PENELOPE Monte Carlo Simulation Research Articles

Characteristic X-ray yields and cross sections of thick targets of Al, Ti, Zr, W and Au induced by keV-electron impact

In this paper, pure thick Al (Z = 13), Ti (Z = 22), Zr (Z = 40), W (Z = 74) and Au (Z = 79) targets are bombarded by electrons in an energy range of 5–27 keV, and the experimental thick-target characteristic X-ray yields of K-shell and L-shell, the X-ray production cross sections and the ionization cross sections of inner shells are presented. The present experimental setup and data processing are improved, specifically, a deflection magnet is installed in front of the X-ray detector to prevent the backscattered electron from entering into the X-ray detector, and the bremsstrahlung background spectra calculated from PENELOPE Monte Carlo simulations are used to deduce the net peak areas. The X-ray detector used in this experiment is the XR-100SDD manufactured by Amptek Inc. with a 25 mm<sup>2</sup> C2 ultra-thin window which can detect the low-energy x-rays down to boron Kα line (0.183 keV). Standard sources (<sup>55</sup>Fe, <sup>57</sup>Co, <sup>137</sup>Cs and <sup>241</sup>Am) with an activity accuracy range of 1%–3% (<i>k</i> = 2), supplied by the Physikalisch-Technische Bundesanstalt, Germany (PTB), are used to perform the detector’s efficiency calibration, and in a low-energy range (< 3.3 keV) the efficiency calibration is accomplished by measuring characteristic X-ray spectra produced by 20 keV electron impacting various thickness solid targets (i.e. by the characteristic peak method). The uncertainty of the detector’s efficiency calibration obtained in this paper is ~1.6%. The experimental thick-target characteristic X-ray yield data with an uncertainty of 1.7%–6.2% are compared with the PENELOPE Monte Carlo simulations, in which the inner-shell ionization cross sections are based on the distorted-wave Born approximation (DWBA) calculations, and they are in good agreement with a difference of less than or ~10%. According to the measured thick-target characteristic x-ray yields, the K-shell ionization cross sections for Al, Ti and Zr and the L-shell X-ray production cross sections for Zr, W and Au are also obtained with an uncertainty of 5%–8% (except for Al due to large K-shell fluorescence yield uncertainty), the difference between the experimental and theoretical data is also less than or ~10%. Moreover, by comparing the thick-target characteristic X-ray yields obtained from the analytical model and the PENELOPE Monte Carlo simulations at the electrons’ incident angles of 45° and 90°, it is found that the degree of agreement between the results from the analytical model and the Monte Carlo simulations at the incident angle of 90° is better than at the incident angle of 45°. Moreover, the contributions of the secondary electrons and bremsstrahlung photons to the characteristic X-ray yield are also given based on the PENELOPE Monte Carlo simulations. As for the elements studied in this paper, for the low ionization threshold energy, the contribution of the secondary electrons is ~2%, and however, for the high ionization threshold energy, the contribution is ~10%–20%. These contributions depend weakly on the energy of the incident electrons and show that these contributions are closely correlated with atomic number.

An artificial X-ray wire test emitter and calculations on the resolution and field of view of X-ray pinhole optics by simulation

The PENELOPE Monte Carlo simulation code was used to evaluate pinhole mask parameters for X-ray backscatter imaging in a security application. This work makes four major contributions: it describes a convenient efficient test object for evaluating X-ray optics, it converts the PENELOPE output into a simulated CCD image, it compactly outlines how image characteristics can be simply and reproducibly quantified, and it gives guidance on suitable materials and geometries for pinhole masks for X-ray imaging that could be applied to more complicated X-ray optics, such as coded masks. A novel test object X-ray emitter with the shape of a thin wire was specifically designed to explore the effect of mask material thickness and pinhole aperture diameter on image quality. Setting the test object to be the X-ray emitter rather than being a passive scatterer increases computational speed. The photon energy distribution of the artificial test object was set flat between selected energy limits to avoid the model being specific to any particular X-ray source technology. The modelled detector is an array of 1040 x 1392 pixels’ area detector inside a lead-lined camera housing. The pixelated detector was modelled by digitising the surface area represented by the PENELOPE phase space file and integrating the energies of the photons impacting each pixel with MATLAB code. The pinhole must be wide enough for sufficient field of view, whilst narrow enough for sufficient spatial resolution and the mask material needs to be thick enough to absorb most X-rays. When the mask material was too thick and the aperture too narrow, a collimation effect occurred. The consequence of excess collimation in a coded aperture is partial coding giving poor image reconstruction. Pure tungsten appears the most versatile material tested, where a 2 mm thickness and 2 mm aperture gives the most appropriate image characteristics for X-ray security imaging.

Dosimetric effects of polyethylene glycol surface coatings on gold nanoparticle radiosensitization

One of the main appeals of using gold nanoparticles (GNPs) as radiosensitizers is that their surface coatings can be altered to manipulate their pharmacokinetic properties. However, Monte Carlo studies of GNP dosimetry tend to neglect these coatings, potentially changing the dosimetric results. This study quantifies the dosimetric effects of including a polyethylene glycol (PEG) surface coating on GNPs over both nanoscopic and microscopic ranges. Two dosimetric scales were explored using PENELOPE Monte Carlo simulations. In microscopic simulations, 500–1000 GNPs, with and without coatings, were placed in cavities of side lengths 0.8–4 µm, and the reduction of dose deposited to surrounding medium within these volumes due to the coating was quantified. Including PEG surface coatings of up to 20 nm thickness resulted in reductions of up to 7.5%, 4.0%, and 2.0% for GNP diameters of 10, 20, and 50 nm, respectively. Nanoscopic simulations observed the dose falloff in the first 500 nm surrounding a single GNP both with and without surface coatings of various thicknesses. Over the first 500 nm surrounding a single GNP, the presence of a PEG surface coating reduced dose by 5–26%, 8–28%, 8–30%, and 8–34% for 2, 10, 20, and 50 nm diameter GNPs, respectively, for various energies and coating thicknesses. Reductions in dose enhancement due to the inclusion of a GNP surface coating are non-negligible and should be taken into consideration when investigating GNP dose enhancement. Further studies should be carried out to investigate the biological effects of these coatings.

Abstract ID: 159 Contribution of coherent and incoherent scatter in grating-based phase-contrast imaging

Aim Grating-based phase-contrast imaging (GB-PCI) is a recent development in X-ray imaging. Using three gratings, a highly periodic intensity pattern is created and measured. The amplitude (visibility) of this pattern is an important system parameter that determines image quality and scatter degrades this system visibility. Two GB-PCI setup aspects are expected to reduce the scatter fraction in the image: grating G2 can be considered an anti-scatter grid while large object-to-detector distances (D) will limit large angle scatter contributions. The aim was to quantify the efficiency with which these two aspects reduce both coherent and incoherent scatter in GB-PCI. Method PENELOPE Monte Carlo simulations were used with a realistic GB-PCI imaging geometry: gratings G1 and G2 had pitches of 4 and 2 μm, respectively, heights of 35 and 26 μm and duty cycles of 0.5. The trenches of the silicon gratings were filled with air in G1 and gold in G2. The G1-to-detector distance was 4.53 cm, while G2 was fixed to the detector. A 40 kVp X-ray tube was simulated in a plane wave geometry. Three different cases were evaluated for two objects, a finger imaged in air and a 5 cm thick PMMA slab. Case 1 had G2 in place and D was 9.5 cm, G2 was removed for case 2 and D was 9.5 cm, while case 3 had G2 removed and D was 4 cm. Results and conclusion Coherent scatter-to-primary ratio (SPRcoh) for the finger was 1.63%, 0.56% and 0.77% for cases 1,2,3 respectively. Similarly the incoherent SPR (SPRincoh) was 0.29%, 0.20% and 0.67%. For the PMMA slab, SPRcoh was 6.65%, 7.67% and 10.50% for cases 1, 2 and 3, while SPRincoh was respectively 2.26%, 3.05% and 7.86%. The uncertainty was 5%. Increasing D from 4 to 9.5 cm reduces both scatter-to-primary ratios (SPRcoh by 27% and SPRincoh by 65%). G2 will only reduce scattered radiation for larger objects with higher exit scatter fractions(consistent with [1] ) due to absorption of primaries and the creation of scatter in the grating.

SU-E-T-102: Determination of Dose Distributions and Water-Equivalence of MAGIC-F Polymer Gel for 60Co and 192Ir Brachytherapy Sources

Purpose: Analyse the water-equivalence of MAGIC-f polymer gel for {sup 60}Co and {sup 192}Ir clinical brachytherapy sources, through dose distributions simulated with PENELOPE Monte Carlo code. Methods: The real geometry of {sup 60} (BEBIG, modelo Co0.A86) and {sup 192}192Ir (Varian, model GammaMed Plus) clinical brachytherapy sources were modelled on PENELOPE Monte Carlo simulation code. The most probable emission lines of photons were used for both sources: 17 emission lines for {sup 192}Ir and 12 lines for {sup 60}. The dose distributions were obtained in a cubic water or gel homogeneous phantom (30 × 30 × 30 cm{sup 3}), with the source positioned in the middle of the phantom. In all cases the number of simulation showers remained constant at 10{sup 9} particles. A specific material for gel was constructed in PENELOPE using weight fraction components of MAGIC-f: wH = 0,1062, wC = 0,0751, wN = 0,0139, wO = 0,8021, wS = 2,58×10{sup −6} e wCu = 5,08 × 10{sup −6}. The voxel size in the dose distributions was 0.6 mm. Dose distribution maps on the longitudinal and radial direction through the centre of the source were used to analyse the water-equivalence of MAGIC-f. Results: For the {sup 60} source, themore » maximum diferences in relative doses obtained in the gel and water were 0,65% and 1,90%, for radial and longitudinal direction, respectively. For {sup 192}Ir, the maximum difereces in relative doses were 0,30% and 1,05%, for radial and longitudinal direction, respectively. The materials equivalence can also be verified through the effective atomic number and density of each material: Zef-MAGIC-f = 7,07 e .MAGIC-f = 1,060 g/cm{sup 3} and Zef-water = 7,22. Conclusion: The results showed that MAGIC-f is water equivalent, consequently being suitable to simulate soft tissue, for Cobalt and Iridium energies. Hence, gel can be used as a dosimeter in clinical applications. Further investigation to its use in a clinical protocol is needed.« less

The light output and the detection efficiency of the liquid scintillator EJ-309

The light output response and the neutron and gamma-ray detection efficiency are determined for liquid scintillator EJ-309. The light output function is compared to those of previous studies. Experimental efficiency results are compared to predictions from GEANT4, MCNPX and PENELOPE Monte Carlo simulations. The differences associated with the use of different light output functions are discussed.

SU-E-T-178: Optically Stimulated Luminescence (OSL) Dosimetry: A Study of A-Al2O3:C Assessed by PENELOPE Monte Carlo Simulation.

To use the Monte Carlo code PENELOPE to study attenuation and tissue equivalence properties of a-Al2O3:C for OSL dosimetry. Mass attenuation coefficients of α-Al2O3 and α-Al2O3:C with carbon percent weight concentrations from 1% to 150% were simulated with PENELOPE Monte Carlo code and compared to mass attenuation coefficients from soft tissue for photon beams ranging from 50kV to 10MV. Also, the attenuation of primary photon beams of 6MV and 10MV and the generation of secondary electrons by α-Al2O3 :C dosimeters positioned on the entrance surface of a water phantom were studied. A difference of up to 90% was found in the mass attenuation coefficient between the pure \agr;-A12O3 and the material with 150% weight concentration of dopant at 1.5 keV, corresponding to the K-edge photoelectric absorption of aluminum. However for energies above 80 keV the concentration of carbon does not affect the mass attenuation coefficient and the material presents tissue equivalence for the beams studied. The ratio between the mass attenuation coefficients for \agr-A12O3:C and for soft tissue are less than unit due to the higher density of the \agr-A12O3 (2.12 g/cms ) and its tissue equivalence diminishes to lower concentrations of carbon and for lower energies due to the relation of the radiation interaction effects with atomic number. The larger attenuation of the primary photon beams by the dosimeter was 16% at 250 keV and the maximum increase in secondary electrons fluence to the entrance surface of the phantom was found as 91% at 2MeV. The use of the OSL dosimeters in radiation therapy can be optimized by use of PENELOPE Monte Carlo simulation to provide a study of the attenuation and response characteristics of the material.

Dosimetry of intensive synchrotron microbeams

Abstract Intensive synchrotron X-ray microbeams form an integral part of microbeam radiation therapy (MRT). MRT is a novel radiation medicine modality being developed for inoperable and otherwise untreatable brain tumours. The extremely high dose rate (∼20 kGy/s), laterally fractionated radiation field and steep dose gradients utilized in this therapy make real-time dosimetry a significant challenge. In order for this treatment to advance to the clinical trial stage of development real-time dosimetry systems must be developed. This paper demonstrates the capabilities of a new dosimetry system based on an epitaxial silicon detector. The system combines high spatial resolution and real-time readout and we have measured the lateral dose profile of the MRT radiation field which incorporates 59 X-ray microbeams. All microbeam peaks and valley regions between two microbeams are clearly resolved. The measured detector response at any point is reproducible to within 0.5% after scaling for the known synchrotron storage ring beam current lifetime. The variation of the lateral dose profile at different depths in a PMMA phantom has been measured with the results compared to those from Penelope Monte Carlo simulations. The trend in the measured response with depth agrees with the simulation data (within the experimental variation of the central five microbeams peaks and valleys measured). However the measured peak-to-valley ratio response is a factor of 4.5 ± 0.1 times lower than that expected. The disagreement was further investigated and shown to be contributed to by charge recombination effects at the low bias voltages used.

Influence of Dental Restorative Materials on ESR Biodosimetry in Tooth Enamel

Using an experimental model and PENELOPE Monte Carlo simulations, the effects of resin and amalgam on the absorbed doses in tooth enamel were studied to evaluate the feasibility of using restored teeth in electron spin resonance (ESR) dose reconstruction. The model consisted of a phantom containing a plate of these restorative materials placed between powered enamel layers exposed to X rays and a ⁶⁰Co beam. The experimental results and simulations agreed, showing that the attenuation produced by amalgam and resin with a thickness of 1, 2, and 4 mm is similar to that produced by the enamel itself in the case of the radiation sources employed. For X rays and ⁶⁰Co γ radiation the attenuation reached almost 100% and 40%, respectively. These results show that for ESR dose reconstruction, the use of all available enamel of a tooth leads to errors in the estimated dose due to attenuation effects in both healthy and restored teeth. Thus the importance of an enamel selection from different sides of the tooth surface to apply ESR dose reconstruction in the case of a practical situation is shown.

SU‐FF‐J‐149: Monte Carlo Simulation of Dose Distribution Around 198 Au Nanoparticles: A Preliminary Study for Treatment Quality Criterion

Purpose: To simulate dose distributions around gold‐198 nanoparticles using the Monte Carlo method and study the relation between nanoparticles concentration and dose homogeneity in a tissue. Method and Materials: nanoparticles with 500 nm of radius immersed in a water phantom were simulated with PENELOPE Monte Carlo simulation code. The spatial resolution in the simulations wasset to 0.32 μm. Gold K‐edge decay (80.7 keV), L1‐edge decay (14.7 keV), L2‐edge decay (13.4 keV) and L3‐edge decay (12 keV) energies were simulated individually. A volumetric dose distribution around the Au particle was obtained and percentage depth dose curves and relative 2D and 3D maps were achieved. Using standards for large radiation field treatments, the region of 80% dose was found to define a dose penumbra region. The necessary distance between adjacent nanoparticles distributed in a tissue was studied to result in a homogeous distribution for each energy decay. Results: The percentage depth dose curve related to K‐edge energy showed a buid‐up region of 120 nm (reference to the center of the nanosphere) and 1% dose at a 3.5 cm from the source. To the L1‐edge, L2‐edge and L3‐edge energies the the 1% dose was found at 1.7 cm, 1.6 cm and 1.5 cm respectively. The distances between particles necessary to achieve dose homogeneity in water are 1.0 cm, 0.7 cm, 0.6 cm and 0.5 cm, respectively to K‐edge, L1‐edge, L2‐edge and L3‐edge. The nanoparticles concentration when the L1‐edge is used is 42% grater then when the K‐edge emission is used. The same relations calculated to L2 and L3 relative to the K‐edge are similar. Conclusions: Monte Carlo simulation is an accurate instrument to study dose distributions around nanoparticles and provides data to define a homogeneity criteria for treatments.

SU‐FF‐T‐18: On the Determination of the Absolute Dose for a 192Ir HDR Source Using a Fricke Dosimeter: Correction Factors for Several Radii Using MC Calculation

Purpose: To determine the absolute dose at the reference point using measurement with Fricke dosimeter at various transverse bisector distances. Method and Materials: In a previous work we showed how the absolute dose from a source can be measured using a Fricke dosimeter. Ring shaped fricke volume of 1 mm width and 3 mm height was used to determine the absolute dose. AlphaOmega HDR source was located at the center of the ring which was 1 cm away from the source, the uncertainty at this point was too high (k=7%). In another second study we demonstrated that there is a size detector dependence and source positioning uncertainty when doing the measurement. We did dose profile measurements along and away from a source using four different detectors (two ionization chambers and two diodes). We showed that the detector size effect goes from 1.3% at 1 cm to 0.2% at 2.5 cm. Therefore PENELOPE Monte Carlo simulations for several radii have been used to determine conversion factors to the reference point. Results: We show in this work that the absorbed dose at various distances away from the source does not have an inverse square law behavior. Therefore, factors have been determined to convert the dose at measured point to the dose at the reference point recommended by AAPM TG‐43. These conversion factors are 4.454, 7.609, 11.288 and 16.003 for the distances 1.5, 2.0, 2.5, and 3.0 cm respectively. Conclusion: Feasibility of determining the absorbed dose using a Fricke dosimeter at the reference point (1 cm distance) was demonstrated in a previous work, however the uncertainty was too high (7%). Measurement at distances greater than 1 cm was demonstrated to lower the uncertainty. PENELOPE MC code was used to obtain the conversion factors at various radii to the reference point.

Microbeam radiation therapy: A Monte Carlo study of the influence of the source, multislit collimator, and beam divergence on microbeams

Microbeam radiation therapy (MRT) is a new oncology method currently under development for the treatment of inoperable pediatric brain tumors. Monte Carlo simulation, or the computational study of radiation transport in matter, is often used in radiotherapy to theoretically estimate the dose required for treatment. However, its potential use in MRT dose planning systems is currently hindered by the significant discrepancies that have been observed between measured and theoretical dose and the PVDR (peak to valley dose ratio). The need to resolve these discrepancies is driven by the desirability of making MRT available to humans in the next few years. This article aims to resolve some of the discrepancies by examining the simplifications adopted in previous MRT Monte Carlo studies, such as the common practice of commencing microbeam transport on the surface of the target which neglects the influence of the distributed synchrotron source, multislit collimator, and the beam divergence between them. This article uses PENELOPE Monte Carlo simulation to investigate the influence of these beamline components upstream of the target on the lateral dose profiles and PVDRs of an array of 25 microbeams. It also compares the dose profiles and PVDRs of a microbeam array produced from a single simulation (full array) to those produced from the superposition of a single microbeam profile (sup array). The effect of modeling the distributed source and the beam divergence was an increase in the absorbed dose in the penumbral and valley regions of the microbeam profiles. Inclusion of the multislit collimator resulted in differences of up to 5 microm in the FWHM of microbeam profiles across the array, which led to minor variations in the corresponding PVDR yields.

Chamber-quality factors in 60Co for three plane-parallel chambers for the dosimetry of electrons, protons and heavier charged particles: PENELOPE Monte Carlo simulations

The IBA-Scanditronix NACP-02, IBA-Wellhöfer PPC-40 and PPC-05 plane-parallel ionization chambers have been simulated with the Monte Carlo code PENELOPE to obtain their chamber- and quality-dependent factors fc,Qo for a 60Co gamma beam. These are applicable to the determination of kQ beam-quality factors for the dosimetry of electron, protons and heavier charged particles beams based on standards of absorbed dose to water. The factor fc,Q is equivalent to the product sw,airp, but it is not subject to the assumed independence of perturbation factors and stopping power (Sempau et al 2004 Phys. Med. Biol. 49 4427–44). The calculations have been carried out using three different 60Co source models: a monoenergetic point source, a point source with a realistic 60Co spectrum and the simulated phase space from a radiotherapy 60Co unit. Both the detailed geometries of the ionization chambers and of the 60Co unit have been obtained from the manufacturers. In the case of the NACP-02 chamber, values of fc,Qo have been compared with those in the IAEA TRS-398 Code of Practice and from other authors, results being in excellent agreement. The PPC-05 and PPC-40 chambers are of relatively new design, and their values have not been calculated before. Within the estimated uncertainty, computed at the 2σ level (95% confidence limit), the results for each of the three chambers appear to be independent of the degree of sophistication of the 60Co source model used. For the NACP-02 chamber this assumption is justified by the excellent agreement between the various models, which occurs at the level of one standard uncertainty. This suggests the possibility of adopting the mean value of the three source models, weighted with the inverse of their corresponding uncertainties, as a better estimate of fc,Qo. A consequence of the above conclusions is that the estimated uncertainty of kQ beam-quality factors of all charged particles referred to 60Co can potentially be decreased considerably using our approach. For example, the estimated relative standard uncertainty of the denominator of kQ, given in TRS-398 as 1.6% for plane-parallel ionization chambers, can be reduced to 0.06% for a NACP chamber using the mean value of fc,Qo given in this work. Similar reductions could be obtained for the combined standard uncertainty of the kQ beam-quality factors of all charged particles, notably electrons.

New measurements and quantitative analysis of electron backscattering in the energy range of neutron β-decay

We report on the first detailed measurements of electron backscattering from plastic scintillator targets, extending our previous work on beryllium and silicon targets. The scintillator experiment posed several additional experimental challenges associated with charging of the scintillator target, and those challenges are addressed in detail. In addition, we quantitatively compare the energy and angular distributions of this data, and our previous data, with electron transport simulations based on the GEANT4 and PENELOPE Monte Carlo simulation codes. The PENELOPE simulation is found globally to give a superior description of the data. Such information is crucial for a broad array of weak-interaction physics experiments, where electron backscattering can give rise to the dominant detector-related systematic uncertainty.