PMMA Slab Research Articles

Commentary

Aim of this work is the study of the preliminary steps to perform a particle treatment of cancer cells inoculated in small animals and to realize a preclinical hadrontherapy facility. A well-defined dosimetric protocol was developed to explicate the steps needed in order to perform a precise proton irradiation in small animals and achieve a highly conformal dose into the target. A precise homemade positioning and holding system for small animals was designed and developed at INFN-LNS in Catania (Italy), where an accurate Monte Carlo simulation was developed, using Geant4 code to simulate the treatment in order to choose the best animal position and perform accurately all the necessary dosimetric evaluations. The Geant4 application can also be used to realize dosimetric studies and its peculiarity consists in the possibility to introduce the real target composition in the simulation using the DICOM micro-CT image. This application was fully validated comparing the results with the experimental measurements. The latter ones were performed at the CATANA (Centro di AdroTerapia e Applicazioni Nucleari Avanzate) facility at INFN-LNS by irradiating both PMMA and water solid phantom. Dosimetric measurements were performed using previously calibrated EBT3 Gafchromic films as a detector and the results were compared with the Geant4 simulation ones. In particular, two different types of dosimetric studies were performed: the first one involved irradiation of a phantom made up of water solid slabs where a layer of EBT3 was alternated with two different slabs in a sandwich configuration, in order to validate the dosimetric distribution. The second one involved irradiation of a PMMA phantom made up of a half hemisphere and some PMMA slabs in order to simulate a subcutaneous tumour configuration, normally used in preclinical studies. In order to evaluate the accordance between experimental and simulation results, two different statistical tests were made: Kolmogorov test and gamma index test. This work represents the first step towards the realization of a preclinical hadrontherapy facility at INFN-LNS in Catania for the future in vivo studies.

The effect of a compression paddle on energy response, calibration and measurement with mammographic dosimeters using ionization chambers and solid-state detectors

A compression paddle is always used in mammography x-ray examinations, in order to improve image quality and reduce patient doses. Although clinical dose measurements should be performed with the paddle to interfere with the x-ray beam, calibration of mammography dosimeters is performed free in air without the presence of the paddle. The paddle hardens the x-ray beam, which has an impact on a dosimeter performance, particularly on high-energy-dependent detectors. Due to the paddle, clinical mammography x-ray systems may exhibit beams with HVL values exceeding those of the IEC 61267 RQR-M series qualities at which dosimeters are usually calibrated. In this study, the influence of the paddle in mammography dosimetry is examined, in Mo/Mo anode/filter x-ray qualities. PMMA slabs of 1, 2 and 3 mm thickness and Al foils of 0.05, 0.10 and 0.15 mm thicknesses were used to simulate the paddles, producing beams with HVL values from 0.28 up to 0.43 mmAl. In these qualities, four solid-state (ST) detectors and three ionizations chambers (IC) were calibrated in terms of Kair and NK and kQ were deduced. The results showed that all IC and two modern-type ST dosimeters have a flat energy response in the above HVL range (less than 3%), so their calibration factor at RQR-M2 quality could be safely used for clinical measurements. Two other ST dosimeters exhibit up to 20% energy response, so differences up to 15% in dose measurement may be observed if the effect of paddle on their performance is ignored. Finally, the need of additional mammographic calibration qualities to the existing IEC 61267 RQR-M series is examined and discussed.

Polymer Optical Fibre for in Situ Monitoring of Gamma Radiation Processes

Abstract Poly Methyl MethAcrylate (PMMA) based plastic optical fibres offer a low cost radiation monitoring solution with a wide dose range, capable of providing real-time, online information. The sensitivity of the fibre to ionising radiation is shown to be dependant on wavelength, with the sensitivity increasing with decreasing wavelength, and so by carefully selecting the monitoring wavelength, the desired sensitivity and dose range can be achieved. The fibres exhibit high sensitivity, up to 0.6dBm−1/kGy, and are capable of monitoring dose ranges between 30Gy and 45kGy. This exceeds the sensing range of all commercially available PMMA slab sensors, allowing it to be used for a wide range of applications.

Monte Carlo characterization of materials for prosthetic implants and dosimetric validation of Pinnacle 3 TPS

In external beam radiotherapy the calculation of dose distribution for patients with hip prostheses is critical. Metallic implants not only degrade the image quality but also perturb the dose distribution. Conventional treatment planning systems do not accurately account for high-Z prosthetic implants heterogeneities, especially at interfaces. The materials studied in this work have been chosen on the basis of a statistical investigation on the hip prostheses implanted in 70 medical centres. The first aim of this study is a systematic characterization of materials used for hip prostheses, and it has been provided by BEAMnrc Monte Carlo code. The second aim is to evaluate the capabilities of a specific treatment planning system, Pinnacle 3, when dealing with dose calculations in presence of metals, also close to the regions of high-Z gradients. In both cases it has been carried out an accurate comparison versus experimental measurements for two clinical photon beam energies (6 MV and 18 MV) and for two experimental sets-up: metallic cylinders inserted in a water phantom and in a specifically built PMMA slab. Our results show an agreement within 2% between experiments and MC simulations. TPS calculations agree with experiments within 3%.

Numerical analysis of the heating process in upward flame spread over thick PMMA slabs

A detailed analysis of the unburned material heat-up during upward flame spread over small slabs of PMMA is provided using a numerical model. The two-dimensional time-dependent Favre-averaged Navier–Stokes equations coupled with sub-models for turbulence, combustion, soot formation, and radiation are solved for the gas phase. The modelling of condensed phase processes is based on the one-dimensional heat conduction equation and pyrolysis is treated as a phase change using the latent heat approach. Pyrolysis heights, x p, up to 0.6 m are considered. Model results agree with data on the pyrolysis front trajectory and heat release rate per unit width, Q ˙ ′ , as a function of x p from experiments in which the lateral air entrainment is expected to have no significant effect on flame spread. It is shown that the average luminous flame height, x fl, defined by the height at which the total wall heat flux is equal to 10 kW/m 2, correlates with Q ˙ ′ 1.25 for Q ˙ ′ < 20 kW / m , whereas it correlates with Q ˙ ′ 2 / 3 for Q ˙ ′ 20 kW / m , as deduced from experiments. Appropriate scaling of the total wall heat flux distribution is found in terms of ξ = ( x - x p ) / ( x fl - x p ) . This scaling allows us to locate the continuous flame region between ξ=0 and 0.4 and the plume region above ξ=1.6, the intermittent flame region being located between the two. Both continuous and intermittent flames are found to play an important role in the process of heating the unburned solid fuel whereas the plume contribution is negligible.

EFFECT OF OPPOSED FLOW ON FLAME SPREAD OVER A FINITE-LENGTH PMMA SLAB IN A TWO-DIMENSIONAL WIND TUNNEL

The ignition delay and subsequent downward flame spread over a finite-length PMMA slab with the effect of radiation included in a two-dimensional wind tunnel are investigated using the opposed flow velocity as a parameter. The gas and solid phase temperatures, preheat length and heat flux are considered in order to examine the flame ignition and spread characteristics. The numerical results reveal that the ignition delay time increases with the opposed flow velocity. However, the flame spread rate varies with the opposed flow velocity in a non-monotonic manner that can be identified as two distinct regimes with a peak value in between. The flame spread rate reaches a maximum at V g = 32cm/s, and then falls, regardless of whether the flow velocity is increasing or decreasing. For V g < 32cm/s, the flame behaviors are dominated by oxygen transport. For V g > 32cm/s, the flame stretch effect controls the flame behavior. Additionally, this work demonstrates that the effect of radiation delays the flame ignition and reduces both the flame strength and the corresponding spread rate. The effects of the opposed flow temperature and thickness of the solid fuel are also investigated. The predicted results indicate that a higher opposed flow temperature or a thinner solid fuel facilitates the ignition and accelerates flame spread. The effect of opposed flow velocity eventually overcomes that of opposed flow temperature as the flow speed is further increased.

Experimental study of beam hardening artifacts in photon counting breast computed tomography

We are implementing an X-ray breast Computed Tomography (CT) system on the gantry of a dedicated single photon emission tomography system for breast Tc-99 imaging. For the breast CT system we investigated the relevance of the beam hardening artifact. We studied the use of a single photon counting silicon pixel detector (0.3 mm thick, 256 × 256 pixel, 55 μ m pitch, bump-bonded to the Medipix2 photon counting readout chip) as detector unit in our X-ray CT system. We evaluated the beam hardening “cupping” artifact using homogeneous PMMA slabs and phantoms up to 14 cm in diameter, used as uncompressed breast tissue phantoms, imaged with a tungsten anode tube at 80 kVp with 4.2 mm Al filtration. For beam hardening evaluation we used a bimodal energy model. The CT data show a “cupping” artifact going from 4% (4-cm thick material) to 18% (14-cm thick material). This huge artifacts is influenced by the low detection efficiency and the charge sharing effect of the silicon pixel detector.

SU-FF-T-190: Dosimetry Verification for Pencil Beam, Clarkson and Equivalent Square Dose Calculation Algorithms

Purpose: To study the accuracy of Brainlab pencil beam and Clarkson algorithms in homogeneous phantoms and heterogeneous geometries. Materials and method: In this study PMMA was used as the homogenous phantom. Different heterogeneous geometries were designed by introducing different materials between the PMMA slabs. In each situation one material is used with different thicknesses: (a) thickness of 2.7 cm and 8.1cm low‐density material and (b) thickness of 3.8 cm and 7.6 cm of high‐density material. The low‐ and high‐density material is placed beneath 6 cm of PMMA. All the phantoms were CT scanned. Six different irregularly shaped beams were designed using a Brainlab multileaf collimator. Calculations were performed for these beams in the homogenous phantom using the Brainlab pencil beam, Clarkson, Modified Clarkson and equivalent square dose calculation algorithms. The percentage depth doses based on different models were compared to measurements. A dose of 150 cGy was specified to a point beneath the inhomogeneous material for the heterogeneous cases. Modified Clarkson, pencil beam and equivalent square algorithms were used to calculate the dose and then compared to measurements. Results: All the models agreed well with measurements; the equivalent square method showed the highest average deviations of 0.55% from measured dose values. In heterogeneous geometries the equivalent square method gave very large deviations (up to 17 %). Clarkson and pencil beam algorithms disagreed with measurements by 4.8 % and 3.8 % for the low‐density material of 2.7 cm thickness, and by 8 % and 6.9 % for the low‐density material of 8.7 cm thickness, respectively. Conclusion: All Brainlab dose calculation models are reliable for homogeneous phantom but some may differ significantly from measurements in heterogeneous geometries, especially in low‐density issues.

SU-FF-T-367: Radiochromic Film and Ion Chamber Dosimetry for Monochromatic X-Rays in PMMA

Purpose: K‐edge capture radiotherapy using monochromatic, keV x‐ray beams necessitates accompanying dosimetry methods. This work compares radiochromic film and ion chamberdosimetry methods potentially suitable for use with monochromatic x‐ray beams. Method and Materials: X‐rays were produced at the LSU CAMD synchrotron by passing a 1.3‐GeV electron beam (≈200‐mA) through a 7‐T superconductingwiggler. The resulting polychromatic beam was passed through a double multilayermonochromator to generate an approximately 0.1×2.8‐cm2, 35‐keV x‐ray beam. A 2.5×2.8‐cm2 broad beam was produced via oscillation of phantom and dosimeters by a triangular waveform. Central‐axis depth dose was measured in a 10×10×12.5‐cm3 PMMA slab phantom using 5.12×5.12‐cm2 GAFChromic® EBT films and an air‐equivalent, cylindrical ion chamber (0.23‐cm3). Films were digitized using the red channel of a flatbed scanner, and pixel values were converted to dose using both 6‐MV x‐ray and 125 I brachytherapy seed calibration curves. 125 I doses were calculated using AAPM TG‐43 formalism. Ion chamber charge readings were converted to dose using the AAPM TG‐61 protocol for kilovoltage x‐ray beam dosimetry.Results: Measurements in a PMMA phantom yielded film depth‐dose curves from film that were 2.5–4.4% higher than those from the ion chamber for depths of 0 to 9 cm when using the 125 I seed calibration. Using the 6‐MV x‐ray dosecalibration for film resulted in doses approximately 35% lower due to a significantly different film calibration curve compared to that using 125 I seeds. Conclusion: These methods should be suitable for future dose measurements required for cell and small animal irradiations. The discrepancy between 6‐MV x‐rays and 125 I seeds is contrary to previously reported results and currently under investigation.

On the flame height definition for upward flame spread

Flame height is defined by the experimentalists as the average position of the luminous flame and, consequently is not directly linked with a quantitative value of a physical parameter. To determine flame heights from both numerical and theoretical results, a more quantifiable criterion is needed to define flame heights and must be in agreement with the experiments to allow comparisons. For wall flames, steady wall flame experiments revealed that flame height may be defined by a threshold value on the wall heat flux. From steady wall flame measurements, three definitions of flame height from wall heat flux are retained: the first is based on the continuous flame while the two others are based on threshold values of 4 kW/m 2 and 10 kW/m 2. These definitions are applied to determine flame heights from a two-dimensional time-dependent CFD model used to describe flame spread on a vertical slab of PMMA. Results show that the predicted flame heights are consistent with the available data of the literature. Defining flame height by threshold values on the wall heat flux of 4 and 10 kW/m 2 allows the correlation of the wall heat flux in terms of ( x− x p)/( x fl− x p), which is the dimensionless characteristic length scale for upward flame spread. It is also found that the continuous flame is not a characteristic length for the heat transfer to the unburnt fuel and is not really appropriate to define flame height in upward flame spread.

Propagation and extinction mechanisms of opposed-flow flame spread over PMMA for different sample orientations

To understand the propagation and the extinction mechanisms of flame spreading along a combustible solid, opposed-flow flame spread along a thick slab of PMMA was experimentally investigated for four different sample orientation angles from 0° (horizontal) to −180° (ceiling flame spread) in several airflow rates. The detailed temperature distribution in the condensed phase during flame spread was measured using holographic interferometry (HI) and infrared thermography (IR). The particle-track laser sheet (PTLS) technique was employed to measure the local entrainment velocity to the flame front at the leading edge. This study found that a flame spread rate with a constant speed is proportional to the net total heat transfer rate. The heat transfer rate from the gas phase, Q y , is about 60% of the total heat transfer rate, Q T , under the no-imposed-flow horizontal flame spread condition ( u ¯ = 0 m / s ). However, the heat transfer rate through the condensed phase, Q x , increases with increasing airflow above 80% of Q T near the extinction limit. The radiative heat loss from the surface, Q R , increases with increasing opposed-flow rate and reaches a maximum of 13% of Q T at u ¯ = 0.65 m / s , while the heat transfer rate in ceiling flame spread is different than in horizontal or downward flame spread under nonopposed and slow-opposed flow conditions. The Q y in nonopposed flow ceiling flame spread is double that in horizontal flame spread, and is 85% of Q T . Despite adequate heat feedback to the condensed phase near the flame leading edge, the flame spread rate decreases rapidly as it approaches the extinction limit. In order to interpret the absence of flame spread or the extinction mechanism, we introduce the Damköhler number including the local air entrainment velocity and the burning rate at finite flame thickness. PTLS showed that the local entrainment velocity at the flame leading edge increased the opposed-flow rate. This leads to a decrease in the Damköhler number. When the Damköhler number reaches a critical value (at most <1), the flame cannot spread, retreats, and then is finally extinguished.

Flame height correlation and upward flame spread modelling

AbstractPrevious work has demonstrated that flame height is one of the two most important parameters determining the rate of vertical flame spread on a wall. Flame spread models rely on empirical flame height correlations of the form Xf=KQ̇′n, but there have been no carefully controlled experiments designed to establish the validity of such correlations and there are no data for values of Q̇′&lt; c.25 kW/m. Two new sets of data are presented here, one based on experiments with 6 mm thick PMMA slabs (heights 25–250 mm, width 50–150 mm) as the fire source, measuring Q̇′ and Xf simultaneously. This set relates to the early stages of a wall fire when Q̇′&lt; 30 kW/m. The other set of data was obtained with a vertical gas‐fired panel which consisted of an array of 14 independent burners arranged to allow the aspect ratio of the burning surface to be varied. The data confirm that the flame height correlates with Q̇′, but reveal (inter alia) that there are two regions, for values of Q̇′ greater than and less than c. 20 kW/m.An existing upward flame spread model was modified to allow these two correlations to be incorporated to predict the development of a small fire on a vertical surface for these two regions, which would provide more realistic modelling information. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Spot scanning using radioactive beams for heavy–ion radiotherapy (in Japanese)

In this thesis we describe a technique for determining the spatial distribution of beams at the Heavy Ion Medical Accelerator in Chiba (HIMAC). A narrow beam of ions will be moved to various positions to irradiate a three‐dimensional conformal field in a patient body. Since is a positron emitter, one can determine the spatial distribution of where the ions terminated with a PET scanner. The intensity of the beam is less than 1% of the beam. We used the more efficient spot scanning for the dose delivery. The spot beam of (346 MeV/n) is produced by a projectile‐fragmentation process of accelerated (430 MeV/n) beams in a Be target 51 mm in thickness. The depth and lateral size of the spot beam are chosen by selection of its momentum and focusing, respectively. In order to deliver the desired dose to the irradiation volume, the depth dose and lateral dose distribution of the spot beam were corrected by taking into consideration its wide momentum spread (2%). A spot beam consists of monochromatic beam fractions of various momenta, which lead to various ranges. Each beam fraction has a different lateral beam size because of an aberration of the lens system used in beam delivery. The dose distribution is measured by a multistrip parallel‐plate ionization chamber by changing the thickness of upstream PMMA slabs. The calculated dose distribution of the irradiation volume of in the PMMA phantom of was in good agreement (±0.2%) with measurements.

Two-dimensional metallic photonic band-gap crystals fabricated by LIGA

Two-dimensional metallic photonic band-gap crystals, consisting of square and triangular lattices of nickel pillars, were fabricated by the LIGA process. In particular, PMMA slabs with a thickness of up to 800 μm were irradiated with synchrotron radiation and the holes produced were then filled with nickel via electroforming. The lattice constant, i.e., the center-to-center distance between the pillars, is either 60 μm or 40 μm for the square and triangular structures respectively. The metal filling ratio is 10% for the square and 20% for the triangular structures. Transmission and reflection measurements demonstrate that the metallic photonic band-gap crystals show a cutoff frequency in the far infrared regime between 2 and 5 THz. It is concluded that LIGA is a promising method for the fabrication of 2-D metallic photonic band-gap structures, which could be potentially used as passive filters in THz devices.

Temperature and Time Dependence of Copolymer Mixtures on Interfacial Adhesion at PS/PMMA Interfaces

The effect of copolymer mixtures on the interfacial adhesion between slabs of PS and PMMA was investigated as a function of composition, time and temperature using the asymmetric double cantilever beam (ADCB) method. The nature of the interface was further probed using atomic force microscopy (AFM) and dynamic secondary ion mass spectroscopy (D-SIMS). The results show that mixtures of graft and block copolymers are much more effective than pure block copolymers in enhancing the interfacial adhesion. The most effective mixture consisted of a block copolymer of molecular weight 70K and a copolymer with two PS grafts of molecular weight 30K. This mixture yielded an interfacial fracture toughness of Gc = 127.5 J/m2 as compared with Gc = 38.2 J/m2 and Gc = 3.5 J/m2 for the pure block and graft copolymer, respectively. Gc at the PS/PMMA interface reinforced only with block copolymer was maximal after an annealing temperature of 150°C for 1 hr. It decreased by an order of magnitude when the temperature was increased to 180°C or the joining time was increased from 1 to 10 hours. Gc at the interface reinforced with a graft/diblock copolymer mixture was also maximum at an annealing temperature of 150°C but it decreased only by a factor of 2 with increasing joining time or temperature. Dynamic Secondary Ion Mass Spectroscopy (DSIMS) data show that this effect may be due to decrease in the diffusion of the copolymer from the interface when the mixture is present, i.e, the diblock copolymer is trapped within the graft copolymer.

Oxidizer Flow Effects on the Flammability of Solid Combustibles

A study is presented on the effect of oxidizer flow characteristics on the piloted ignition and opposed flame spread of a slab of PMMA heated by an external radiant flux. The objective is to establish the basis for a potential new lest method that may be used to determine the flammability performance of solid materials in terms of their ignition delay, critical heat flux for ignition, and flame-spread rate, for varied oxidizer flow conditions. The proposed Forced-flow Ignition and flame-Spread Test (FIST) follows the concepts of the LIFT (ASTM E 1321-93), and thus consists of a combination of ignition delay and flame spread experiments as a function of an externally applied radiant flux, but incorporates controlled forced convection as the predominant mechanism for the gas-phase transport of heat and mass. PMMA slabs were tested to assess the applicability of the methodology, and results are presented for radiant fluxes ranging from 0 to 35 kW/m2, forced oxidizer flow velocities of 1.0, 1.75, and 2.5 m/s. and natural convection. Results of the variation of ignition delay as a function of free stream oxygen concentration (18 to 45%) are also presented. Following the LIFT methodology, the ignition delay and flame spread data are used to assemble flammability diagrams for PMMA at different oxidizer flow velocities. Additional natural convection tests were performed in the LIFT apparatus with the purpose of establishing a base line for comparison of results. It is shown that the resulting FIST and LIFT flammability diagrams are similar. However, the diagrams as well as the parameters extracted from them depend on the oxidizer flow velocity, resulting in families of flow-dependent flammability diagrams. As a result of the use of forced convection, it is considered that the FIST is suitable for characterizing solid material flammability in mixed regimes ranging from natural to predominantly forced convection, and at different oxygen concentrations. This could provide a means to more accurately rank the relative flammability of solid combustible materials that would be used in environments where the oxidizer flow differs from air in natural convection, such as areas with significant air currents, vitiated environments, or microgravity applications (space based facilities).

The Effect of Carbon Black and Colloidal Silica Fillers on the Fracture Toughness at Polymethylmethacrylate Interfaces

ABSTRACTWe have measured the fracture toughness of PMMA slabs with carbon black and colloidal silica fillers placed at the interface. The results show that the fracture toughness decreases linearly with carbon black concentration. The fracture toughness scales as annealing time, t1/2indicating that it is diffusion limited. Addition of Colloidal Silica drastically reduces the fracture toughness regardless of concentration and annealing time. These results indicate that carbon black introduces physical cross links which decrease dynamics whereas colloidal silica produces permanent chemical cross links which prevent diffusion across the interface.

Film dosimetric verification of the Voxel Monte Carlo (VMC) algorithm with electron beam dose distributions

Monte Carlo (MC) methods have the potential to predict radiation-therapy doses more accurately than any conventional technique, but normal MC simulations are very time consuming. Therefore, a fast MC code (Voxel Monte Carlo; VMC) was developed especially for radiation therapy purposes and experiments with the comparable precision were performed to demonstrate its accuracy. In the present study the dose distributions measured with film dosimetry in a cylindrical phantom were compared with calculations derived by VMC. The phantom consisted of 18 circular shaped PMMA slabs with a diameter of 20 cm and a thickness of approx. 1 cm. The films were placed between the slabs, and the whole phantom was irradiated with electron beams of different energies (6MeV, 10MeV, 18MeV). The measured optical density distributions were then converted into dose distributions using characteristic curves of the film. Taking into account experimental uncertainties and statistical calculation fluctuations, agreement was found between measurements and VMC simulations with a maximal deviation of 3 mm on isodose curves for 18MeV.

A Comparison of Measured and Calculated Photon Backscatter from Dosemeter Calibration Phantoms

Personnel dosemeter calibration for the determination of operational quantities used in individual monitoring requires placement on a phantom that provides a reasonable approximation to the backscatter properties of the part of the body on which it is worn. The operational quantities of interest for personnel dosimetry are defined for a person and for calibration in a 30 cm X 30 cm X 15 cm ICRU four-element tissue phantom. However, since ICRU tissue is not readily available, a phantom of alternative composition must be used for calibration. The extent to which phantom composition affects dosemeter response was measured with a widely used personnel thermoluminescence dosemeter (TLD) system. Measured backscatter results from PMMA and ISO water-filled phantoms over the photon energy range 35 to 662 keV are reported and compared with calculated backscatter factors (BSFs) for an ICRU tissue slab. Measured PMMA slab backscatter agreed with calculated ICRU tissue slab BSFs to within 2-3% for the NIST X ray spectra used in this study.

TSEE Measurement of the Dose of 90Sr/90Y Beta Radiation Behind Absorbers of Different Atomic Number

With the thermally stimulated exoelectron emission (TSEE) of BeO thin film detectors the dose in the beta radiation field of a 90 Sr- 90 Y point source behind absorbers of different atomic number and thickness was determined. The absorbers consisted of either tissue-equivalent materials, aluminium, copper or gold. Their thickness ranged from 0 to 200 mg.cm -2 . The TSEE detectors and absorbers were placed on a PMMA slab phantom at perpendicular radiation incidence. For Cu absorbers a Cu slab phantom was additionally used, for comparison. Increasing atomic number of the absorber causes stronger dose buildup behind an absorber thickness of 20-40 mg.cm -2 which can be attributed to the differences of mass scattering power.