Spherical Ionization Chambers Research Articles

Experimental and Monte Carlo Evaluation of Spherical Ionization Chambers’ Response to 137Cs Radioactive Source

Measurements were performed for the calibration of an ionization chamber in the secondary standard dosimetry laboratory of IFIN-HH. A 10-liter spherical ionization chamber was calibrated using a 1-literspherical reference standard ionization chamber. The ionization chamber’s response to 137Cs was also evaluated computationally by MCNP and FLUKA Monte Carlo codes. The models of the experimental setup are validated and can be used for further studies, especially in cases where the real experiment is hard or impossible to perform

The NMIJ air kerma primary standard for high energy x-ray beams in 300–450 kV

Accurate radiation dosimetry is required for radiation protection in various environments. Therefore, dosemeters and dose-rate meters must be calibrated in standard radiation fields. The National Metrology Institute of Japan (NMIJ) expands the energy range of x-ray reference field measurement up to 450 kV using a cylindrical graphite-walled cavity ionization chamber. Departure from the condition of the Spencer-Attix cavity theory was evaluated by comparing the measurement results obtained using the cavity ionization and the free-air ionization chambers, which are used as the primary standard up to a tube voltage of 250 kV. The calibration coefficients found using the spherical ionization chamber were in good agreement with those obtained by the free-air ionization chamber within relative standard uncertainties (k = 1) for N-200 and N-250 x-ray fields. Consistent calibration coefficients were obtained in the energy range 300–450 kV.

Technical Note: Self-shielding evaluation and radiation leakage measurement of a jawless ring gantry linac with a beam stopper.

To characterize the shielding design and leakage radiation from a newly released ring gantry linac (Halcyon, Varian Medical Systems). To assess the radiation leakage surrounding headshield and the radiation level after the beam stopper, measurements were made with GafChromic films. To evaluate the in-room radiation levels, the radiation leakage in the isocenter plane was measured with a large volume spherical ionization chamber (Exradin A6, Standard Imaging). A lead enclosure was constructed to shield the chamber from the low energy scatter radiation from the room. The radiation level at multiple locations was measured with the MLC fully closed and gantry at 0, 45, 90, 135, 180, 225, 270, and 315 degrees. The leakage radiation passing through multiple concrete slabs with various thickness was recorded in a narrow beam geometry to determine the tenth value layer (TVL). A uniform leakage (<0.05%) at 1 m from electron beam line was measured surrounding the linac head with the maximum leakage measured at the top of the head enclosure. The highest radiation level (<0.08%) was measured near the edge of the beam stopper when projected to the measurement plane. The maximum radiation levels due to the head leakage at 15 locations inside the treatment room were recorded and a radiation map was plotted. The maximum leakage was measured at points that along the electron beam line while the gantry at 90 or 270 degree and at the end of head enclosure (0.314%, 0.4m from electron beamline). The leakage TVL value is found to be 226mm in a narrow beam geometry with the concrete density of 2.16g/cm3 or 134.6lb/cu.ft. An overall uniform leakage was measured surrounding linac head. The beam stopper shields the primary radiation with the highest valued measured near the edge of beam stopper. The leakage TVL values are derived and less than the values reported for conventional C-arm linac.

DEVELOPMENT OF A NEW SECONDARY STANDARD FOR H*(10).

A secondary standard for ambient dose equivalent, H*(10), is necessary for the dissemination of the unit Sievert (Sv), but there is no such standard commercially available currently. Furthermore, the measurement of H*(10) instead of calculating H*(10) from air kerma and conversion coefficients is needed for unknown radiation fields. We developed a prototype of a new secondary standard for H*(10) based on a spherical 1l ionization chamber for air kerma. This chamber was modified with copper wires at the inner surface to adjust the response of the chamber according to H*(10). Additionally, a Makrolon shell and an aluminium coating were added to optimize the response at energies below 50keV. The prototype fulfils the requirements given in ISO 4037-2 in the energy range from 12keV to 7MeV. In combination with an electrometer, it can be used as area dosemeter, suitable for pulsed fields and for low energy radiation.

Shutdown dose rate measurements after the 2016 Deuterium-Deuterium campaign at JET

The EUROfusion Work Package JET3 programme, established to enable the technological exploitation of the JET experiments over the next years, includes, within the NEXP subproject, a novel Shutdown Dose Rate (SDR) experiment. Considering its ITER-relevance, SDR experiments at JET represent a unique opportunity to validate the numerical tools for ITER nuclear analysis, through the comparison between numerical predictions and measured quantities (C/E). Within this framework, two active gamma dosimeters based on spherical air-vented ionization chambers (ICs) have been installed in ex-vessel positions close to the horizontal ports of the tokamak in Octants 1 and 2. The first JET campaign exploited in the novel SDR experiment is the latest 5-week Deuterium-Deuterium campaign (c36b), which achieved the best results in recent years in terms of high power operation. The present work is dedicated to the analysis of dose rate measurements carried out during this campaign and after shutdown. Proper correction factors are evaluated and applied to the instrument reading, while influence quantities and error sources are analyzed in order to calculate the overall experimental uncertainty.

Characterization of the energy spectrum of a 137Cs irradiator through measurements using a pulse-mode detector

137Cs irradiators are used for many different applications including food processing, radiobiological experiments, and calibrations of radiation detectors at standards labs. Air-kerma modulation techniques including varying source size, adding attenuation, or varying source-to-detector distance, combined with the geometry of the collimation in the irradiator, may change the energy spectrum of the irradiator. This is unaccounted for during beam characterization, which is completed using a spherical ionization chamber with a minimal energy dependence in the 662 keV 137Cs gamma-ray emission region. This work incorporated a high-purity germanium (HPGe) detector collimated using a lead aperture to measure the energy spectrum of a 5 Ci source in a Hopewell Designs G-10 dual source irradiator with 4.08 cm and 5.54 cm of attenuating material. The measured pulse-height distributions (PHDs) were compared with simulations in MCNP6 to provide validation of the geometry of the irradiator model. PHD measurements were corrected for detector response using MCNP and the true spectrum was obtained using Gold's deconvolution method. Due to the fluence limitations and collimation of the HPGe detector, measurements represented the primary photon energy spectrum. The impact of scattered photons on the energy spectrum was investigated using MCNP6 simulations. In addition, simulations were run to determine the effects of attenuation and source-to-detector distance (SDD) on the energy spectrum. Comparisons of spectra were made in terms of mean energy. Simulated and measured PHDs showed good agreement when comparing spectral features including FWHM of the 662 keV photopeak, mean energy, and the total area under the curve. After deconvolution of the PHDs, the primary energy spectrum mimicked an encapsulated 137Cs source with emission of only the 662 keV photopeak. For 5.54 cm of attenuation, simulations showed that if scattered photons were taken into account, the average energy decreased from 662 keV to 656 keV, which was a 2% difference. Simulations showed that as attenuation increased from 0 cm to 5.54 cm, the mean energy of the spectrum increased 9% from 605 keV to 656 keV. The SDD did not affect the mean energy. This work used spectrum measurements to validate a 137Cs irradiator model in MCNP6, which represents a method that is more sensitive to geometry errors than conventional PDD and profile comparisons. Using the validated model, simulations were run to improve the understanding of the output of the Hopewell G-10 137Cs irradiator.

Reference air kerma rate calibration system for high dose rate Ir-192 brachytherapy sources in Taiwan

Ir-192 sources are widely used in brachytherapy and the number of treatments is around seven thousand for the use of the high dose rate (HDR) Ir-192 brachytherapy source per year in Taiwan. Due to its physical half-life of 73.8 days, the source should be replaced four times per year to maintain the HDR treatment mode (DDEP, 2005; Coursey et al., 1992). When doing this work, it must perform the source dose trace to assure the dose accuracy. To establish the primary measurement standard of reference air kerma rate(RAKR) for the HDR Ir-192 brachytherapy sources in Taiwan, the Institute of Nuclear Energy Research (INER) fabricated a dual spherical graphite-walled cavity ionization chambers system to directly measure the RAKR of the Ir-192 brachytherapy source.In this system, the ion-charge was accumulated by the two ionization chambers and after correction for the ion recombination, temperature, atmosphere pressure, room scattering, graphite-wall attenuation, air attenuation, source decay, stem effect, and so on. The RAKR of the Ir-192 source was obtained in the ambient conditions of 22°C and one atmosphere. The measurement uncertainty of the system was around 0.92% in 96% confidence level (k=2.0). To verify the accuracy of the result, the source calibration comparison has been made at the National Radiation Standard Laboratory (NRSL) of INER and Physikalisch-Technische Bundesanstalt (PTB, Germany) in 2015. The ratio of the measurement results between INER and PTB, INER/PTB, was 0.998±0.027 (k=2) which showed good consistency and the performance of the system was verified.

The preparation of the Shutdown Dose Rate experiment for the next JET Deuterium-Tritium campaign

The assessment of the Shutdown Dose Rate (SDR) due to neutron activation is a major safety issue for fusion devices and in the last decade several benchmark experiments have been conducted at JET during Deuterium-Deuterium experiments for the validation of the numerical tools used in ITER nuclear analyses. The future Deuterium-Tritium campaign at JET (DTE2) will provide a unique opportunity to validate the codes under ITER-relevant conditions through the comparison between numerical predictions and measured quantities (C/E). For this purpose, a novel SDR experiment, described in the present work, is in preparation in the frame of the WPJET3-NEXP subproject within EUROfusion Consortium. The experimental setup has been accurately designed to reduce measurement uncertainties; spherical air-vented ionization chambers (ICs) will be used for on-line ex-vessel decay gamma dose measurements during JET shutdown following DT operations and activation foils have been selected for measuring the neutron fluence near ICs during operations. Active dosimeters (based on ICs) have been calibrated over a broad energy range (from about 30keV to 1.3 MeV) with X and gamma reference beam qualities. Neutron irradiation tests confirmed the capability of active dosimeters of performing on-line decay gamma dose rate measurements, to follow gamma dose decay at the end of neutron irradiation as well as insignificant activation of the ICs.

A new approach for the evaluation of the effective electrode spacing in spherical ion chambers

Proper determination of the effective electrode spacing (deff) of an ion chamber ensures proper determination of its collection efficiency either in continuous or in pulsed radiation in addition to the proper evaluation of the transit time. Boag's method for the determination of deff assumes the spherical shape of the internal electrode of the spherical ion chambers which is not always true, except for some cases, its common shape is cylindrical. Current work provides a new approach for the evaluation of the effective electrode spacing in spherical ion chambers considering the cylindrical shape of the internal electrode. Results indicated that deff values obtained through current work are less than those obtained using Boag's method by factors ranging from 12.1% to 26.9%. Current method also impacts the numerically evaluated collection efficiency (f) where values obtained differ by factors up to 3% at low potential (V) values while at high V values minor differences were noticed. Additionally, impacts on the evaluation of the transit time (τi) were obtained. It is concluded that approximating the internal electrode as a sphere may result in false values of deff, f, and τi.

Efficiencies of Some Spherical Ion Chambers in Continuous and Pulsed Radiation: A Numerical Evaluation.

SummaryBackgroundEvaluation of the collection efficiencies of ion chambers is a necessity for the proper evaluation of radiation quantities in different applications. Overall collection efficiency is the product of three different values: collection efficiencies considering contributions of, volume recombination, back-diffusion loss and initial recombination, the later may be neglected at low charge rates.Material/MethodsFive common spherical ion chambers of different volumes and specifications were included in this study for the evaluation of volume recombination collection efficiency and back diffusion collection efficiency for continuous and pulsed radiation and at different values of the applied polarizing potential. Through current work there is an attempt of focusing on how the selection of ion chamber dimensions may affect the overall collection efficiency in addition to the proper selection of other influencing parameters.ResultsCollection efficiencies considering volume recombination (fv) for five spherical ion chambers of common types were evaluated for continuous and pulsed radiation over a wide range of polarizing potential. The relation between the ion chamber volume and its evaluated collection efficiencies were studied for both continuous and pulsed radiation; transit time values for the ion chambers included in this study were evaluated at different values of applied potential. Also, collection efficiencies considering diffusion current values (fd) were evaluated for the five chambers, and plotted versus the applied polarizing potential.ConclusionsThrough this study it was feasible to evaluate numerically the collection efficiencies of some spherical ion chambers considering volume recombination and back diffusion effects; the affecting parameters were studied and highlighted.

TU‐F‐CAMPUS‐T‐01: Dose and Energy Spectra From Neutron Induced Radioactivity in Medical Linear Accelerators Following High Energy Total Body Irradiation

Purpose:To assess the risk posed by neutron induced activation of components in medical linear accelerators (linacs) following the delivery of high monitor unit 18 MV photon beams such as used in TBI.Methods:Gamma spectroscopy was used to identify radioisotopes produced in components of a Varian 21EX and an Elekta Synergy following delivery of photon beams. Dose and risk estimates for TBI were assessed using dose deliveries from an actual patient treatment. A 1 litre spherical ion chamber (PTW, Germany) has been used to measure the dose at the beam exit window and at the total body irradiation (TBI) treatment couch following large and small field beams with long beam‐on times. Measurements were also made outside of the closed jaws to quantify the benefit of the attenuation provided by the jaws.Results:The radioisotopes produced in the linac head have been identified as 18⁷W, ⁵⁶Mn, 2⁴Na and 2⁸Al, which have half‐lives from between 2.3 min to 24 hours. The dose at the beam exit window following an 18 MV 2197 MU TBI beam delivery was 12.6 µSv in ten minutes. The dose rate at the TBI treatment couch 4.8 m away is a factor of ten lower. For a typical TBI delivered in six fractions each consisting of four beams and an annual patient load of 24, the annual dose estimate for a staff member at the treatment couch for ten minutes is 750 µSv. This can be further reduced by a factor of about twelve if the jaws are closed before entering the room, resulting in a dose estimate of 65 µSv.Conclusion:The dose resulting from the activation products for a representative TBI workload at our clinic of 24 patients per year is 750 µSv, which can be further reduced to 65 µSv by closing the jaws.

Sci-Fri AM: Mountain - 03: Current status of the NRC primary standard for 192 Ir HDR brachytherapy sources

The Canadian primary standard for 192Ir HDR brachytherapy sources has been recently revised in a more accurate manner allowing for more realistic uncertainty estimation. Air-kerma strength Sk is derived from measurements of the source's output using a graphite-walled spherical ionization chamber (2S) at several distances. Traceability to NRC primary standards for the 192Ir calibration coefficient Nk is insured by estimating it as the inverse arithmetic mean of the inverse of the calibration coefficients for a 137Cs beam and the medium energy x-ray beam quality N250, both of which are traceable to NRC primary standards. The multiple-distance method is combined with a non-linear least squares fit to determine St, while at the same time removing the effects of room scatter and position offset. The previously used shadow-cone method for directly measuring the room scatter is found to be inadequate due to the increased scatter contribution from the lead cone itself, especially at short source-detector distances. Rather than including the reported 1% difference in source strength for 192Ir HDR sources of different construction into the total uncertainty, users are cautioned that the calibration coefficient provided by NRC is only valid for a microSelectron V2 model. A comprehensive uncertainty budged shows that the total one sigma uncertainty of the standard is actually 0.6% rather than the previously assumed 1.2%. NRC measured Sk agrees within 0.03% of the manufacturer's value.

Recombination Parameters of some Fabricated Ionization Chambers

Some pancake and spherical type ionization chambers of various size have been designed and fabricated for absolute air kerma measurement in 60Co and 137Cs ?-ray fields at the Primary Standard Dosimetry Laboratory (PSDL), National Institute of Advanced Industrial Science &amp; Technology (AIST), Tsukuba, Japan. Values of recombination parameters A and m2g of these ionization chambers are obtained using a method proposed by De Almeida and Niatel and adopted by Boutillon. For absolute air kerma measurement, it is important to obtain accurate signal currents by correcting it for ion losses using the values of A and m2g of each of the ionization chambers. It has been observed from the measurements that the recombination parameters for pancake ionization chamber are smaller than spherical ionization chambers and for spherical type ionization chamber recombination parameter values depend on the dimensions of the electrodes and also the size of the chambers. DOI: http://dx.doi.org/10.3329/bjmp.v4i1.14694 Bangladesh Journal of Medical Physics Vol.4 No.1 2011 101-106

Gas spherical ionization chamber

A pressurized ionization chamber detector able to measure radioactive sources in internal 2π or 4π geometry was built in order to characterize alpha and beta radioactive sources, i.e. to calibrate these sources by a relative method, and to test the behavior of gas mixtures in pressurized-gas radiation detectors. The detector we made is of spherical shape and works by collecting in a quasi-uniform electric field the ionization charges resulting from the interaction of ionizing radiation with gas in the sensitive volume of the chamber. An ionizing current proportional to the activity of the radioactive source to be measured is obtained. Pressure inside the detector in standard conditions is maximum 6 × 105 Pa at 22 °C and the maximum allowed voltage is 3000 V. It is presented as a spherical ionization chamber with gas under pressure, geometrical shape, dimensions and structural characteristics, functional and technical parameters and preliminary experimental results obtained from radiation measurements in the laboratory SALMROM of the IFIN-HH.

SU-E-J-24: Measurement of Skin Dose toward Real-Time Tumor Tracking Irradiation in MHI-TM2000 (VERO): A Preliminary Study

Purpose: The purpose of this study was to estimate skin dose from real‐time X‐ray fluoroscopyimaging to do clinical trials for real‐time tumor irradiation in MHI‐TM2000 system (Vero). Methods: First, exposure dose with backscatter was measured using a single set of kilovoltage X‐ray tube and a flat panel detector; a cylindrical ionization chamber (DC300, IBA); and a water‐equivalent plate phantom. The chamber was rigidly attached on the plate phantom of 200 mm in thickness. Then, the chamber was positioned to 150 mm upper from the isocenter. Subsequently, exposure dose to an irradiation field of 171 mm × 226 mm was measured at a speed of 5 fps with 200 mA of the X‐ray tube current; and 40∼120 kVp of the X‐ray tube voltage, respectively. The nominal exposure time was set to 5 ms. Next, half‐value layer of aluminum was measured using a spherical ionization chamber (A4, Exradin) to calculate the effective energy and absorbed dose conversion factors of soft tissue. Then, the skin dose was calculated from the exposure dose and absorbed dose conversion factors. Results: There were good liner correlations between the X‐ray tube current and skin doses; and therefore, estimated skin dose from the X‐ray fluoroscopy with 1 mAs and 100 kV was 140 μGy. Furthermore, estimated skin dose for one successive minute exposure at a speed of 5 fps with 0.1 mAs and 100 kV was 4 mGy. The skin doses in the MHI‐TM2000 were greater by a factor of 1.4∼1.6 than those in Elekta Synergy system with the tube voltage of 100 or 120 kV. Conclusions: We have observed that the skin dose is a little larger than in the Synergy system while the filtration to the X‐ray source has not been applied to the MHI‐TM2000. To perform real‐time tumor tracking, further investigation may be required. This research was in part sponsored by Mitsubishi Heavy Industries, Ltd., Japan.

Development of ion chambers for the measurement of low energy synchrotron radiation

Measurement of radiation fields due to low energy synchrotron radiation (SR) needs special considerations. The photoelectric interactions, which are mainly responsible for signal generation in this case, result in electron emission in a direction nearly perpendicular to the direction of incident photon. A free air ionisation chamber was designed and used for measurement of intensity of SR in the beam line and a methodology has been developed to measure SR fields away from the beam line using large volume ion chambers. Studies with spherical and cylindrical ion chambers in X-ray beams obtained at a tube potential of 15 kV (3 mm Be inherent filtration) showed that the chamber sensitivity at low energy strongly depended on curvature of the chamber wall especially in thick walled chambers. Our studies further showed that large volume thin and plane walled (0.01 mm thick aluminium wall) chambers could be used for measurement of SR fields away from the beam lines, with reasonably good accuracy (±10%) by calibrating them against the free air ion chamber through a similar chamber of smaller dimensions.

Development of a High Pressure Xe Ionization Chamber for Environmental Radiation Spectroscopy

A High Pressure Xenon ionization chamber is a promising radiation detector for environmental radiation measurement due to its radiation hardness, its physical rigidity, and its capability of operation at a high temperature up to about 170 °C. A cylindrical high pressure xenon ionization chamber, which was configured with a shielding mesh to improve its energy resolution, was designed on the basis of an electron transfer simulation code (EGSnrc) to extract an optimal density of Xe gas and a thickness of the chamber wall. An electron drift simulation code, Garfield, which was coupled with a Maxwell electric filed calculator, was also employed for the electron drift simulations due to the geometry of the shielding mesh. Shielding inefficiency was also calculated. A spherical ionization chamber was also designed and fabricated to monitor environmental radiation. A noble gas system was constructed to create a noble gas with a high purity and to inject the noble gas up to 60 atm. The combination of an oxygen absorbent (Oxisorb), a molecular sieve, and a high temperature getter can minimize the electro-negative impurities, such as the O2 and N2 gas, to below about several ppb levels. Preliminary tests such as leakage currents, saturation currents, and gas leak test were performed. The performance of the two fabricated ionization chambers at a low dose rate was tested by using a conventional shadow technique with a NIST certified 33.52 MBq 226Ra source in the calibration room at KAERI.

TH‐C‐AUD A‐02: The Air‐Kerma Strength Standard for 192Ir HDR Sources at the University of Wisconsin ADCL

Purpose: To present a compilation of a seven‐year study of measurements using several HDR afterloaders and to provide a comprehensive analysis of the various published methodologies for interpolating between NIST standards to determine the air‐kerma calibration coefficient for . Ultimately an update of the current interim standard will be considered. Method and Materials: An acrylic apparatus for performing the seven‐distance measurement technique equipped with laser alignment was used to acquire all datasets. Measurements were performed with an Exradin A3 spherical ion chamber that had been calibrated at NIST for beam qualities of M250 as well as . A total of four different afterloaders were measured during multiple trials and a comparison of the results was made to assess any trends in measurements due to source geometry. Recently published interpolation methods were compared to the method used in the original establishment of the interim standard in 1991 with proper accounting of the revisions in the NIST air‐kerma standards in 2003. Three different methods for solving the non‐linear system of equations were compared to assess stability and minimize uncertainty. Results: Depending on the interpolation method, deviations of −1.12% to −0.37% from the long‐standing air‐kerma calibration factor were observed. In comparing the measurements from the last seven years, (2000–2007), to the well chamber transfer standards, (1991), certain trends between various source models were identified, but the overall effect was found to be in the range of — 0.95% to 0.18%. Conclusion: Based on the data recorded it is reasonable to assume, given the uncertainty in the method, that a single calibration factor would indeed be appropriate for all source models. The possibility of a formal update to the standard will be considered.

Performance of a high-pressure xenon ionization chamber for environmental radiation monitoring

High-pressure xenon (HPXe) ionization chambers are ideal for use in uncontrolled environments, as a detector's response has been shown to be uniform over large temperature ranges ( 20 – 170 ∘ C ) . A cylindrical HPXe ionization chamber, which was configured with a shielding mesh to improve its energy resolution, was designed on the basis of an EGSnrc simulation code to extract an optimal density of Xe gas and a thickness of the chamber's wall. A Garfield, which was coupled with a Maxwell electric field calculator, was also employed for the electron drift simulations due to the geometry of the adapted shielding mesh. Shielding inefficiency was also calculated. A spherical ionization chamber was also designed to monitor an environmental radiation level and the responses for low dose rates with the fabricated HPXe ionization chamber were compared. A noble gas system was constructed to create a noble gas with a high purity and to inject the noble gas at up to 60 atm. The combination of an oxisorb, a molecular sieve, and a high-temperature getter can minimize the electro-negative impurities, such as the O 2 and N 2 gas, to below about several ppb levels. Preliminary tests such as leakage currents, saturation currents, and gas leak tests were performed. The performance of the two fabricated ionization chambers at a low dose rate was tested by using a conventional shadow technique with a NIST certified 0.906 mCi 226Ra standard source in a calibration room at KAERI.

Analytical form of current-voltage characteristic of parallel-plane, cylindrical and spherical ionization chambers with homogeneous ionization

The elementary processes taking place in the formation ofcharged particles and their flow in parallel-plane, cylindrical andspherical geometry cases of ionization chamber are considered. On thebasis of particles and charges balance a differential equationdescribing the distribution of current densities in the ionizationchamber volume is obtained. As a result of the differential equationsolution an analytical form of the current-voltage characteristic ofan ionization chamber with homogeneous ionization is obtained. For theparallel-plane case comparision with experimental data is performed.