Small Photon Research Articles

Magnetic fields alter strong-field ionization

When a strong laser pulse induces the ionization of an atom, momentum conservation dictates that the absorbed photons transfer their momentum $p_{\gamma}=E_{\gamma}/c$ to the electron and its parent ion. Even after 30 years of studying strong-field ionization, the sharing of the photon momentum between the two particles and its underlying mechanism are still under debate in theory. Corresponding experiments are very challenging due to the extremely small photon momentum ($~10^{-4}$ a.u.) and their precision has been too limited, so far, to ultimately resolve the debate. Here, by utilizing a novel experimental approach of two counter-propagating laser pulses, we present a detailed study on the effects of the photon momentum in strong-field ionization. The high precision and self-referencing of the method allows to unambiguously demonstrate the action of the light's magnetic field on the electron while it is under the tunnel barrier, confirming theoretical predictions, disproving others. Our results deepen the understanding of, for example, molecular imaging and time-resolved photoelectron holography.

A Monte Carlo study on the PTW 60019 microDiamond detector.

Data on the output correction factor for small photon beam dosimetry of the microDiamond detector manufactured by the company PTW can be found in a variety of papers. Referring either to measurements or to Monte Carlo (MC) calculations, they show substantial disagreements particularly at very small fields. This work reports results of a further MC study aiming at a better understanding of how specific properties of the microDiamond detector are influencing its output correction factor and whether this can explain at least some of the disagreements. In this study the method of a fluence-based decomposition of the dose conversion factor was used which is considered as a useful tool to understand the response of a detector in nonreference conditions. This decomposition method yields the following three factors: (a) the stopping power ratio water to diamond, (b) a perturbation factor pint taking into account all fluence changes in the transition from a small water voxel at the point of dose determination to the bare diamond detector, and (c) a perturbation factor pext taking into account all additional fluence changes in the fully simulated diamond detector caused by the material and design details outside the sensitive volume. Monte Carlo calculated output correction factors were obtained for Co-60, 6MV and 10MV photon beams showing that the maximum variation with field size remained in the order of 2% for quadratic field sizes larger than about 0.3cm. For field sizes smaller than about 0.5cm a clear under-response is obtained at all three radiation qualities in agreement with all known MC calculations, however, in contrast to some measured result. The shape of the output correction factor can be well explained by an opposite mode of action between under-response expressed by the perturbation factor pint and over-response expressed by the perturbation factor pext where the first one is mainly influenced by volume averaging, and the second one by a back scatter effect of electrons from the diamond substrate into the sensitive volume. The response of microDiamond detector can be well described under various measuring conditions by the dose conversion factor and the dependency of its fluence-based subfactors on detector characteristics. Monte Carlo simulations offer an improvement in the understanding particularly of small-field effects by relating the output correction factor to spectral fluence changes in the sensitive volume of the detector. The most significant influence factors are the finite size of the active volume and the presence of the high-density diamond substrate causing a field size-dependent backscattering. These perturbations are opposite in their effects. The diamond in the sensitive volume itself and in particular its density has almost no influence. Scattering of results at very small field sizes can be explained by different gradients of dose profiles around the beam axis at identical full width half maximum (FWHM) field size parameters and by possible deviations of the radius of the sensitive volume from the nominal radius. The backscattering effect also has an influence on the determination of profiles and for very small field sizes on the response at different rotation angles.

The effective point of measurement for depth-dose measurements in small MV photon beams with different detectors.

The effective point of measurement (EPOM) of cylindrical ionization chambers differs from their geometric center. The exact shift depends on chamber construction details, above all the chamber size, and to some degree on the field-size and beam quality. It generally decreases as the chamber dimensions get smaller. In this work, effective points of measurement in small photon fields of a range of cylindrical chambers of different sizes are investigated, including small chambers that have not been studied previously. In this investigation, effective points of measurement for different ionization chambers (Farmer type, scanning chambers, micro-ionization chambers) and solid state detectors were determined by measuring depth-ionization curves in a 6MV beam in field sizes between 2×2cm2 and 10×10cm2 and comparing those curves with curves measured with plane-parallel chambers. It was possible to average the results to one shift per detector, as the results were sufficiently independent of the studied field sizes. For cylindrical ion chambers, shifts of the EPOM were determined to be between 0.49 and 0.30 times the inner chamber radius from the reference point. We experimentally confirmed the previously reported decrease of the EPOM shift with decreasing detector size. Highly accurate data for a large range of detectors, including new very small ones, were determined. Thus, small chambers noticeably differ from the 0.5-times to 0.6-times the inner chamber radius recommendations in current dosimetry protocols. The detector-individual EPOMs need to be considered for measurements of depth-dose curves.

Engineering Quantum States of Matter for Atomic Clocks in Shallow Optical Lattices.

We investigate the effects of stimulated scattering of optical lattice photons on atomic coherence times in a state-of-the art ^{87}Sr optical lattice clock. Such scattering processes are found to limit the achievable coherence times to less than 12s (corresponding to a quality factor of 1×10^{16}), significantly shorter than the predicted 145(40)s lifetime of ^{87}Sr's excited clock state. We suggest that shallow, state-independent optical lattices with increased lattice constants can give rise to sufficiently small lattice photon scattering and motional dephasing rates as to enable coherence times on the order of the clock transition's natural lifetime. Not only should this scheme be compatible with the relatively high atomic density associated with Fermi-degenerate gases in three-dimensional optical lattices, but we anticipate that certain properties of various quantum states of matter-such as the localization of atoms in a Mott insulator-can be used to suppress dephasing due to tunneling.

Intercalibration of a polycrystalline 3D diamond detector for small field dosimetry

In medical radiation dosimetry, the use of small photon fields is almost a prerequisite for high precision localized dose delivery to delineated target volumes. The accurate measurement of standard dosimetric quantities in such situations depends on the size of the detector with respect to the field dimensions. Polycrystalline diamond devices with 3-dimensional structures are produced by using laser pulses to create graphitic paths in the diamond bulk. By fabricating very narrow and close-by columnar electrodes perpendicular to the detector surface, it is possible to create arrays of 3-D cells (pixels) with a very small sensitive volume. In order to present a solution to the problem of the detector size for small field dosimetry the 3-D technology aims to a new highly segmented larger polycrystalline diamond dosimeter to obtain field profiles in a single measurement, reducing the uncertainty of the delivered dose. To this purpose a 3D all carbon detector with an array of 9 3-D pixels has been produced. Due to the heterogeneous structure of the polycrystalline diamond substrate, it was necessary to study the response of each pixel under a standard field photon beam (10 x 10 cm2). For 6 pixels of the array was demonstrated that they present different sensitivities to the radiation beam, but the response is linear and stable hence different calibration factors can be applied to obtain an overall detector response and reduce the uncertainty of the delivered dose.

Measurement of 6 MV small field beam profiles – comparison of micro ionization chamber and linear diode array with monte carlo code

The objective of this study is to analyze small field photon beams acquired with commonly available detectors. Beam profiles of 6 MV photons from the Siemens Primus Linear Accelerator were measured with a micro ion chamber (IC CC01, IBA) and linear diode array (LDA-99SC, IBA). Data was acquired using a water phantom for small fields (0.5×0.5 cm2 to 4×4 cm2) at depth of maximum dose, 5 cm and 10 cm. Profiles were also generated with EGSnrc Monte Carlo code. Measured and simulated profiles were compared in terms of percentage difference of the area under the simulated and measured profiles (PD), ratio of the measured to simulated dose at the point of maximum deviation within the central region of profile (R), full width half maximum (FWHM) and penumbra. For field sizes ≥1×1 cm2, the maximum PD is 3.17 % and 2.87 % for IC and LDA respectively, whereas R is in the range of 0.95-1.05 for IC and 0.99-1.05 for LDA. LDA measured FWHM and penumbra are also in better agreement with the simulated results. This study demonstrated that LDA can be used for acquisition of beam profiles for field size as low as 1×1 cm2.

Measurements of Qf СйП, fmsr by Using Different Detectors in Radiation Qclin, Qmse Therapy Systems

AbstractBackground: It worth to be mentioned that the use of small size photon beam is frequently used in modern radio-therapy to treat brain tumors and functional disorders.Aim of Study: To measure output factors (OFs) and cal-culate the field factor ( ƒclin, ƒmsr ) of small fields andV clin, Qmsestudy its suitability for small field relative dosimetry.Material and Methods: Numerous detectors were used for measuring the output factors for 6 MV photon beams by a CyberKnife®. To normalize different detector responses for the same field configuration, a correction factor was calculated for each detector by simulating the radiation delivery using Monte Carlo (MC) methods. Detectors used in the study were PTW60019 MicroDiaond, PTW 60018 Silicon diode, PTW31018 MicroLion, and Exradin W1 Scintillator. Field factors were calculated using Alfonso formula. Output factors for a CyberKnife were measured in circular fields with the diameters range from 5mm to 60mm. Measurements were made in a water tank at a 1.5cm in term of depth and at 80 cm for source-to-axis distance.Results: The results of the current study show that the output measured by the detectors Micro Diamond and Exradin W1 Scintillator (PSD) were within the uncertainties of the Monte Carlo simulations for all the beam cones. The silicon diode detector was over-responding, while the MicroLion was under-responding. We found that an accurate dosimeter could be the MicroDiamond and Exradin W1 Scintillators in small field dosimetry.Conclusion: We found that at small collimator settings only the synthetic microdiamond (PTW60019) and Exradin W1 scintillation detectors can be used as relative dosimeters without applying any correction factor.

Do all the linear accelerators comply with the ICRU 91's constraints for stereotactic body radiation therapy treatments?

Recent technological developments in linear accelerators (linacs) and their imaging systems have made it possible to routinely perform stereotactic radiotherapy (SRT) treatments. To ensure the security and quality of the treatments, national and international recommendations have been written. This review focuses on the recommendations of the report 91 of the International Commission on Radiation Units (ICRU) on stereotactic treatments with small photon beams and proposes to answer the question of the eligibility of the commercially available accelerators for the treatment of extra-cranial SRT (SBRT). The ICRU 91 report outlines important features needed to respect the constraints, which are high intensity photon beam, integrated image-guidance, high mechanical accuracy of the linac, multileaf collimator with reduced leaf width, bundled motion management and bundled 6 Dimensional “robotic” couch tabletop. Most of the contemporary linacs meet these recommendations, in particular, stereotactic dedicated linacs, or modern gantry-based linacs equipped with 3 dimensional cone-beam CT imaging and 2D-stereoscopic planar imaging. Commercially available ring-based linacs have some limitations: they offer only coplanar treatments, and couch movements are limited to translations and, some have limited imaging equipment and no ability to manage intrafraction motion. However, for performing SBRT, non-coplanar irradiations are not mandatory, contrarily to intracranial stereotactic irradiations. Furthermore, patients’ rotations can be corrected, thanks to real-time adaptive radiotherapy available on MRI-linacs. Finally, significant improvements are expected in the short term to compensate the weaknesses of the current devices.

Blocking Characteristics of Photoconductive Switches Based on Semi-Insulating GaP and GaN

This article presents results of research work aimed at manufacturing photoconductive semiconductor switches (PCSSs) based on semi-insulating (SI) gallium phosphide (GaP) and gallium nitride (GaN). Currently, the work is in progress to determine the optimal values of PCSS parameters. In this article, the parameters of the selected semiconductor materials used for making PCSSs, the device operation principle, and possible areas of use are presented. The paper demonstrates the construction of test PCSSs based on SI GaP and SI GaN and results of blocking characteristics measurements without the illumination, as well as with illumination with a small photon flux. Further research directions are presented also.

Photon Distribution Function in Weakly Coupled Maxwellian Plasma

It is shown that the photon distribution for fully ionized plasma with weak interparticle interaction asymptotically corresponds to the Planck distribution at small photon momenta k. The number of photons decreases with increasing k in comparison with the Planck distribution. Calculation of the asymptotics for large k requires consideration of the interaction effects.

Three-dimensional characterization of the active volumes of PTW microDiamond, microSilicon, and Diode E dosimetry detectors using a proton microbeam.

PurposeThe purpose of this work is the three‐dimensional characterization of the active volumes of commercial solid‐state dosimetry detectors. Detailed knowledge of the dimensions of the detector’s active volume as well as the detector housing is of particular interest for small‐field photon dosimetry. As shown in previous publications from different groups, the design of the detector housing influences the detector signal for small photon fields. Therefore, detailed knowledge of the active volume dimension and the surrounding materials form the basis for accurate Monte Carlo simulations of the detector.MethodsA 10 MeV proton beam focused by the microbeam system of the Physikalisch‐Technische Bundesanstalt was used to measure two‐dimensional response maps of a synthetic diamond detector (microDiamond, type 60019, PTW Freiburg) and two silicon detectors (microSilicon, type 60023, PTW Freiburg and Diode E, type 60017, PTW Freiburg). In addition, the thickness of the active volume of the new microSilicon was measured using the method developed in a previous study.ResultsThe analysis of the response maps leads to active area of 1.18 mm2 for the Diode E, 1.75 mm2 for the microSilicon, and 3.91 mm2 for the microDiamond detector. The thickness of the active volume of the microSilicon detector was determined to be (17.8 ± 2) µm.ConclusionsThis study provides detailed geometrical data of the dosimetric active volume of three different solid‐state detector types.

Effects of Ionizing Radiation Detector Characteristics on the Results of Measurements of Percent Depth Doses in Small Photon Fields

Studies of the dosimetric parameters of small fields of ionizing radiation are extremely important in current clinical dosimetry. This article considers analysis of plots of percent depth dose in small photon fields obtained using an Elekta Synergy medical linear electron accelerator with energies of 6 and 10 MeV using ionization chambers with different active volumes and a semiconductor detector. Comparative data for the following parameters are presented: depth of maximum dose, surface dose, dose at a depth of 10 cm, and various other aspects of the plots. These data were found to depend on the type of detector used in identical measurement conditions. The main features of the detection of dose distributions by different detectors are noted and conclusions are drawn regarding the advantages of using detectors with the smallest available active volumes in clinical dosimetry.

Modeling the primary source intensity distribution: reconstruction and inter-comparison of six Varian TrueBeam sources

The primary source size is one of the most important beam model parameters in small photon fields. In this work we apply a recently suggested reconstruction technique to characterize the primary source of 6 Varian TrueBeam (TB) linacs. A series of photon fluence profile measurements were performed on 6 Varian TB linacs in the crossplane and inplane orientation using radiochromic film in air and a 2 mm Pb foil as a build-up layer. An image reconstruction algorithm was then applied, based on the maximum likelihood expectation-maximization (MLEM) algorithm, to estimate the source distribution. The method iteratively ray-traces photons from the source plane to the measurement plane to extract source profile corrections. The technique was first benchmarked using a Monte Carlo (MC) model of a Varian TrueBeam with known input Gaussian source sizes. The robustness of the suggested technique was also tested by randomly sampling different combinations of source and field size values and repeating the reconstruction. At the MC benchmarking stage the MLEM reconstruction algorithm was capable of reproducing the Gaussian shape with a RMSE less than 4.0%, while the reconstructed source size (FWHM) and field size were determined with an accuracy level of 0.14 mm and 0.10 mm respectively. Experimentally, the reconstructed TB sources presented FWHM values between 1.02–1.5 mm (–0.18 mm) and 1.08–1.42 mm (–0.13 mm) in the crossplane and inplane orientations respectively. All TB sources studied in this work can be considered symmetric within uncertainties with the exception of one. The source distribution presented systematic deviations from a Gaussian distribution mostly in the lower tail region. Multi-parameter functional forms, such as Pearson VII or double Gaussian presented improvements in modeling the source in this region, but increase the model complexity. The reconstructed sources measured in this work can serve as reference values for commissioning beam models in small fields and set upper and lower thresholds values of the expected source size for a TB linac.

The effect of SSD, Field size, Energy and Detector type for Relative Output Factor measurement in small photon beams as compared with Monte Carlo simulation

Abstract Introduction: Small fields photon dosimetry is associated with many problems. Using the right detector for measurement plays a fundamental role. This study investigated the measurement of relative output for small photon fields with different detectors. It was investigated for three-photon beam energies at SSDs of 90, 95, 100 and 110 cm. As a benchmark, the Monte Carlo simulation was done to calculate the relative output of these small photon beams for the dose in water. Materials and Methods: 6, 10 and 15 MV beams were delivered from a Synergy LINAC equipped with an Agility 160 multileaf collimator (MLC). A CC01 ion chamber, EFD-3G diode, PTW60019 microdiamond, EBT2 radiochromic film, and EDR2 radiographic film were used to measure the relative output of the linac. Measurements were taken in water for the CC01 ion chamber, EFD-3G diode, and the PTW60019. Films were measured in water equivalent RW3 phantom slabs. Measurements were made for 1 × 1, 2 × 2, 3 × 3, 4 × 4, 5 × 5 and a reference field of 10 × 10 cm2. Field sizes were defined at 100cm SSD. Relative output factors were also compared with Monte Carlo (MC) simulation of the LINAC and a water phantom model. The influence of voxel size was also investigated for relative output measurement. Results and Discussion: The relative output factor (ROF) increased with energy for all fields large enough to have lateral electronic equilibrium (LEE). This relation broke down as the field sizes decreased due to the onset of lateral electronic disequilibrium (LED). The high-density detector, PTW60019 gave the highest ROF for the different energies, with the less dense CC01 giving the lowest ROFs. Conclusion: These are results compared to MC simulation, higher density detectors give higher ROF values. Relative to water, the ROF measured with the air-chamber remained virtually unchanged. The ROFs, as measured in this study showed little variation due to increased SSDs. The effect of voxel size for the Monte Carlo calculations in water does not lead to significant ROF variation over the small fields studied.

The effect of an air gap on a 2D monolithic silicon detector for relative dosimetry

Purpose: to evaluate the impact of an air gap on the Magic Plate (MP512) response and optimize this gap for relative dosimetry in photon and electron beams. Materials and methods: MP512 is a 2D monolithic silicon detector manufactured on a p-type substrate. The array consists of 512 pixels with 0.5 × 0.5 mm2 size and 2 mm pitch with an overall dimension of 52 × 52 mm2. The signal ratio (SR) as a function of beam size and the percentage were measured with MP512 in 6 MV and 10 MV photon beams. The enhanced dynamic wedge (EDW) beam profile measurements were performed for 6 MV photon beams. In this work the signal ratio is defined as the ratio of central axis MP512 reading for field sizes ranging from 0.5 × 0.5 cm2 to 10 × 10 cm2 and for the reference square field of side 10 cm at a depth of 10 cm in solid water phantom. The measurements were performed with an air gap immediately above the detector array of 0.5, 1.0, 1.2, 2.0 and 2.6 mm, respectively. The PDD was measured for field sizes 2× 2 cm2, 5× 5 cm2 and 10× 10 cm2 by scanning the MP512 from the depth of 0.5 cm to 10 cm. The beam profiles were measured for Varian linac enhanced dynamic wedge (EDW) angles of 15̂, 45̂ and 60̂ for field size 5× 5 cm2. The PDD for 6, 12 and 20 MeV electron beams were performed for a standard applicator providing 10× 10 cm2 field size. Results: the signal ratio measured with MP512 reduces with increasing air gap above the detector. The strongest effect of the air gap size was observed for small fields of 0.5× 0.5 cm2 and 1× 1 cm2 while the effect was negligible within ± 2% (1 standard deviation) for field sizes larger than 4× 4 cm2. The signal ratio measured with MP512 with air gaps of 0.5 mm and 1.2 mm showed a good agreement with signal ratio measured with the EBT3 film (within ± 2%) and MOSkinTM for 6 MV and 10 MV, respectively. Similar results were observed for the PDD measurement for field size 5× 5 cm2 and 10× 10 cm2. The PDD measured with M512 was in good agreement with Markus Ionization chamber (IC) within ± 1.6% (1 standard deviation) for 6 MV and ± 1.5% (1 standard deviation) for 10 MV. The PDD discrepancy for 2× 2 cm2 was within ± 3% of the EBT3 for both photon energies. The EDW dose profile matched well with the EBT3 for the air gap of 0.5 mm within ± 2% (1 standard deviation) for all wedge angles. The PDD measured by electron beams demonstrated no significant effect of the air gap size above MP512 for all energies. The results showed similar variations (within ± 3%) compared to Markus IC for both 0.5 mm and 2.6 mm gap. Conclusion: the MP512 diode array was demonstrated to be suitable as an in-phantom dosimeter for QA in small radiation treatment fields. The study shows that air gap size has a significant effect on small field photon beam dosimetry due to a loss of electronic equilibrium. The small air gaps of 0.5 mm and 1.2 mm were the best air gaps for 6 MV and 10 MV, respectively. The effect of the air gap in electron beam fields is not significant due to the fact that an electronic equilibrium is fully established.

Characterization of a novel 3D printed patient specific phantom for quality assurance in cranial stereotactic radiosurgery applications

In single-isocenter stereotactic radiosurgery/radiotherapy (SRS/SRT) intracranial applications, multiple targets are being treated concurrently, often involving non-coplanar arcs, small photon beams and steep dose gradients. In search for more rigorous quality assurance protocols, this work presents and evaluates a novel methodology for patient-specific pre-treatment plan verification, utilizing 3D printing technology.In a patient’s planning CT scan, the external contour and bone structures were segmented and 3D-printed using high-density bone-mimicking material. The resulting head phantom was filled with water while a film dosimetry insert was incorporated. Patient and phantom CT image series were fused and inspected for anatomical coherence. HUs and corresponding densities were compared in several anatomical regions within the head. Furthermore, the level of patient-to-phantom dosimetric equivalence was evaluated both computationally and experimentally. A single-isocenter multi-focal SRS treatment plan was prepared, while dose distributions were calculated on both CT image series, using identical calculation parameters. Phantom- and patient-derived dose distributions were compared in terms of isolines, DVHs, dose-volume metrics and 3D gamma index (GI) analysis. The phantom was treated as if the real patient and film measurements were compared against the patient-derived calculated dose distribution.Visual inspection of the fused CT images suggests excellent geometric similarity between phantom and patient, also confirmed using similarity indices. HUs and densities agreed within one standard deviation except for the skin (modeled as ‘bone’) and sinuses (water-filled). GI comparison between the calculated distributions resulted in passing rates better than 97% (1%/1 mm). DVHs and dose-volume metrics were also in satisfying agreement. In addition to serving as a feasibility proof-of-concept, experimental absolute film dosimetry verified the computational study results. GI passing rates were above 90%.Results of this work suggest that employing the presented methodology, patient-equivalent phantoms (except for the skin and sinuses areas) can be produced, enabling literally patient-specific pre-treatment plan verification in intracranial applications.

Evaluating small field dosimetry with the Acuros XB (AXB) and analytical anisotropic algorithm (AAA) dose calculation algorithms in the eclipse treatment planning system

AbstractBackground:An increasing number of external beam treatment modalities including intensity modulated radiation therapy, volumetric modulated arc therapy (VMAT) and stereotactic radiosurgery uses very small fields for treatment planning and delivery. However, there are major challenges in small photon field dosimetry, due to the partial occlusion of the direct photon beam source’s view from the measurement point, lack of lateral charged particle equilibrium, steep dose-rate gradient and volume averaging effect of the detector response and variation of the energy fluence in the lateral direction of the beam. Therefore, experimental measurements of dosimetric parameters such as percent depth doses (PDDs), beam profiles and relative output factors (ROFs) for small fields continue to be a challenge.Materials and Methods:In this study, we used a homogeneous water phantom and the heterogeneous anthropomorphic stereotactic end-to-end verification (STEEV) head phantom for all dose measurements and calculations. PDDs, lateral dose profiles and ROFs were calculated in the Eclipse Treatment Planning System version 13·6 using the Acuros XB (AXB) and the analytical anisotropic algorithms (AAAs) in a homogenous water phantom. Monte Carlo (MC) simulations and measurements using the Exradin W1 Scintillator were also accomplished for four photon energies: 6 MV, 6FFF, 10 MV and 10FFF. Two VMAT treatment plans were generated for two different targets: one located in the brain and the other in the neck (close to the trachea) in the head phantom (CIRS, Norfolk, VA, USA). A Varian Truebeam linear accelerator (Varian, Palo Alto, CA, USA) was used for all treatment deliveries. Calculated results with AXB and AAA were compared with MC simulations and measurements.Results:The average difference of PDDs between W1 Exradin Scintillator measurements and MC simulations, AAA and AXB algorithm calculations were 1·2, 2·4 and 3·2%, respectively, for all field sizes and energies. AXB and AAA showed differences in ROF of about 0·3 and 2·9%, respectively, compared with W1 Exradin Scintillator measured values. For the target located in the brain in the head phantom, the average dose difference between W1 Exradin Scintillator and the MC simulations, AAA and AXB were 0·2, 3·2 and 2·7%, respectively, for all field sizes. Similarly, for the target located in the neck, the respective dose differences were 3·8, 5·7 and 3·5%.Conclusion:In this study, we compared dosimetric parameters such as PDD, beam profile and ROFs in water phantom and isocenter point dose measurements in an anthropomorphic head phantom representing a patient. We observed that measurements using the W1 Exradin scintillator agreed well with MC simulations and can be used efficiently for dosimetric parameters such as PDDs and dose profiles and patient-specific quality assurance measurements for small fields. In both homogenous and heterogeneous media, the AXB algorithm dose prediction agrees well with MC and measurements and was found to be superior to the AAA algorithm.

The DSM theory for tip electron-atom bremsstrahlung at [formula omitted] MeV

The Dirac–Sommerfeld-Maue (DSM) theory for electron bremsstrahlung at the short-wavelength limit, based on a point-Coulomb description of the target nucleus, is tested against the Dirac partial-wave (DW) theory at collision energies up to 35 MeV. It is found that for the small photon angles, which contribute to the angle-integrated bremsstrahlung yield, the screening effects are at most in the percent region. Predictions are made for the nuclear charge dependence of the singly differential bremsstrahlung cross section at 500 MeV, where experiments are planned in the near future.

EP-1754 High-resolution assessment of dose calculations in small MV photon beams on and off central axis

EP-1754 High-resolution assessment of dose calculations in small MV photon beams on and off central axis

Influence of thermal radiation on hot cluster decay rates and abundances

The influence of radiative cooling on the unimolecular decay rates of free, hot clusters and molecules with unspecified excitation energies is quantified. Two different regimes, defined by the magnitude of the energy of the photons emitted, are identified and the boundary between them is given. The boundary is determined in terms of the photon emission rate constants and thermal properties of the particles. Also the abundance spectra are calculated for the continuous cooling case, corresponding to small photon energies. The two regimes correspond to continuous cooling and single photon quenching of the unimolecular decay. The radiative effect can be parametrized by a redefinition of the time each individual cluster has available to undergo evaporation, expressed by an effective radiative time constant.