Tissue Equivalent Proportional Counter Research Articles

Modelling of the Silicon-On-Insulator microdosimeter response within the International Space Station for astronauts’ radiation protection

Astronauts are exposed to high-energy cosmic radiation which may have harmful health effects. At the altitude of the International Space Station (ISS), the main radiation sources are Galactic Cosmic Rays (GCRs), Solar Particle Events (SPEs) and trapped protons of the Van Allen Belts. The radiation field mainly consists of protons, helium nuclei and heavy ions with energies up to hundreds of GeV/n. A powerful approach to determine the effect of space radiation on astronauts is microdosimetry. The Centre for Medical Radiation Physics is active in the development of Silicon-On-Insulator (SOI) microdosimeters, as an alternative to Tissue Equivalent Proportional Counters (TEPCs) for radiation protection purposes. SOI microdosimeters are portable and do not require a high-voltage power supply. They consist of a matrix of silicon Sensitive Volumes (SV), which mimic the dimensions of biological cells.In this study, we investigated for the first time the response of the 3D “Mushroom” microdosimeter, a type of SOI microdosimeter in the Columbus module of the ISS. Tissue-equivalent microdosimetric spectra of GCRs, SPEs, and trapped protons were obtained to estimate the dose equivalent delivered to the astronauts. Results demonstrate a non-negligible production of secondary particles due to the propagation of space radiation through the wall of the Columbus and the microdosimeter. A number of heavy ions were detected with high lineal energies, these events pose a significant hazard in terms of radiation protection.Moreover, the dose evaluation shows a good agreement with experimental data found in the literature, confirming the suitability of our Geant4 model and the feasibility of using the SOI microdosimeter for ISS astronauts’ personal dosimetry.

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.

Preparation and study of a conductive nanocomposite applicable for the wall material of tissue-equivalent proportional counters used in radiation protection

In the present work a tissue-equivalent (TE) nanocomposite is prepared by mixing polyamide 6 (PA6), polyethylene (PE) and carbon nanotubes (CNTs) to be a substitution for the expensive and difficult to obtain A-150 plastic as the wall material of tissue-equivalent proportional counters (TEPCs). It is found that 3% of CNTs in the nanocomposite produced 3 ×10−6S cm−1 electrical conductivity comparable with that of A-150 and the soft tissue. Also, TEM picture shows the electrical network formation in the continuous PA phase and the interface of PA and PE phases. Determination of the density and the weight percentage of elements of this nanocomposite reveal that the obtained values are similar to A-150 and the soft tissue. On the other hand, simulations carried out by Geant4 toolkit show that the fluence of secondary charged particles of the gamma rays or neutrons inside the gas cavity originating from A-150 and the nanocomposite (3% CNT) is similar. In addition, microdosimetric distributions derived from a TEPC with the nanocomposite wall agree well with a TEPC with A-150 wall. Hence, the frequency-mean and dose-mean lineal energies obtained by the two TEPCs are also compatible. A maximum difference 3% is observed between the frequency-mean lineal energies. Also, the dose-mean lineal energies differ up to 10%. Consequently, the dose-equivalent determined by the two above TEPCs is found similar. Finally, it is concluded that the fabricated nanocomposite can be a reliable substitution for A-150 plastic as a wall material of TEPCs in the neutrons and gamma ray measurements.

MONTE CARLO-BASED INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTION OF HIGH ENERGY BRACHYTHERAPY SOURCES.

FLUKA-based Monte Carlo calculations were carried out to study microdosimetric distributions in air and in water for encapsulated high energy brachytherapy sources (60Co, 137Cs, 192Ir and 169Yb) by simulating a Tissue Equivalent Proportional Counter (Model LET1/2) having sensitive diameter of 1. 27 cm for a site size of 1 μm. The study also included microdosimetric distributions of bare sources. When the sources are in air, for a given source, the source geometry does not affect the y¯F and y¯D values significantly. When the encapsulated 192Ir, 137Cs and 60Co sources are in water, y¯F and y¯D values increase with distance in water which is due to degradation in the energy of photons. Using the calculated values of y¯D, relative biological effectiveness (RBE) was obtained for the investigated sources. When 60Co, 137Cs and 192Ir sources are in water, RBE increases from 1.03 ± 0.01 to 1.17 ± 0.01, 1.24 ± 0.01 to 1.46 ± 0.02 and 1.50 ± 0.01 to 1.75 ± 0.03, respectively, when the distance was increased from 3-15 cm, whereas for 169Yb, RBE is about 2, independent of distance in water.

Measurement of environmental radiation and analysis of microdosimetric spectra of cosmic rays during the 35th expedition to Indian station Bharati, Antarctica

Tissue equivalent proportional counter (TEPC) model HAWK FWAD-3 was used for the evaluation of absorbed dose, dose equivalent rate (ḢTEPC) and microdosimetric spectra measured (i) in the mixed radiation fields formed mainly by cosmic rays onboard aircraft during the flights between Mumbai and the Indian station Bharati (Progress air base), two polar flights at Antarctica, and (ii)ground level environmental radiation field at locations situated in three continents, namely Antarctica, Africa and Asia with different latitudes. The study also includes evaluation of absorbed dose rate, dose equivalent rate of cosmic rays and terrestrial radiation using active detectors namely RADEYE-G and BERTHOLD Rem counter. An effort was made to compare the measured dose equivalent rate by TEPC for the commercial flight routes to Antarctica with the values calculated by the software code EPCARD and a good agreement was observed. The 35th expedition to Bharati, Antarctica showed higher values of dose equivalent at the pole (Larsemann Hills region, 69°24′37.60″S, 76°11′01.70″E) than the world average for an evaluation period of six months during summer.

THE COMPLEXITY OF QUANTITIES IN RADIATION DOSIMETRY: THE ISSUE OF RADIATION QUALITY.

When Harald Rossi developed a new radiation measurement instrument more than 60 years ago-the tissue equivalent low-pressure proportional counter, also called Rossi Counter-his initial intention was to provide a technique to measure LET. He realized soon that the measurements provided insight into the stochastic nature of the interactions of ionizing radiation with matter and that the measured results represented an alternative to characterize radiation quality in terms of stochastic or microdosimetric quantities. Progress in cellular and molecular radiation biology and in simulating charged particle tracks in biological entities are now essential features and have extended the original delineation of the field of microdosimetry. This general field of microdosimetry, as part of an interdisciplinary approach for the study of molecular mechanisms and cellular radiation effects, continues to be an important area of radiation research, documented by 13 previous Microdosimetry Symposia and, again, at this Symposium. However, the quantification of radiation quality remains a subject of practical importance. In fact, the issue has gained in importance due to the application of ions for therapy and due to the need for protection from exposures to complex and high-energy radiation fields, for example, to cosmic radiation in civil aviation and space missions. In practice pragmatic, empirical approaches are being used which has led to a complex set of dosimetric quantities and terms.

Microdosimetry on nanometric scale with a new low-pressure avalanche-confinement TEPC

The tissue equivalent proportional counter (TEPC) is the most accurate device for measuring the microdosimetric properties of a particle beam, nevertheless no detailed information on the track structure of the impinging particles can be obtained, since the lower operation limit of common TEPCs is about 0.3 μm. On the other hand, the pattern of particle interactions is measured by track-nanodosimetry, which derives the single-event distribution of ionization cluster size at the nanometric scale. Anyway, only three nanodosimeters are available worldwide. A feasibility study for extending the performances of TEPC down to the nanometric region was performed and a novel avalanche-confinement TEPC was designed and constructed. This detector is constituted by a cylindrical chamber, based on a three-electrode structure, connected to a vacuum and gas flow system to ensure a continuous replacement of the tissue equivalent gas, thus allowing to simulate different biological site sizes in the range 300-25 nm. This TEPC can be calibrated by exploiting a built-in alpha source and a miniaturized solid-state detector as a trigger. Irradiations with photons, fast neutrons and two hadron beams demonstrated the good performances of the device. A satisfactory agreement with FLUKA simulations was obtained.

A FPGA-based software for microdosimetric data processing

The tissue equivalent proportional counter (TEPC) is the most accurate device for measuring microdosimetric properties of particle beams. Since microdosimetric quantities span over several decades, the electronic and acquisition chain should meet specific requirements. In order to cover the wide dynamic range of the signals generated by the TEPC, the output signal from the preamplifier is fed in parallel to three linear amplifiers which shape and amplify the signal with different gains. Very low-energy deposition events are filtered in the high-gain stage, and high-energy deposition events are processed in the low-gain stage. A new system with high acquisition performance and compact hardware was developed for this purpose. The analog-to-digital conversion is performed by a commercial acquisition system that includes a FPGA. Thanks to the FPGA a parallel high-speed acquisition on three channels can be performed. The software merges signals together from the three electronic chains and computes a real time microdosimetric spectrum giving a prompt information about the irradiation field. This acquisition system, which performs analog-to-digital conversion and signal processing at a sampling rate up to 15 MS/s, was tested by irradiating a TEPC with an Am-Be fast neutron field, an intense quasi-monoenergetic neutron beam and a 62 MeV/u helium ion beam.

INVESTIGATION OF APPLICABILITY OF PURE PROPANE GAS FOR MICRODOSIMETRY AT NEUTRON FIELDS: A MONTE CARLO STUDY.

Applicability of pure propane gas for microdosimetric measurements in neutron fields was investigated using the FLUKA Monte Carlo code. Monoenergetic neutrons in the energy range 1 keV-20 MeV and the ISO-neutron sources such as 241Am-Be, 241Am-B, 252Cf and 252Cf + D2O were considered in the present study. The tissue-equivalent proportional counter (TEPC) simulated in the study was LET-1/2 (by Far West Technology) with site sizes 1, 2 and 8 μm. The study demonstrates that for a given site size, the TEPC filled with tissue-equivalent propane and pure propane gases produce similar microdosimetric distributions when the density of pure propane gas is lowered appropriately. For the ISO-neutron sources, the density of propane gas requires scaling by a factor 0.85. For the monoenergetic neutrons, depending upon the neutron energy, the values of scaling factors are in the range of 0.58-0.93.

MICRODOSIMETRY AT NANOMETRIC SCALE WITH AN AVALANCHE-CONFINEMENT TEPC: RESPONSE AGAINST A HELIUM ION BEAM.

The tissue-equivalent proportional counter (TEPC) is the most accurate device for measuring the microdosimetric properties of a particle beam but, since the lower operation limit of common TEPCs is ~0.3 μm, no detailed information on the track structure of the impinging particles can be obtained. The pattern of particle interactions at the nanometric level is measured directly by only three different nanodosimeters worldwide: practical instruments are not yet available. In order to partially fill the gap between microdosimetry and track-nanodosimetry, a low-pressure avalanche-confinement TEPC was designed and constructed for simulating tissue-equivalent sites down to the nanometric region. The present paper aims at describing the response of this TEPC in the range 0.3 μm-25 nm to a 62 MeV/n 4He ion beam. The experimental results, for depths near the Bragg peak, show good agreement with FLUKA simulations and suggest that, for smaller depths, the distribution is highly influenced by secondary electrons.

Design and preliminary test of a multi-element tissue-equivalent proportional counter based on the gas electron multiplier

In order to monitor the intense neutron and gamma radiation, multi-element tissue equivalent proportional counters (TEPC) based on the standard gas electron multiplier (GEM) and ceramic thick GEM (THGEM) foils were designed and respectively tested by implanted 55Fe and 241Am sources which would also be used as calibration sources. The performance testing of GEM-TEPC/THGEM-TEPC detectors at various parameters showed that the detectors could work well. The THGEM-TEPC was also used to measure the linear energy spectra in 241Am–Be and intense (108 ncm−2s−1) neutron radiation fields. The results displayed that THGEM-TEPC was available to do microdosimetric research or be a dosimeter in intense neutron/gamma field.

Out-of-field doses from secondary radiation produced in proton therapy and the associated risk of radiation-induced cancer from a brain tumor treatment

PurposeTo determine out-of-field doses produced in proton pencil beam scanning (PBS) therapy using Monte Carlo simulations and to estimate the associated risk of radiation-induced second cancer from a brain tumor treatment. MethodsSimulations of out-of-field absorbed doses were performed with MCNP6 and benchmarked against measurements with tissue-equivalent proportional counters (TEPC) for three irradiation setups: two irradiations of a water phantom using proton energies of 78–147 MeV and 177–223 MeV, and one brain tumor irradiation of a whole-body phantom. Out-of-field absorbed and equivalent doses to organs in a whole-body phantom following a brain tumor treatment were subsequently simulated and used to estimate the risk of radiation-induced cancer. Additionally, the contribution of absorbed dose originating from radiation produced in the nozzle was calculated from simulations. ResultsOut-of-field absorbed doses to the TEPC ranged from 0.4 to 135 µGy/Gy. The average deviation between simulations and measurements of the water phantom irradiations was about 17%. The absorbed dose contribution from radiation produced in the nozzle ranged between 0 and 70% of the total dose; the contribution was however small in absolute terms. The absorbed and equivalent doses to the organs ranged between 0.2 and 60 µGy/Gy and 0.5–151 µSv/Gy. The estimated lifetime risk of radiation-induced second cancer was approximately 0.01%. ConclusionsThe agreement of out-of-field absorbed doses between measurements and simulations was good given the sources of uncertainties. Calculations of out-of-field organ doses following a brain tumor treatment indicated that proton PBS therapy of brain tumors is associated with a low risk of radiation-induced cancer.

GNP enhanced responses in microdosimetric spectra for 192Ir source

The potential radiosensitizing effects of gold nanoparticles (GNPs) in radiation therapy have been extensively studied. GNPs loaded in the tumor produce a large number of low-energy secondary electrons which contribute to the local dose enhancement and elevate the relative biological effectiveness (RBE). Thus GNP enhanced responses in 192Ir, a widely used brachytherapy source, are the subject of interest. These radiosensitizing effects can be examined through comparisons between microdosimetric spectra of secondary electrons resulting from the 192Ir in the presence and absence of GNPs in cells. Measurements of single-event lineal energy spectra give indications of a relative therapeutic advantage. In the present work, a mini tissue equivalent proportional counter (TEPC) was constructed to measure the microdosimetric spectra in cells containing and without GNPs for 192Ir source. A plug, either contained a 50 nm-thick gold foil coated on its inner surface or without the foil, was inserted into the TEPC wall during measurements. Analyses of the measured un-normalized and normalized dose lineal energy spectra provided information on the enhancements in absorbed dose and RBE. The dose enhancement was 1.27 and 1.37 for 1 and 0.5 μm-diameter sites, respectively. The increase in dose enhancement with decreasing site diameter revealed the sharp fall-off in the local dose enhancement. Applying the biological weighting function method, the RBE enhancement was 1.32 based on the lineal energy spectra of 1 μm site.

Slowing-down and stopped charged particles cause angular dependence for absorbed dose measurements

The space radiation environment is dominated by heavy charged particles with atomic numbers ranging from 1 to 93, with broad energy spectra that exceed 10 GeV per nucleon. Despite advances in space radiation modeling and transport, radiation detectors continue to provide critical data for understanding risks of health effects to astronauts in space. In the past, NASA relied on tissue-equivalent proportional counters and passive devices for operational dosimetry; however, in recent years, pixel detectors providing detailed information about the radiation environment through analysis of charged particle tracks have been demonstrated in space. These next-generation detectors, based on Timepix read-out technology, require special analysis considerations that were not necessary or possible for previous dosimetry tools. The impacts of slowing-down and stopped ions on absorbed dose measurements must be explicitly modeled to understand variations with detector orientation. The purpose of the present study is to conclusively demonstrate that while absorbed dose measurements of penetrating charged particles are independent of detector orientation, slowing-down and stopped particles can result in charged particle absorbed dose measurements that are dependent on detector orientation. Monte Carlo simulations of an unshielded detector, irradiated at selected orientations by different kinetic energy domains with fluence spectra representative of two historical solar particle events, are presented to demonstrate the dependence of absorbed dose measurements. Next, results from Monte Carlo simulations of the same energy domains and fluence spectra, isotropically impinging on an anisotropic shield configuration about the detector, are shown, to exhibit the potential for observing varying absorbed doses under realistic environment and shielding conditions. Finally, slowing-down and stopped proton data acquired with Timepix-based detectors at the Tandem Van de Graaff at Brookhaven National Laboratory are used to demonstrate the effect via accelerator-based measurements.

In-field and out-of-file application in 12C ion therapy using fully 3D silicon microdosimeters

This paper presents recent development of Silicon on Insulator (SOI) detectors for microdosimetry at the Centre for Medical Radiation Physics (CMRP) at the University of Wollongong. A new CMRP SOI microdosimeter design, the 3D mushroom microdosimeter is presented. Modification of SOI design and changes to the fabrication processes have led to improved definition of the microscopic sensitive volumes (SV), and thus to better modelling of the deposition of ionizing energy in a biological cell. The electrical and charge collection properties of the devices have been presented in previous works. In this study, the response of the microdosimeters in monoenergetic and spread out Bragg peak therapeutic 12C ion beam at Heavy Ion Medical Accelerator in Chiba (HIMAC, Japan) are presented. Derived relative biological effectiveness (RBE) in 12C ion radiation therapy matches the tissue equivalent proportional counter (TEPC) well, along with outstanding spatial resolution. The use of SOI technology in experimental microdosimetry offers simplicity (no gas system or HV supply), high spatial resolution, low cost, high count rates capabilities for beam characterisation and quality assurance (QA) in charged particle therapy.

A LiF and BeO TLD based microdosimeter for space radiation risk assessment of astronauts

The ratio of thermoluminescence glow curve area of BeO and LiF dosimeters was found to be proportional to average LET and quality factor (Q) of impinging mixed radiations. Using this phenomenon and widely available Thermoluminescence Dosimeter TLD-700 (7LiF: Mg,Ti) and BeO (Thermolux 995) chips a TLD-Microdosimeter (LiBe-14) emulating a much larger gas-filled Tissue Equivalent Proportional Counter (TEPC) was developed. The TEPC is an essential device of space radiation dosimetry widely used by international space agencies. The LiBe-14 is capable of assessing the LETTissue (5–300 keV/μm), quality factor Q (1–30) and associated dose equivalent H (0.1–1000 mSv) of any mixed radiation fields of interest, including space radiations predominant in Low Earth Orbit (LEO) environment. The TLD microdosimeter was calibrated using the secondary radiation fields produced by bombarding a 25 cm × 25 cm × 35 cm polystyrene phantom with 81, 119, 150, 177, 201 and 231 MeV protons from a Proton Therapy Medical Cyclotron. The TLD pair (BeO/LiF) was attached to the TEPC and placed lateral to the proton beam. The characteristics of space radiation inside the spacecraft are complex. Hence, personal dosimetry of astronauts in the space habitat is performed using “multi-element” dosimeters made of different types of TLD and CR-39 plastic nuclear track detector (PNTD). The TLD and PNTD are used to assess the sparsely (low LET) and densely (high LET) ionising radiation component respectively. This report elucidates the feasibility of LiBe-14 microdosimeter for the estimation of overall dose equivalent and “risk of exposure induced death” (REID) of astronauts working in LEO space stations.

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.

Instrument intercomparison in the high-energy field at the CERN-EU reference field (CERF) facility and comparison with the 2017 FLUKA simulations

This paper discusses an instrument intercomparison performed in the high-energy field at the CERF facility at CERN between 2015 and 2017 (October 2015, May 2016, November 2016 and June 2017). Measurements were performed in several reference exposure locations with the CERN extended-range Bonner Sphere Spectrometer (BSS), a Berthold LB4611, the LINUS rem counter from CERN, the LUPIN rem counter from ELSE Nuclear, the FHT 762 Wide Energy Neutron Detection Instrument (WENDI-II) from Thermo Scientific, the LUDLUM MODEL 42–41 PRESCILA NEUTRON PROBE and two models of Tissue-equivalent proportional counters (TEPCs) from Far West Technologies. All results are compared with the latest FLUKA reference values from simulations performed in 2017.