Gas-filled Ionization Chambers Research Articles

First experimental validation of silicon-based sensors for monitoring ultra-high dose rate electron beams

Monitoring Ultra-High Dose Rate (UHDR) beams is one of the multiple challenges posed by the emergent FLASH radiotherapy. Technologies (i.e., gas-filled ionization chambers) nowadays used in conventional radiotherapy are no longer effective when applied to UHDR regimes, due to the recombination effect they are affected by, and the time required to collect charges. Exploiting the expertise in the field of silicon sensors’ applications into clinics, the medical physics group of the University and INFN Torino is investigating thin silicon sensors as possible candidates for UHDR beam monitoring, exploiting their excellent spatial resolution and well-developed technology. Silicon sensors of 30 and 45 µm active thicknesses and 0.25, 1 and 2 mm2 active areas were tested at the SIT ElectronFlash machine (CPFR, Pisa) on 9 MeV electron beams, featuring a pulse duration of 4 µs, a frequency of 1 Hz, and a dose-per-pulse ranging from 1.62 to 10.22 Gy/pulse. The silicon sensors were positioned at the exit of the ElectronFlash applicator, after a solid water build-up slab, and were readout both with an oscilloscope and with a multi-channel front-end readout chip (TERA08). A response linearity extending beyond 10 Gy/pulse was demonstrated by comparison with a reference dosimeter (FlashDiamond), thus fulfilling the first requirement of a potential application in UHDR beam monitoring.

Beam Monitors for Tomorrow: The Challenges of Electron and Photon FLASH RT

The emergent FLASH RadioTherapy (RT) uses ultrahigh dose-rate irradiation (up to 107 Gy/s instantaneous dose-rate in each μs pulse) to deliver a single high dose of irradiation in a very short time (less than 200 milliseconds). Pre-clinical studies at ultrahigh dose-rates recently showed an increased ratio between tumoricidal effect and normal tissue toxicity (therapeutic index), compared to conventional RT at standard Gy/min dose-rates. If confirmed by biological in vivo validations, this could represent a breakthrough in cancer treatment. However, the reliability and the accuracy of experimental studies are nowadays limited by the lack of detectors able to measure online the beam fluence at FLASH dose-rates. The behaviour of standard beam monitors (gas-filled ionization chambers) is compromised by the volume recombination caused by the amount of charges created per unit volume and unit time, due to the large dose-rate. Moreover, due to the lack of proper monitoring devices and to the uncertainties of its future applications, very few facilities are able to deliver at present FLASH irradiations. In this contribution, we report about the physical and technological challenges of monitoring high and ultra-high dose-rates with electrons and photon beams, starting from the pre-clinical and clinical constraints for new devices. Based on the extensive experience in silicon detectors for monitoring applications in RT with external beams, the work then investigates silicon sensors as a possible option to tackle such extreme requirements and a rugged thin and large (e.g. 10×10 cm2) flat detector (silicon-based sensor + readout electronics) is therefore outlined. This study aims at presenting the FLASH-RT dosimetry problem and analysing the possibilities for a silicon sensor to be employed as sensing device for several FLASH scenarios, including some ideas on the readout part. However, more detailed simulations and studies are demanded to delineate more precisely the technical choices to be undertaken in order to tackle the clinical accuracy required on the beam fluence, typically a few %, during photon and electron high and ultra-high irradiations, the required minimal perturbation of the beam and the high level of radiation resistance.

Gas-Filled Ionization Chamber and Secondary-Emission Detector for Measuring γ-Ray Dose Rate

The design of a KGK-2 γ-ray dose rate detector is described. The basic characteristics of the detector are presented. The readings of a KGK-2 gas-filled ionization chamber detector and VED-2 secondary-electron emission detector during power startup of BR-K1 and BR-1M reactors are compared. It is shown that by using KGK-2 and VED-2 detectors in combination the γ-ray dose rate can be measured in the interval 1 μGy/sec to 10 GGy/sec with the linear sections of the regulation curves overlapping to at least four decimal orders.

Modeling ion recombination in liquid ionization chambers - Improvement and analysis of the two-dose-rate method.

The use of liquid ionization chambers can provide useful information to endeavors with radiation dosimetry for highly modulated beams. Liquid ionization chambers may be particularly suitable for computed tomography applications where conventional ionization chambers do not present a high enough sensitivity for the spatial resolution required to characterize common X-ray beams. Due to the sensitivity, which leads to high charge densities, liquid ionization chambers can suffer from large recombination losses leading to degradation in signal to dose rate linearity. To solve this problem, a two-dose-rate method for general recombination correction has been proposed for liquid ionization chambers. However, the valid range of recombination losses that the method can accurately account for has been found to vary depending on radiation quality. The present work provides an in-depth analysis of the performance of the two-dose-rate method. Furthermore, the soundness of applying gas theory to liquids is investigated by using the two-dose-rate method. In the present work, the two-dose-rate method for general recombination correction of liquid ionization chambers used in continuous beams is studied by employing theory for gas-filled ionization chambers. An approximate relation for the general collection efficiency containing a material-specific parameter that is traceable to liquids has been derived for theoretical and experimental investigation alongside existing theory. Furthermore, the disassociation between initial and general recombination in the method is analyzed both theoretically and experimentally. The results indicate that liquids and gases share general recombination characteristics, where the liquids investigated (isooctane and tetramethylsilane) to a large extent mimic the behavior theoretically expected in gases. Furthermore, it is shown that the disassociation between initial and general recombination in the two-dose-rate method is an approximation that depends on the relation between initial recombination and the collecting electric field strength at the dose rates used. Due to the approximation used to separate initial and general recombination the valid range of collection efficiencies for the two-dose-rate method will not only depend on the model used to describe general recombination but also on the type of liquid and radiation beam quality. As there is no robust theory for initial recombination in liquids to apply, the valid range of general collection efficiencies for the two-dose-rate method should be experimentally evaluated for each radiation dosimetry application.

Multi-electrode ionization chamber energy loss calculator (MICE)

A gas-filled ionization chamber is well suited for low energy nuclear physics applications such as measuring beam intensities and detecting heavy recoils from nuclear reactions. Recently developed multi-electrode ionization chamber shows great performance in terms of the energy resolution and the maximum counting rate. It requires, however, iterative energy loss calculations for certain applications mainly due to its complex geometry. For easier energy loss calculations, a stopping power calculation code called the Multi-electrode Ionization Chamber Energy Loss Calculator (MICE) has been developed by utilizing the Java programming language. The development procedure, usage, and application of the MICE are reported.

Derivation of formulas to calculate the saturation correction of ionization chambers in pulsed beams of short, nonvanishing pulse durations.

Gas-filled ionization chambers are the most important radiation detectors in radiotherapy. The collected charge at the electrodes does not represent the total released charge due to the unavoidable recombination processes. This needs to be considered for precise dose measurements. A quantitative description and correction of the recombination effects is established for two cases: continuous radiation exposure and pulsed radiation fields of single pulses with vanishing pulse duration. This work derives formulas for calculating the saturation correction for pulsed beams of nonvanishing pulse duration. Recursive formulas are derived describing the spatio-temporal development of the charge density distributions in plane-parallel ionization chambers starting at neglected recombination. Pulse duration dependent effective chamber parameters are calculated, by comparing the coefficients of a series expansion for small recombination effects. These parameters are used afterward in the known formula for the saturation correction factor for pulsed irradiation with increased recombination effects which was established with the assumption of vanishing pulse duration. The formulas should be valid for pulse durations shorter than half the collection time of the chamber. They allow calculating the saturation correction factor for pulsed beams of nonvanishing pulse duration and arbitrary pulse dose, if chamber, beam, and filling gas parameters are known. The filling gas parameters could be determined from experimental data. The calculation results are in good agreement with already published experimental and simulated results for a Roos chamber. The new formulas can be applied to determine the expected saturation correction for pulsed beams of different pulse duration and pulse dose including new beams at accelerators of new technologies. More experimental validation by using other chambers and an extension of the new formalism to pulse durations longer than half the collection time of the chamber is desired.

Construction and commissioning of a position-sensitive ionization chamber

A position-sensitive ionization chamber has been constructed and commissioned at the Physics Department of Sungkyunkwan University to extract position information on incident charged particles for future nuclear reaction measurements. By utilizing the newly-designed position-sensitive anodes and the previously-commissioned portable gas-filled ionization chamber by Chae et al., position information on incident particles could be obtained. The device was tested with an 241Am α-emitting source, and the standard deviation of the fitted Gaussian distribution was measured to be 1.76 mm when a collimator with a 2 mm hole was used.

Derivation of a formula describing the saturation correction of plane-parallel ionization chambers in pulsed fields with arbitrary repetition rate

Gas-filled ionization chambers are widely used radiation detectors in radiotherapy. A quantitative description and correction of the recombination effects exists for two cases, for continuous radiation exposure and for pulsed radiation fields with short single pulses. This work gives a derivation of a formula for pulsed beams with arbitrary pulse rate for which the prerequisites of the two existing descriptions are not fulfilled. Furthermore, an extension of the validity of the two known cases is investigated.The temporal evolution of idealized charge density distributions within a plane parallel chamber volume is described for pulsed beams of vanishing pulse duration and arbitrary pulse repetition rate. First, the radiation induced release, movement and collection of the charge carriers without recombination are considered. Then, charge recombination is calculated basing on these simplified charge distributions and the time dependent spatial overlap of positive and negative charge carrier distributions. Finally, a formula for the calculation of the saturation correction factor is derived by calculation and simplification of the first two terms of a Taylor expansion for small recombination.The new formula of saturation correction contains the two existing cases, descriptions for exposure by single pulses and continuous irradiation, as limiting cases. Furthermore, it is possible to determine the pulse rate range for which each of the three descriptions is applicable by comparing the dependencies of the new formula with the two existing cases.As long as the time between two pulses is lower than one third of the collection time of the chamber, the formalism for a continuous exposure can be used. The known description for single pulse irradiation is only valid if the repetition rate is less than 1.2 times the inverse collection time. For all other repetition rates in between the new formula should be used. The experimental determination by Jaffe diagrams can be applied for all repetition rates which are not single pulse irradiation but will result in a small deviation from the correct saturation correction value.

The multi-purpose hard X-ray beamline BL10 at the DELTA storage ring.

The layout and the characteristics of the hard X-ray beamline BL10 at the superconducting asymmetric wiggler at the 1.5 GeV Dortmund Electron Accelerator DELTA are described. This beamline is equipped with a Si(111) channel-cut monochromator and is dedicated to X-ray studies in the spectral range from ∼4 keV to ∼16 keV photon energy. There are two different endstations available. While X-ray absorption studies in different detection modes (transmission, fluorescence, reflectivity) can be performed on a designated table, a six-axis kappa diffractometer is installed for X-ray scattering and reflectivity experiments. Different detector set-ups are integrated into the beamline control software, i.e. gas-filled ionization chambers, different photodiodes, as well as a Pilatus 2D-detector are permanently available. The performance of the beamline is illustrated by high-quality X-ray absorption spectra from several reference compounds. First applications include temperature-dependent EXAFS experiments from liquid-nitrogen temperature in a bath cryostat up to ∼660 K by using a dedicated furnace. Besides transmission measurements, fluorescence detection for dilute sample systems as well as surface-sensitive reflection-mode experiments are presented.

Understanding current signals induced by drifting electrons

Consider an electron drifting in a gas toward a collection electrode. A common misconception is that the electron produces a detectable signal only upon arrival at the electrode. In fact, the situation is quite the opposite. The electron induces a detectable current in the electrode as soon as it starts moving through the gas. This induced current vanishes when the electron arrives at the plate. To illustrate this phenomenon experimentally, we use a gas-filled parallel-plate ionization chamber and a collimated 241Am alpha source, which produces a track of a fixed number of ionization electrons at a constant distance from the collection electrode. We find that the detected signal from the ionization chamber grows with the electron drift distance, as predicted by the model of charge induction, and in conflict with the idea that electrons are detectable upon arrival at the collection plate.

Construction of a fast ionization chamber for high-rate particle identification

A new gas-filled ionization chamber for high count rate particle identification has been constructed and commissioned at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL). To enhance the response time of the ionization chamber, a design utilizing a tilted entrance window and tilted electrodes was adopted, which is modified from an original design by Kimura et al. [1]. A maximum counting rate of ~700,000 particles per second has been achieved. The detector has been used for several radioactive beam measurements performed at the HRIBF.

Development of a portable gas-filled ionization chamber

A new portable gas-filled ionization chamber has been designed and constructed at the Physics Department of Sung Kyun Kwan University. To overcome the maximum count rate of ∼105 particles per second of a conventional ionization chamber, which utilizes a Frisch grid, and to enhance the portability of a detector, we adopted the design of multiple electrodes and modified it from the original designs by Kimura et al. and Chae et al. The new design utilizes a stack of multiple electrodes installed perpendicular to the optical beam axis. This configuration provides a fast response time for the detector, which is essential for high-rate counting. The device has been tested with a 241Am (t1/2 = 432.2 years) radioactive α source, which mainly emits 5.486-MeV (branching ratio of 85%) and 5.443-MeV (branching ratio of 13%) α particles. An energy resolution of 6.3% was achieved.

電子に対する制限質量衝突阻止能の計算・検索ソフトウェアの開発

In external radiotherapy, the absorbed doses are measured using an ionization process in a gas-filled ionization chamber and estimated by the extended cavity theory. The calculation requires both the W-value of cavity gas and the restricted mass collision stopping powers (L/ρ) for the gas and medium. ICRU Report 37 gives us the data regarding the mass collision stopping powers (S/ρ) for several elements and chemical compounds or mixtures. However, there are no detailed data for L/ρ. In this study, we developed an in-house program to calculate the L/ρ for arbitrary substances by the use of the equation described in ICRU Report 37. With this program, we can search the calculated L/ρ easily. When we calculated L/ρ for chemical compounds and mixtures with implementation of Bragg's additivity rule, a large error of density effect corrections was observed. Therefore, our program adopted both the mean excitation energy and density effect correction obtained by ESTAR, which was developed by Berger et al. With adoption of these values, the calculation accuracy of our program was improved. Our program is useful to search L/ρ for radiation dosimetry in radiotherapy.

Experimentelle Untersuchung der Sättigungskorrektion einer PTW Roos-Ionisationskammer in gepulsten Strahlungsfeldern mit hoher Pulsdosis bei verschiedenen Pulsdauern

In gas-filled ionization chambers as radiation detectors, the collection of the charge carriers is affected by the recombination effect. In dosimetry this effect must be accounted for by the saturation correction factor k S. The physical description of the correction factor by Boag, Hochhäuser and Balk for pulsed radiation is well established. However, this description is only accurate when the pulse length is short compared to the collection time of the ionization chamber. In this work experimental investigations of the saturation correction factor have been made for pulses of 4 μs up to pulse doses of about 230 mGy, and the theory of Boag, Hochhäuser and Balk was again confirmed. For longer pulses, however, the correction factor decreases and at a pulse duration of about 200 μs reaches 75% of the value valid for short pulses. This reduced influence of the ion recombination is interpreted by the reaction kinetics of ion recombination as a second-order reaction. This effect is negligible for PTW Roos chambers at clinical linear accelerators with 4 μs pulse duration for pulse doses up to 120 mGy.

Standard neutron source based on the F-1 reactor and standard calibration facility – effective tool for determining the sensitivity of gas-filled ionization chambers

Standard neutron source based on the F-1 reactor and standard calibration facility – effective tool for determining the sensitivity of gas-filled ionization chambers

A study of very thin DLC foils as a gas barrier

Measurements of vacuum tightness and mechanical strength of diamond-like carbon (DLC) foils in the thickness range of 1–7 μg cm −2 have been performed with a purpose to evaluate suitability of foils as a gas barrier. Hydrogen and argon at pressures from 10 −2 Pa to 20 kPa were used as test gases. The permeation rate specified as conductance density was found for the best sample of self-supporting foil to be around 1.5×10 −3 l and 3.3×10 −4 l s −1 cm −2 for H 2 and Ar, respectively. Conductance density of the same foils mounted on the frames with a mesh along the apertures as support was about twice higher than that for the self-supporting ones, likely due to the mechanical imperfections of the foil assemblies of the first ones. On the other hand, mesh-supported foils as thin as 3 μg cm −2 and of 5 mm in diameter were withstanding the pressure of up to 18 kPa, while self-supporting foils of the same thickness ruptured at around 1.2 kPa. There was no observed relation between thickness of the foil and its mechanical properties and permeation rate. This suggests that rather tears and pinholes present in foils are the limiting factors of the foil–vacuum tightness and strength. Results obtained in the studies, presented in this work, demonstrate the ability of very thin DLC to isolate a high vacuum beam line from a gas cell in a variety of applications and ability to withstand the gas pressure relevant, in particular, to some gas-filled ionization chambers.

Making MUSIC: A multiple sampling ionization chamber

A multiple sampling ionization chamber (MUSIC) was developed for use in conjunction with the Atlas scattering chamber (ATSCAT). This chamber was developed to study the (α,p) reaction in stable and radioactive beams. The gas filled ionization chamber is used as a target and detector for both particles in the outgoing channel (p+ beam particles for elastic scattering or p+residual nucleus for (α,p) reactions). The MUSIC detector is followed by a Si array to provide a trigger for anode events. The anode events are gated by a gating grid so that only (α,p) reactions where the proton reaches the Si detector result in an anode event.The MUSIC detector is a segmented ionization chamber. The active length of the chamber is 11.95in. and is divided into 16 equal anode segments (3.5in.×0.70in. with 0.3in. spacing between pads). The dead area of the chamber was reduced by the addition of a Delrin snout that extends 0.875in. into the chamber from the front face, to which a mylar window is affixed. 0.5in. above the anode is a Frisch grid that is held at ground potential. 0.5in. above the Frisch grid is a gating grid. The gating grid functions as a drift electron barrier, effectively halting the gathering of signals. Setting two sets of alternating wires at differing potentials creates a lateral electric field which traps the drift electrons, stopping the collection of anode signals. The chamber also has a reinforced mylar exit window separating the Si array from the target gas. This allows protons from the (α,p) reaction to be detected. The detection of these protons opens the gating grid to allow the drift electrons released from the ionizing gas during the (α,p) reaction to reach the anode segment below the reaction.

Traceability on Radon Measurements at the JAEA Ningyo-toge

The study on the establishment and maintenance of the traceability on radon measurements at the Ningyo-toge Environmental Engineering Center of the Japan Atomic Energy Agency (JAEA Ningyo-toge) is illustrated in this paper. The primary standard is a radium solution provided by the National Bureau of Standard (present the National Institute of Standards and Technology) in the USA, and the secondary standard is the method with gas-filled ionization chambers calibrated with the solution. The radon reference chamber is utilized to provide reference atmospheres in calibration experiments for other monitors. Through the intercomparison experiments among the international reference institutes, it was confirmed that the reliability and consistency of the secondary standard of the JAEA Ningyo-toge have been retained since 1984. It shows that the calibration and measurement techniques associated with the traceability system constructed have been maintained well at the JAEA Ningyo-toge. Since there is no reference field as a national standard and no method regulated by the Japanese Industrial Standard, this paper provides not only reliability and accuracy of the radon measurements at the JAEA Ningyo-toge, but also useful information for the standardization of radon measurements in Japan.

ラドン測定におけるガス封入型電離箱法の諸特性について

The radon measurement with a gas-filled ionization chamber is generally performed at many laboratories in Japan as a standard method. In this paper, the characteristics of the method have been examined as follows. The wall effects against the different filled gases, nitrogen and dry air, have been evaluated with Monte Carlo simulation. The columnar recombination has been also evaluated from two different ways; one is the common evaluation from the relationship between inverse of ionization current and inverse of applied voltage; another is the evaluation from the relationship between the ionization current in the different gas pressure and the wall effect expected in each pressure.Both results have been agreed with each other. As the total, the efficiencies of ionization chamber, volume of which is 1.5 L, are estimated to be 0.02706 Bq.fA-1 for nitrogen gas of 93.3 kPa and 0.02701 Bq.fA-1 for air of 101.3 kPa, and the values are very close to the conversion factor on the literatures within 3% of difference. Furthermore the effect by humidity has been preliminarily estimated, and the effect is almost -3% per 1 % of absolute humidity but it can be easily removed by the suitable dryer. These examinations have proven the method with a gas filled ionization chamber has high accuracy for radon measurement. An 241Am disk source was used with same methods to confirm the results above mentioned.

A Channel for Neutron Flux Measurement

The design principles of a channel for measuring fluxes of thermal neutrons in a nuclear-reactor control and protection system are considered. The channel consists of a fission chamber, a set of boron-containing gas-filled ionization chambers, and an electronic unit that ensures a counting rate of fission pulses of up to ∼1 × 106 s–1. It is shown that the widest linear range is achieved in channels equipped with a fission chamber and a gas-filled ionization chamber with electrodes coated with natural boron. The channel allows for measurements of the thermal-neutron flux density in a range of 0.5 to 5 × 1011 cm–2 s–1.