Polyethylene Slabs Research Articles

Silicon strip detector for a novel 2D dosimetric method for radiotherapy treatment verification

The aim of this work is to characterize a silicon strip detector and its associated data acquisition system, based on discrete electronics, to obtain in a near future absorbed dose maps in axial planes for complex radiotherapy treatments, using a novel technique. The experimental setup is based on two phantom prototypes: the first one is a polyethylene slab phantom used to characterize the detector in terms of linearity, percent depth dose, reproducibility, uniformity and penumbra. The second one is a cylindrical phantom, specifically designed and built to recreate conditions close to those normally found in clinical environments, for treatment planning assessment. This system has been used to study the dosimetric response of the detector, in the axial plane of the phantom, as a function of its angle with respect to the irradiation beam. A software has been developed to operate the rotation of this phantom and to acquire signals from the silicon strip detector. As an innovation, the detector was positioned inside the cylindrical phantom parallel to the beam axis. Irradiation experiments were carried out with a Siemens PRIMUS linac operating in the 6MV photon mode at the Virgen Macarena Hospital. Monte Carlo simulations were performed using Geant4 toolkit and results were compared to Treatment Planning System (TPS) calculations for the absorbed dose-to-water case. Geant4 simulations were used to estimate the sensitivity of the detector in different experimental configurations, in relation to the absorbed dose in each strip. A final calibration of the detector in this clinical setup was obtained by comparing experimental data with TPS calculations.

Tandem collimators for the JET tangential gamma-ray spectrometer

The tangential gamma-ray spectrometer (TGRS) of the JET tokamak fusion facility is an important diagnostics for investigating the fast particle evolution. A well defined field of view for the TGRS diagnostics is essential for its proper operation and this is to be determined by a rather complex system of collimators and shields both for the neutron and gamma radiations. A conceptual design for this system has been carried out with the main design target set to maximize the signal-to-background ratio at the spectrometer detector, the ratio being defined in terms of the plasma emitted gamma radiation and the gamma-ray background. As a first phase of the TGRS diagnostics upgrade a set of two tandem collimators has been designed with the aim of determining a quasi-tangential field of view through JET tokamak plasmas. A modular design of the tandem system has been developed in order to allow for the construction of different configurations for deuterium and deuterium–tritium discharges. The internal structure of the collimators consists of nuclear grade lead and high density polyethylene slabs arranged in an optimized pattern. The performance of a simplified geometry of the tandem collimator configuration has been evaluated by neutron and photon transport calculations and the numerical results show that the design parameters can be attained.

Experimental study of the secondary neutron angular spectra emitted from polyethylene slabs with D-T neutron source

An experimental method of measuring the secondary neutron angular spectrum from polyethylene slab assembly with D-T neutron source based on the proton-recoil method is developed. In order to ensure the energy linearity of the detector and obtain a good neutron-gamma discrimination capability in the case of a low effective neutron threshold (0.5MeV), neutron energy spectrum is measured in two parts: one is in a high energy region and the other in a lower energy region. The pulse shape discrimination (PSD) and the pulse height distribution (PH) two-dimensional information of the secondary neutron and the associated gamma rays are recorded event-by-event simultaneously. The selection and the deduction of the gamma ray events and the joining of the two-segment energy spectra are carried out by using the off-line data analysis programs written in the ROOT data analysis framework. Furthermore, an artificial neural network technique is used to accomplish the unfolding of the neutron spectra successfully. The experimental results of the second neutron angular spectra from 9cm and 18cm thick polythene slab assemblies with D-T neutron source in an energy region of 0.5 MeV to 15 MeV are obtained. The Monte Carlo calculations of the experimental models are accomplished by using the MCNP5 code with ENDF-VI data library. The experimental results are in agree ment with the calculation results within the experimental uncertainties.

SU-FF-T-188: Dose Verification for IMRT DMLC Delivery Using An EPID Imager

Purpose To reconstruct dose distributions using EPID images acquired during treatment. Method and Materials This study used a Varian aS500 EPID imager located at SDD = 150 cm and operated in integration mode. The absolute dose of non-uniformity corrected EPID images was cross-calibrated with ion chamber measurements in the slab phantom for experiments. A method based on Monte Carlo simulation was developed to estimate dose spread within the imager and scatter radiation from the phantom/patient to EPID. The scatter calculation accounts for imager composition, treatment fluence, patient anatomy and air gap using convolution/superposition algorithm. The primary radiation reaching EPID was extracted by removing the scatter part from the measurement. The fluence without patient was then obtained by correcting for the patient attenuation estimated from the CT images. The dose distribution was calculated by convolution of the reconstructed fluence with the dose deposition kernel from Monte Carlo simulation. For verification we compared our reconstructed results with TPS (Varian Eclipse™) calculation and EDR2 film measurements for different IMRT beams and gantry angles. A polyethylene slab phantom (25×25×15 cm3) was used for measurements with SSD = 93.7 cm and film insert depth of 5.30 cm. Comparison using profiles, isodoses as well as the gamma evaluation (criteria: 3%/3 mm) was presented. Results Our results indicate that the isodose curves of reconstructed dose overlap well with film measurement. The agreement is better than that between TPS calculation and film measurement. The 2D gamma evaluation between EPID reconstructed doses and film measurements shows a maximum γ of 1.70, a mean of 0.42 and 97.6% points with γ< 1. Close comparisons of their profiles reconfirm the results. Conclusion We reconstructed the dose distributions using EPID images acquired during treatment and it was verified with film dose measurements. The proposed method provides a reliable tool for IMRT verification.

WE‐E‐L100J‐02: A Phantom Study of Contrast‐Enhanced Dual Energy Mammography (CEDEM)

Purpose: To optimize and study the effectiveness of CEDEM using simple breast‐equivalent phantoms in the experimental setting. Method and Materials: The experimental evaluation of CEDEM was performed using a dedicated cone‐bream breast CT scanner in our laboratory, with the system operating in stationary imaging mode. A series of physical phantoms approximating the anatomical complexity of the breast were fabricated. Polyethylene and water were used for the adipose and glandular aspects of the breast phantom, respectively, due to their similar x‐ray attenuation characteristics. Holes of various sizes were randomly drilled on the polyethylene slabs to create an “anatomical” noise pattern. Rectangular cells (cuvettes) filled with various iodine concentrations were placed in a polyethylene slab to maintain a homogeneous projected iodine thickness (mg/cm2). When imaged experimentally, the polyethylene slab with five cuvettes of various iodine concentrations was overlaid with a number of the slabs with pseudo‐anatomical noise distributions while immersed in a plastic container filled with water. Data were acquired at 55 kVp and 100 kVp (suggested by computer optimization) with additional elemental filters (0.1 – 0.3 mm copper filtration), and dual‐energy subtracted images were produced. Results: Preliminary results were acquired at 55 kVp and 100 kVp (0.3 mm additional copper filtration). The sensitivity to iodine contrast agent in the dual‐energy subtracted image (signal‐difference‐to‐noise ratio (SDNR) per mg/cm2) was evaluated and found to be linear with concentration. The influence of the breast phantom thickness, composition, and dose on contrast enhancement at different kVp combinations will be presented. Conclusion: The effectiveness of CEDEM with an anatomical‐complex phantom was evaluated. The results indicate that the dual‐energy subtracted image can enhance the iodine contrast agent with background anatomical structures. This approach may be useful for the evaluation of the kinetic curve in contrast enhanced breast imaging procedures.

Sol–gel derived hybrid coatings for the improvement of scratch resistance of polyethylene

Organic–inorganic hybrid materials were prepared through the sol–gel approach starting from tetraethoxysilane (TEOS), as silica precursor, and triethoxysilane terminated polymers; before gelling the solutions were applied to polyethylene (PE) films and slabs by spin-coating, without any previous surface pre-treatment of the substrate, and finally the coatings were thermally cured at 60 °C for 24 h. Among the various polymers used to prepare the coatings, only polyethylene-b-poly(ethylene glycol) copolymers gave good results in terms of adhesion to the PE substrates, and hybrid coatings with different organic–inorganic ratios were prepared. As suggested by visual inspection and SEM investigation, and confirmed by the critical loads derived from scratch tests, a good adhesion of the coating to the PE substrates was obtained, probably due to the presence of PE segments in the organic phase of the coating. Transparency as well as SEM and DSC data were in agreement with the formation of a nanostructured hybrid coating, with a high level of interpenetration between organic and inorganic domains. It was also observed that these hybrid coatings are able to improve significantly the scratch resistance and slightly increase the wettability with respect to uncoated PE. This approach to the surface-properties modification of PE appears as a simple and convenient method for the functionalization of PE substrates.

Response of RadFET dosimeters to high fluences of fast neutrons

We present irradiation experiments carried out on RadFETs in the high-intensity T2 neutron beam at the CRC-UCL in Belgium. The aim of the test was to characterize the neutron response of RadFETs in view of their use as an integrated part of a radiation-monitoring sensor for the CERN Large Hadron Collider (LHC) experiments. Two types of RadFETs were investigated up to a total neutron fluence of 3/spl times/10/sup 14/ cm/sup -2/ corresponding to a deposited dose of 744 Gy in silicon. The responses of bare devices to neutrons are compared to the commonly used reference measurements with gamma rays. It is found that the gamma ray calibration cannot directly be adopted to convert the RadFET signals into neutron dose. In a second experiment, the influence of a plastic packaging, simulated by polyethylene slabs of different thicknesses, was tested in the neutron beam and compared to GEANT4 Monte Carlo simulations. An increase of the RadFET neutron sensitivity by a factor of up to 7 due to the packaging is found. The influence of these findings on the conception of the radiation-monitoring sensor is discussed.

Simulations of an accelerator-based shielding experiment using the particle and heavy-ion transport code system PHITS

In order to estimate the biological effects of HZE particles, an accurate knowledge of the physics of interaction of HZE particles is necessary. Since the heavy ion transport problem is a complex one, there is a need for both experimental and theoretical studies to develop accurate transport models. RIST and JAERI (Japan), GSI (Germany) and Chalmers (Sweden) are therefore currently developing and bench marking the General-Purpose Particle and Heavy-Ion Transport code System (PHITS), which is based on the NMTC and MCNP for nucleon/meson and neutron transport respectively, and the JAM hadron cascade model. PHITS uses JAERI Quantum Molecular Dynamics (JQMD) and the Generalized Evaporation Model (GEM) for calculations of fission and evaporation processes, a model developed at NASA Langley for calculation of total reaction cross sections, and the SPAR model for stopping power calculations. The future development of PHITS includes better parameterization in the JQMD model used for the nucleus–nucleus reactions, and improvement of the models used for calculating total reaction cross sections, and addition of routines for calculating elastic scattering of heavy ions, and inclusion of radioactivity and burn up processes. As a part of an extensive bench marking of PHITS, we have compared energy spectra of secondary neutrons created by reactions of HZE particles with different targets, with thicknesses ranging from <1 to 200 cm. We have also compared simulated and measured spatial, fluence and depth–dose distributions from different high energy heavy ion reactions. In this paper, we report simulations of an accelerator-based shielding experiment, in which a beam of 1 GeV/n Fe-ions has passed through thin slabs of polyethylene, Al, and Pb at an acceptance angle up to 4°.

PD inference for the early detection of electrical treeing in insulation systems

Partial discharge (PD) measurements constitute one of the most promising tools for electrical insulation diagnosis. This paper describes how a procedure based on PD measurements can provide early detection of electrical trees in polymeric insulation systems. Such an application relies upon a new methodology, which provides enhanced tools for the identification of PD generating defects. Tree inference is carried out stepwise. Acquired signals are primarily separated according to their waveform, thus achieving data sets related to a specific PD typology. Then, fuzzy algorithms are applied to PD height and phase derived quantities belonging to these homogeneous data sets, in order to assign a membership degree to specific output categories. If the data set is relevant to an internal defect, a further analysis is performed in order to establish whether or not this defect is a treed region. The algorithm described in this paper was developed resorting to tests performed on artificial test specimens and electrical apparatus. In particular, the rules to detect the presence of electrical trees were derived from experiments carried out on needle-plane objects, constituted by slabs of cross-linked polyethylene (XLPE) where a needle is inserted and partially extracted in order to generate a cavity in front of the needle tip. Tests were also performed on cables having artificial defects., as well as on other insulation systems, such as high frequency transformers. Applications of the proposed approach to MV cables and to HV transformers show that electrical trees can be detected successfully before final breakdown.

Are neutrons responsible for the dose discrepancies between Monte Carlo calculations and measurements in the build-up region for a high-energy photon beam?

This study presents measured neutron dose using a neutron dosimeter in a water phantom and investigates a hypothesis that neutrons in a high-energy photon beam may be responsible for the reported significant dose discrepancies between Monte Carlo calculations and measurements at the build-up region in large fields. Borated polyethylene slabs were inserted between the accelerator head and the phantom in order to remove neutrons generated in the accelerator head. The thickness of the slab ranged from 2.5 cm to 10 cm. A lead slab of 3 mm thickness was also used in the study. The superheated drop neutron dosimeter was used to measure the depth–dose curve of neutrons in a high-energy photon beam and to verify the effectiveness of the slab to remove these neutrons. Total dose measurements were performed in water using a WELLHOFER WP700 beam scanner with an IC-10 ionization chamber. The Monte Carlo code BEAM was used to simulate an 18 MV photon beam from a Varian Clinac-2100EX accelerator. Both EGS4/DOSXYZ and EGSnrc/DOSRZnrc were used in the dose calculations. Measured neutron dose equivalents as a function of depth per unit total dose in water were presented for 10 × 10 and 40 × 40 cm2 fields. The measured results have shown that a 5–10 cm thick borated polyethylene slab can reduce the neutron dose by a factor of 2 when inserted between the accelerator head and the detector. In all cases the measured neutron dose equivalent was less than 0.5% of the photon dose. In order to study if the ion chamber was highly sensitive to the neutron dose, we have investigated the disagreement between the Monte Carlo calculated and measured central-axis depth–dose curves in the build-up region when different shielding materials were used. The result indicated that the IC-10 chamber was not highly sensitive to the neutron dose. Therefore, neutrons present in a high-energy photon beam were unlikely to be responsible for the reported discrepancies in the build-up region for large fields.

Dynamic plasma pressure measurements

An interferometric registration scheme for dynamic plasma pressure measurements has been developed and tested. The sensitive element of this scheme is an acoustic quartz rod. A description of the method is presented. The main features of the proposed scheme are the absence of a calibration procedure, the small size of the sensitive area, and the possibility of separating the studied object from the measuring setup. Experimental results for a capillary discharge plasma in a polyethylene slab with three different diameters of the channel, namely, 1.5, 2, and 3 mm are shown and illustrate the feasibility of the approach. Pressures in the range 30–600 bar were recorded.

Shielding for neutron scattered dose to the fetus in patients treated with 18 MV x-ray beams.

Neutrons are associated with therapeutic high energy x-ray beams as a contaminant that contributes significant unwanted dose to the patient. Measurement of both photon and neutron scattered dose at the position of a fetus from chest irradiation by a large field 18 MV x-ray beam was performed using an ionization chamber and superheated drop detector, respectively. Shielding construction to reduce this scattered dose was investigated using both lead sheet and borated polyethylene slabs. A 7.35 cm lead shield reduced the scattered photon dose by 50% and the scattered neutron dose by 40%. Adding 10 cm of 5% borated polyethylene to this lead shield reduced the scattered neutron dose by a factor of 7.5 from the unshielded value. When the 5% borated polyethylene was replaced by the same thickness of 30% borated polyethylene there was no significant change in the reduction of neutron scatter dose. The most efficient shield studied reduced the neutron scatter dose by a factor of 10. The results indicate that most of the scattered neutrons present at the position of the fetus produced by an 18 MV x-ray beam are of low energy and in the thermal to 0.57 MeV range since lead is almost transparent to neutrons with energies lower than 0.57 MeV. This article constitutes the first report of an effective shield to reduce neutron dose at the fetus when treating a pregnant woman with a high energy x-ray beam.

Neutron Leakage From Polyethylene Slit

A measurement of neutron dose rate on iron-polyethylene shielding structure was carried out by 252Cf source. Simulated geometry was slit-like opening of polyethylene in iron slab and polyethylene slab shielding.These experiment was done at research facility of Hazama Co,. Iron slab and polyethylene slab thickness were 10 cm each. A gap of the polyethylene was simulated. Neutron REM-counter, polyethylene covered BF3 counter (STUDSVIK 2202-D), was used for measurement of streaming neutron dose equivalent. The solid state track detector (SSTD), allyl-diglycol-carbonate, were used for measurement of fast neutron dose equivalent in the range of 170Kev to 15Mev.The experimental data was obtained against gap width, source location and detector location.Obtained data shows strong correlation between dose rate and above parameters.These data was investigated in the view of to make use of actual facility design and compared with calculation such as MCNP4B.From the result of gap streaming experiment and calculation, we obtained allowable gap width as 6mm for this case (10cm polyethylene thickness).

JSSTDL-300 : The Standard Shielding Cross Section Library Based on JENDL-3.2

A common group cross-section library JSSTDL based on Japanese Evaluated Nuclear Data Library, Version 3 Revision 2 (JENDL-3.2) has been updated in Japan Atomic Energy Research Institute (JAERI) under the cooperation of the Working Group on Standard Group Constants affiliated by the Subcommittee of Group Constants of Japanese Nuclear Data Committee (JNDC). Up to now, JSSTDL-295 based on JENDL-3.2 was developed for shielding applications, however some users pointed out a difficulty of the JSSTDL-295 for applying configurations including polyethylene slabs. It was caused from inadequate group structure for the thermal energy region and the weighting function. In the update version of JSSTDL (JSSTDL-300), we increased number of thermal energy groups from 5 to 10, and also determined the energy boundaries much carefully. The JSSTDL-300 was tested with various shielding benchmark problems, and the validity was confirmed. The integral test scheme established in JNDC was applied in this study. In the present paper, we describe the result of integral test of JSSTDL-300 for applying benchmarks of Japanese-American Shielding Program for Experimental Research (JASPER). It has been concluded that JSSTDL-300 is highly applicable for shielding applications such as configurations in which thermal neutrons were enhanced.

Bulk media assay using backscattered Pu–Be neutrons

Spectral yields of elastically backscattered Pu–Be neutrons measured for graphite, water, polyethylene, liquid nitrogen, paraffin oil, SiO2, Al, Fe, and Pb slabs show a definite correlation with the energy dependence of the elastic scattering cross sections, σEL(En). The C, N and O can be identified by the different structures in their σEL(En) functions. The integrated spectral yields versus thickness exhibit saturation for each sample. The interrogated volume is limited by the presence of hydrogen in the sample.

Accelerated weathering‐induced degradation of unprotected multilayer polyethylene slabs. I

AbstractThe in‐depth degradation of unprotected low density polyethlene (LDPE) induced by photooxidation was studied by following the carbonyl content and other by‐product groups as well as the retention of mechanical properties as a function of time and distance from the surface. The multilayer slabs were prepared by hot pressing of nine LDPE film layers. Weathering of the samples was performed in a Q.U.V. apparatus running dry at 55°C for different time periods. The carbonyl content, as measured by FTIR spectra, showed a parabolic symmetry around the center layer which was least affected. Oxygen barriers placed on one or both surfaces of the slabs significantly reduced the photooxidation. The retention of mechanical properties followed a similar profile as that of the carbonyl. However, the outermost layers, especially those that were close to the UV‐radiation source were slightly more affected. The results of this study lead to the conclusion that photooxidation is autocatalytic and is affected primarily by diffusion of oxygen. © 1995 John Wiley &amp; Sons, Inc.

Total neutron cross section of Teflon between 0.00038 and 590 eV

The neutron total cross section of Teflon has been measured for energies between 3.8 × 10 −4 eV and 5.9 × 10 2 eV by the transmission method with pulsed neutron time-of-flight techniques. In order to cover that wide energy range, two independent measurements were performed on samples with different thicknesses and employing as neutron moderator a conventional polyethylene slab for the epithermal region and a liquid nitrogen cooled paraffin block for the cold/thermal region. Due to some inhomogeneities in the sample material, the transmission data were normalized using the best available values of the scattering lengths and incoherent cross sections for carbon and fluorine.

A Fundamental Experimental Study of the Mechanisms of Warpage and Shrinkage of Polyethylene in Rotational Molding

Abstract An off-line experimental simulation of cooling and warpage of rotationally molded parts is described. Slabs of polyethylene rotationally molded parts containing thermocouples and monitored by displacement transducers are placed on a metal surface representing a mold wall. This is cooled by varying methods such as simulated water showers, fan blown air, and natural convection cooling. Temperature profiles in the mold wall and through the polymer are determined as a function of time. The position of crystallization and the development of warpage are observed. Warpage is attributed to thermal quench stresses and release of the polymer from the mold surface.

Optimum Arrangement to Minimize Total Dose Rate of Iron-Polyethylene Shielding System

The shielding experiments using iron-polyethylene slab shields with a 252Cf neutron source were carried out to find out an optimum arrangement to minimize the total dose rate; that is composed of neutron and secondary γ-ray dose rates. The total thickness of the iron slabs was fixed at 32 cm, while a variety of thickness and location of a polyethylene slab in the iron slabs were employed as a parameter. The minimum dose point (i.e. optimum shielding arrangement) was observed when the polyethylene slab was located at approximately 20 cm depth from the source side in the arrangements. The ratio of the minimum dose rate obtained for the optimum arrangement to the maximum for the worst arrangement became 1/1.3 for the polyethylene slab of 1-cm-thick, 1/2.0 for 3-cm-thick, 1/2.9 for 6-cm-thick, 1/3.9 for 10-cm-thick and 1/3.6 for 14-cm-thick. The appearance of the optimum arrangement for the total dose rates, and changing profiles of the secondary γ-ray as well as the neutron dose rates were reproduced for the...

Optimum arrangement to minimize total dose rate of iron-polyethylene shielding system.

The shielding experiments using iron-polyethylene slab shields with a 252Cf neutron source were carried out to find out an optimum arrangement to minimize the total dose rate; that is composed of neutron and secondary γ-ray dose rates. The total thickness of the iron slabs was fixed at 32 cm, while a variety of thickness and location of a polyethylene slab in the iron slabs were employed as a parameter. The minimum dose point (i.e. optimum shielding arrangement) was observed when the polyethylene slab was located at approximately 20 cm depth from the source side in the arrangements. The ratio of the minimum dose rate obtained for the optimum arrangement to the maximum for the worst arrangement became 1/1.3 for the polyethylene slab of 1-cm-thick, 1/2.0 for 3-cm-thick, 1/2.9 for 6-cm-thick, 1/3.9 for 10-cm-thick and 1/3.6 for 14-cm-thick. The appearance of the optimum arrangement for the total dose rates, and changing profiles of the secondary γ-ray as well as the neutron dose rates were reproduced for the typical two cases of the polyethylene slab thickness by the Monte Carlo calculations with the splitting technique.