Light yield and field dependence measurement in PandaX-II dual-phase xenon detector

The dual-phase xenon time projection chamber (TPC) is one of the most sensitive detector technology for dark matter direct search, where the energy deposition of incoming particle can be converted into photons and electrons through xenon excitation and ionization. The detector response to signal energy deposition varies significantly with the electric field in liquid xenon. We study the detector's light yield and its dependence on the electric field in the PandaX-II dual-phase detector containing 580 kg liquid xenon in the sensitive volume. From our measurements, the light yield at electric fields from 0 V/cm to 317 V/cm is obtained for energy depositions up to 236 keV.

Primary Ionization Density Produced by Charged Fragments in the Working Volume of the Fission Chambers

The question of the spatial distribution of ion pairs created by 235U fission fragments in the active volume of the fission chamber has been studied. The formulas of the spatial distribution of ion pairs in cylindrical fission chambers are proposed, which allows you to evaluate correctly the density of ion pairs in any point in the sensitive volume of the fission chamber

Assessment of Delivery Quality Assurance for Stereotactic Radiosurgery With Cyberknife

PurposeThe purpose of this study is to establish and assess a practical delivery quality assurance method for stereotactic radiosurgery with Cyberknife by analyzing the geometric and dosimetric accuracies obtained using a PTW31016 PinPoint ionization chamber and EBT3 films. Moreover, this study also explores the relationship between the parameters of plan complexity, target volume, and deliverability parameters and provides a valuable reference for improving plan optimization and validation.MethodsOne hundred fifty cases of delivery quality assurance plans were performed on Cyberknife to assess point dose and planar dose distribution, respectively, using a PTW31016 PinPoint ionization chamber and Gafchromic EBT3 films. The measured chamber doses were compared with the planned mean doses in the sensitive volume of the chamber, and the measured planar doses were compared with the calculated dose distribution using gamma index analysis. The gamma passing rates were evaluated using the criteria of 3%/1 mm and 2%/2 mm. The statistical significance of the correlations between the complexity metrics, target volume, and the gamma passing rate were analyzed using Spearman’s rank correlation coefficient.ResultsFor point dose comparison, the averaged dose differences (± standard deviations) were 1.6 ± 0.73% for all the cases. For planar dose distribution, the mean gamma passing rate for 3%/1 mm, and 2%/2 mm evaluation criteria were 94.26% ± 1.89%, and 93.86% ± 2.16%, respectively. The gamma passing rates were higher than 90% for all the delivery quality assurance plans with the criteria of 3%/1 mm and 2%/2 mm. The difference in point dose was lowly correlated with volume of PTV, number of beams, and treatment time for 150 DQA plans, and highly correlated with volume of PTV for 18 DQA plans of small target. DQA gamma passing rate (2%/2 mm) was a moderate significant correlation for the number of nodes, number of beams and treatment time, and a low correlation with MU.ConclusionPTW31016 PinPoint ionization chamber and EBT3 film can be used for routine Cyberknife delivery quality assurance. The point dose difference should be within 3%. The gamma passing rate should be higher than 90% for the criteria of 3%/1 mm and 2%/2 mm. In addition, the plan complexity and PTV volume were found to have some influence on the plan deliverability.

Isotope Exchange Kinetics in Clay-Water System

<p>Knowledge of reactions at solid/liquid and solid/gas interfaces is of great importance in the study of all adsorption phenomena. Techniques that enable a study of molecules (liquid or gaseous) adsorbed onto a surface may be divided into two categories: (a) those that upset the equilibrium between molecules in the gaseous (or liquid) phase above the solid surface and molecules actually adsorbed onto it, and (b) those that do not. Those techniques that do not disturb this equilibrium will give results that would be expected to have greater reliability than those obtained from techniques that upset this equilibrium (for example by heating or by affecting one component of the equilibrium by titration, precipitation etc.) In an endeavour to study the properties of water adsorbed onto various substances such as clay, wool and textile fibres without affecting the equilibrium the technique of isotopic exchange has been developed. Essentially the procedure is to take a closed adsorber system in equilibrium with a gas (or liquid), part of which is in the sensitive region of a geiger counter, and to add a very small amount of radioactively labelled gas (or liquid) to the system. The adsorber is placed in the bottom of a geiger counter out of the sensitive volume and a known fraction of gas (or liquid) is in the sensitive volume. As the system is at equilibrium there is continuous exchange between the adsorbed molecules on the sample and the molecules in the gaseous (or liquid) state. Thus, when a very small amount, by weight, of the radioactively labelled gas (or liquid) is added to the system, exchange will take place with the non-radioactive molecules adsorbed on the surface of the material under study. Thus radioactivity will be removed from the sensitive volume of the geiger counter and adsorbed onto the surface of the material, and so the specific activity (count rate), as measured with the geiger counter, will drop. The advantage of this technique is that the equilibrium between the adsorbed molecules and the free gas (or liquid) is not disturbed. The actual amount of radioactive material added is so minute that there is no effective change in the concentration of the free gas (or liquid).</p>

Isotope Exchange Kinetics in Clay-Water System

<p>Knowledge of reactions at solid/liquid and solid/gas interfaces is of great importance in the study of all adsorption phenomena. Techniques that enable a study of molecules (liquid or gaseous) adsorbed onto a surface may be divided into two categories: (a) those that upset the equilibrium between molecules in the gaseous (or liquid) phase above the solid surface and molecules actually adsorbed onto it, and (b) those that do not. Those techniques that do not disturb this equilibrium will give results that would be expected to have greater reliability than those obtained from techniques that upset this equilibrium (for example by heating or by affecting one component of the equilibrium by titration, precipitation etc.) In an endeavour to study the properties of water adsorbed onto various substances such as clay, wool and textile fibres without affecting the equilibrium the technique of isotopic exchange has been developed. Essentially the procedure is to take a closed adsorber system in equilibrium with a gas (or liquid), part of which is in the sensitive region of a geiger counter, and to add a very small amount of radioactively labelled gas (or liquid) to the system. The adsorber is placed in the bottom of a geiger counter out of the sensitive volume and a known fraction of gas (or liquid) is in the sensitive volume. As the system is at equilibrium there is continuous exchange between the adsorbed molecules on the sample and the molecules in the gaseous (or liquid) state. Thus, when a very small amount, by weight, of the radioactively labelled gas (or liquid) is added to the system, exchange will take place with the non-radioactive molecules adsorbed on the surface of the material under study. Thus radioactivity will be removed from the sensitive volume of the geiger counter and adsorbed onto the surface of the material, and so the specific activity (count rate), as measured with the geiger counter, will drop. The advantage of this technique is that the equilibrium between the adsorbed molecules and the free gas (or liquid) is not disturbed. The actual amount of radioactive material added is so minute that there is no effective change in the concentration of the free gas (or liquid).</p>

Repeatability and Reproducibility of Microdosimetry With a Mini-TEPC

Experimental microdosimetry measures the energy deposited in a microscopic sensitive volume (SV) by single ionizing particles traversing the SV or passing by. The fundamental advantage of experimental microdosimetry over the computational approach is that the first allows to determine distributions of energy deposition when information on the energy and nature of the charged particles at the point of interest is incomplete or fragmentary. This is almost always the case in radiation protection applications, but discrepancies between the modelled and the actual scenarios should be considered also in radiation therapy. Models for physical reality are always imperfect and rely both on basic input data and on assumptions and simplifications that are necessarily implemented. Furthermore, unintended events due to human errors or machine/system failures can be minimized but cannot be completely avoided.Though in proton radiation therapy (PRT) a constant relative biological effectiveness (RBE) of 1.1 is assumed, there is evidence of an increasing RBE towards the end of the proton penetration depth. Treatment Planning Systems (TPS) that take into account a variable linear energy transfer (LET) or RBE are already available and could be implemented in PRT in the near future. However, while the calculated dose distributions produced by the TPS are routinely verified with ionization chambers as part of the quality assurance program of every radiotherapy center, there is no commercial detector currently available to perform routine verification of the radiation quality, calculated by the TPS through LET or RBE distributions. Verification of calculated LET is required to make sure that a complex robustly optimized plan will be delivered as planned. The scientific community is coming to conclusion that a new domain of Quality Assurance additionally to the physical dose verification is required, and microdosimetry can be the right approach to address that. A first important prerequisite is the repeatability and reproducibility of microdosimetric measurements. This work aims at studying experimentally the repeatability and reproducibility of microdosimetric measurements performed with a miniaturized Tissue Equivalent Proportional Counter (mini-TEPC) in a 62 MeV proton beam. Experiments were carried out within 1 year and without propane gas recharging and by different operators. RBE was also calculated by applying the Loncol’s weighting function r(y) to microdosimetric distributions. Demonstration of reproducibility of measured microdosimetric quantities y¯F, y¯D and RBE10 in 62 MeV proton beam makes this TEPC a possible metrological tool for LET verification in proton therapy. Future characterization will be performed in higher energy proton beams.

A new imaging technology based on Compton X-ray scattering

A feasible implementation of a novel X-ray detector for highly energetic X-ray photons with a large solid angle coverage, optimal for the detection of Compton X-ray scattered photons, is described. The device consists of a 20 cm-thick sensitive volume filled with xenon at atmospheric pressure. When the Compton-scattered photons interact with the xenon, the released photoelectrons create clouds of secondary ionization, which are imaged using the electroluminescence produced in a custom-made multi-hole acrylic structure. Photon-by-photon counting can be achieved by processing the resulting image, taken in a continuous readout mode. Based on Geant4 simulations, by considering a realistic detector design and response, it is shown that photon rates up to at least 1011 photons s−1 on-sample (5 µm water-equivalent cell) can be processed, limited by the spatial diffusion of the photoelectrons in the gas. Illustratively, if making use of the Rose criterion and assuming the dose partitioning theorem, it is shown how such a detector would allow obtaining 3D images of 5 µm-size unstained cells in their native environment in about 24 h, with a resolution of 36 nm.

Towards high spatial resolution tissue-equivalent dosimetry for microbeam radiation therapy using organic semiconductors

Spatially fractionated ultra-high-dose-rate beams used during microbeam radiation therapy (MRT) have been shown to increase the differential response between normal and tumour tissue. Quality assurance of MRT requires a dosimeter that possesses tissue equivalence, high radiation tolerance and spatial resolution. This is currently an unsolved challenge. This work explored the use of a 500 nm thick organic semiconductor for MRT dosimetry on the Imaging and Medical Beamline at the Australian Synchrotron. Three beam filters were used to irradiate the device with peak energies of 48, 76 and 88 keV with respective dose rates of 3668, 500 and 209 Gy s−1. The response of the device stabilized to 30% efficiency after an irradiation dose of 30 kGy, with a 0.5% variation at doses of 35 kGy and higher. The calibration factor after pre-irradiation was determined to be 1.02 ± 0.005 µGy per count across all three X-ray energy spectra, demonstrating the unique advantage of using tissue-equivalent materials for dosimetry. The percentage depth dose curve was within ±5% of the PTW microDiamond detector. The broad beam was fractionated into 50 microbeams (50 µm FHWM and 400 µm centre-to-centre distance). For each beam filter, the FWHMs of all 50 microbeams were measured to be 51 ± 1.4, 53 ± 1.4 and 69 ± 1.9 µm, for the highest to lowest dose rate, respectively. The variation in response suggested the photodetector possessed dose-rate dependence. However, its ability to reconstruct the microbeam profile was affected by the presence of additional dose peaks adjacent to the one generated by the X-ray microbeam. Geant4 simulations proved that the additional peaks were due to optical photons generated in the barrier film coupled to the sensitive volume. The simulations also confirmed that the amplitude of the additional peak in comparison with the microbeam decreased for spectra with lower peak energies, as observed in the experimental data. The material packaging can be optimized during fabrication by solution processing onto a flexible substrate with a non-fluorescent barrier film. With these improvements, organic photodetectors show promising prospects as a cost-effective high spatial resolution tissue-equivalent flexible dosimeter for synchrotron radiation fields.

Towards Absolute Dose Measurement In MRI-LINAC And GAMMA KNIFE®: Design And Construction Of An MR-Compatible Water Calorimeter

The purpose of this work was to design and build a portable 4⁰C stagnant Water Calorimeter (WC) for dual use in MRI-linac and Gamma Knife® in addition to conventional radiotherapy linacs. WC determines radiation energy absorbed in a sensitive volume absolutely and directly through measuring radiation-induced temperature rise (related to the medium’s specific heat capacity). To assist with the design process, several parameters involved in calorimeter tank design including tank dimensions, a variety of insulation material and thicknesses, unique cooling design structures to sustain WC at 4⁰C, as well as the calorimeter vessel design were simulated, and the results on heat gain/loss at the point of measurement was evaluated. Based on the optimizations, a calorimeter tank was built, and one single set of initial measurements in a conventional clinical linear accelerator was performed.