Bragg Peak Shift Research Articles

SU‐E‐J‐201: Investigation of MRI Guided Proton Therapy

Purpose:Image‐guided radiation therapy has been employed for cancer treatment to improve the tumor localization accuracy. Radiation therapy with proton beams requires more on this accuracy because the proton beam has larger uncertainty and dramatic dose variation along the beam direction. Among all the image modalities, magnetic‐resonance image (MRI) is the best for soft tissue delineation and real time motion monitoring. In this work, we investigated the behavior of the proton beam in magnetic field with Monte Carlo simulations.Methods:A proton Monte Carlo platform, TOPAS, was used for this investigation. Dose calculations were performed with this platform in a 30cmx30cmx30cm water phantom for both pencil and broad proton beams with different energies (120, 150 and 180MeV) in different magnetic fields (0.5T, 1T and 3T). The isodose distributions, dose profiles in lateral and beam direction were evaluated. The shifts of the Bragg peak in different magnetic fields for different proton energies were compared and the magnetic field effects on the characters of the dose distribution were analyzed.Results:Significant effects of magnetic field have been observed on the proton beam dose distributions, especially for magnetic field of 1T and up. The effects are more significant for higher energy proton beam because higher energy protons travel longer distance in the magnetic field. The Bragg peak shift in the lateral direction is about 38mm for 180MeV and 11mm for 120MeV proton beams in 3T magnetic field. The peak positions are retracted back for 6mm and 2mm, respectively. The effect on the beam penumbra and dose falloff at the distal edge of the Bragg peak is negligible.Conclusion:Though significant magnetic effects on dose distribution have been observed for proton beams, MRI guided proton therapy is feasible because the magnetic effects on dose is predictable and can be considered in patient dose calculation.

SU-E-T-550: Range Effects in Proton Therapy Caused by Systematic Errors in the Stoichiometric Calibration

Purpose: The procedure for proton treatment planning involves the conversion of the patient's X-ray CT from Hounsfield units into relative stopping powers (RSP), using a stoichiometric calibration curve (Schneider 1996). In clinical practice a 3.5% margin is added to account for the range uncertainty introduced by this process and other errors. RSPs for real tissues are calculated using composition data and the Bethe-Bloch formula (ICRU 1993). The purpose of this work is to investigate the impact that systematic errors in the stoichiometric calibration have on the proton range. Methods: Seven tissue inserts of the Gammex 467 phantom were imaged using our CT scanner. Their known chemical compositions (Watanabe 1999) were then used to calculate the theoretical RSPs, using the same formula as would be used for human tissues in the stoichiometric procedure. The actual RSPs of these inserts were measured using a Bragg peak shift measurement in the proton beam at our institution. Results: The theoretical calculation of the RSP was lower than the measured RSP values, by a mean/max error of - 1.5/-3.6%. For all seven inserts the theoretical approach underestimated the RSP, with errors variable across the range of Hounsfield units. Systematic errors for lung (average of two inserts), adipose and cortical bone were - 3.0/-2.1/-0.5%, respectively. Conclusion: There is a systematic underestimation caused by the theoretical calculation of RSP; a crucial step in the stoichiometric calibration procedure. As such, we propose that proton calibration curves should be based on measured RSPs. Investigations will be made to see if the same systematic errors exist for biological tissues. The impact of these differences on the range of proton beams, for phantoms and patient scenarios, will be investigated. This project was funded equally by the Engineering and Physical Sciences Research Council (UK) and Ion Beam Applications (Louvain-La-Neuve, Belgium).

Highly Radiation Sensitive Type IA FBGs for Future Dosimetry Applications

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Apparent lattice expansion in ordered nanoporous silica during capillary condensation of fluids

Ordered nanoporous silica materials containing cylindrical pores on a two-dimensional hexagonal lattice are known to deform upon adsorption and condensation of fluids. These sorption strains can be measured byin situsmall-angle X-ray scattering by analysing the shift of the Bragg peaks from the ordered pore lattice. Besides the real lattice deformation due to the interaction of the solid pore walls with the fluid, a hitherto unexplained apparent lattice expansion is found experimentally in SBA-15 nanoporous silica in a narrow pressure region for three different fluids (pentane, perfluoropentane and water). It is shown that the Bragg peak shift in this region results partly from a subtle contrast effect. The pore form factor changes during capillary condensation owing to the sequential filling of pores according to their diameter. Together with the structure factor from a lattice of finite size, this leads to an effective shift of the Bragg peaks, resulting in the measured apparent lattice strain. Two simple models are presented, which aim to quantitatively describe these apparent strains. The first model employs cylindrical pores with a size distribution on a two-dimensional hexagonal lattice. Filling of pores is achieved by changing the contrast between the pores of a certain size with respect to the surrounding matrix, and the peak shift is calculated from the power spectrum of this lattice. The second model derives analytical expressions for the peak shifts as a function of pore filling fraction by using a Taylor expansion of the pore form factor and a uniform pore size distribution. Both models are able to reproduce the experimental results satisfactorily, providing the possibility to separate these apparent lattice strains from real pore wall deformation in nanoporous materials.

259 TIME-OF-FLIGHT METHOD FOR NEUTRON REJECTION IN PROMPT GAMMA IMAGING OF BEAM RANGE AND DENSITY CHANGES IN PROTON THERAPY

A therapeutic proton beam penetrating a patient generates a large number of prompt photons as well as neutrons, resulting from proton-induced nuclear reactions. This allows for obtaining longitudinal beam-profile images via the detection of selected prompt-gamma rays emitted perpendicularly to the beam direction. The significant neutron background on top of the prompt photons signal was observed. Thus, the goal of presented research is to propose the novel in-situ time-of-flight (TOF) technique for neutron background rejection in order to get the optimum signal from prompt-gamma rays escaping the irradiated object. Density heterogeneities within the patient affect the position of the Bragg peak. Due to potential morphological changes the Bragg peak position may vary during the course of treatment, which may lead to clinically significant under-dosage in the tumor or over-dosage in the organs-at-risk. The Bragg peak shifts due to target morphological modifications, whichmay occur in realistic radiotherapy scenarios (e.g. tumor or normal tissue swelling), can be detected with the presented method to within a few millimeters. We show a clinically relevant correlation between the prompt-gamma profiles and the target (patient) density.

Gamma-Radiation Effects on Bragg Gratings Written by Femtosecond UV Laser in Ge-Doped Fibers

We studied the effect of Co60 gamma-radiation on the position and the amplitude of fiber Bragg gratings. These gratings were written with a femtosecond UV laser in three different fibers, both intrinsically photosensitive and hydrogen sensitized. We have found that in contrast with the results obtained on gratings written with low-intensity UV lasers, the Bragg peak shift under irradiation is almost independent on fiber type, fiber sensitization or grating amplitude. The grating strength behavior depends on the fiber type.

SU‐FF‐T‐411: Boundary Study of Bragg Peak Shift and Bragg Peak Degradation in Proton Dose Calculation

Purpose: The purpose of this work is to study the Bragg peak shifts and degradation caused by density and boundary changes in proton beam dose calculation Method and Material: Proton beam delivery provides promising dose characteristics as radiation dose can conform tightly to tumor while sparing surrounding healthy tissues. Proton particles deposit energy in a narrow range around the Bragg peak and as such the dose calculation is more challenging for that the Bragg peak is sensitive to tissue density, tissue composition and organ boundaries along the proton track path. We simulated a few scenarios to study the proton Bragg peak shift due to density and Bragg peak degradation due to change and boundary changes. The calculation of the three dimension dose matrix was performed using a 2 × 2 × 1 mm3 voxels in the depth peak dose range in water phantom after some rough simulation for the dose peak estimation. Results: Bragg peak shift at the iso‐center slice were found to follow a linear relationship with the density of heterogeneity insert based on our simulations with density ranging [0.4 2.0] g/cm∧3 which we studied. Bragg peak degradation and proton dose changed significantly due to low density and small beams size. Proton dose degraded when high energy proton beam irradiated to low density material. Proton dose degraded also when small beam with beam radius at several mm range. Conclusion: Proton dose calculation depends on many factors as Bragg peak is sensitive to tissue density and composition. Besides that, there exist several scenarios causing Bragg peak shift due to density change, causing Bragg peak degradation due to low density and small proton beam. The Monte Carlo simulation is a very accurate solution to provide precise dose distribution in inhomogeneous structures by simulating transport and energy deposition.

SU‐FF‐T‐420: A Track Repeating Algorithm for Carbon Therapy Dose Calculation

Purpose: In radiation therapy, the use of heavy charged particles offers numerous advantages (narrow Bragg peak and sharp penumbra) over electron and X‐ray beams. Conventional dose algorithms are fast but less accurate for heterogeneous patient anatomy for small fields and beam scanning. Accurate dose calculations based Monte Carlo simulations are time consuming. Our goal is to develop a fast Monte Carlo dose calculation algorithm using a track repeating method for carbon therapy treatment planning and dose verification. Method and Materials: Using the GEANT 4 Monte Carlo code we recorded the tracks of mono‐energetic (300MeV/nuclon) carbon ions in a homogeneous water phantom. For every carbon history, the incident carbon ion and all secondary particles are transported in discrete steps and all parameters for each step are recorded in a database. A program is developed to adjust the step length and energy deposition in different materials along the particle path in a heterogeneous phantom. Based on the energy and angle of an incident carbon either the whole history or parts of tracks will be repeated with proper angular rotation. Results: We first demonstrated the applicability of the track repeating method for mono‐energetic carbon beams (100–300 MeV/nucleon) by comparing the calculated dose distributions in homogeneous and heterogeneous phantoms with those simulated by two established Monte Carlo codes (GEANT 4 and FLUKA). As a second step we confirmed that the Bragg peak shift and energy deposition changes in bone and soft tissue are accurately reproduced. As a final test we applied the algorithm to calculate the dose distribution in patient geometry built from CT data. Conclusion: Our track repeating technique for carbon ions is extremely efficient (11‐100 times faster than FLUKA) opening the possibility for both fast and clinically accurate dose calculations for advanced carbon therapy using beam scanning and intensity modulation.

The measurement of internal strain in core–shellNi3Si(Al)–SiOx nanoparticles

Internal defects and strain in nanoparticles can influence their properties and thereforemeasuring these values is relevant. Powder diffraction techniques (neutron andsynchrotron) are successfully used to characterize internal strain in the core–shellNi3Si(Al)–SiOx nanoparticles havingmean diameters of ∼80 nm. The nanoparticles, which are strain-free after extraction from the bulkalloys, develop internal strain on heating. Both micro- and macro-strains can bemeasured from the analysis of Bragg peak shift and broadening. It is identifiedthat differences in thermal expansion coefficient of the metallic core and theamorphous shell of the nanoparticles, as well as partial disordering of theL12 ordered core phase, are the main causes of strain evolution. Synchrotron measurements alsodetected partial crystallization of the amorphous silica shell.

Foetal dose conversion coefficients for ICRP-compliant pregnant models from idealised proton exposures

Protection of pregnant women and their foetus against external proton irradiations poses a unique challenge. Assessment of foetal dose due to external protons in galactic cosmic rays and as secondaries generated in aircraft walls is especially important during high-altitude flights. This paper reports a set of fluence to absorbed dose conversion coefficients for the foetus and its brain for external monoenergetic proton beams of six standard configurations (the antero-posterior, the postero-anterior, the right lateral, the left lateral, the rotational and the isotropic). The pregnant female anatomical definitions at each of the three gestational periods (3, 6 and 9 months) are based on newly developed RPI-P series of models whose organ masses were matched within 1% with the International Commission on Radiological Protection reference values. Proton interactions and the transport of secondary particles were carefully simulated using the Monte Carlo N-Particle eXtended code (MCNPX) and the phantoms consisting of several million voxels at 3 mm resolution. When choosing the physics models in the MCNPX, it was found that the advanced Cascade-Exciton intranuclear cascade model showed a maximum of 9% foetal dose increase compared with the default model combination at intermediate energies below 5 GeV. Foetal dose results from this study are tabulated and compared with previously published data that were based on simplified anatomy. The comparison showed a strong dependence upon the source geometry, energy and gestation period: the dose differences are typically less than 20% for all sources except ISO where systematically 40-80% of higher doses were observed. Below 200 MeV, a larger discrepancy in dose was found due to the Bragg peak shift caused by different anatomy. The tabulated foetal doses represent the latest and most detailed study to date offering a useful set of data to improve radiation protection dosimetry against external protons.

X-ray diffraction study of the damage induced in yttria-stabilized zirconia by swift heavy ion irradiations

The lattice damage was investigated by x-ray diffraction techniques in yttria-stabilized zirconia single crystals with the (100) or (110) orientation upon irradiation with swift heavy ions (from 100-MeV C to 2.6-GeV U) in a broad electronic stopping power range (from about 0.3 to 48 keV nm−1). The θ-2θ scans show that no amorphization or change to a new crystalline phase occurs regardless of the ion and crystal features. However, the rocking curves (ω scans) and reciprocal space mappings show evidence of the mosaicity of the crystals, which is produced above a threshold electronic stopping power between 18 and 27 keV nm−1. This threshold is in agreement with our previous Rutherford backscattering spectroscopy/channeling spectroscopy data. Two kinds of damage phenomena are found: (i) nuclear-collision induced clusters of point defects which generate Bragg peak shifts and broadening in the 2θ-ω and θ-2θ scans, and (ii) electronic-excitation induced lattice damage yielding broad peaks in the ω scans above the stopping power threshold at high fluences.

Stabilization of Fiber Bragg Gratings Against Gamma Radiation

The effect of gamma-radiation on the spectral characteristics of FBGs has been studied experimentally. The FBGs were fabricated in optical fibers with a GeO2 concentration in the core in a range from several to 99 mol.%. Various pre- and post-fabrication treatments were applied with the aim to improve the radiation tolerance of the FBGs. The best result was obtained for a Type IIa annealed grating written in SM310 fiber, where the Bragg peak shift saturated at a 12 pm level and an amplitude change of 0.04 dB with the maximal dose of about 50 kGy.

Repulsive interlamellar interaction induced by addition of colloidal particles

The effects of colloidal particles confined between lamellar membrane slits on interlamellar interactions have been investigated by small-angle neutron scattering. On addition of colloidal particles to a lamellar phase composed of a non-ionic surfactant, the first lamellar peak becomes sharper and higher-order peaks appear. Thus the colloidal particles suppress undulation fluctuations of lamellar membranes by their steric hindrance, which results in a repulsive interlamellar interaction. As the interlamellar distance decreases, the position of the Bragg peak shifts towards higher q [where q is the magnitude of scattering vector, given by q = (4\pi/\lambda)\sin\theta, where 2\theta is the scattering angle and λ is the wavelength] and the peak intensity weakens. This tendency is completely opposite to the behavior of non-ionic surfactant lamellar phases, where the interlamellar interaction is governed by the Helfrich interaction. A phenomenological free-energy model is proposed based on the restriction of membrane fluctuations by colloidal particles. This model describes the experimental results well.

Ultra-high-angle double-crystal X-ray diffractometry (U-HADOX) for determining a change in the lattice spacing: experiment.

An experiment is reported on ultra-high-angle double-crystal X-ray diffractometry (U-HADOX) at the Bragg angle theta up to 89.5 degrees, coupled with white X-rays from a conventional source. It is shown that the Bragg-peak shift associated with a change in the lattice spacing increases in proportion to tantheta as predicted; the relative accuracy of a change in the spacing to 10(-8) is attained. The performance is demonstrated for the determination of the linear thermal-expansion coefficient alpha of silicon and strontium titanate SrTiO(3), a value with four significant figures being determined with the data in a temperature range narrower than 1 K; the value of alpha for silicon (2.621 +/- 0.003) x 10(-6) K(-1) at 300.4 K is compared with those in the literature based on experiments using the Bond method of X-ray diffraction and macroscopic thermal expansion. The advantages and characteristics of U-HADOX including the coupling with synchrotron X-rays are discussed.

Dose-rate dependencies in gamma-irradiated fiber Bragg grating filters

Wavelength-division multiplexed fiber-optic links could find applications in various radiation environments. The space industry, for example, could benefit from radiation-tolerant high-speed optical links for the next generation of observation satellites. For applications in the nuclear energy industry, the use of passive optical devices also needs to be evaluated at high dose rates and total doses. Our previous results have shown that in-fiber Bragg grating filters are key components in radiation-resistant passive optical links, as they showed a remarkably high radiation-acceptance level in terms of total absorbed dose. The dose-rate dependencies also need to be evaluated. We therefore studied gamma-irradiated fiber Bragg grating filters at several dose rates ranging from 1 to 25 kGy/h. We show, for the first time, that the dose rate does not affect the full-width at half-maximum of the Bragg peak and that the saturating Bragg peak shift remains.

Radiation-resistant WDM optical link for thermonuclear fusion reactor instrumentation

The future International Thermonuclear Experi- mental Reactor (ITER) is a complex installation that will require permanent monitoring and frequent maintenance operations. The high-gamma dose rates, the high neutron fluence, and other radiological hazards call for the use of remote-handled equipment. The management of heavy umbilicals connecting the control systems with the remote tools is therefore a key issue. Multiplexing signals can relieve the cable-handling difficulties. In this respect, the intrinsic wavelength division multiplexing (WDM) capabilities of fiber-optic technology make it a very promising candidate for integration in ITER instrumentation links. However, the radiation hardness of a complete WDM optical link still needs to be assessed. In this paper, as a first step toward the development of a rad-hard WDM optical link, we report on irradiations of different parts of a typical WDM optical link. We present our irradiation results on COTS fiber-optic devices, including WDM single-mode couplers, which remain operational up to MGy dose levels while the channel drift observed in narrow-band couplers compromises their use in WDM multiplexers. The intrinsic wavelength encoding of fiber Bragg grating (FBG) sensors makes them ideal candidates for WDM fiber-optic sensor networks. Therefore, we also investigated the -radiation response of FBGs written in germanosilicate fibers. We irradiated such sensors up to MGy dose levels. At a total dose of 0.1 MGy, saturation of the radiation-induced Bragg peak shift has been observed, evidencing the potential radiation hardness of FBG-based devices in highly radioactive environments. To illustrate wavelength multiplexing in sensing, we discuss our preliminary results on a new multicomponent force sensor design based on eight multiplexed FBG sensors intended for use at the end effector wrist of remote-controlled robots. Finally, we present the in-reactor irradiation results of standard Corning Ge-doped fiber up to GGy dose levels. Index Terms—Fiber Bragg grating (FBG) sensor, gamma radia- tion, International Thermonuclear Experimental Reactor (ITER), nuclear robotics, optical fiber sensor, radiation effects, wavelength division multiplexing (WDM).

Neutron strain scanning in bimetallic tubes: experimental and Monte Carlo simulation results

Neutron strain scanning measurements have been performed on straight and curved parts of a component manufactured from bimetallic tubes. Instrumental and sample-related effects contributing to non-strain induced Bragg peak shifts have been corrected for by means of Monte Carlo simulations which incorporate details of the various diffractometer elements.

Pressure stabilization and piezoelectric properties of polycrystalline perovskite PbZn1/3Nb2/3O3–PbTiO3

By using high-pressure heat treatment, we synthesized the single perovskite phase of polycrystalline (1−x)PbZn1/3Nb2/3O3–xPbTiO3 which was reported to show giant piezoelectricity in crystal forms. Piezoelectric constant of the polycrystalline materials at room temperature was found to become maximum at the rhombohedral side (x=8%) of morphotropic phase boundary. Microscopic strain under the electric field (E) along different crystallographic directions was identified by observing the x-ray Bragg peak shift of polycrystalline samples with E.

Irreversible Structural Transformation of five fold i-AlPdMn Quasicrystals after Ion Bombardment and Annealing

AbstractFive fold i-AlPdMn surface prepared under UHV by ion bombardment and annealing was so far considered to be bulk terminated. This result was substantially based on a quantitative LEED analyses [1]. Analysis of the specular rod in a X ray diffraction experiment at grazing incidence supported this result [2]. We present a new study of this surface by high resolution X ray diffraction at normal incidence. In this Bragg configuration the diffraction peak 18 – 29 for instance is at a photon energy of 2.873keV, the 72 – 116 reflection at 5.725keV. This results in an analyzed thickness of the sample surface of a few micrometers.The surface was cleaned by ion bombardment. During annealing (T≅880K), we clearly observed the progressive disappearance of the initial Bragg peak characteristic of the as cast bulk sample. Conversely a new Bragg peak grows at an energy position shifted by 1eV compared to the position of the original Bragg peak. This is a clear signature for an irreversible structural transformation which takes place on at least the micron thickness. On the transformed surface, both, a LEED pattern and a RHEED pattern, characteristic for a five fold surface were easily obtained.This high resolution experiment (the relative Bragg peak shift is 3ׁ10−4) was reproduced on samples from different initial compositions. This shows that five fold i-AlPdMn surface changes after preparation by ion bombardment and annealing at 900K on a micrometer thickness. This is not consistent with the conclusion that the surface is simply terminated by a cut of the original bulk. We conclude that a reorganization process of the quasicrystalline structure during annealing proceeds in the surface vicinity (probed depth is close to a few microns).

Deformation induced transformations and grain boundary thickness in nanocrystalline B2 FeAl

Precise measurement of fundamental Bragg peak shifts during milling of nanocrystalline ordered B2 Fe 60Al 40 has allowed for the first time, the deconvolution of the 110 fundamental Bragg peak intensities of the b.c.c. disordered regions and of the ordered B2 regions from the start and before full disappearance of the latter. The evolution of the lattice parameter a 0 of the b.c.c. solid solution with milling time shows two characteristics. First a jump to higher values from the initial a 0 of the B2 phase, with a Δ a 0 change of the order of 1% corresponding to a volume per atom Δ V expansion of about 3%. Subsequently, a 0 continues to increase slowly with further milling at constant grain size D and in the absence of any B2 phase. This continuing change of a 0 with further milling up to at least 180 min is attributable to a reduction of chemical short-range order (CSRO) or the number of Al–Fe heteroatomic “bonds”. The appearance of two well-defined maxima in the hyperfine field (HF) distributions derived from the Mössbauer spectra indicated the presence of two ferromagnetic environments contributing to the broadened Mössbauer resonance sextet signal. The evolution of the second component of this Mössbauer signal scales with the grain size. Using the mean grain size D derived from X-ray peak profiles and TEM pictures together with the grain boundary thickness d gb of 1.25 nm determined by Fultz et al. ( J. appl. Phys., 1996, 7, 127) for b.c.c. Fe-based alloys, the fraction of grain boundary atoms n gb/ n total was estimated and found to be consistent with the fraction of Fe atoms contributing to the lower HF component of the Mössbauer sextet signal. The grain boundary atom count using both methods confirms that grain boundaries in materials nanocrystallized by heavy deformation are nearly as dense as in the bulk.