Chamber Response Research Articles

The Mechanical Response of a Magma Chamber With Poroviscoelastic Crystal Mush

AbstractImproved understanding of the impact of crystal mush rheology on the response of magma chambers to magmatic events is critical for better understanding crustal igneous systems with abundant crystals. In this study, we extend an earlier model by Liao et al. (2018); https://doi.org/10.1029/2018jb015985 which considers the mechanical response of a magma chamber with poroelastic crystal mush, by including poroviscoelastic rheology of crystal mush. We find that the coexistence of the two mechanisms of poroelastic diffusion and viscoelastic relaxation causes the magma chamber to react to a magma injection event with more complex time‐dependent behaviors. Specifically, we find that the system’s short‐term evolution is dominated by the poroelastic diffusion process, while its long‐term evolution is dominated by the viscoelastic relaxation process. We identify two post‐injection timescales that represent these two stages and examine their relation to the material properties of the system. We find that better constraints on the poroelastic diffusion time are more important for the potential interpretation of surface deformation using the model.

Correction of the measured current of a small-gap plane-parallel ionization chamber in proton beams in the presence of charge multiplication

PurposeThe aims of this work are to study the response of a small-gap plane-parallel ionization chamber in the presence of charge multiplication and suggest an experimental method to determine the product of the recombination correction factor (ks) and the charge multiplication correction factor (kCM) in order to investigate the latter. MethodsExperimental data were acquired in scanned proton beams and in a Cobalt-60 beam. Measurements were carried out using an IBA PPC05 chambers of which the electrode gap is 0.6mm. The study is based on the determination of Jaffé plots by operating the chambers at different voltages. Experimental results are compared to theoretical equations describing initial and volume recombination as well as charge multiplication for continuous and pulsed beams. ResultsResults obtained in protons and Cobalt-60 with the same PPC05 chamber indicate that the charge multiplication effect is independent of the beam quality, while results obtained in different proton beams with two different PPC05 chambers show that the charge multiplication effect is chamber dependent. ConclusionsThe approach to be taken when using a small-gap plane-parallel ionization chamber with a high voltage (e.g. 300V or 500V) for reference dosimetry in scanned proton beams depends on which correction factors were applied to the chamber response during its calibration in terms of absorbed dose to water:-if ks and kCM are applied during the calibration, the user can use a linear extrapolation method (e.g. two-voltage method or 3VL-method) to determine the product of ks and kCM in the user beam.-if ks and kCM are not applied during the calibration: the user can neglect kCM and determine ks in the user beams using a linear interpolation of the chamber response.In both cases, it is recommended to use the ionization chamber at the same operating voltage used during its ND,w-calibration.Another solution consists of operating the PPC05 chamber at a lower voltage (e.g. 50V) with larger ks and smaller kCM and determining the product of both factors with higher accuracy using a linear extrapolation method.

Effects on skin dose from unwanted air gaps under bolus in an MR-guided linear accelerator (MR-linac) system

Bolus is commonly used in MV photon radiotherapy to increase superficial dose and improve dose uniformity for treating shallow lesions. However, irregular patient body contours can cause unwanted air gaps between a bolus and patient skin. The resulting dosimetric errors could be exacerbated in MR-Linac treatments, as secondary electrons generated by photons are affected by the magnetic field. This study aimed to quantify the dosimetric effect of unwanted gaps between bolus and skin surface in an MR-Linac. A parallel-plate ionization chamber and EBT3 films were utilized to evaluate the surface dose under bolus with various gantry angles, field sizes, and different air gaps. The results of surface dose measurements were then compared to Monaco 5.40 Treatment Planning System (TPS) calculations. The suitability of using a parallel-plate chamber in MR-Linac measurement was validated by benchmarking the percentage depth dose and output factors with the microDiamond detector and air-filled ionization chamber measurements in water. A non-symmetric response of the parallel-plate chamber to oblique beams in the magnetic field was characterized. Unwanted air gaps significantly reduced the skin dose. For a frontal beam, skin dose was halved when there was a 5 mm gap, a much larger difference than in a conventional linac. Skin dose manifested a non-symmetric pattern in terms of gantry angle and gap size. The TPS overestimated skin dose in general, but shared the same trend with measurement when there was no air gap, or the gap size was larger than 5 mm. However, the calculated and measured results had a large discrepancy when the bolus-skin gap was below 5 mm. When treating superficial lesions, unwanted air gaps under the bolus will compromise the dosimetric goals. Our results highlight the importance of avoiding air gaps between bolus and skin when treating superficial lesions using an MR-Linac system.

Numerical Investigation on Hydrodynamic Energy Conversion Performance of Breakwater-Integrated Oscillating Water Column-Wave Energy Converters

Various wave energy converters (WECs) have been developed to utilize ocean wave energy resources. Among them, an oscillating water column (OWC) device is a promising wave energy converter that has undergone several pilot tests in the ocean with various prototype devices. Recently, a breakwater-integrated OWC-WEC system was proposed to improve the economic feasibility by reducing the construction and installation costs of the OWC chamber structure. In this study, a numerical investigation was conducted to simulate the hydrodynamic energy conversion problem for a breakwater-integrated OWC-WEC. The boundary value problem with the Laplace equation was solved directly by using a three-dimensional numerical wave tank based on a finite element method in the time domain. In the present numerical method, an efficient numerical model for the tetrapod (TTP), which is an armored rubble mound breakwater, was introduced using multiple blocks and numerical damping zone. Further, numerical investigations were performed on the effects of breakwater integration, TTP layer modeling, and the application of the numerical damping zone, and their features in terms of the wavefield and the OWC chamber responses were analyzed. Finally, the validity of the present numerical method was examined by comparing it with the experimental data.

Calibration and measurement of X-ray personal dose equivalent with a Hp(10) ionization chamber

The value of the personal dose equivalent at a 10 mm depth is to characterize the energy deposition of strong penetrating radiation in the human body and is derived by measurement of air kerma and application of conversion coefficients from the ISO report. However, the conversion coefficients depend strongly on the photon energy and angles of incidence for low-energy photons. In order to overcome the problem that the conversion coefficient of low energy rays changes greatly due to the small change of energy, a secondary standard ionization chamber was used to measure the personal dose equivalent directly. A matched reference field was established with 20-250 kV X-rays and correction factors with the Hp (10) chamber were calculated under these radiation qualities with different angles of incidence. The results showed that the differences were almost 22.7 % of correction factors for the low energy photons at angles of incidence 0?. With the conversion coefficient recommended in ISO 4037-3-2019, the performance of the chamber response with respect to Hp (10) in the energy range from 33 keV to 208 keV was within about 10 %, and in the energy range from 12 keV to 208 keV and for angles of incidence between 0? and 75? was within about ?19 %.

Acoustic characterization of combustion chambers in reciprocating engines: An application for low knocking cycles recognition

In this paper the acoustic response of a combustion chamber is studied by assuming different pressure field excitation. The viscous effects on the combustion chamber and the finite impedance of the walls have been modeled with a first order system, which damps the resonance oscillation created by combustion. The characterization of the acoustic response of the combustion chamber has been used to identify the source of the excitation in order to distinguish normal combustion from knock. Two engines, a conventional spark ignited (SI) and a turbulent jet ignition (TJI) engine, were used, fueled with gasoline and compressed natural gas (CNG), respectively. The pressure fluctuations in the combustion chambers are analyzed and a pattern recognition system identifies the most likely source of excitation. This new criteria for knock identification permits a more consistent differentiation between knocking and no-knocking cycles, independent on the amplitude of the phenomenon, thus allowing the improvement for knock control algorithms, specially with combustion modes which heavily excite resonance, such as turbulent jet ignition or homogeneous charge compression ignition (HCCI).

Approximation and analysis of transient responses of a reverberation chamber by pulsed excitation

Abstract. Reverberation chambers show transient behaviour when excited with a pulsed signal. The field intensities can in this case be significantly higher than in steady state, which implies that a transient field can exceed predefined limits and render test results uncertain. Effects of excessive field intensities of short duration may get covered and not be observable in a statistical analysis of the field characteristics. In order to ensure that the signal reaches steady state, the duration of the pulse used to excite the chamber needs to be longer than the time constant of the chamber. Initial computations have shown that the pulse width should be about twice as long as the time constant of the chamber to ensure that steady state is reached. The signal is sampled in the time domain with a sampling frequency according to the Nyquist theorem. The bandwidth of the input signal is determined using spectral analysis. For a fixed stirrer position, the reverberation chamber, wires, connectors, and antennas can jointly be considered as a linear time-invariant system. In this article, a procedure will be presented to extract characteristic signal properties such as rise-time, transient overshoot and the mean value in steady state from the system response. The signal properties are determined by first computing the envelope of the sampled data using a Hilbert transform. Subsequent noise reduction is achieved applying a Savitzky–Golay filter. The point where steady state is reached is then computed from the slope of the envelope by utilising a cumulative histogram. The spectral analysis is not suitable to examine the transient behaviour and determine the time constants of the system. These constants are computed applying the method of Prony, which is based on the estimation of a number of parameters in a sum of exponential functions. An alternative to the Prony Method is the Time-Domain Vector-Fit method. In contrast to the first mentioned variant, it is now also possible to determine the transfer function of the overall RC system. Differences and advantages of the methods will be discussed.

Small-cavity chamber dose response in megavoltage photon beams coupled to magnetic fields

In MRgRT, dosimetry measurements are performed in the presence of magnetic fields. For high-resolution measurements, small-cavity ionization chambers are required. While Monte Carlo simulations are essential to determine dosimetry correction factors, models of small-chambers require careful validation with experimental measurements. The aim of this study is to characterize small-cavity chamber response coupled to magnetic fields. Small-cavity chambers (PTW31010, PTW31016, PTW31021 and PTW3022) are irradiated by a 6 MV photon beam for 9 magnetic field strengths between -1.5 T and +1.5 T. The chamber axis is orientated either parallel or perpendicular to the irradiation beam, with the magnetic field always perpendicular to the beam. MC simulations are performed in EGSnrc. The sensitive volume of the chambers is reduced to account for the inefficiency adjacent to the guard electrode (dead volume) based on COMSOL calculations of electric potentials. The magnetic field affects the chamber response by up to 4.1% and 4.5% in the parallel and perpendicular orientations, respectively, compared to no magnetic field. The maximal difference in dose response between experiments and simulations is up to 6.1% and 4.5% for parallel and perpendicular orientation, respectively. When the dead volume is removed, which accounts for the 15%-23% of the nominal volume, the difference, in most cases, is within the stated uncertainties. Nevertheless, for a particular chamber, the reduced nominal volume barely improved the agreement between the experimental and calculated relative response (4.53% to 4.13%). This disagreement may be due to the imperfect chamber geometry model, as was found from microCT images. A detailed uncertainty analysis is presented. The characterization of small-cavity ion chamber response coupled to magnetic fields is complex. Small differences between real and model chamber geometry that normally would be insignificant become an issue in the presence of magnetic fields. Accurate characterization of the nominal volume is essential for small-cavity ion chamber modelling.

Linearity and rate capability measurements of RPC with semi-insulating crystalline electrodes operating in avalanche mode

The intrinsic rate capability and the ageing properties of the Resistive Plate Chambers are closely related to the electrodes material and to the front-end electronics threshold. The development of a low noise pre-amplifier led us to improve the intrinsic rate capability of High Pressure Laminate (bakelite) RPCs up to ∼10 kHz/cm2, nevertheless the effective rate is significantly limited by electrodes ageing. To further improve the effective rate capability new materials are investigated. A Resistive Plate Chamber with crystalline semi-insulating Gallium Arsenide electrodes has been characterized with high energy electrons beam at the Beam Test Facility (BTF), (INFN National Laboratory of Frascati, Italy). The response of the Resistive Plate Chamber to multiple bunched electrons was measured operating the detector in avalanche mode. The intrinsic rate capability has been also measured operating the detector in a uniform high energy gamma radiation field at the GIF++ facility (EHN1 of SPS, CERN).

Direct determination of for cylindrical ionization chambers in a 6 MV 0.35 T MR-linac

To ensure accurate reference dosimetry with ionization chambers in magnetic resonance linear accelerators (MR-linacs), the influence of the magnetic field on the response of the ionization chambers must be considered. The most direct method considering the influence of magnetic fields in dosimetry is to apply an appropriate absorbed-dose-to-water primary standard. At PTB, a new water calorimeter has been designed which is capable to determine Dw,Q in an MR-linac. The new device allows the direct calibration of ionization chambers in terms of absorbed dose to water for MR-linac irradiation conditions. Hence, the correction factors can be determined which replace the current radiation-quality dependent correction factors for dosimetry in the presence of magnetic fields. In cooperation with Heidelberg University Hospital, factors were measured at the 6 MV 0.35 T Viewray MR-linac for different cylindrical ionization chambers with sensitive volumes ranging from 0.015 cm3 to 0.65 cm3. The chambers were placed both perpendicular and parallel in respect to the magnetic field. Standard uncertainties of about 0.5% were achieved.

Dosimetry in magnetic fields with dedicated MR-compatible ionization chambers.

MR-integrated radiotherapy requires suitable dosimetry detectors to be used in magnetic fields. This study investigates the feasibility of using dedicated MR-compatible ionization chambers at MR-integrated radiotherapy devices. MR-compatible ionization chambers (Exradin A19MR, A1SLMR, A26MR, A28MR) were precisely modeled and their relative response in a 6MV treatment beam in the presence of a magnetic field was simulated using EGSnrc. Monte Carlo simulations were carried out with the magnetic field in three orientations: the magnetic field aligned perpendicular to the chamber and beam axis (transverse orientation), the magnetic field parallel to the chamber as well as parallel to the beam axis. Monte Carlo simulation results were validated with measurements using an electromagnet with magnetic field strength upto 1.1T with the chambers in transverse orientation. The measurements and simulation results were in good agreement, except for the A26MR ionization chamber in transverse orientation. The maximum increase in response of the ionization chambers observed was 8.6% for the transverse orientation. No appreciable change in chamber response due to the magnetic field was observed for the magnetic field parallel to the ionization chamber and parallel to the photon beam. Polarity and recombination correction factor were experimentally investigated in the transverse orientation. The polarity effect and recombination effect were not altered by a magnetic field. This study further investigates the response of the ionization chambers as a function of the chambers' rotation around their longitudinal axis. A variation in response was observed when the chamber was not rotationally symmetric, which was independent of the magnetic field.

An inter-comparison between accuracy of EGSnrc and MCNPX Monte Carlo codes in dosimetric characterization of intraoperative electron beam

BackgroundIonometric dosimetry in IOERT is a complicated process, due to the sophisticated beam setup and the necessity for dedicated protocols for ion chamber response correction. On the other hand, the Monte Carlo (MC) technique can easily overcome such limitations and be considered as an alternative dosimetry approach. This paper presents a comparative analysis of two widely used MC codes, EGSnrc and MCNPX, for intraoperative electron beam dosimetry. MethodThe head of LIAC12, a dedicated IOERT accelerator, was modeled by both mentioned MC codes. Then, the percentage depth dose (PDD) curves, transverse dose profiles (TDPs), and output factor (OF) values were accordingly calculated within the water phantom. To realize the accuracy of MC codes in dosimetric characterization of intraoperative electron beam, their results were finally compared with those measured by corresponding ionometric dosimetry for all forms of electron energy/applicator size. ResultsA good agreement was observed between the simulated and measured PDDs/TDPs for both considered MC codes, such that the calculated gamma index values were always lower than unity for both considered MC codes. Nevertheless, the lower gamma index values were found in the case of the EGSnrc code. The maximum difference between the measured and calculated OF was obtained as 2.3% and 3.1% for EGSnrc and MCNPX code, respectively. ConclusionsAlthough both studied MC codes showed compatible results with the measured ones, EGSnrc code has superior accuracy in this regard and can be considered as a more reliable toolkit in Monte Carlo-based commissioning of dedicated IOERT accelerators.

Assessing the suitability of a two-dimensional array for routine quality assurance checks of flatness and symmetry.

Quality assurance of radiotherapy linear accelerator beams demands the use of equipment with high resolution and reliability of reproducible results over time. This study examines the suitability of a commercially available two-dimensional ionization chamber array-the StarCheck array (PTW, Frieburg, Germany) to measure symmetry and flatness for both photon and electron beams. The study was conducted over a period of 4years whereby the reliability of the array could be established. The reproducibility, uniformity of chamber response, and comparison of both photon and electron profiles acquired with the StarCheck array to that of the water-tank were examined. The most significant result was that across all profiles acquired using the StarCheck array, a defective chamber was detected, manifested by a 'dip' in the cross-plane profile at the same position. Assuming all detectors are functional, StarCheck was shown to be within 1% of the water-tank results for both flatness and symmetry. The detector array exhibited many calibration and detector issues over the period in which it has been used in the department. Furthermore, PTW recommends that recalibration of the array should to be performed every 2years at either PTW, Freiburg, or at a qualified calibration laboratory. Therefore, the department requires a backup device for the recalibration time, approximately 4months as worst case.

Beam monitor calibration of a synchrotron-based scanned light-ion beam delivery system

PurposeThis paper presents the implementation and comparison of two independent methods of beam monitor calibration in terms of number of particles for scanned proton and carbon ion beams. MethodsIn the first method, called the single-layer method, dose-area-product to water (DAPw) is derived from the absorbed dose to water determined using a Roos-type plane-parallel ionization chamber in single-energy scanned beams. This is considered the reference method for the beam monitor calibration in the clinically relevant proton and carbon energy ranges. In the second method, called the single-spot method, DAPw of a single central spot is determined using a Bragg-peak (BP) type large-area plane-parallel ionization chamber. Emphasis is given to the detailed characterization of the ionization chambers used for the beam monitor calibration. For both methods a detailed uncertainty budget on the DAPw determination is provided as well as on the derivation of the number of particles. ResultsBoth calibration methods agreed on average within 1.1% for protons and within 2.6% for carbon ions. The uncertainty on DAPw using single-layer beams is 2.1% for protons and 3.1% for carbon ions with major contributions from the available values of kQ and the average spot spacing in both lateral directions. The uncertainty using the single-spot method is 2.2% for protons and 3.2% for carbon ions with major contributions from the available values of kQ and the non-uniformity of the BP chamber response, which can lead to a correction of up-to 3.2%. For the number of particles, an additional dominant uncertainty component for the mean stopping power per incident proton (or the CEMA) needs to be added. ConclusionThe agreement between both methods enhances confidence in the beam monitor calibration and the estimated uncertainty. The single-layer method can be used as a reference and the single-spot method is an alternative that, when more accumulated knowledge and data on the method becomes available, can be used as a redundant dose monitor calibration method. This work, together with the overview of information from the literature provided here, is a first step towards comprehensive information on the single-spot method.

Quantifying temperature-equilibrium time using temperature analysis inside a Farmer ionization chamber.

In this study, we propose a methodology for temperature determination of the temperature and pressure correction factor, PTP, by analyzing the temperature distribution of the modeled ionization chamber taking into account the thermal effect of a water phantom on neighboring materials in the process. Additionally, we present an appropriate temperature-equilibrium time for conducting measurements. The temporal response in the cavity is acquired at 20-s intervals using a Farmer ionization chamber and an electrometer. The initial temperature of the water phantom is 20–25°C with continuous heating/cooling. The temporal response is measured until temperature equilibrium is confirmed, specifically when a temperature difference of 1–5°C is observed between the ionization chamber and the water phantom. Using an ionization-chamber model, temperature distribution is simulated between 20 and 25°C with various parameters set to receive heating and cooling from surrounding media. The results suggest that the temporal response of the ionization chamber essentially coincides with the temperature change at the tip and middle; moreover, the predicted temperature change for temporal response and the simulated temperature of water are different by ~0.16°C at the tip and ~0.79°C at the bottom. Overall, the temperature-equilibration time for absorbed dosimetry is affected by two factors: the cavity wall and the stem side of the cavity; moreover, 400 s is required to obtain complete temperature equilibrium in the water phantom. This analytical study supports the experimental value obtained in previous research. Therefore, analytical representation of the temperature distribution in the ionization chamber is possible.

On the stability of well-type ionization chamber source strength calibration coefficients.

To investigate the variability and stability of brachytherapy source strength calibration coefficients for air-communicating and pressurized well-type ionization chambers among high-dose rate (HDR), low-dose rate (LDR), and electronic brachytherapy (EBT) source qualities. These qualities of an ionization chamber are important features to assess given their role in maintaining traceability to a primary national standards laboratory and facilitating efficacious patient care in brachytherapy. The calibration records from the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) customer well-type ionization chamber database were retrospectively analyzed for calibrations performed between 1996 and 2019. A statistical analysis was performed and differentiated among calibration type to quantify the distribution of chamber calibration coefficients among several chamber models. Distributions were quantified based on their moments and by quantile analysis. For LDR calibrations, chamber response was further differentiated by seed type to study the variability in seed dependence within and across chamber models. In addition to these metrics, the calibration lineage at the UWADCL was used to assess the stability of these calibration coefficients among chamber model and source type based on the ratio of subsequent calibration coefficients. The distribution of brachytherapy source strength calibration coefficients for a particular chamber model is not necessarily normally distributed and is sensitive to changes in the machining tolerances or design of the chamber model. Calibration source quality also influenced the distributions of calibration coefficients for a chamber model; the air-kerma rate calibration coefficients for EBT sources were the most variable followed by LDR and then HDR source types. The stability of a chamber's source strength calibration coefficient exhibited a similar dependence on the source quality. Air-communicating and pressurized chambers exhibited an average stability between subsequent calibrations of 0.2% and 3.0%, respectively, for HDR calibrations, but could exhibit more than double this variability characteristic of their leptokurtic distributions. For LDR calibrations, the spread and stability in a model's calibration ratio toward another seed type is extensive and notable. For some seed types and chamber models exhibit variations of <0.5% while others exceeded 2.0%. Furthermore, the magnitude of this dependence is outside the variability of the source's source strength due to manufacturing, which was determined from the manufacturer and NIST source strength intercomparison records at the UWADCL. As a result, establishing and disseminating calibration conversion factors among different source and chamber models is not advised as it would substantially increase the uncertainty in a clinical user's determination of source strength. The calibration of a well-type ionization chamber is unique to the chamber, source, and source holder. Attempting to generalize source strength calibration coefficients among chambers of the same model or source type is impractical given the variation in response observed across the calibration history of the UWADCL. Attempting to quantitate and transfer calibration coefficients in such a relative sense may significantly degrade the uncertainty relative to specifically calibrating a chamber for an individual source.

The challenge of ionisation chamber dosimetry in ultra-short pulsed high dose-rate Very High Energy Electron beams

High dose-rate radiotherapy, known as FLASH, has been shown to increase the differential response between healthy and tumour tissue. Moreover, Very High Energy Electrons (VHEEs) provide more favourable dose distributions than conventional radiotherapy electron and photon beams. Plane-parallel ionisation chambers are the recommended secondary standard systems for clinical reference dosimetry of electrons, therefore chamber response to these high energy and high dose-per-pulse beams must be well understood. Graphite calorimetry, the UK primary standard, has been employed to measure the dose delivered from a 200 MeV pulsed electron beam. This was compared to the charge measurements of a plane-parallel ionisation chamber to determine the absolute collection efficiency and infer the ion recombination factor. The dose-per-pulse measured by the calorimeter ranged between 0.03 Gy/pulse and 5.26 Gy/pulse, corresponding to collection efficiencies between 97% and 4%, respectively. Multiple recombination models currently available have been compared with experimental results. This work is directly applicable to the development of standard dosimetry protocols for VHEE radiotherapy, FLASH radiotherapy and other high dose-rate modalities. However, the use of secondary standard ionisation chambers for the dosimetry of high dose-per-pulse VHEEs has been shown to require large corrections for charge collection inefficiency.

CyberKnife reference dosimetry: An assessment of the impact of evolving recommendations on correction factors and measured dose.

Specialized treatment machines such as the CyberKnife, TomoTherapy, or the GammaKnife, utilize flattening filter free (FFF) photon beams and may not be able to generate a 10cm x 10cm reference field. A new Code of Practice has recently been published (IAEA TRS483) to give recommendations for these machines. Additionally, some standard laboratories provide measured beam quality correction factors for the user's reference chamber, which can be used instead of the published tabulated beam quality correction factors. The purpose of this study was first to assess how different recommendations, as outlined below, affect the reference dosimetry at the CyberKnife, and second, to assess the impact of using measured rather than tabulated beam quality correction factors on measured dose. Following recommendations in TRS398, three field chambers (IBA CC04, Exradin A19, and Exradin A12S) were cross-calibrated with a user's reference chamber (IBA FC65-G), which was calibrated in a Cobalt-60 (Co-60) beam by a primary standards laboratory. The chamber response was corrected for influence quantities such as temperature, pressure, ion recombination, polarity, and beam quality. Additionally, correction factors for volume averaging and differences due the FFF beam spectrum were determined for the CyberKnife beam. Three different methods were utilized - TRS398; Intermediate (i.e. TRS398 with additional published recommendations); and TRS483. The measurements were undertaken in a 10cm×10cm field defined by jaws for a uniform flattened (WFF) 6MV photon beam of a Varian TrueBeam linear accelerator (linac) with a source to detector distance (SDD) of 100cm, and in a 60mm diameter circular field for a 6MV flattening filter free (FFF) Accuray CyberKnife beam with SDD of 80cm. All measurement was performed at 10cm deep in a full scatter phantom as defined in TRS398. Differences between the three methods in volume averaging correction factors ranged from 0.01% to 0.45% depending on the chamber assessed. As expected, an increased chamber length leads to a larger correction factor. The differences in beam spectrum correction factors range from 0.09% to 0.3%. Negligible differences in beam quality correction factors were observed; however, differences up to 1% were found between measured and tabulated values. Differences in cross-calibrated chamber calibration coefficients range from 0.05% to 0.51% depending on the chamber assessed. Differences in measured dose are up to 0.87% between Method TRS398 and Intermediate, again chamber dependent, and 0.28% between Method Intermediate and TRS483. Using chambers cross-calibrated in the linac beam can lead to differences in measured dose per Monitor Unit (MU) in the CyberKnife beam of approximately 0.5% between chambers. Using Method Intermediate vs using recommendations given in TRS483 led to a difference of 0.28% in measured dose per MU, which is due to differences in volume averaging and beam spectrum correction factors. Using TRS483 is recommended as the cross-calibration is done in the CyberKnife beam and accounts for its specific reference conditions. It will also ensure consistency between different centers. The measured beam quality correction factors agree within the uncertainties with the tabulated values.

Monte Carlo and water calorimetric determination of kilovoltage beam radiotherapy ionization chamber correction factors

The in-phantom calibration method for radiotherapy kilovoltage x-ray beams requires ionization chamber correction factors. The overall ionization chamber correction factor accounts for changes in the chamber response due to the displacement of water by the chamber cavity and wall, the presence of the stem and the change in incident photon energy and angular distribution in the phantom to that in air. A waterproof sheath, if required, is accounted for in a sheath correction factor. The aim of this study is to determine chamber correction factors through Monte Carlo (MC) simulations and water calorimetry measurements. Correction factors are determined for the PTW TM30013, NE2571, IBA FC65-G, IBA FC65-P and Exradin A12 ionization chambers. They are compared to experimental values obtained at the German national metrology institute Physikalisch-Technische Bundesanstalt (PTB) with their water calorimetry-based absorbed dose to water primary standard and at other national metrological institutes. An uncertainty analysis considers the contributions to the uncertainty on the chamber correction factors from the field size, photon cross sections, photon fluence spectra and chamber wall and central electrode dimensions. The MC calculated chamber correction factors are within 2.2% of unity with a standard uncertainty of 0.3%. For the 50 kV and 100 - 140 kV radiation beam qualities, the calculated correction factors deviate from the measured correction factors (with a standard uncertainty of 1%) by up to 2.6%. The calculated chamber correction factors for the PTW TM30013 and Exradin A12 are consistent with those derived from the BIPM kilovoltage primary standard. The inconsistencies between the calculated and experimental chamber correction factors indicate the need to further investigate the accuracy of kilovoltage absorbed dose to water primary standards and the use of MC simulations to determine kilovoltage beam chamber correction factors.

Liquid petroleum gas flame in a double-swirl gas turbine model combustor: Lean blow-out, pollutant, preheating

In this paper, lean blow-out limits in a double swirl GT model combustor were investigated experimentally for liquid petroleum gas fuel. The lean blow-out curve was extracted for different combustor configurations. While burner could operate reasonably under ultra-lean conditions, two different sets of operating conditions, one with a low flow rate and another one with high flow rate, are identified and studied in terms of lean blow-out and pollutant. Results showed that while the flame structure was similar in both cases, the chamber responses to geometrical changes and also preheating are minimal at the low flow rate. That means confinement and injector type have desirable effects on stability borders but not for the low flow rate. The channeled injector shifted down the lean blow-out limit around 28% at high flow rate. Measurements on the combustor exhaust gas composition and temperature indicate a region with relatively complete combustion and reasonable temperature and a very low level of exhaust NOx pollutants (i. e., below 10 ppm) at about 25-50% above the lean blow-out. In this operating envelope, a burner power increment led to a higher exhaust average temperature and combustion efficiency, while NOx formation decreased. Preheating the inlet air up to 100?C results in an improvement in burner stability in about 10%, but NOx production intensifies more than three times. Results indicate that the lean blow-out limit is con-figured more by the burner design and aerodynamic aspects rather than the fuel type.