Miniature Ionization Chamber Research Articles

Gamma-Heating and Gamma Flux Measurements in the JSI TRIGA Reactor: Results and Prospects

The neutron field of various irradiation positions of the TRIGA Mark II reactor of the Jožef Stefan Institute (JSI) has been thoroughly characterized by neutron activation dosimetry and miniature fission chambers techniques. In order to have a fully validated calculation scheme to analyze and plan experiments, the gamma field also has to be experimentally validated. The 10-year long collaboration between CEA and JSI is a perfect framework to carry out such a study, and measurements of the gamma field started in late 2016. Several measurement techniques were investigated in in-core and ex-core positions. Online measurements were carried out using miniature ionization chambers (ICs) manufactured by the CEA and PTW Farmer. Positional dependence was studied, showing a decrease in the delayed gamma contribution to the total gamma flux with increasing distance from the reactor core center. To characterize the gamma dose in the core, as well as in the periphery, thermoluminescent and optically stimulated luminescent detectors (TLDs and OSLDs, respectively) were tested. These detectors are commonly used at CEA to measure the gamma dose in a given material, in order to study the nuclear heating in various core elements (control rod, baffle, structural material). Different filters were used in order to assess an integrated dose ranging from a few Gy up to several kGy. The comparisons of experimental results against calculations performed with the JSIR2S code package show a very good agreement. The feasibility of such measurements demonstrates the complementarity between measurements with dosimetry and ICs from low to very high gamma-dose environment, such as in material testing reactors.

Gamma-heating and gamma flux measurements in the JSI TRIGA reactor, results and prospects

The neutron field of various irradiation positions of the TRIGA Mark II reactor of the Jožef Stefan Institute has been thoroughly characterized by neutron activation dosimetry and miniature fission chambers techniques. In order to have a fully validated calculation scheme to analyze and plan experiments, the gamma field also has to be experimentally validated. The 10-year long collaboration between CEA and JSI is a perfect framework to carry out such a study, and measurements of the gamma field started in late 2016. Several measurement techniques were investigated in in-core and ex-core positions. On-line measurements were carried out using miniature ionization chambers manufactured by the CEA and PTW Farmer ionization chambers. Positional dependence was studied, showing a decrease in the delayed gamma contribution to the total gamma flux with increasing distance from the reactor core center. To characterize the gamma dose in the core, as well as in the periphery, thermo- and optically stimulated luminescent detectors were tested. These detectors are commonly used at CEA to measure the gamma dose in a given material in order to study the nuclear heating in various core elements (control rod, baffle, structural material). Different filters were used in order to assess an integrated dose ranging from a few Gy up to several kGy. The feasibility of such measurements demonstrates the complementarity between measurements with dosimetry and ionization chambers from low to very high gamma-dose environment, such as in material testing reactors.

Investigation of an Online Reactor Neutron Spectrum Measurement Method with Ionization Chambers

In this paper, an online reactor neutron spectrum measurement method is presented. The basic theory of this method is based on the unfolding of few-channel data, in which three miniature ionization chambers are applied. The neutron spectrum can be unfolded with the count rates and response functions of the three detectors through an unfolding program. In order to investigate the feasibility of this method, simulation tests have been performed with the reference neutron spectra and neutron spectra from the China LEAd-based Reactor (CLEAR). The research results show that this method can provide an alternative means for an online neutron spectrum measurement in the reactors. This method is suitable to be applied in fast neutron reactors due to the miniature size of ionization chambers and fission threshold of 238U.

Delayed Gamma Measurements in Different Nuclear Research Reactors Bringing Out the Importance of Their Contribution in Gamma Flux Calculations

Neutron and gamma flux levels are key parameters in nuclear research reactors. In Material Testing Reactors, such as the future Jules Horowitz Reactor, under construction at the French Alternative Energies and Atomic Energy Commission (CEA Cadarache, France), the expected gamma flux levels are very high (nuclear heating is of the order of 20 W/g at 100 MWth). As gamma rays deposit their energy in the reactor structures and structural materials it is important to take them into account when designing irradiation devices. There are only a few sensors which allow measurements of the nuclear heating [12]; a recent development at the CEA Cadarache allows measurements of the gamma flux using a miniature ionization chamber (MIC) [3]. The measured MIC response is often compared with calculation using modern Monte Carlo (MC) neutron and photon transport codes, such as TRIPOLI-4 and MCNP6. In these calculations only the production of prompt gamma rays in the reactor is usually modelled thus neglecting the delayed gamma rays. Hence calculations and measurements are usually in better accordance for the neutron flux than for the gamma flux. In this paper we study the contribution of delayed gamma rays to the total MIC signal in order to estimate the systematic error in gamma flux MC calculations. In order to experimentally determine the delayed gamma flux contributions to the MIC response, we performed gamma flux measurements with CEA developed MIC at three different research reactors: the OSIRIS reactor (MTR — 70 MWth at CEA Saclay, France), the TRIGA MARK II reactor (TRIGA — 250 kWth at the Jozef Stefan Institute, Slovenia) and the MARIA reactor (MTR — 30 MWth at the National Center for Nuclear Research, Poland). In order to experimentally assess the delayed gamma flux contribution to the total gamma flux, several reactor shut down (scram) experiments were performed specifically for the purpose of the measurements. Results show that on average about 30 % of the MIC signal is due to the delayed gamma rays. In this paper we describe experiments in each of the three reactors and how we estimate delayed gamma rays with MIC measurements. The results and perspectives are discussed.

Brightness measurement of an electron impact gas ion source for proton beam writing applications.

We are developing a high brightness nano-aperture electron impact gas ion source, which can create ion beams from a miniature ionization chamber with relatively small virtual source sizes, typically around 100 nm. A prototype source of this kind was designed and successively micro-fabricated using integrated circuit technology. Experiments to measure source brightness were performed inside a field emission scanning electron microscope. The total output current was measured to be between 200 and 300 pA. The highest estimated reduced brightness was found to be comparable to the injecting focused electron beam reduced brightness. This translates into an ion reduced brightness that is significantly better than that of conventional radio frequency ion sources, currently used in single-ended MeV accelerators.

Measurements of miniature ionization chamber currents in the JSI TRIGA Mark II reactor demonstrate the importance of the delayed contribution to the photon field in nuclear reactors

The characterization of experimental locations of a research nuclear reactor implies the determination of neutron and photon flux levels within, with the best achievable accuracy. In nuclear reactors, photon fluxes are commonly calculated by Monte Carlo simulations but rarely measured on-line. In this context, experiments were conducted with a miniature gas ionization chamber (MIC) based on miniature fission chamber mechanical parts, recently developed by the CEA (French Atomic Energy and Alternative Energies Commission) irradiated in the core of the Jožef Stefan Institute TRIGA Mark II reactor in Ljubljana, Slovenia. The aim of the study was to compare the measured MIC currents with calculated currents based on simulations with the MCNP6 code. A discrepancy of around 50% was observed between the measured and the calculated currents; in the latter taking into consideration only the prompt photon field. Further experimental measurements of MIC currents following reactor SCRAMs (reactor shutdown with rapid insertions of control rods) provide evidence that over 30% of the total measured signal is due to the delayed photon field, originating from fission and activation products, which are untreated in the calculations. In the comparison between the measured and calculated values, these findings imply an overall discrepancy of less than 20% of the total signal which is still unexplained.

Evaluation of Rectal Dose During High-Dose-Rate Intracavitary Brachytherapy for Cervical Carcinoma

High-dose-rate intracavitary brachytherapy (HDR-ICBT) for carcinoma of the uterine cervix often results in high doses being delivered to surrounding organs at risk (OARs) such as the rectum and bladder. Therefore, it is important to accurately determine and closely monitor the dose delivered to these OARs. In this study, we measured the dose delivered to the rectum by intracavitary applications and compared this measured dose to the International Commission on Radiation Units and Measurements rectal reference point dose calculated by the treatment planning system (TPS). To measure the dose, we inserted a miniature (0.1 cm 3) ionization chamber into the rectum of 86 patients undergoing radiation therapy for cervical carcinoma. The response of the miniature chamber modified by 3 thin lead marker rings for identification purposes during imaging was also characterized. The difference between the TPS-calculated maximum dose and the measured dose was <5% in 52 patients, 5–10% in 26 patients, and 10–14% in 8 patients. The TPS-calculated maximum dose was typically higher than the measured dose. Our study indicates that it is possible to measure the rectal dose for cervical carcinoma patients undergoing HDR-ICBT. We also conclude that the dose delivered to the rectum can be reasonably predicted by the TPS-calculated dose.

Efficiency of the focusing channel of the Belok Station in the Sibir-2 storage ring

The efficiency of the focusing systems of the Belok station on the synchrotron radiation beam from the bending magnet in the Sibir-2 storage ring has been experimentally determined using the IK-12 and IK-30 miniature ionization chambers developed at the Shubnikov Institute of Crystallography and calibrated with an FDUK-100UV photodiode developed at the Ioffe Physical Technical Institute.

Establishing radiation therapy treatment planning effects involving implantable pacemakers and implantable cardioverter‐defibrillators

Recent improvements to the functionality and stability of implantable pacemakers and cardioverter‐defibrillators involve changes that include efficient battery power consumption and radiation hardened electrical circuits. Manufacturers have also pursued MRI‐compatibility for these devices. While such newer models of pacemakers and cardioverter‐defibrillators are similar in construction to previously marketed devices – even for the recent MRI‐compatible designs currently in clinical trials – there is increased interest now with regard to radiation therapy dose effects when a device is near or directly in the field of radiation. Specifically, the limitation on dose to the device from therapeutic radiation beams is being investigated for a possible elevation in limiting dose above 200 cGy. We present here the first‐ever study that evaluates dosimetric effects from implantable pacemakers and implantable cardioverter‐defibrillators in high energy X‐ray beams from a medical accelerator. Treatment plan simulations were analyzed for four different pacemakers and five different implantable cardioverter‐defibrillators and intercompared with direct measurements from a miniature ionization chamber in water. All defibrillators exhibited the same results and all pacemakers were seen to display the same consequences, within only a a±1.8% deviation for all X‐ray energies studied. Attenuation, backscatter, and lateral scatter were determined to be −13.4%, 2.1% and 1.5% at 6 MV, and −6.1%, 3.1% and 5.1% at 18 MV for the defibrillator group. For the pacemaker group, this research showed results of −15.9%, 2.8% and 2.5% at 6 MV, and −9.4%, 3.4% and 5.7% at 18 MV, respectively. Limited results were discovered from scattering processes through computer modeling. Strong verification from measurements was concluded with respect to simulating attenuation characteristics. For IP and ICD leads, measured dose changes were less than 4%, existing as attenuation processes only, and invariant with regard to X‐ray energy.PACS number: 87.53.Bn, 87.53.Dq, 87.53.Tf, 87.66.Jj

Dosimetric measurements of a new electronic brachytherapy surface applicator

Dosimetric measurements of a new electronic brachytherapy surface applicator

WE-D-224C-09: Dosimetric Evaluation of Proton Field Matching by a Robotic Patient Positioner

Purpose: The objective of this study is to evaluate the accuracy from a dosimetry point of view with which proton field matching can be accomplished. Method and Materials: This proton therapy center currently treats patients with a nominal 208 MeV proton beam in a single Fixed Horizontal Beam Line (FHBL) room that employs a double passive beam spreading system with a fixed range modulator. The FHBL room takes advantage of a novel robotic patient positioner system (PPS) providing 6 degrees of freedom with a specified accuracy of 300 microns when transiting up to a 200 kg payload. To investigate field matching doses three methods are used: calculations from a treatment planning system, film dosimetry measurements and scans using a miniature ionization chamber. The field delivery for this study consists of four proton fields matched by the robotic PPS. The measurements are performed in a solid water phantom for the film and a water phantom for the ionization chamber scans Results: The planned dose delivery for the central field is normalized to 100%. The average relative dose values in the field junctions are 128%, 135% and 130% for the ionization chamber scans, film dosimetry and treatment planning calculation, respectively. In this case this was thought to be clinically acceptable as the volume lay entirely within the target. Conclusion:Proton therapy is suitable for treating volumes of relatively large area and shallow depth. In this way the ranged property of the protons can spare underlying structures. The sharp proton penumbra and precision of a robotic patient positioner simplifies field matching. Further study is underway with a half beam block to investigate the range of junction doses that can be expected under a variety of field matching conditions.

Miniature ionization chamber detector developed for X-ray microprobe measurements.

A windowless small ionization chamber detector has been developed for monitoring the intensity of the microbeam at the ID18F microprobe end-station of the European Synchrotron Radiation Facility. The small dimensions of the ionization chamber (10 mm along the beam direction and 5 mm perpendicular to it) make it possible to place it very close to the sample. A pinhole of diameter 50 microm was used for defining the entrance window of the ionization chamber; thus the small counter can be used as an order-selecting aperture while measuring simultaneously the intensity after the aperture. In the present work the technical characteristics, such as the current-voltage curve, stability and linearity, of the small monitor have been tested.

Effects of beamline components (undulator, monochromator, focusing device) on the beam intensity at ID18F (ESRF)

The ID18F microprobe end-station of the European Synchrotron Radiation Facility (ESRF) is dedicated to precise and reproducible quantitative X-ray fluorescence analysis in the ppm level with ⩽5% accuracy for elements of Z⩾19 and micron-size spatial resolution. In order to fulfill this requirement the precise monitoring and normalization of the intensity variation of the focused micro-beam is necessary. The various effects influencing the intensity variation, hence the stability of the μ-beam, were investigated by placing different detectors (miniature ionization chamber, photodiodes) into the monochromatic beam. The theoretical statistical error of the measured signal in each detector was estimated on the basis of the absorption and e −–ion-pair production processes and was compared with the measured statistical errors.

A quality assurance test tool for high dose-rate remote afterloading brachytherapy units.

A QA test tool is designed to quantitatively measure the HDR source positioning error, and to facilitate quick and dependable HDR timer linearity test and daily output constancy check. The test tool consists of two concentric disks. The lower disk has a cutout for inserting an HDR catheter, and the upper disk accepts a diode or miniature ionization chamber and can rotate relative to the lower disk. Ionization readings from the source (transferred to the center of the disks) are obtained at two rotational positions of the upper disk which houses the detector. The ratio of the readings is used to determine the source-positioning error of the HDR unit relative to the nominal source position by a simple triangulation principle. Experimental measurements confirm that the QA test tool is sensitive to approximately 0.2 mm variance in source positioning errors. In addition, the QA test tool is suitable for other common HDR QA tests such as the source travel step size test, the daily HDR unit output constancy check, and the timer linearity test. Its simple and robust design permits routine clinical use and provides a high confidence level in the accurate operation of HDR units.

The252Cf(sf) Neutron Spectrum in the 5- to 20-MeV Energy Range

The 252Cf neutron spectrum is measured at high energies with a miniature ionization chamber and two different NE-213 neutron detectors. The gamma-ray background and the main cosmic background caused by muons were suppressed by applying efficient pulse-shape discrimination. On the basis of two-dimensional spectroscopy of the neutron time-of-flight and scintillation pulse height, the sliding bias method is used to minimize experimental uncertainties.The experimental data, corrected for several systematic influences, confirm earlier results that show negative deviations from a reference Maxwellian distribution with a 1.42-MeV spectrum “temperature” for neutron energies above 6 MeV.Experimental results of this work are compared with various statistical model approaches to the 252Cf(sf) neutron spectrum.

The patient equivalence of the RANDO phantom for cobalt gamma rays.

The RANDO2 “average man” or “average woman” phantoms are especially designed for use in connection with radiation dosimetry studies. The construction of the phantom, which has been discussed in detail in the literature (1, 2), consists of a special thermosetting synthetic rubber which is molded around a natural human skeleton. Lungs are simulated with a special foam reported to have a density of 0.32. During the past three years a RANDO “average man” phantom has been in use at Walter Reed General Hospital in the development of a number of special dosimetry technics using 60Co gamma rays and 2 MVp x rays. In all of these studies the exposure in the phantom was measured with film or miniature ionization chambers. In the application of the measurements to treatment planning we have assumed that the phantom was “patient equivalent,” i.e., that an exposure at a depth and in any particular anatomical site in the phantom would accurately predict the exposure under similar conditions in the patient. This assumpti...

Radium dosage patterns in the treatment of uterine carcinoma.

In the radium treatment of endometrial carcinoma using Heyman's packing technique, variations in shape and size of malignant uteri become very important in evaluation of the dose delivered to uterine walls and the adjacent structures. Because of these difficulties, an unsatisfactory method of stating dosage in milligram-hours of radium has been retained in gynelogic curie therapy.An experimental program to determine the distribution of radiation along the periphery of the uterine wall as well as to structures of anatomic importance was recently completed. Multiple measurements were performed in different sizes of uterine models using Sievert miniature ionization chambers, lithium fluoride dosimeters and slow films. A limited computer program was written to check the pattern of experimental measurements. The results were satisfactory and an extensive computer program was started to develop a system of complete radium dosage patterns for uterine cavities of different size and shape and their associated radi...

Measurement of very high dose rate of ionization radiation by ionometric method.

A miniature ionization chamber of air-equivalent material in the form of a thimble is described. By means of this ionization chamber in connection with a Siemens-universal-dosimeter-system it is possible to measure a dose rate up to 1.55 million R per minute.

Dental x-ray exposure of sites within the head and neck

Measurements made with miniature ionization chambers and a phantom head have indicated the magnitude of the radiation exposure at selected sites within the head and neck. With few exceptions, the exposures made with 65 and 90 KVP were comparable, and all were relatively low compared with earlier studies reported in the dental literature.

Miniature Ionization Chambers for Measurements in Body Cavities

The construction of miniature ionization chambers is described and an instance of their use is given.