielding the radiation dose to the skin by introducing a thin layer of high density material (high Z powder) inside the balloon catheter. In this case, the metalpowders may be controlled by a relatively weak external magnetic field applied externally to the patient to form an internal shielding layer inside the balloon in the region of concern, thus avoiding radiation damage to the skin.Results: Our preliminary results indicate that if, for example, the skin‐to‐balloon spacing is only 4 mm, when about 2 mm thin layer of ironpowders arranged internally under that segment of surface of the MammoSite balloon will decrease the skindose to an acceptable level. Our laboratory tests show that only a slight magnetic filed is required to bring such a small amount of ferrous powders to controllable configuration. Conclusion: The suggested approach will improve cosmetic outcome for all APBI patients treated, in addition to increasing survival expectancy and minimizing negative side effects.

Internally Damped CCC for Accurate Measurements of Small Electrical Currents

We have developed and tested an internally damped cryogenic current comparator (CCC) with a main 1:20 000 winding ratio for the accurate measurement of small electrical currents. The key features of the CCC are the use of resistive CuNi wire for the windings and brass internal shields for additional damping of the internal resonance of the CCC. Inductance measurements of the CCC overlapping tube at 300 and 77 K, as well as the superconducting quantum interference device (SQUID) noise spectra of the complete CCC setup at 4 K, confirm the effectiveness of the damping of the CCC self-resonance with respect to other designs. At 4 K, the CCC is remarkably stable in measurements with external feedback mode; even mains-operated current sources can be used without unlocking the SQUID null detector of the measurement bridge. First measurements of currents in the range of 100-300 pA using this CCC are performed, resulting in deviations from the expected value in the range of about 20-50 muA/A.

Dosimetric evaluation of internal shielding in a high-dose-rate skin applicator

Purpose: The Valencia HDR skin applicators are accessories of the microSelectron HDR afterloading system (Nucletron B.V., Veenendaal, Netherlands), these are cup shaped to limit the dose to the irradiation area. There are two Valencia applicators with 2 and 3cm in diameter. The typical prescription depth is 3mm. In some patients the lesion is located at the eyelid. The motivation of this work is to evaluate the potential backscatter and electron contamination of an internal shielding to reduce the dose to the ocular globe.

Radiation survey around a Liac mobile electron linear accelerator for intraoperative radiation therapy

The aim of this study was to perform a detailed analysis of the air kerma values around a Liac mobile linear accelerator working in a conventional operating room (OR) for IORT.The Liac delivers electron beams at 4, 6, 8 and 10 MeV. A radiation survey to determine photon leakage and scatter consisted of air kerma measurements on a spherical surface of 1.5 m radius, centered on the titanium exit window of the accelerating structure. Measurements were taken using a 30 cm3 calibrated cylindrical ion chamber in three orthogonal planes, at the maximum electron energy. For each point, 10 Gy was delivered. At selected points, the quality of x‐ray radiation was determined by using lead sheets, and measurements were performed for all energies to investigate the energy dependence of stray radiation. The photon scatter contribution from the metallic internal patient‐shielding in IORT, used to protect normal tissues underlying the target, was also evaluated. At seven locations outside the OR, the air kerma values derived from in‐room measurements were compared to measurements directly performed using a survey meter. The results, for a delivered dose of 10 Gy, showed that the air kerma values ranged from approximately 6 μGy (upper and rear sides of the Liac) to 320 μGy (lateral to beam stopper) in the two orthogonal vertical planes, while values lower than 18 μGy were found in the horizontal plane. At 10 MeV, transmission behind 1 cm lead shield was found to be 42%. The use of internal shielding appeared to increase the photon scatter only slightly. Air kerma values outside the OR were generally lower than 1 mGy for an annual workload of 200 patients. Thus, the Liac can safely work in a conventional OR, while the need for additional shielding mainly depends on patient workload. Our data can be useful for centers planning to implement an IORT program using a mobile linear accelerator, permitting radiation safety personnel to estimate in advance the shielding required for a particular workload.PACS number: 87.55.ne, 87.56.bd

Conceptual design of the ECH upper launcher system for ITER

The challenge of developing the conceptual design of the ECH Upper Launcher system for MHD control in the ITER plasmas has been tackled by team of European Associations together with the European Domestic Agency (“F4E”). The launcher system has to meet the following requirements: (a) a mm-wave system extending from the interface to the transmission line up to the target absorption zone in the plasma and performing as an intelligent antenna; (b) a structural system integrating the mm-wave system and ensuring sufficient thermal and nuclear shielding; (c) port plug remote handling and testing capability ensuring high port plug system availability. The paper describes the reference launcher design. The mm-wave system is composed of waveguide and quasi-optical sections with a front steering system. An automated feedback control system is developed as a concept based on an assimilation procedure between predicted and diagnosed absorption location. The structural system consists of the blanket shield module, the port plug frame, and the internal shield for appropriate neutron shielding towards the launcher back-end. The specific advantages of a double walled structure are discussed with respect to adequate baking, to rigidity towards launcher deflection under plasma-generated loads and to removal of thermal loads, including nuclear ones. Basic studies of remote handling (RH) to validate design development are initiated using a virtual reality simulation backed by experimental validation, for which a launcher handling test facility (LHT) is set up as a full scale experimental site allowing furthermore thermohydraulic studies with ITER blanket water parameters.

Improvements to the Leake neutron detector II: Extension to 10 GeV energy

We report on an improved design of neutron ambient dose equivalent (DE) detector, SNS-LINUS, consisting of a spherical polyethylene detector with internal shields of boron and lead based on the LINUS model thus extending the response to the GeV region. The computed Monte Carlo neutron ambient DE energy response is within a factor ±2.4 from 0.1 keV to 10 GeV with a detector weight of 7.8 kg that is significantly lower than other LINUS designs. The ambient DE response in the 50 keV–10 MeV range is within a factor ±1.36 (i.e. ±30%). The main applications are likely to be around particle accelerators, in aircraft or in spacecraft. The improved response in the 20 MeV energy region should make it attractive for use with fission sources. The computed responses of the SNS NGREM and LINUS detectors to pulsed neutrons and limits for their prudent use in portable monitors in such fields are given. The effect of large gamma bursts on the measurement of pulsed neutrons is discussed with recommendations made to enable operation up to 10 μGy of gamma radiation per burst. We propose a method of extending the dynamic range for pulsed neutron detection in which the neutron decay within the moderator is used to make measurements at a time at which the count rate losses are acceptable.

Monte Carlo simulation of backscatter from lead for clinical electron beams using EGSnrc

In electron radiotherapy of superficial lesions in the eyelid, lip, buccal mucosa, ear, and nose, backscattered electrons are produced from the lead shield used to protect the critical tissue underneath the tumor. In this study, the backscattered electrons, produced by clinical electron beams using a Varian 21 EX linear accelerator, were studied using Monte Carlo simulations. The electron backscatter factor (EBF), defined as the ratio of dose at the tissue-lead interface to the dose at the same point without the presence of backscatter, was calculated using the Monte Carlo EGSnrc-based code. The calculated EBFs were verified with measurements using metal-oxide-semiconductor field effect transistor detectors. The effect of the (1) initial electron beam energy, (2) thickness of bolus over the lead shield, (3) beam's angle of incidence, and (4) presence of an aluminum sheet used to absorb backscattered electrons, on the EBF, were studied. It is found that for lead shielding positioned at any fixed depth, the EBF decreases with an increase in initial electron beam energy (4-16 MeV). In addition, for depths within the electron practical range, Rp, and at a particular beam energy, the EBF increases with depth (or thickness of the treatment volume). When the electron beam angle increases from 0 degrees to 5 degrees, the EBF only decreases slightly (<4%) for all energies. The influence of the beam obliquity on the EBF is important when the treatment surface is not flat and perpendicular to the central beam axis. The use of an aluminum sheet to reduce backscattered electrons was also investigated. For a relatively low electron beam energy (4 MeV), a 2 mm aluminum sheet can reduce backscattering by 31%. While the electron beam energy increased, less backscattered electrons were produced and therefore removed by the same thickness of aluminum (only about 6% for 16 MeV). The Monte Carlo calculated EBFs from this study, characterized by the electron beam energy, depth of bolus above the lead shield, beam obliquity, and presence of an aluminum sheet, may provide important clinical information for radiation oncology staff when considering the effect of electron backscatter on radiotherapy using internal shielding.

The use of directional interstitial sources to improve dosimetry in breast brachytherapy

The purposes of this study were to investigate the feasibility of improving dosimetry with temporary low-dose-rate (LDR) multicatheter breast implants using directional 125I (iodine) interstitial sources and to provide a comparison of a patient treatment plan to that achieved by conventional high-dose-rate (HDR) interstitial breast brachytherapy. A novel 125I source emitting radiation in a specified direction has been developed. The directional sources contain an internal radiation shield that greatly reduces the intensity of radiation in the shielded direction. The sources have a similar dose distribution to conventional nondirectional sources on the unshielded side. The treatment plan for a patient treated with HDR interstitial brachytherapy with 192Ir (iridium) was compared with a directional 125I treatment plan using the same data set. Several dosimetric parameters are compared including target volume coverage, volume receiving 50%, 100%, and 150% of the prescription dose (V50, V100, and V150, respectively), dose homogeneity index (DHI), and the skin surface areas receiving 30%, 50%, and 80% of the prescription dose (S30, S50, and S80, respectively). The HDR and LDR prescription doses were 34 Gy in ten fractions delivered over five days and 45 Gy in 108 h, respectively. Similar and excellent target volume coverage was achieved by both directional LDR and HDR plans (99.2% and 97.5%, respectively). For a 170 cm3 target volume, the dosimetric parameters were similar for LDR and HDR: DHI was 0.82 in both cases, V100 was 214.4 cm3 and 225.7 cm3, and V150 was 39.1 cm3 and 40.4 cm3, respectively. However, with directional LDR, significant reductions in skin dose were achieved: S30 was reduced from 100.6 to 62.5 cm2, S50 from 50.6 to 16.1 cm2, and S80 from 2 cm2 to zero. The reduction in V50 for the whole breast was more than 100 cm3 (386.1 cm3 for LDR versus 489.2 cm3 for HDR). In this case study, compared with HDR, directional interstitial LDR 125I sources allow similar dose coverage to the subcutaneous target volume while lowering the skin dose due to a more conformal dose distribution and quicker falloff beyond the target. The improved dose distribution provided by directional interstitial brachytherapy might enable partial breast treatment to tumors closer to the skin or chest wall or in relatively small breasts.

Radiation shield analyses in support of the FS design for the ITER ECRH launcher

This paper is devoted to the radiation shield analyses for the front steering (FS) design of the electron cyclotron resonance heating (ECRH) launcher installed in the ITER upper port. The neutron fluxes, nuclear heat density, helium production, and neutron displacements rate have been calculated by Monte Carlo N-Particle (MCNP) transport code. Three-dimensional MCNP models of the FS ECRH launcher have been produced with high accuracy directly from the CAD geometry models by means of the McCad interface programme. A shielding analysis has been performed to estimate the material composition and optimum length required for the launcher's internal shield. Neutron streaming calculations have been done using the MCNP point detectors method. Variance reductions in nuclear responses of particles track-length estimations were achieved by the optimized weight windows, particles splitting, and Russian roulette techniques. Nuclear design parameters of the FS ECRH launcher have been evaluated by the neutronics analyses with dedicated computational procedures. The analyses show that safety issues and radiation shielding requirements in the FS design are fully satisfied to radiation design limits specified for ITER project.

TU-EE-A1-05: The Use of Directional Interstitial Sources to Reduce Skin Dose in Breast Brachytherapy

Purpose: To investigate the feasibility of reducing the skindose with temporary LDR multicatheter breast implants with the use of directional 125I interstitial sources in comparison to conventional HDR interstitial breast brachytherapy.Method and Materials: The treatment plan for a patient treated with HDR interstitial brachytherapy with 192Ir was compared to a directional 125I treatment plan in the same dataset. Directional sources contain an internal radiation shield that greatly reduces the intensity of radiation in the shielded direction. They have a similar dose distribution to non‐directional sources on the unshielded side. Several dosimetric parameters are compared including target volume coverage, dose homogeneity index, and the skin surface areas receiving 30%, 50% and 80% of the prescription dose (S30, S50 and S80, respectively). The HDR prescription dose was 34 Gy in 10 fractions. Results: Similar excellent target coverage was achieved by both directional LDR and HDR (99.2% and 97.5%, respectively). Moreover, for a 170‐cc target volume, the dose homogeneity index was 0.82 for both LDR and HDR (V100 was 211.4 cc or 225.7 cc, and V150 was 39.1 cc or 40.4 cc, respectively). However, with directional LDR, the following reductions in skindose may be achieved: S30 is reduced from 100.6 cm2 to 62.6 cm2, S50 from 50.6 cm2 to 16.1 cm2, and S80 at 2 cm2 to null. The reduction in V50 for the whole breast is more than 100 cm3 (386.1 cc vs. 489.2 cc). Conclusion: As compared to HDR, directional interstitial 125I sources allow similar dose coverage to the subcutaneous target, while significantly lowering the skindose due to a quicker fall‐off beyond the target. Directional LDR sources can produce a similar dose homogeneity index, but the biological characteristics are more tolerable to the patient and can potentially reduce the risk of late skin and subcutaneous toxicity.

Internal Barium Shielding to Minimize Fetal Irradiation in Spiral Chest CT: A Phantom Simulation Experiment

To use a phantom to prospectively examine the attenuating effect of barium sulfate as an internal shield to protect the fetus. In an adult-size phantom, 1- and 2-cm-thick acrylic slabs containing 315 or 630 mL of water, 2% or 40% barium sulfate suspension, and a 1-mm lead sheet were placed under the diaphragm. In 17 experiments, fetal dose was measured by using thermoluminescent dosimeters that were placed immediately under (near field) and 10 cm below (far field) the water slab (eight experiments), barium sulfate slab (eight experiments), and lead sheet (one experiment). In a pulmonary embolism protocol, the phantom was scanned with single-detector spiral computed tomography (CT) at 130 kVp and 230 mAs. The control radiation dose was 3.60 mSv+/-0.54 (standard deviation) with the water slab at near field, where the uterus dome is at near term, and 0.507 mSv+/-0.07 with the water slab at far field, the uterus position during early gestation. Scattered radiation was attenuated 13% and 21% with 2% barium sulfate and 87% and 96% with 40% barium sulfate, as calculated in the near and far fields, respectively, and 99% with the 1-mm lead sheet. The extrapolated attenuations for 5%-40% barium sulfate suspensions indicated that beyond a 30% suspension, attenuation increased further only slightly. Study results in the phantom experiment suggest that fetal irradiation during maternal chest CT can be reduced substantially with barium shielding.

Gamma dose from activation of internal shields in IRIS reactor

The International Reactor Innovative and Secure is a modular pressurised water reactor with an integral design. This means that all the primary system components, such as the steam generators, pumps, pressuriser and control rod drive mechanisms, are located inside the reactor vessel, which requires a large diameter. For the sake of better reliability and safety, it is desirable to achieve the reduction of vessel embrittlement as well as the lowering of the dose beyond the vessel. The former can be easily accomplished by the presence of a wide downcomer, filled with water, which surrounds the core region, while the latter needs the presence of additional internal shields. An optimal shielding configuration is under investigation, for reducing the ex-vessel dose due to activated internals and for limiting the amount of the biological shielding. MCNP 4C calculations were performed to evaluate the neutron and the gamma dose during operation and the 60Co activation of various shields configurations. The gamma dose beyond the vessel from activation of its structural components was estimated in a shutdown condition, with the Monte Carlo code FLUKA 2002 and the MicroShield software. The results of the two codes are in agreement and show that the dose is sufficiently low, even without an additional shield.

Simulation and Properties of the Shielded Twined Helical Deflecting System

A helical traveling-wave deflection system for wide-band cathode-ray tube is considered. The system contains coaxial helices intertwined in a bifilar fashion. Besides external shields, internal shields are used. The model of the system, containing segments of multiconductor lines, is proposed. The retardation factor and input impedance of the system are considered. Possibilities to improve properties of the system are revealed.

Preparation of nonconducting infrared-absorbing thin films

A simple procedure for preparing colloidal “black” bismuth films is introduced, which leaves the target cold and does not pollute the recipient. The Bi evaporation occurs in a closed box in the evaporation chamber with an internal radiation shield. The bismuth is evaporated from a tantalum boat at a residual air pressure of 2×102Pa. The resulting films with a thickness of about 10μm are structureless down to a spatial resolution of 5.6μm, they become electrically insulating after 48h storage time in air, and they show an IR absorbance of above 70% in the 3–5μm wavelength range. The films are easily removable in an ultrasonic water bath. Thus, these films are ideally appropriate to increase the IR emissivity of microelectronic structures in microthermal infrared failure analysis investigations such as lock-in thermography, as is demonstrated in an application example. The application of this film may improve the thermographic detection limit of heat sources below metallized areas by up to a factor of 10, leading to a saving in acquisition time by a factor of 100.

Structural integration studies for the ITER ECRH Upper Launcher

The launcher structural design integration is presented for the ‘8-beamline’ remote steering (RS) reference launcher. It covers the design of the blanket shield module (BSM) which closes the gap in the regular blanket structure and of the internal neutron shield in the main launcher structure. Nuclear heating rates are provided by nuclear calculations; including heat loads from the mm-wave components, a conceptual design is defined for the cooling system fed by the ITER blanket cooling water. The thermo-mechanical performance of the first wall panel which is welded to BSM side walls is proven by FEM modelling. The disassembly of the port plug and its internals is based on an axial access scheme. Boundary conditions to neighbouring ITER systems are quantified in terms of shut-down dose levels for hands-on access, helium generation in the vacuum vessel and dimensions of the cut-out required in the lower regular blanket module. The adaptation of the shielding and cooling configuration is discussed for the two advanced beamline variants which are the 6 beamline RS dogleg option and the front steering (FS) launcher.

Development of a cryogenic detection concept for GNO

The Gallium-Neutrino-Experiment at GNO measures solar neutrinos radiochemically via the reaction 71Ga(ν,e)71Ge. In order to reduce statistical and systematic uncertainties in detecting the decay of 71Ge to 71Ga, a favourable way could be the use of high-resolution cryogenic detectors instead of the presently integrated miniaturized proportional counters. After the successful development of a highly efficient 4π-detector, optimization of thermal Ge-deposition and decoupling of deposition and detection, present activity concentrates on low background considerations, enhancement of external and internal shielding of the cryostat and a long-term run with artificially activated 71Ge to prove the feasibility of ‘cryoGNO’ in our Underground Laboratory in Garching.

SYSTEM TRADE-OFF STUDY AND OPTO-THERMO-MECHANICAL ANALYSIS OF A SUNSHIELD ON THE MSC OF THE KOMPSAT-2

The Multi-Spectral Camera (MSC) is the payload of KOMPSAT-2 which is designed for earth imaging in optical and near-infrared region on a sun-synchronous orbit. The telescope in the MSC is a Ritchey-Chretien type with large aperture. The telescope structure should be well stabilized and the optical alignment should be kept steady so that best images can be achieved. However, the MSC is exposed to adverse thermal environment on the orbit which can give impacts on optical performance. Solar incidence can bring non-uniform temperature rise on the telescope tube which entails unfavorable thermal distortion. Three ways of preventing the solar radiation were proposed, which were installing external mechanical shield, internal shield, and maneuvering the spacecraft. After trade-off study, internal sun shield was selected as a practical and optimal solution to minimize the effect of the solar radiation. In addition, detailed designs of the structure and sunshield were produced and analyses have been performed. The results were assessed to verify their impacts to the image quality. It was confirmed that the internal sunshield complies with the requirements and would improve image quality.

A universal marx generator with double shielding

The described Marx generator is deemed universal for the easiness with which one can change the number of capacitors of a multiplying stage and the number of the multiplying stages themselves, as well as change a mixture of gases both inside the discharge column and outside of it, i.e., under the shielding housing of the generator. The second internal shield is the copper cap covering the discharge column. The presence of double shielding allows the generator to be used simultaneously with the operation of low-current equipment.

Supershielding: Confinement of Magnetic Fields

In open-shield systems, we have derived current conditions that lead to remarkable magnetic-field confinement, which we call ``supershielding.'' The freedom to vary both primary and shield currents is important in solving the conditions. Solutions leading to very effective shielding are illustrated by the simple system of two parallel flat strips. Generalizations include other geometries, internal shielding, and electric-field confinement. An industrial coil design serves as a reference for practical issues. We discuss mathematical limits, via a ``seesaw'' approach, to ``perfect shielding.''