Ionization Chamber Research Articles

Long-term performance monitoring of a-Si 1200 electronic portal imaging device for dosimetric applications.

Recently, dosimetri applications of the electronic portal imaging device (EPID) in radiotherapy have gained popularity. Confidence in the robust and reliable dosimetric performance of EPID detectors is essential for their clinical use. This study aimed to evaluate the dosimetric performance of the a-Si 1200 EPID and assess the long-term stability of its response. Weekly measurements were performed on two clinically used TrueBeam linear accelerators (linacs) equipped with a-Si 1200 EPID detectors over a 2-year period. They included dark and flood calibration fields, and EPID response to an open field corrected for the long-term machine output drift measured with the secondary absolute dosimeters: an ion chamber and an ion chamber array. All measurements were performed using five photon beam energies and two imaging modes: continuous and dosimetry. The measurements were analyzed for constancy and the presence of long-term trends. Comparisons were made between the two linacs for each beam energy. Pixel sensitivity matrices (PSM) were determined semi-annually and analyzed for long-term constancy for both treatment machines. The long-term variation of the dark and flood field signals, integrated across the EPID plane, over the entire observation period did not exceed 0.17% and 0.79%, respectively. The output-corrected EPID response showed long-term variation from 0.28% to 0.36%, depending on beam energy, while the short-term variation was 0.04%-0.07% for EPID and 0.02%-0.06% for secondary dosimeters. The long-term variation of secondary dosimeters was 0.2%-0.3%. PSMs were found to be stable to within 1% for 97.8% of pixels and 2% for 100% of pixels. Techniques to monitor and assess the long-term performance of the a-Si 1200 EPID as a dosimeter were developed and implemented using two TrueBeam linacs. The long-term variation of the EPID response was within clinical tolerance indicated in AAPM TG-142 report, and the detector was shown to be stable and reproducible for routine clinical dosimetry.

8096 Optimization of Radiation Safety Release Instructions After Iodine-131 Treatment for Differentiated Thyroid Cancers

Abstract Disclosure: A.H. Smith: None. T. Greist: None. Introduction: Iodine-131 (I-131) is a common therapy for treatment of differentiated thyroid cancers (DTC); however its radioactivity poses a risk to public health. There is wide inter-facility variation in release instructions to minimize incident exposure to other individuals. Isolation measures to reduce gamma exposure may pose harm to an individual's mental or physical health, especially in new parents or patients who require caregivers. Most studies estimating incident exposure utilize cumulative dosimetry to estimate duration of safety release instructions, but this relies on expert assumptions of the clearance of I-131, which is altered in patients who have had their thyroid removed. Measuring real-time longitudinal gamma exposure would optimize recommendations to balance the radiation safety risks with the risks associated with isolation. Objective: Quantify real-time longitudinal incident gamma radiation exposure to others from patients with DTC status post thyroidectomy treated with I-131. Methods: Patients were asked to measure exposure rates at a distance of one meter three times a day for fourteen days, using a portable ion chamber at their homes. Exposure rates over time were used to form an exponential decay model and to estimate the time after which cumulative exposure is below a reasonably low threshold. Results: 32 patients were included in the study. The average effective half-life was 15.8 hours. Exposure readings over time were best fit with an exponential decay model using one decay rate, rather than a model that uses separate extrathyroidal and thyroidal decay rates. Among patients administered less than 114 mCi, cumulative gamma exposure is less than 100 mR after 24 hours of isolation. Between 114 and 163 mCi, cumulative gamma exposure is less than 100 mR after 48 hours of isolation. Conclusion: Cumulative gamma radiation exposure at one meter was found to be low enough to consider re-evaluating isolation protocols and safety release instructions for long-term distancing in patients status post thyroidectomy treated with I-131. Presentation: 6/1/2024

Dosimetric saturation effect study and correction calculation method of ionization chamber at ultra-high dose rate (FLASH)

Backgroundand Purpose: FLASH radiotherapy has aroused strong interest among researchers, but how to monitor dose in real time and lack of generally accepted ion correction model are one of the challenges. This study is based on previous research work, the dosimetric verification of FLASH ionization chamber was performed using a 200 keV electron beam irradiation platform at an ultra-high dose rate. At the same time, the finite element program is written to analyze and calculate the ion correction factor. In addition, the results are compared with the calculation results of Boag model. MethodsIn this study, the pressure of the sensitive volume in the detector was adjusted to 16 mbar for the purpose of dosimetry of dose rates in excess of 100 Gy/s. In order to monitor the response of the detector, the beam frequency and pulse width were adjusted accordingly. However, due to the saturation effect of the ionization chamber, the processes of electron ion pair drift, attachment, recombination and diffusion in the sensitive volume were modelled on the basis of the relevant physical principles. Finally, the correction factor was calculated by the finite element analysis. ResultsThe experimental results demonstrate that the FLASH ionization chamber is capable of meeting the requirements of dose measurement and beam monitoring of the electron beam at ultra-high dose rates. Furthermore, the analytical model is able to more accurately describe the saturation effect and calculate the correction factor. ConclusionIn this paper, the method of reducing air pressure is employed for the purpose of monitoring the dose of ultra-high dose rate. Simultaneously, the finite element method was employed to analyze the physical process of electron–ion pairs within the chamber and to calculate the ion correction factor analytically. A comparison with the Boag model indicates that the proposed approach is effective. However, the results exhibit a certain degree of divergence from experimental outcomes. This discrepancy may be attributed to the influence of the input parameters, which require further calibration to enhance the accuracy and the robustness of the model.

Overview of the neutron diagnostic systems for the SPARC tokamak.

Neutron measurement is the primary tool in the SPARC tokamak for fusion power (Pfus) monitoring, research on the physics of burning plasmas, validation of the neutronics simulation workflows, and providing feedback for machine protection. A demanding target uncertainty (10% for Pfus) and coverage of a wide dynamic range (>8 orders of magnitude going up to 5 × 1019 n/s), coupled with a fast-track timeline for design and deployment, make the development of the SPARC neutron diagnostics challenging. Four subsystems are under design that exploit the high flux of direct DT and DD plasma neutrons emanating from a shielded opening in a midplane diagnostic port. The systems comprise a set of ∼15 flux monitors, mainly ionization chambers and proportional counters for measurement of the neutron yield rate, two independent foil activation systems for measurement of the neutron fluence, a spectrometric radial neutron camera for poloidal profiling of the plasma emissivity, and a high-resolution magnetic proton recoil spectrometer for measurement of the core neutron spectrum. Together, the four systems ensure redundancy of sensors and methods and aim to provide high resolutions of time (10ms), space (∼7cm), and energy (<2% at 14MeV). This paper presents the broader objectives behind the preliminary design of the SPARC neutron diagnostics and discusses the ongoing studies on neutronics, detector comparisons, prototyping, and integration with the unique infrastructure of SPARC. Engineering details of the four subsystems and the concepts for in situ neutron calibration are also highlighted.

Quantitative spatial visualization of X-ray irradiation via redox reaction by dynamic nuclear polarization magnetic resonance imaging

The dose of X-ray irradiation is commonly measured by point assessment with an ionization chamber dosimeter. However, to achieve spatially accurate delivery of X-ray to avoid the exposure to normal tissues, an accurate imaging method for spatially and quantitatively detecting exposure is required. Herein, we present a novel method to visualize X-ray exposure using low-field dynamic nuclear polarization magnetic resonance imaging (DNP-MRI) with nitroxyl radical tempol as the chemical dosimeter. In this system, gel phantoms containing glutathione (GSH) and the paramagnetic tempol radical were used to monitor the deposited X-ray-irradiation via the redox reaction. The tempol radical level was evaluated by DNP-MRI whose signal intensity was linearly correlated with the radical concentration. The radical level in the presence of GSH decreased in proportion to the dose of X-irradiation deposited. In an imaging experiment simulating clinical radiotherapy, we used a clinical linear accelerator with a radiotherapy planning software to confirm the utility of the exposure imaging. The X-ray exposure and its distribution were clearly visualized on the gel phantom image acquired by DNP-MRI. The results were consistent with those specified in the radiotherapy plan where the intensity of the radiation beam was modulated. This exposure estimation will be useful for determining an accurate irradiation field and reducing off-target exposure in clinical settings.

Validation of MLC leaf open time calculation methods for PSQA in adaptive radiotherapy with tomotherapy units.

Treatment delivery safety and accuracy are essential to control the disease and protect healthy tissues in radiation therapy. For usual treatment, a phantom-based patient specific quality assurance (PSQA) is performed to verify the delivery prior to the treatment. The emergence of adaptive radiation therapy (ART) adds new complexities to PSQA. In fact, organ at risks and target volume re-contouring as well as plan re-optimization and treatment delivery are performed with the patient immobilized on the treatment couch, making phantom-based pretreatment PSQA impractical. In this case, phantomless PSQA tools based on multileaf collimator (MLC) leaf open times (LOTs) verifications provide alternative approaches for the Radixact® treatment units. However, their validity is compromised by the lack of independent and reliable methods for calculating the LOT performed by the MLC during deliveries. To provide independent and reliable methods of LOT calculation for the Radixact® treatment units. Two methods for calculating the LOTs performed by the MLC during deliveries have been implemented. The first method uses the signal recorded by the build-in detector and the second method uses the signal recorded by optical sensors mounted on the MLC. To calibrate the methods to the ground truth, in-phantom ionization chamber LOT measurements have been conducted on a Radixact® treatment unit. The methods were validated by comparing LOT calculations with in-phantom ionization chamber LOT measurements performed on two Radixact® treatment units. The study shows a good agreement between the two LOT calculation methods and the in-phantom ionization chamber measurements. There are no notable differences between the two methods and the same results were observed on the different treatment units. The two implemented methods have the potential to be part of a PSQA solution for ART in tomotherapy.

Dosimetric calibration of anatomy-specific ultra-high dose rate electron irradiation platform for preclinical FLASH radiobiology experiments.

FLASH radiation therapy (RT) offers a promising avenue for the broadening of the therapeutic index. However, to leverage the full potential of FLASH in the clinical setting, an improved understanding of the biological principles involved is critical. This requires the availability of specialized equipment optimized for the delivery of conventional (CONV) and ultra-high dose rate (UHDR) irradiation for preclinical studies. One method to conduct such preclinical radiobiological research involves adapting a clinical linear accelerator configured to deliver both CONV and UHDR irradiation. We characterized the dosimetric properties of a clinical linear accelerator configured to deliver ultra-high dose rate irradiation to two anatomic sites in mice and for cell-culture FLASH radiobiology experiments. Delivered doses of UHDR electron beams were controlled by a microcontroller and relay interfaced with the respiratory gating system. We also produced beam collimators with indexed stereotactic mouse positioning devices to provide anatomically specific preclinical treatments. Treatment delivery was monitored directly with an ionization chamber, and charge measurements were correlated with radiochromic film measurements at the entry surface of the mice. The setup for conventional dose rate irradiation utilized the same collimation system but at increased source-to-surface distance. Monte Carlo simulations and film dosimetry were used to characterize beam properties and dose distributions. The mean electron beam energies before the flattening filter were 18.8 MeV (UHDR) and 17.7 MeV (CONV), with corresponding values at the mouse surface of 17.2 and 16.2 MeV. The charges measured with an external ion chamber were linearly correlated with the mouse entrance dose. The use of relay gating for pulse control initially led to a delivery failure rate of 20% (±1 pulse); adjustments to account for the linac latency improved this rate to<1/20. Beam field sizes for two anatomically specific mouse collimators (4 × 4 cm2 for whole-abdomen and 1.5 × 1.5 cm2 for unilateral lung irradiation) were accurate within<5% and had low radiation leakage (<4%). Normalizing the dose at the center of the mouse (∼0.75cm depth) produced UHDR and CONV doses to the irradiated volumes with>95% agreement. We successfully configured a clinical linear accelerator for increased output and developed a robust preclinical platform for anatomically specific irradiation, with highly accurate and precise temporal and spatial dose delivery, for both CONV and UHDR irradiation applications.

Implementing a Phase II Quality Control Protocol for a High Precision 137Cs Dosimetry Calibration Irradiator.

In medical physics, rigorous quality assurance and quality control protocols are vital for precise dose delivery applications. In many health physics applications, the allowable uncertainty for various processes is often greater than that of medical physics due to looser safety ties. This results in less demand for quality control and uncertainty analyses, since these may not be needed. However, certain applications can benefit from a comprehensive quality control program, as it may yield important insights, such as air kerma monitoring in dosimetry calibrations for environmental and low-dose applications. By implementing a thorough quality control program tailored to specific contexts and needs, uncertainties associated with dose measurements can be quantified with greater accuracy and reliability. This proactive approach not only ensures the integrity of data collected but also enhances understanding of the measured doses. For these reasons, a comprehensive quality control initiative was implemented following documented failures in a 137Cs dosimetry calibration irradiator. This involved systematic charge collection using NIST-traceable ion chambers to observe long-term changes. A Phase I quality control protocol was previously implemented, which employed Shewhart control charts and Nelson's rules to analyze various datasets subgrouped under different conditions. This study addresses the development of a Phase II protocol, which focuses more on uncertainty quantification of systematic errors and irradiator changes, and air kerma precision for dosimetry calibrations. A designed experiment was performed to identify how much systematic errors influence the air kerma. Emphasis was placed on stricter quality assurance protocols, continuous data collection, and additional control charts to monitor short-term changes, such as exponentially weighted moving average control charts. A pre-irradiation control process was implemented to verify that the total air kerma met the measurement quality objective and to show how various uncertainties were applied before calibration. This study indicates how uncertainty is applied given observed air kerma measurements from the irradiator. Ongoing efforts aim to streamline the quality control procedure, ensure consistent data collection, and assess its impact on dosimetry applications.

Irradiation system for biological experiments on BL14B1 at SPring-8

We present an irradiation system for biological experiments on BL14B1 beam line of SPring-8, where enables to use white and monochromatic X-rays (from 5 to 90 keV). An effective energy of white X-rays is assessed as 120 keV and their average dose rate is determined as approximately 170 Gy/s using a l-alanine dosimeter. Also, we speak about dosimetry of monochromatic X-rays using an optical stimulated luminescence detector. Average dose rates of monochromatic X-rays with 33.0, 33.2, 33.4 and 34.0 keV are estimated as 0.099 ± 0.006 Gy/s. The irradiation dose of white and monochromatic X-rays, of course, can be monitored using ionization chamber. When we use monochromatic X-rays with 33.2 keV, where is the K-shell absorption edge of iodine, Auger electrons are emitted, leading significant improvement of biological effectiveness. Thus, we have estimated the absorbed dose low-energy secondaries, such as Auger electrons and characteristic X-rays, emitted from iodine as 1.3 × 10−2 Gy/mM/min at BL14B1, depending on iodine concentration.

Cathode-less RF plasma thruster design and optimisation for an atmosphere-breathing electric propulsion (ABEP) system

Atmosphere-breathing electric propulsion (ABEP) is a concept that ingests residual atmospheric gases as a source of propellant for an electric thruster, removing the need for onboard propellant storage. This would enable continuous low-thrust drag compensation, extending the lifetime of spacecraft in Very-Low Earth Orbit (VLEO); <250 km. VLEO is an appealing region for spacecraft operations, enabling new remote sensing missions with improved radiometric performance and spatial resolution, whilst reducing size, mass and power requirements, as well as mission cost. A preliminary design review and optimisation is therefore conducted for an ABEP system that uses the cathode-less radio frequency (RF) plasma thruster from Technology for Innovation & Propulsion (T4i) S.p.A. This removes the issue of thruster erosion by means of magnetic confinement and offers reduced susceptibility to varying atmospheric composition. A semi-empirical oxygen-nitrogen global source model (GSM) has been developed which considers the volume-averaged flux, momentum, and energy balance of the RF discharge. This includes a detailed chemistry model for the complex electron-molecular reactions and energy-loss channels of air plasma in the ionisation chamber. The GSM is coupled to an analytical model of flux balance for an air intake, verified by Direct Simulation Monte-Carlo (DSMC) simulation, to consider its design for maximum collection efficiency. This is then utilised in a robust multi-objective optimisation of the ABEP system, accounting also for spacecraft aerodynamics and power requirements.

Evaluation of the detector linearity response of the DR system in different x-ray bean filters

In the recent past, at Brazilian radiology services, direct digital radiography (DR) systems are replacing Computed Radiography (CR). This situation provides several significant advantages such as improving digital image quality, patient effective dose reduction, and improved speed of examinations performed. To evaluate DR systems, various organizations have published protocols with guidelines concerning quality assurance and acceptance tests for DRs. In these protocols, there are methodologies for evaluating several parameters, such as the detector linearity response (DLR). To test the DLR, different methodologies are proposed by the organizations, such as the application of different additional filters on the X-ray beams spectrum to promote the X-ray doses suitable to produce the latent image in the DR. The objective of this work was to evaluate the results of different protocol methodologies for carrying out the DLR test of a DR flat panel. An analogic radiology equipment, a primary ionization chamber, a DR system and images unprocessed were used in this study. As indicated by the protocols, the spectra of the X-rays were modified by additional filters of copper (Cu) with aluminum (Al); Al; Cu; and polymethylmethacrylate (PMMA). The spectra of the X-rays were modified to achieve the exposure range of 0.1 to 7 mR. The results showed that the DLR curves adjustment were R²&gt;0.99, for a logarithmic equation, type y=a.ln(x)+b. For the filters of 0.5 mmCu with 1.0 mmAl; 11.5 mmAl; 1.0 mmCu; 21 mmAl the mean pixel values showed 0.9% of the coefficient of variation (COV), while for 10 cm PMMA filters, the COV increased to 2.7%. The study showed that methodologies using Al, Cu and Al with Cu filters DLR have similar responses while using PMMA, the response was slightly different.

Development of novel ionization chambers for reference dosimetry in electron flash radiotherapy

AbstractBackgroundReference dosimetry in ultra‐high dose rate (UHDR) beamlines is significantly hindered by limitations in conventional ionization chamber design. In particular, conventional chambers suffer from severe charge collection efficiency (CCE) degradation in high dose per pulse (DPP) beams.PurposeThe aim of this study was to optimize the design and performance of parallel plate ion chambers for use in UHDR dosimetry applications, and evaluate their potential as reference class chambers for calibration purposes. Three chamber designs were produced to determine the influence of the ion chamber response on electrode separation, field strength, and collection volume on the ion chamber response under UHDR and ultra‐high dose per pulse (UHDPP) conditions.MethodsThree chambers were designed and produced: the A11‐VAR (0.2–1.0 mm electrode gap, 20 mm diameter collector), the A11‐TPP (0.3 mm electrode gap, 20 mm diameter collector), and the A30 (0.3 mm electrode gap, 5.4 mm diameter collector). The chambers underwent full characterization using an UHDR 9 MeV electron beam with individually varied beam parameters of pulse repetition frequency (PRF, 10–120 Hz), pulse width (PW, 0.5–4 µs), and pulse amplitude (0.01–9 Gy/pulse). The response of the ion chambers was evaluated as a function of the DPP, PRF, PW, dose rate, electric field strength, and electrode gap.ResultsThe chamber response was found to be dependent on DPP and PW, and these dependencies were mitigated with larger electric field strengths and smaller electrode spacing. At a constant electric field strength, we measured a larger CCE as a function of DPP for ion chambers with a smaller electrode gap in the A11‐VAR. For ion chambers with identical electrode gap (A11‐TPP and A30), higher electric field strengths were found to yield better CCE at higher DPP. A PW dependence was observed at low electric field strengths (500 V/mm) for DPP values ranging from 1 to 5 Gy at PWs ranging from 0.5 to 4 µs, but at electric field strengths of 1000 V/mm and higher, these effects become negligible.ConclusionThis study confirmed that the CCE of ion chambers depends strongly on the electrode spacing and the electric field strength, and also on the DPP and the PW of the UHDR beam. A significant finding of this study is that although chamber performance does depend on PW, the effect on the CCE becomes negligible with reduced electrode spacing and increased electric field. A CCE of ≥95% was achieved for DPPs of up to 5 Gy with no observable dependence on PW using the A30 chamber, while still achieving an acceptable performance in conventional dose rate beams, opening up the possibility for this type of chamber to be used as a reference class chamber for calibration purposes of electron FLASH beamlines.

Simulation of gamma reference field at Institute for Nuclear Science and Technology using Monte Carlo method

This paper presents the characterization of two gamma reference fields of 137Cs and 60Co sources at the Institute for Nuclear Science and Technology. The characterization of the fields in terms of gamma fluence Φ, mean energy, EΦ, kerma weighted mean energies, Eₖ, air kerma, Eair, were determined at various distances from the source center by Monte Carlo simulation using MCNP6. The air kerma results were compared with the measurements obtained by using a calibrated ionization chamber. The discrepancy between the simulated and measured air kerma was less than 4.5%. The scatterred component was also simulated and calculated. The results of both methods showed that the contribution of the scattered components to the gamma reference field is less than 3%. This contribution comply with the international standard criteria of ISO 4037 (&lt;5%). The results confirmed that the characterization of the gamma reference field could be determined using the simulation code.

Real-time delivered dose assessment in carbon ion therapy of moving targets

Abstract Purpose:&amp;#xD;Real-time adaptive particle therapy is being investigated as a means to maximize the treatment delivery accuracy. To react to dosimetric errors, a system for fast and reliable verification of the agreement between planned and delivered doses is essential. This study presents a clinically feasible, real-time 4D-dose reconstruction system, synchronized with the treatment delivery and motion of the patient, which can provide the necessary feedback on the quality of the delivery.&amp;#xD;Methods:&amp;#xD;A GPU-based analytical dose engine capable of millisecond dose calculation for carbon ion therapy has been developed and interfaced with the next generation of the Dose Delivery System (DDS) in use at Centro Nazionale di Adroterapia Oncologica (CNAO). The system receives the spot parameters and the motion information of the patient during the treatment and performs the reconstruction of the planned and delivered 4Ddoses. After each iso-energy layer, the results are displayed on a graphical user interface by the end of the spill pause of the synchrotron, permitting verification against the reference dose.&amp;#xD;The framework has been verified experimentally at CNAO for a lung cancer case based on a virtual phantom 4DCT. The patient’s motion was mimicked by a moving Ionization Chambers (ICs) 2D-array.&amp;#xD;Results:&amp;#xD;For the investigated static and 4D-optimized treatment delivery cases, real time dose reconstruction was-achieved with an average pencil beam dose calculation speed up to more than one order of magnitude smaller than the spot delivery. The reconstructed doses have been benchmarked against offline log-file based dose reconstruction with the TRiP98 treatment planning system, as well as QA measurements with the ICs 2D-array, where an average gamma-index passing rate (3%/3mm) of 99.8% and 98.3%, respectively, were achieved.&amp;#xD;Conclusion:&amp;#xD;This work provides the first real-time 4D-dose reconstruction engine for carbon ion therapy. The framework integration with the CNAO DDS paves the way for a swift transition to the clinics.&amp;#xD;

A Novel Shielding Device for Cardiac Cath Labs

Abstract This research evaluates the effectiveness of a large specialized cardiac catheterization laboratory shielding device (SCCLSD) placed perpendicular to the patient compared to traditional shielding methods in reducing occupational exposure to scattered x rays, contributing to the ongoing enhancement of radiation safety in the cardiac catheterization laboratory (CCL) setting. An experimental setup involving an anthropomorphic phantom on the catheterization table simulated radiation scatter from a patient. Measurements were taken systematically at various grid points and heights in the CCL using a Fluke 451P ion chamber while mimicking a real interventional scenario. In-air peak exposure rates were analyzed at head, chest, and waist heights in the anteroposterior (AP) position. Results demonstrated that the SCCLSD provided a superior radiation shadow and effective whole-body radiation exposure reduction compared to conventional shielding devices. Considering that conventional shielding requires staff to wear lead aprons, an effective dose equivalent correction factor was applied for exposure measurements without the SCCLSD. Even after the correction factor, the SCCLSD continued outperforming lead aprons and offered whole-body protection, including the head and arms, which is typically neglected with conventional shielding. The SCCLSD also reduces exposure to the eyes, aligning with lower occupational exposure recommendations from ICRP and NCRP. However, proper CCL staff positioning is important in maximizing the effectiveness of the SCCLSD. Future research avenues may explore exposure rates at different C-arm angles to more completely assess the SCCLSD’s impact on occupational exposure.

Long-term stability of a PTW 34070 large-area parallel ionization chamber in clinical proton scanning beams.

In the modeling of beam data for proton therapy planning systems, absolute dose measurements are performed utilizing a Bragg peak chamber (BPC), which is a parallel-plate ionization chamber. The long-term stability of the BPC is crucial for ensuring accurate absolute dose measurement. The study aims to assess the long-term stability of the BPC in clinical proton pencil beam scanning delivery. The long-term stability evaluation focused on the BPC-Type 34070 (PTW Freiburg, Germany), utilizing clinical proton scanning beams from December 2022 to November 2023. Monthly investigations were conducted to evaluate the response and cross-calibration factor of the BPC and a reference chamber, employing the spread-out Bragg peak (SOBP) field. Additionally, assessments were made regarding the BPC's response to monoenergetic beams, along with an examination of the impact of polarity and ion recombination on the BPC. The response and cross-calibration factor of the BPC varied up to 1.9% and 1.8%, respectively, while the response of the reference chamber remained within a 0.5% range. The BPC's response to the mono-energetic beams varied up to 2.0% across all energies, demonstrating similar variation trends in both the SOBP field and mono-energetic beams. Furthermore, the variations in polarity and ion recombination effect remained stable within a 0.4% range throughout the year. Notably, the reproducibility of the BPC remained high for each measurement conducted, whether for the SOBP field or mono-energetic beams, with a maximum deviation observed at 0.1%. The response and cross-calibration factor of the BPC demonstrated significant variations, with maximum changes of 1.9% and 1.8%, respectively. However, the reproducibility of the BPC remained consistently high for each measurement. It is recommended that when conducting absolute dose measurements using a BPC, its response should be compared and corrected against the reference chamber for each measurement.

Development and Testing of a Helicon Plasma Thruster Based on a Magnetically Enhanced Inductively Coupled Plasma Reactor Operating in a Multi-Mode Regime

A disruptive Electric Propulsion system is proposed for next-generation Low-Earth-Orbit (LEO) small satellite constellations, utilizing an RF-powered Helicon Plasma Thruster (HPT). This system is built around a Magnetically Enhanced Inductively Coupled Plasma (MEICP) reactor, which enables acceleration of quasi-neutral plasma through a magnetic nozzle. The MEICP reactor features an innovative design with a multi-dipole magnetic confinement system, generated by neodymium iron boron (NdFeB) permanent magnets, combined with an azimuthally asymmetric half-wavelength right (HWRH) antenna and a variable-section ionization chamber. The plasma reactor is followed by a solenoid-free magnetic nozzle (MN), which facilitates the formation of an ambipolar potential drop, enabling the conversion of electron thermal energy into ion beam energy. This study explores the impact of an inhomogeneous magnetic field on the heating mechanism of the HPT and highlights its multi-mode operation within a pulsed power range of 200 to 500 W of RF. The discharge state, characterized by high-energy electron-excited ions and low-energy excited neutral particles in the plasma plume, was analyzed using optical emission spectroscopy (OES). The experimental testing campaign, conducted under pulsed power excitation, reveals that, as RF input power increases, the MEICP reactor transitions from inductive (H-mode) to wave coupling (W-mode) discharge modes. Spectrograms, electron temperature, and plasma density measurements were obtained for the Helicon Plasma Thruster within its operational envelope. Based on OES data, the ideal specific impulse was estimated to exceed 1000 s, highlighting the significant potential of this technology for future LEO/VLEO space missions.

Investigating the relationships between radon and the geology of the Sterkfontein cave

Radon, a naturally occurring radioactive gas, has become a subject of increasing interest and concern, particularly in the context of subterranean environments such as caves. This research investigates these dynamics in the Sterkfontein Cave in South Africa, which has formed in the karst geology of the Cradle of Humankind, Gauteng. Additionally, it set out to compile a radon map for the cave, identifying potential radon hotspots. Twenty-four electret ion chambers were placed in the tourist section of the cave and left for a period of 24 h. The radon concentrations were found to be between 53 Bq/m3 and 2770 Bq/m3. Three regions within the cave exhibited elevated radon concentrations, with these occurrences being linked to phosphatic deposits. A subterranean lake also concentrates radon gas in the lower areas of the cave. While the cave's average radon concentration of 427 Bq/m3 exceeds the World Health Organization's (WHO) hazardous level of 300 Bq/m3, occupational exposure remains minimal during a typical cave tour. Consequently, there is no discernible risk during an average tour through the cave.

Technical Note: Investigating of dosimetric leaf gap and leaf transmission factor variations across gantry and collimator angles in volumetric modulation arc therapy.

This study investigates the influence of gantry and collimator angles on the dosimetric leaf gap (DLG) and leaf transmission factor (LTF) in a Varian LINAC equipped with rounded-end multi-leaf collimators (MLCs). While Varian guidelines recommend DLG measurements at zero degrees for both gantry and collimator, this research aims to address the knowledge gap by assessing DLG and LTF variations at different gantry and collimator angles. Measurements were conducted using a Varian TrueBeam LINAC with a Millennium 120-leaf MLC and Eclipse TPS version 16.1. The beams utilized in this study had energies of 6 MV, 10 MV, 6 FFF, and 10 FFF. LTF and DLG were determined using ionization chambers in solid water phantoms at various gantry angles (0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°). For each gantry angle, measurements were also taken at various collimator angles (0°, 45°, 90°, and 315°). Dosimetric impacts were evaluated through VMAT Picket Fence tests and patient-specific verification using portal dosimetry for 10 clinical VMAT plans. LTF values showed no significant variation across gantry and collimator angles. However, DLG values exhibited notable differences depending on the gantry angle and were independent of the collimator angle. The highest DLG value was observed at a gantry angle of 270 degrees, while the lowest was at 90 degrees. The AXB DLGAverage (averaging seven measurements of DLGs at different gantry angles) model demonstrated the best agreement between measured and calculated dose distributions, indicating the importance of considering averaged DLG values across multiple gantry angles for accurate dose calculations. Our study highlights the variability of DLG with gantry angle alterations, contrary to Varian guidelines recommending DLG measurements at zero gantry angle only. We advocate for utilizing an averaged DLG value from measurements across multiple gantry angles, as outlined in our methodology.

Angular distribution measurements of neutron elastic scattering on natural carbon

The angular distributions of neutron elastic scattering on natural carbon were studied in the fast neutron energy region between 1 and 8 MeV. The experiments were carried out at the white neutron source of the Geel Electron Linear Accelerator (GELINA) facility by using thin and thick natural carbon samples. This work demonstrates the need for using thin samples to avoid strong multiple scattering effects. Neutrons and γ rays from scattering were detected using the ELastic and Inelastic Scattering Array (ELISA), a setup consisting of 32 liquid organic scintillators. The n−γ separation was achieved via pulse-shape analysis. For each sample a different approach in methodology is studied, one based on a global response function model and another one based on a per-detector model. The detectors are placed at eight different detection angles between 16∘ and 164∘ with respect to the neutron beam direction, allowing the simultaneous calculation of both the differential and the integral cross section by implementing the Gauss-Legendre quadrature rule. The neutron flux was measured with a U235 ionization chamber. The angular distributions were extracted relative to the U235(n,f) cross section. The results are compared with other experimental data available in the EXFOR library, along with the most recent nuclear data evaluations. The angle-integrated cross sections are in excellent agreement with the nuclear data evaluations and for the angular distributions, ENDF/B-VIII.0 is better reproducing the experimental data in all eight detection angles. Published by the American Physical Society 2024