Spherical Tissue-equivalent Proportional Counter Research Articles

Prediction of fast neutron spectra with a spherical TEPC using a machine-learning algorithm

For many years, new unfolding methods based on machine learning have been studied and developed to improve the prediction capabilities of fluence spectra in neutron fields. These methods are mainly applied to traditional devices used for specific measurements like Bonner spheres, activation detectors or liquid scintillators. In this paper, we attempt to develop a new method based on the unfolding of the fluence spectrum from the micro-dosimetric spectrum measured by a tissue-equivalent proportional counter (TEPC). This type of counter is commonly used for neutron kerma measurements and quality factor assessments, but has never been employed as a neutron spectrometer. This work focuses on the fast neutron region, which is an extremely relevant subject in various fields of nuclear energy. We have tested different machine-learning models to define an unfolding algorithm that can be used to reconstruct the energy distribution of the fluence for spectra of various origins, ranging between 50 keV and 20 MeV.

Predictive model for H*(10) derived from microdosimetric quantities with a TEPC detector, for neutron spectra with a mean energy from 50 keV to 14 MeV

In this paper, using a spherical Tissue-Equivalent Proportional Counters (TEPC), a new correlation between the ambient dose equivalent for pure neutron spectra H∗(10) and microdosimetric quantities is proposed. A fitting regression built upon a learning base of 272 miscellaneous spectra allows to predict this operational quantity. Data to build this predictive model are drawn from MCNP6.1 simulations. This new model reveals a satisfactory accuracy in the prediction with, in majority, discrepancies within 5% for common spectra with a mean energy ranging over 50 keV to 14 MeV. These observations could offer the prospect to use the TEPC as reference device for measuring H∗(10), in the covered range of spectra.

Characterization of a new tissue equivalent proportional counter for dosimetry of neutron and photon fields: comparison of measurements and Monte Carlo simulations

The tissue equivalent proportional counter (TEPC) is widely recognized as an important dosimetric technique particularly for complex radiation fields. The Korea Astronomy and Space Science Institute (KASI) has recently developed a new spherical TEPC to monitor the space radiation environment in the low earth orbit. The purpose of this study is to examine the performance of the TEPC against standard photon (137Cs) and neutron (252Cf) sources through ground-based measurements and Monte Carlo simulations prior to its actual implementation. Lineal energy distributions, microdosimetric spectra and dosimetric quantities for a 2 µm simulated site in pure propane gas were determined for both sources. Both the measured and calculated 137Cs spectra were shown to occur below 11 keV µm−1 that is the typical range covered by photon sources. Complete coincidence of their electron edge regions was also observed. Meanwhile, the proton edge from the measured 252Cf spectra was found to be in good agreement with those from the simulated ones and the literature. The gamma, recoil proton and heavy ions peaks expected for neutron sources were well defined, albeit deviations in the gamma region. The absorbed dose and dose equivalent for both irradiation conditions were also successfully obtained. The dose equivalent for 252Cf was found to be ten times the absorbed dose whereas it remained the same for 137Cs. The discrepancies observed in the low lineal energy region for both irradiation conditions were caused by intrinsic limitations on the experimental set-up and simulation configurations. This mainly contributed to a difference in the measured and calculated dose mean lineal energies of about 4.1% and 8.7% for the photon and neutron cases, respectively. Nevertheless, fair consistency with published data suggested that our TEPC could adequately reproduce the expected microdosimetric distributions for complex radiation fields.

The microdosimetric extension in TOPAS: development and comparison with published data

Microdosimetric energy depositions have been suggested as a key variable for the modeling of the relative biological effectiveness (RBE) in proton and ion radiation therapy. However, microdosimetry has been underutilized in radiation therapy. Recent advances in detector technology allow the design of new mico- and nano-dosimeters. At the same time Monte Carlo (MC) simulations have become more widely used in radiation therapy. In order to address the growing interest in the field, a microdosimetric extension was developed in TOPAS. The extension provides users with the functionality to simulate microdosimetric spectra as well as the contribution of secondary particles to the spectra, calculate microdosimetric parameters, and determine RBE with a biological weighting function approach or with the microdosimetric kinetic (MK) model. Simulations were conducted with the extension and the results were compared with published experimental data and other simulation results for three types of microdosimeters, a spherical tissue equivalent proportional counter (TEPC), a cylindrical TEPC and a solid state microdosimeter. The corresponding microdosimetric spectra obtained with TOPAS from the plateau region to the distal tail of the Bragg curve generally show good agreement with the published data.

THE DETERMINATION OF NEUTRON FLUENCE TO ABSORBED DOSE CONVERSION COEFFICIENTS AND RELATIVE BIOLOGICAL EFFECT BASED ON MICRODOSIMETRY MEASUREMENTS.

A tissue-equivalent proportional counter (TEPC) is a reference detector to measure microdosimetric quantities. A conventional spherical TEPC and a novel TEPC based on a ceramic thick gas electron multiplier (THGEM) foil were developed to carry out microdosimetric measurements of lineal energy spectra in monoenergetic and 252Cf/241Am-Be neutron radiation fields, and the absorbed dose values had been derived. In order to go further in radiobiology and therapy, the fluence to absorbed dose conversion coefficients in neutron fields were also determined. According to the dose distribution in lineal energy, the neutron relative biological effect (RBE) values were also calculated using an empirical procedure applying biological weighting functions.

Technical Note: Effect of magnetic fields on the microdosimetry of carbon-ion beams.

To investigate the influence of magnetic fields on the microdosimetry of carbon-ion beams and the scaling effect of tissue equivalent proportional counter (TEPC) defined as the change of energy deposition in a TEPC from that in a microscopic scale region of interest due to the presence of a magnetic field in combination with the TEPC larger physical dimensions. Geant4-based Monte Carlo simulations were conducted to calculate the microdosimetric quantities for carbon-ion beams with different initial energies (10-290MeV/u) under magnetic fields of various strengths (0.5-3T). The calculations were performed for a 1μm spherical volume made of tissue, and for spherical TEPCs of 1 and 10mm in diameter. Then, values of dose-averaged lineal energy (yD ) were acquired for the different scenarios to analyze the effect of magnetic fields on the microdosimetry of carbon-ion beams and the scaling effect of TEPC. The yD values and lineal energy spectra in the 1μm spherical tissue volume for the scenarios without magnetic field and with magnetic fields of different strengths and directions remained nearly the same for the various energy carbon-ion beams. However, compared with those of the 1μm spherical tissue volume, an increase of of yD values and an obvious shift of the lineal energy spectra for the TEPCs of 1 and 10mm in diameter under magnetic fields were found. The application of magnetic fields under 3T has no significant influence on the microdosimetric results of carbon-ion beams. However, there is definitely a scaling effect when using TEPC for microdosimetric study, which alters the reading of TEPC in the presence of magnetic fields. Novel methods to correct the reading of TEPC or scaling effect-resistant microdosimetric measurement detectors are urgently needed to perform experimental microdosimetric studies under magnetic fields.

Assessment of mixed neutron and photon radiation ratios using a tissue equivalent proportional counter on KSTAR

Tokamaks have a mixed radiation field consisting of neutrons, neutron-induced gamma rays, and hard X-rays due to runaway electrons. Neutron and gamma-ray diagnostics for characterization of the mixed radiation fields of tokamak in such an environment is essential for determining neutron emission rate as well as for understanding behaviors of fast ions in plasma operations.The experiment using a tissue equivalent proportional counter (TEPC) based on a microdosimetric technique has performed with the aiming of investigating its application to tokamaks through the measurement of dosimetric quantities at the KSTAR tokamak. For the work, a Bemjamin type of spherical TEPC with a 0.318 cm tissue equivalent wall of Shonka type A-150 plastic has applied to define lineal energy boundaries for secondary particles caused by each radiation component. The TEPC has placed on a deck at a distance of about 20 m from KSTAR tokamak. The mixture distributions have obtained from the measurement of lineal energy spectra. The mixture ratio of neutrons and gammas in the KSTAR tokamak has evaluated to be around 1:4. The present result shows not only the availability of the developed TEPC for the assessment of neutron and gamma components in the tokamak, but also its applicability for fusion reactors even though the application of TEPC on tokamak is still at experimental stage.

Development of an advanced two-dimensional microdosimetric detector based on THick Gas Electron Multipliers.

The THick Gas Electron Multiplier (THGEM)-based tissue-equivalent proportional counter (TEPC) has been proven to be useful for microdosimetry due to its flexibility in varying the gaseous sensitive volume and achieving high multiplication gain. Aiming at measuring the spatial distribution of radiation dose for mixed neutron-gamma fields, an advanced two-dimensional (2D) THGEM-TEPC was designed and constructed at McMaster University which will enable us to overcome the operational limitation of the classical TEPCs, particularly for high-dose rate fields. Compared to the traditional TEPCs, anode wire electrodes were replaced by a THGEM layer, which not only enhances the gas multiplication gain but also offers a flexible and convenient fabrication for building 2D detectors. The 2D THGEM TEPC consists of an array of 3 × 3 sensitive volumes, equivalent to nine individual TEPCs, each of which has a dimension of 5 mm diameter and length. Taking the overall cost, size and flexibility into account, to process nine detector signals simultaneously, a multi-input digital pulse processing system was developed by using modern microcontrollers, each of which is coupled with a 12-bit sampling ADC. Using the McMaster Tandetron 7 Li(p,n) accelerator neutron source, both fundamental detector performance, as well as neutron dosimetric response of the 2D THGEM-TEPC, has been extensively investigated and compared to the data acquired by a standard spherical TEPC. It was shown that the microdosimetric response and the measured absorbed dose rate of the 2D THGEM detector developed in this study are comparable to the standard 1/2" TEPC which is commercially available. This study proved that the 2D TEPC based on the THGEM technology can be effectively used in microdosimetry studies and is a promising detector for measuring the absorbed dose rate distribution over an area in mixed radiation fields. This unique small gas cavity detector opens new possibilities in applications for high-intensity mixed radiation fields as well as in nanodosimetry.

CONSTRUCTION AND TEST OF A PORTABLE TISSUE-EQUIVALENT PROPORTIONAL COUNTER SYSTEM

Tissue-equivalent proportional counter (TEPC) is a very promising dosemeter for radiation protection in mixed radiation fields. In this article, a portable TEPC system including a spherical homemade TEPC detector and the compact low-power integrated circuit prototype was constructed and tested in gamma and neutron radiation fields. The microdosimetric spectra results displayed that the portable TEPC system could deal with four orders of magnitude voltage pulse signals. And the measured dose equivalent values agreed with reference values, which means the portable TEPC system had fulfilled the test and satisfied the requirement for radiation monitor.

THE RESPONSE OF A MULTI-ELEMENT TISSUE EQUIVALENT PROPORTIONAL COUNTER IN ACCELERATOR BASED HIGH-ALTITUDE NEUTRON FIELDS.

The response of a multi-element tissue equivalent proportional counter (METEPC) was investigated at the high energy neutron facility EU-CERF, in Prevessin, France. This facility was established specifically to provide reference neutron fields typical of those found at high altitude. The METEPC measurements were conducted along with a commercially available spherical tissue equivalent proportional counter (TEPC) used for comparison. The measured microdosimetric spectra were analyzed and compared for various measurement locations, with substantially different neutron energy distributions resulting from the use of iron or concrete shielding. The absorbed dose rate measured by the two counters in the dominant lower-energy neutron field locations from the iron-shielded target was <27%. A significant difference in the range of 32-58% was observed between the two counters in the higher-energy neutron field produced by the concrete-shielded target. Interestingly, due to a combination of factors, including the geometry and structure of the METEPC, values of dose-equivalent derived from the METEPC and the TEPC were in closer agreement (within 30%) and are well within acceptable limits for neutron monitoring.

FLUKA-BASED MONTE CARLO INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTIONS OF TELECOBALT BEAM.

FLUKA-based Monte Carlo calculations of microdosimetric distributions in water phantom involving a walled spherical Tissue-Equivalent Proportional Counter filled with tissue-equivalent propane gas have been studied for an indigenously developed telecobalt machine. The simulated site size considered in the study was 2 μm. In the Monte Carlo calculations, field size was varied from 10 cm × 10 cm to 35 cm × 35 cm and the depth was varied as 5-20 cm. The study also includes calculation of microdosimetric distributions with a 30° wedge filter. The efficiency of the calculations was improved up to a factor of 26 by choosing appropriate cut off values for production and transport of electrons. The calculated microdosimetric distributions of telecobalt machine is distinctly different from that of a bare 60Co source which is attributed to the influence of scattered photons from the machine head and the water phantom.

The spatial dose rate and energy characterization of a P 385 D-D Neutron Generator using a nested neutron spectrometer and a tissue equivalent proportional counter

A nested neutron spectrometer (NNS) and a tissue equivalent proportional counter (TEPC) were used to measure the spatial dose equivalent and energy neutron fluence distribution of a P 385 D-D Thermo Fisher Scientific Neutron Generator located in the mixed field dosimetry facility at the University of Ontario Institute of Technology. The P 385 Neutron Generator uses the D-D nuclear reaction to generate neutrons with an expected average energy of 2.5 MeV. The NNS uses a He-3 proportional counter to measure the neutron capture count rate as a function of nested cylindrical moderator thickness. Using the NNS and associated unfolding software, the energy neutron fluence distribution at various locations from the P 385 neutron generator were determined along with other radiation protection quantities of interest. Measurements of the dose equivalent rate were also conducted at the same locations using a 5-inch spherical TEPC and compared to the NNS results. The TEPC was further used to measure the dose equivalent as a function of radial distance from the neutron generator target and also for different P 385 operating conditions.

Microdosimetric Measurements in Gamma and neutron Fields with a Tissue Equivalent Proportional Counter Based on a Gas Electron Multiplier.

A multi-element tissue-equivalent proportional counter (TEPC), based on a single gas electron multiplier (GEM) foil of standard geometry, has been constructed with 16 cylindrical sensitives volumes. In this article, the design of this novel counter is described and first microdosimetric measurements are presented. To study the response of the GEM-TEPC to both low and high linear energy transfer radiation fields, the microdosimetric spectra due to a 137Cs gamma-ray source and to fast neutrons from 7Li(d,n)8Be reaction have been measured using pure propane gas at low pressure, in order to simulate a tissue site of about 1 µm equivalent size. The comparison with spectra measured with a spherical TEPC and with a mini-TEPC demonstrates promising properties for application of the GEM-TEPC for microdosimetric applications.

An analytical comparison of gas gain in spherical, cylindrical and hemispherical low-pressure proportional counters intended for use in experimental microdosimetry

Traditionally experimental microdosimetry has employed low pressure single cavity spherical Tissue Equivalent Proportional Counters (TEPCs). Multi-Element Tissue Equivalent Proportional Counters (METEPCs) with numerous cylindrical cavities have been constructed in order to increase sensitivity per unit volume; however existing METEPC designs are prohibitively complex and sensitive to motion and audible noise. This work proposes a novel hemispherical element with a wire-less anode ball as a solution to these issues. The gas gain characteristics of this hemispherical METEPC element were analyzed first for a single hemispherical TEPC to evaluate performance relative to current cylindrical and spherical counter designs that have been demonstrated experimentally to perform very well. This gain analysis evaluated relative avalanche size and the uniformity in maximum gain for electrons originating throughout the gas cavities of each of the three counters. Radial gas gain distributions for each counter were determined using both theoretical potential distributions as well as analytical equipotential distributions generated with ANSYS Maxwell (V. 14.0) to solve the Townsend equation. It was found that the hemispherical counter exhibits completely uniform gas gain for electrons approaching the anode from all directions and its avalanche region occupies only 3.5×10−3% of the entire gas cavity volume, whereas in the cylindrical and spherical counters the avalanche occupies 0.6% and 0.12% of the total respective gas cavity volumes. These analytical gas gain results are promising, suggesting that the hemisphere should exhibit uniform signal amplification throughout the gas cavity and if the recommended follow-up experimental work demonstrates the hemispherical counter works as anticipated it will be ready to be incorporated into an METEPC design.

2-D GEM-BASED TEPC FOR NEUTRON PROTECTION DOSIMETRY

A tissue equivalent proportional counter (TEPC) based on a gas electron multiplier (GEM) with a 2-dimensional (2-D) readout board for neutron dosimetry was developed and tested. This 2-D design of the anode structure presents multiple advantages over the traditional spherical TEPC design. This type of detector does not need a central wire to provide amplification as in traditional proportional counter design, making it possible to achieve higher efficiency (or sensitivity). Each charge-collecting pad in the 2-D structure represents a single miniature TEPC that can simulate the tissue size of a fraction of a micrometre. Such measurements could provide more precise dose-equivalent levels. In addition, the 2-D anode pattern provides the ability to reconstruct the 2-D projection of the ionizing particle’s track and it paves the way for the development of a time projection chamber for the full 3-D track reconstruction. This would prove an invaluable tool in future microdosimetry research. The simple design of ...

SINGLE ANODE TRIPLE GEM TISSUE EQUIVALENT PROPORTIONAL COUNTER AS THE BASIS FOR A PERSONAL NEUTRON DOSIMETER.

This paper reports on a tissue-equivalent proportional counter (TEPC) based on a triple gas electron multiplier structure, with a single pad readout, as a basis for a personal neutron dosimeter. Its dosimetric response was studied using the 252Cf neutron source at the Health Physics Generator Facility of the Canadian Nuclear Laboratories. Measured lineal energy spectra were found to be in agreement with numerical simulations performed with Monte Carlo N-Particle eXtended (MCNPX). Both simulations and measurements showed that the mean pathlength of secondary charged particles in the TEPC gas was best represented by the thickness of the drift region of the device. It was determined that the Cauchy Theorem, used to calculate the mean chord length in spherical and cylindrical TEPCs, overestimated the simulated mean chord length by nearly a factor of two. Important operational characteristics of the device were investigated, including gas gain, sensitivity and dosimetric response, as functions of tissue-equivalent gas pressure. This work demonstrates that the proposed design can serve as the basis for a personal neutron dosimeter device, which would satisfy the angular dosimetric response criteria of the personal dosimeter standard IEC61526.

Lineal energy calibration of a spherical TEPC.

Tissue-equivalent proportional counters (TEPCs) do not always allow built-in calibration alpha-particle sources, and the lineal energy calibration of these counters must be performed with an external radiation able to penetrate the detector walls. The irradiation field can be used for calibration if a particular marker point of known lineal energy is identified in the measured spectrum. This point is often identified with the proton edge, which corresponds to the maximum energy deposited by protons in the given volume. If the proton edge cannot be identified precisely in the measured spectrum, a gamma source can be used instead, identifying the maximum lineal energy due to electrons (e-edge). The technique was already described and applied for cylindrical TEPCs, allowing a calibration with an overall uncertainty smaller than 5 % (Conte et al. Lineal energy calibration of mini tissue equivalent gas-proportional counters (TEPC). AIP Conf. Proc. 1530, 171-178 (2013)). In the present work, this study was repeated for spherical detectors. First a marker point was identified in the microdosimetric spectrum of a (137)Cs gamma source, then a precise value of lineal energy was assigned to it. Gas pressures were varied to simulate diameters from 0.5 and 3 µm at density 1 g cm(-3). A simple power equation is given for allowing calibration of TEPCs filled with C3H8-TE gas at different pressures, using an external (137)Cs gamma source.

Description and verification of an algorithm for obtaining microdosimetric quantities for high-LET radiation using a single TEPC without pulse height analysis.

Microdosimetric spectra of single event distributions have been used to provide estimates of quality factors for radiation protection of high-LET radiation. In situations with high-dose rates it becomes difficult to measure, record and store energy deposition from single events. An alternative approach is to store random energy deposition events in a sequence of fixed time intervals that does not require identifying from single events. This can be accomplished with a single detector without pulse height analysis. We show the development of the algorithm using expectation analysis of the statistical estimators for moments of lineal energy: ȳf and ȳD. The method was tested using Monte Carlo simulations based on single event distributions measured with spherical tissue equivalent proportional counters where the event sizes spanned more than two orders of magnitude. The evaluation included testing at various mean numbers of events per interval (i.e., dose rate) and numbers of intervals (i.e., total duration). Results of the expectation analysis and Monte Carlo simulation showed that the algorithm corrects for the excess dispersion due to the random number of events in each time interval when the underlying dose rate is constant. It also converges to the correct value when there is a linear trend in dose rate of the duration of the measurement process. Although this system is not applicable for pulsed radiation fields it proved to be robust when applied to measured distributions with single event spectra (PuBe neutrons, Fe ions at 1,000 MeV/nucleon and a power function distribution of single event sizes) with a coefficient of variation of 25% for estimates of ȳD using 100 sampling intervals and 10% using 400 sampling intervals.

Sensitivity and uncertainty in the measurement of H*(10) in neutron fields using an REM500 and a multi-element TEPC

The REM500 is a commercial instrument based on a tissue-equivalent proportional counter (TEPC) that has been successfully deployed as a hand-held neutron monitor, although its sensitivity is regarded by some workers as low for nuclear power plant radiation protection work. Improvements in sensitivity can be obtained using a multi-element proportional counter design in which a large number of small detecting cavities replace the single large volume cavity of conventional TEPCs. In this work, the authors quantify the improvement in uncertainty that can be obtained by comparing the ambient dose equivalent measured with a REM500, which utilises a 5.72 cm (2(1/4) inch) diameter Rossi counter, with that of a multi-element TEPC designed to have the sensitivity of a 12.7 cm (5 inch) spherical TEPC. The results obtained also provide some insight into the influence of other design features of TEPCs, such as geometry and gas filling, on the measurement of ambient dose equivalent.

TEPC gas gain measurements in propane

Knowledge of the gas gain is important to optimise the design and the operating characteristics of tissue-equivalent proportional counters (TEPCs), especially for simulated sites smaller than 1 µm. TEPC area monitors of the order of centimetres must operate at very low gas pressure to simulate micrometric volumes, consequently the Townsend theory cannot be applied: effects related to the presence of an electric-field gradient become important and must be considered. A detailed description of the electron avalanche formation is complex, but in most practical cases an analytical formula can be used. The so-called gradient-field model includes three characteristic constants of the counting gas, which were already experimentally determined for propane-tissue equivalent (TE) and dimethyl ether (DME) gases. The aim of this work is to measure the gas-dependent parameters for propane gas. Preliminary results obtained with a spherical TEPC are presented.