Chamber Response Research Articles

PD-0811 Lateral dose response of an ionization chamber in an external magnetite field

PD-0811 Lateral dose response of an ionization chamber in an external magnetite field

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 used to protect a 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.

Development of a dosimetry system for therapeutic X-rays using a flexible amorphous silicon thin-film solar cell with a scintillator screen.

To evaluate the dosimetric characteristics and applications of a dosimetry system composed of a flexible amorphous silicon thin-film solar cell and scintillator screen (STFSC-SS) for therapeutic X-rays. The real-time dosimetry system was composed of a flexible a-Si thin-film solar cell (0.2-mm thick), a scintillator screen to increase its efficiency, and an electrometer to measure the generated charge. The dosimetric characteristics of the developed system were evaluated including its energy dependence, dose linearity, and angular dependence. Calibration factors for the signal measured by the system and absorbed dose-to-water were obtained by setting reference conditions. The application and correction accuracy of the developed system were evaluated by comparing the absorbed dose-to-water measured using a patient treatment beam with that measured using the ion chamber. The responses of STFSC-SS to energy, field size, depth, and source-to-surface distance (SSD) were more dependent on measurement conditions than were the responses of the ion chamber, although the former dependence was due to the scintillator screen, not the solar cell. The signals of STFSC-SS were also dependent on dose rate, while the responses of solar cell alone and scintillator screen were not dependent on dose rate. The scintillator screen reduced the output of solar cell at 6 and 15 MV by 0.60 and 0.55%, respectively. The different absorbed dose-to-water measured using STFSC-SS for patient treatment beam differed by 0.4% compared to those measured using the ionization chamber. The uncertainties of the developed system for 6 and 15 MV photon beams were 1.8 and 1.7%, respectively, confirming the accuracy and applicability of this system. The thin-film solar cell-based detector developed in this study can accurately measure absorbed dose-to-water. The increased signal resulting from the use of the scintillator screen is advantageous for measuring low doses and stable signal output. In addition, this system is flexible, making it applicable to curved surfaces, such as a patient's body, and is cost-effective.

Precise activity measurements of medical radionuclides using an ionization chamber: a case study with Terbium-161

Background161Tb draws an increasing interest in nuclear medicine for therapeutic applications. More than 99% of the emitted gamma and X-rays of 161Tb have an energy below 100 keV. Consequently, precise activity measurement of 161Tb becomes inaccurate with radionuclide dose calibrators when using inappropriate containers or calibration factors to account for the attenuation of this low energy radiation. To evaluate the ionization chamber response, the sample activity must be well known. This can be performed using standards traceable to the Système International de Référence, which is briefly described as well as the method to standardize the radionuclides.MethodsIn this study, the response of an ionization chamber using different container types and volumes was assessed using 161Tb. The containers were filled with a standardized activity solution of 161Tb and measured with a dedicated ionization chamber, providing an accurate response. The results were compared with standardized solutions of high-energy gamma-emitting radionuclides such as 137Cs, 60Co, 133Ba and 57Co.ResultsFor the glass vial type with an irregular glass thickness, the 161Tb measurements gave a deviation of 4.5% between two vials of the same type. The other glass vial types have a much more regular thickness and no discrepancy was observed in the response of the ionization chamber for these type of vials. Measurements with a plastic Eppendorf tube showed stable response, with greater sensitivity than the glass vials.ConclusionIonization chamber measurements for low-energy gamma emitters (< 100 keV), show deviation depending on the container type used. Therefore, a careful selection of the container type must be done for activity assessment of 161Tb using radionuclide dose calibrators. In conclusion, it was highlighted that appropriate calibration factors must be used for each container geometry when measuring 161Tb and, more generally, for low-energy gamma emitters.

Constraints on Absolute Magma Chamber Volume From Geodetic Measurements of Trapdoor Faulting at Sierra Negra Volcano, Galapagos

AbstractMagma chamber volume is critical for volcano hazards assessment and forecasting. Standard geodetic methods constrain volume change, not the total volume. Here, we show that the deformation response of the magma chamber to trapdoor faulting events at Sierra Negra volcano, Galapagos, depends on the product of the absolute chamber volume and the magma compressibility. Bubble‐free magma provides the lower limit on compressibility, thus an upper bound on the chamber volume of 13.6–20.6 km3, depending on fault dip. We estimate an upper limit on compressibility using a conduit model relating volatile content to lava fountain height, compared with observations from the 2005 eruption, constrained by volatile content of olivine melt inclusions. This yields a lower bound on chamber volume roughly half the upper bound. We find that the best fitting trapdoor fault is near‐vertical; reverse dips are slightly favored (88°).

A direct test of Auger cascade induced nucleation from heavy element contamination in C3F8 bubble chambers

Understanding and quantifying the gamma-induced bubble nucleation background in clean nuclear recoil detection bubble chambers is of utmost importance to bubble chamber based dark matter searches. We present data confirming the hypothesis that large Auger cascades from high-Z elements such as iodine and xenon dramatically increase the response of C3F8 bubble chambers to gamma rays. These tests, performed with a small calibration bubble chamber filled with C3F8+\U0001d4aa(10) ppm xenon, show that the probability of bubble nucleation scales with the rate of xenon inner-shell vacancies, reaching values >10% per K-Shell vacancy for Seitz thresholds of interest to future dark matter searches in bubble chambers. We also place an upper limit on bubble nucleation probability for argon Auger events, relevant to large future bubble chambers which may contain some residual atmospheric argon after the active fluid fill.

Evaluation of Two-Voltage and Three-Voltage Linear Methods for Deriving Ion Recombination Correction Factors in Proton FLASH Irradiation

Ultrahigh dose-rate (FLASH) proton therapy is of great interest due to potential reduced normal tissue toxicities without compromising tumor-killing effect compared to current clinical proton practices. However, the ionization chamber response to proton beams under ultrahigh dose rates (&#x003E;40 Gy/s) has not been thoroughly investigated. In this study, four different ion chambers (PTW 34045 Advanced Markus, PPC-40, CC-04, and CC-13 from IBA Dosimetry) were irradiated with 230 MeV proton beams at 1.5, 63.7, and 127.6 Gy/s dose rates. Theoretical values of ion recombination correction factor (<inline-formula> <tex-math notation="LaTeX">${k}_{s}$ </tex-math></inline-formula>) were calculated from saturation curves using Niatel&#x2019;s model. The theoretical <inline-formula> <tex-math notation="LaTeX">${k}_{s}$ </tex-math></inline-formula> values were compared to the values using the two-voltage (2V) method from standard dosimetry protocols and the three-voltage linear (3VL) method proposed by Rossomme <i>et al.</i> Both parallel plate chambers and CC-04 demonstrated adequate ion collection efficiency at the highest dose rate. For these three chambers, there is no statistically significant difference between theoretical <inline-formula> <tex-math notation="LaTeX">${k}_{s}$ </tex-math></inline-formula> values and those calculated with 2V and 3VL methods. However, significant ion recombination correction was found in CC-13 (<inline-formula> <tex-math notation="LaTeX">${k}_{s}&gt;1.50$ </tex-math></inline-formula>) when dose rate reached 63.7 Gy/s. The assumption of insignificant initial recombination in standard dosimetry protocols also underestimated the ion recombination effect in this scenario.

Monte Carlo calculated calibration settings for commercially available nuclear medicine ionization chambers.

A method for calculating nuclear medicine ionization chamber (NMIC) calibration settings with a Monte Carlo model is presented and validated against physical measurements. This work provides Monte Carlo-calculated calibration settings for select isotopes with no current manufacturer recommendations and a method by which NMIC manufacturers or standards laboratories can utilize highly detailed specifications to calculate comprehensive lists of calibration settings for generalisotopes. A Monte Carlo model of a Capintec PET series NMIC was developed and used to calculate the chamber response to relevant radioactive decay products over an energy range relevant to nuclear medicine. The photon detection efficiency (PDE) of a high purity germanium (HPGe) detector was modeled and physically validated to facilitate measurements of NMIC calibration settings with HPGe detector spectroscopy. Modeled NMIC response to various isotopes was compared against spectroscopic measurements and National Institute of Standards and Technology (NIST)-validated calibration settings to validate the Monte Carlo-calculated NMIC calibrationsettings. HPGe detector PDE was validated against the physical measurements to within at confidence and used to measure calibration settings, which produced activity readings , , , and different than those validated by NIST for C, F, Ga, and Cu respectively. The Monte Carlo model of the NMIC reproduced measured calibration settings to within at confidence for isotopes with a sufficiently small yield of low energy photons. A method of calculating NMIC calibration settings with Monte Carlo modeling has been developed and validated against HPGe detector spectroscopy. NMIC manufacturers or standards laboratories can use more detailed specifications of the chamber geometries to extend the applicability of this method to a wider range of isotopes.

Shaking Table Test of a Transfer-Purge Chamber in Nuclear Island Structure.

The transfer-purge chamber is an operation room for nuclear fuel transport and purging in a nuclear power plant, which has a demand for structural reliability and radiation protection. The transfer-purge chamber has features such as large curvature, heavy concrete, long overhang, and irregular cross-sections, and it is constructed of double steel plates reinforced concrete (SC) structure. This study performed shaking table tests for a 1:4.5 scale model of the transfer-purge chamber. Three sets of ground motions were input in the scale model in the horizontal and vertical directions to study its structural reliability and seismic performance. Acceleration response and strain response of the structure were analyzed to evaluate the dynamic characteristics of the transfer-purge chamber under the ground motion. The results show that the transfer-purge chamber has great stiffness and short periods. The periods slightly increase with the rise of intensity of seismic ground motions. Under the excitation of ground motions, the dynamic response of the transfer-purge chamber is slight. No obvious deformation or damage occurred on the transfer-purge chamber, and cracking in concrete or buckling on steel plate did not appear. The transfer-purge chamber has excellent seismic performance, and it is sufficiently safe and reliable from a structural perspective.

Uniformity of response of Uranium fission chambers used as neutron beam monitors

Uranium fission chambers are commonly used for neutron beam monitoring in neutron research facilities. The challenges brought with the renewing or commissioning of new neutron facilities encourage to minimize the interaction between the monitor and the neutron beam. In order to characterize both their detection efficiency and transmission factor, several fission chambers, have been scanned on the CT1 monochromatic beam line of the ILL. Results show that the monitor transparency, measured at the center of the monitor, varies from 90% to 96%, and that the detection efficiency varies by as much as 50% over the active area.

Does the ventricle limit cardiac contraction rate in the anoxic turtle (Trachemys scripta)? I. Comparison of the intrinsic contractile responses of cardiac chambers to the extracellular changes that accompany prolonged anoxia exposure.

Multiple lines of evidence suggest that an inability of the ventricle to contract in coordination with the pacemaker during anoxia exposure may suppress cardiac pumping rate in anoxia-tolerant turtles. To determine under what extracellular conditions the ventricle could be the weak link that limits cardiac pumping, we compared, under various extracellular conditions, the intrinsic contractile properties of isometrically-contracting ventricular and atrial strips obtained from 21 °C- to 5 °C- acclimated turtles (Trachemys scripta) that had been exposed to either normoxia or anoxia (16 h at 21 °C; 12 days at 5 °C). We found that combined extracellular anoxia, acidosis, and hyperkalemia (AAK), severely disrupted ventricular, but not right or left atrial, excitability and contractibility of 5 °C anoxic turtles. However, combined hypercalcemia and heightened adrenergic stimulation counteracted the negative effects of AAK. We also report that the turtle heart is resilient to prolonged diastolic intervals, which would ensure that contractile force is maintained if arrhythmia were to occur during anoxia exposure. Finally, our findings reinforce that prior temperature and anoxia experiences are central to the intrinsic contractile response of the turtle myocardium to altered extracellular conditions. At 21 °C, prior anoxia exposure preconditioned the ventricle for anoxic and acidosis exposure. At 5 °C, prior anoxia exposure evoked heightened sensitivity of the ventricle to hyperkalemia, as well as all chambers to combined hypercalcemia and increased adrenergic stimulation. Overall, our findings show that the ventricle could limit cardiac pumping rate during prolonged anoxic submergence in cold-acclimated turtles if hypercalcemia and heightened adrenergic stimulation are insufficient to counteract the negative effects of combined extracellular anoxia, acidosis, and hyperkalemia.

Monte Carlo calculation of detector perturbation and quality correction factors in a 1.5 T magnetic resonance guided radiation therapy small photon beams

Objective. With future advances in magnetic resonance imaging-guided radiation therapy, small photon beams are expected to be included regularly in clinical treatments. This study provides physical insights on detector dose-response to multiple megavoltage photon beam sizes coupled to magnetic fields and determines optimal orientations for measurements. Approach. Monte Carlo simulations determine small-cavity detector (solid-state: PTW60012 and PTW60019, ionization chambers: PTW31010, PTW31021, and PTW31022) dose-responses in water to an Elekta Unity 7 MV FFF photon beam. Investigations are performed for field widths between 0.25 and 10 cm in four detector axis orientations with respect to the 1.5 T magnetic field and the photon beam. The magnetic field effect on the overall perturbation factor (P MC) accounting for the extracameral components, atomic composition, and density is quantified in each orientation. The density (P ρ ) and volume averaging (P vol) perturbation factors and quality correction factors () accounting for the magnetic field are also calculated in each orientation. Main results. Results show that P vol remains the most significant perturbation both with and without magnetic fields. In most cases, the magnetic field effect on P vol is 1% or less. The magnetic field effect on P ρ is more significant on ionization chambers than on solid-state detectors. This effect increases up to 1.564 ± 0.001 with decreasing field size for chambers. On the contrary, the magnetic field effect on the extracameral perturbation factor is higher on solid-state detectors than on ionization chambers. For chambers, the magnetic field effect on P MC is only significant for field widths <1 cm, while, for solid-state detectors, this effect exhibits different trends with orientation, indicating that the beam incident angle and geometry play a crucial role. Significance. Solid-state detectors’ dose-response is strongly affected by the magnetic field in all orientations. The magnetic field impact on ionization chamber response increases with decreasing field size. In general, ionization chambers yield closer to unity, especially in orientations where the chamber axis is parallel to the magnetic field.

Early Right Heart Chambers Reverse Remodeling in Patients Operated in Adulthood for Congenital Lesions Associated with Right Heart Chambers Enlargement.

Progressive right heart chambers dilatation is frequent in the adult congenital heart disease (ACHD) population. We evaluated the immediate and mid-term response of right heart chambers to surgery performed in adulthood for lesions associated with right heart chambers enlargement. Thirty-six adult patients with lesions associated with right heart chambers enlargement submitted to surgery were studied. We collected echocardiographic data of right ventricle (RV) mid-diameter, right atrial volume indexed, RV systolic pressure, and tricuspid annular plane systolic excursion (TAPSE) prior to surgery (T0), at 2 to 5 days (T1), and 3 to 6 months (T2) after surgery. At T1, we observed a significant decrease of RV mid-diameter (47.2 ± 8.4 vs. 39.6 ± 7.4 mm, P < .001), right atrial volume indexed (45.6 ± 26.6 vs. 27.2 ± 11 ml/m2, P < .001), and RV systolic pressure (39 ± 14.8 vs. 32.8 ± 11.3 mm Hg, P = .03). At T2, a further significant deviation in the rate of RV diameter (39.6 ± 7.4 vs. 34.5 ± 5.1 mm, P < .001), in RV systolic pressure (32.8 ± 11.3 vs. 25.3 ± 5 mm Hg, P = .03) and TAPSE (13.9 ± 3.2 vs. 15.8 ± 2.6 mm, P < .001) was observed. Positive right heart chambers remodeling occurs as early as in the immediate post-operative period in most ACHD patients operated for lesions associated with right heart chambers enlargement.

Online Recombination Correction in Dosimetry with Arrays of Ionization Chambers: Application to FFF and UHDR Radiotherapy

Recombination of charge carriers can affect the response of ionization detectors used for the dosimetry of radiotherapy fields. In this work, we present a method for correcting online the recombination effects in arrays of ionization chambers irradiated with time-varying dose rates. The method is based on the characterization of the dose rate/recombination response of the detector, and the measurement of the instant ionization current in the detector, rather than the integrated charge. The proposed method was investigated with simulations of the response of different air and liquid ionization chambers in situations where recombination can be large. In addition, we experimentally investigated the application of the method with an in-house-developed liquid-filled ionization chamber. The proposed online correction method can compensate for recombination losses and seems feasible to implement in the software of ionization arrays/detectors used for the dosimetry of radiotherapy fields.

Effect of well chamber altitude pressure corrections for cesium Blu 131 Cs and CivaDot 103 Pd brachytherapy sources.

Previous publications have described how the standard temperature and pressure correction will overcorrect measurements with a low-energy photon low-dose rate brachytherapy source at low ambient air pressures. To account for this effect, an additional correction factor is applied after the standard temperature and pressure correction. This additional correction is dependent on the source being measured and the chamber it is measured in. Well chamber corrections for two sources and findings regarding aspects that may affect the altitude response of the sources are presented. A purpose-built pressure vessel was constructed previously, which could achieve pressures ranging from 74.661 to 106.66kPa (560-800mmHg). Three Cesium Blu sources (131 Cs) from Isoray Inc. and three CivaDots (103 Pd) from CivaTech Oncology Inc. were tested over this pressure range in increments of 2.7kPa (20mmHg) in three HDR 1000 Plus chambers, and the Cesium Blu sources were also tested in two IVB 1000 chambers. Both chamber models are air communicating well-type ionization chambers produced by Standard Imaging Inc. Multiple runs of each source/chamber combination were completed, corrected with the standard temperature and pressure correction, normalized to the result at 101.325kPa, and averaged with runs of the same combination. The chamber response was also simulated using MCNP6 to validate the experimental results. Measurements of both sources in all chambers followed the expected power dependence on ambient pressure as seen in previous studies. The Cesium Blu source, however, demonstrated a significant difference in response in the HDR 1000 Plus chamber versus the IVB 1000 chamber. For an altitude correction factor of the form, PA =k1 (P)k 2 , new coefficients are proposed for both sources for pressure units of kPa and mmHg. The Monte Carlo calculated chamber response agreed with the experimental results within 2% for all sources and chambers at all pressures. Altitude correction coefficients for two new low-energy photon low-dose rate brachytherapy sources are provided. The directional dependence of the CivaDot has no bearing on its dependence on pressure; however, the difference in construction materials from other 103 Pd sources leads to unique correction coefficients. The higher energy of the Cesium Blu source with respect to 103 Pd and 125 I sources yields a difference in correction factors depending on which model chamber is used for air-kerma strength calculations. Clinics must be careful to select the correct pair of coefficients for the chamber model they used.

Quantification of the temperature equilibrium time of the cavity in parallel-plate-type ionization chambers by thermal analysis.

Temperature corrections are necessary to account for the varying mass of air in the cavity volume of a vented ionization chamber. The temporal response resulting from temperature changes in a cylindrical and/or Farmer-type ionization chamber, which is the standard dosimeter, has been thoroughly discussed by some researchers. The purpose of this study was to characterise and analyse the dependence of the cavity air temperature of the parallel-plate-type ionization chamber on changes in the ambient temperature. Ionization chambers NACP-02 (IBA Dosimetry, GmbH) and Advanced Markus TN34045 (PTW, Freiburg) were modelled using thermal analysis software to present the temperature equilibrium time and the entire ionization chamber temperature distribution. The temporal response of each ionization chamber was measured for comparing the calculation results of the thermal analysis. The ionization chamber cavities of NACP-02 and TN34045 reached complete equilibrium in 670 and 750 s, respectively. Heat transfer occurred faster at the centre of the front wall of TN34045 than at the outside of the centre except for the edges. Further, the non-uniformity of temperature in the cavity was in the range of 24.2–24.8°C for NACP-02 and 23.7–24.4°C for TN34045 at 200 s after the ionization chamber was installed in the water phantom. The previous proposal to wait for about 15 mins after submerging the chamber in a water phantom before the measurement is demonstrated to be appropriate for parallel-plate-type ionization chambers.

The role of the construction and sensitive volume of compact ionization chambers on the magnetic field-dependent dose response.

The magnetic-field correction factors of compact air-filled ionization chambers have been investigated experimentally and using Monte Carlo simulations up to 1.5T. The role of the nonsensitive region within the air cavity and influence of the chamber construction on its dose response have been elucidated. The PTW Semiflex 3D 31021, PinPoint 3D 31022, and Sun Nuclear Cooperation SNC125c chambers were studied. The factors were measured at the experimental facility of the German National Metrology Institute (PTB) up to 1.4T using a 6 MV photon beam. The chambers were positioned with the chamber axis perpendicular to the beam axis (radial); and parallel to the beam axis (axial). In both cases, the magnetic field was directed perpendicular to both the beam axis and chamber axis. Additionally, the sensitive volumes of these chambers have been experimentally determined using a focused proton microbeam and finite element method. Beside the simulations of factors, detailed Monte Carlo technique has been applied to analyse the secondary electron fluence within the air cavity, that is, the number of secondary electrons and the average path length as a function of the magnetic field strength. A nonsensitive volume within the air cavity adjacent to the chamber stem for the PTW chambers has been identified from the microbeam measurements and FEM calculations. The dose response of the three investigated ionization chambers does not deviate by more than 4% from the field-free case within the range of magnetic fields studied in this work for both the radial and axial orientations. The simulated for the fully guarded PTW chambers deviate by up to 6% if their sensitive volumes are not correctly considered during the simulations. After the implementation of the sensitive volume derived from the microbeam measurements, an agreement of better than 1% between the experimental and Monte Carlo factors for all three chambers can be achieved. Detailed analysis reveals that the stem of the PTW chambers could give rise to a shielding effect reducing the number of secondary electrons entering the air cavity in the presence of magnetic field. However, the magnetic field dependence of their path length within the air cavity is shown to be weaker than for the SNC125c chamber, where the length of the air cavity is larger than its diameter. For this chamber it is shown that the number of electrons and their path lengths in the cavity depend stronger on the magnetic field. For clinical measurements up to 1.5T, the required corrections of the three chambers could be kept within 3% in both the investigated chamber orientations. The results reiterate the importance of considering the sensitive volume of fully guarded chambers, even for the investigated compact chambers, in the Monte Carlo simulations of chamber response in magnetic field. The resulting magnetic field-dependent dose response has been demonstrated to depend on the chamber construction, such as the ratio between length and the diameter of the air cavity as well as the design of the chamber stem.

Simulating the time projection chamber responses at the MPD detector using generative adversarial networks

High energy physics experiments rely heavily on the detailed detector simulation models in many tasks. Running these detailed models typically requires a notable amount of the computing time available to the experiments. In this work, we demonstrate a new approach to speed up the simulation of the Time Projection Chamber tracker of the MPD experiment at the NICA accelerator complex. Our method is based on a Generative Adversarial Network – a deep learning technique allowing for implicit estimation of the population distribution for a given set of objects. This approach lets us learn and then sample from the distribution of raw detector responses, conditioned on the parameters of the charged particle tracks. To evaluate the quality of the proposed model, we integrate a prototype into the MPD software stack and demonstrate that it produces high-quality events similar to the detailed simulator, with a speed-up of at least an order of magnitude. The prototype is trained on the responses from the inner part of the detector and, once expanded to the full detector, should be ready for use in physics tasks.

Determination of an ionization chamber response using quality index for kilovoltage x-ray beams

This study aims to investigate the energy response of a Farmer-type ionization chamber for kilovoltage x-ray beams. The chamber response in free air and at depths of 1 to 15 cm in a water phantom was calculated using the Monte Carlo method. The response in the phantom was evaluated using a beam quality conversion factor, kQx, for x-ray beams Qx to a 60Co beam. The response was expressed as a function of the effective energy or a half-value layer (HVL, mm Al) for x-ray beams with given quality index (QI) values of 0.4, 0.5, and 0.6 and for monoenergetic photon beams. QI is defined as the ratio of the effective energy to the tube voltage. The response for 25 fluence spectra of x-ray beams was calculated for the beam qualities with HVL of 1.39-10.4 mm Al and tube voltages of 50-137.6 kVp, as classified by QI. The chamber response calculated in free air for the given QI values agreed with 1% of that provided by secondary standard laboratories. The kQx values depended on depth at the lower energy levels. The depth dependence was less than 1% for an HVL (mm Al) of more than 6 mm. The dose to water obtained from kQx and a 60Co absorbed dose-to-water calibration coefficient ND,wC60o was within 1.5% at depths of 1 to 15 cm with that determined by the AAPM TG-61 for HVL of 2.9 to 10.8 mm Al. This method can reduce the uncertainty of doses in the phantom without any correction using PQ,cham and [(μ‾en/ρ)airw]w, and can directly determine the absorbed dose to water from the ionization chamber reading for medium-energy x-ray beams, including kilovoltage x-ray CT and image-guided cone beam CT in radiotherapy.

MR-guided proton therapy: Impact of magnetic fields on the detector response.

PurposeTo investigate the response of detectors for proton dosimetry in the presence of magnetic fields.Material and MethodsFour ionization chambers (ICs), two thimble‐type and two plane‐parallel‐type, and a diamond detector were investigated. All detectors were irradiated with homogeneous single‐energy‐layer fields, using 252.7 MeV proton beams. A Farmer IC was additionally irradiated in the same geometrical configuration, but with a lower nominal energy of 97.4 MeV. The beams were subjected to magnetic field strengths of 0, 0.25, 0.5, 0.75, and 1 T produced by a research dipole magnet placed at the room’s isocenter. Detectors were positioned at 2 cm water equivalent depth, with their stem perpendicular to both the magnetic field lines and the proton beam’s central axis, in the direction of the Lorentz force. Normality and two sample statistical Student’s t tests were performed to assess the influence of the magnetic field on the detectors’ responses.ResultsFor all detectors, a small but significant magnetic field‐dependent change of their response was found. Observed differences compared to the no magnetic field case ranged from +0.5% to −0.7%. The magnetic field dependence was found to be nonlinear and highest between 0.25 and 0.5 T for 252.7 MeV proton beams. A different variation of the Farmer chamber response with magnetic field strength was observed for irradiations using lower energy (97.4 MeV) protons. The largest magnetic field effects were observed for plane‐parallel ionization chambers.ConclusionSmall magnetic field‐dependent changes in the detector response were identified, which should be corrected for dosimetric applications.