Mean Quality Factor Research Articles

Designing an ultra-compact SOI waveguide with a high nonlinear coefficient for generating optical frequency combs and demultiplexing combs

In this paper, an ultra-compact high capability silicon-based waveguide with a high nonlinear coefficient and engineered dispersion was designed for frequency combs generation and demultiplexing of these combs. In the first part of the structure, a silicon waveguide with two zero-dispersion wavelengths, 1552[Formula: see text]nm and 2350[Formula: see text]nm, and a length of 4[Formula: see text]mm was used to generate optical frequency combs using the four-wave mixing method. Considering the high nonlinear coefficient of [Formula: see text] in the waveguide, an input pump power of 500[Formula: see text]mW was applied to the waveguide at near-zero-dispersion wavelengths of 1552[Formula: see text]nm. The output comb spectrum of the waveguide was obtained after the length of 4[Formula: see text]mm with a spectral bandwidth of 400[Formula: see text]nm and a free spectral range of 5[Formula: see text]nm. Additionally, by similar pumping at wavelengths of 1553[Formula: see text]nm and 1555[Formula: see text]nm to this waveguide, optical frequency combs with a free spectral range of 2[Formula: see text]nm and a spectral bandwidth of 150[Formula: see text]nm were obtained. In the second part of the structure, to isolate the optical frequency combs, the given waveguide was expanded to generate an SOI demultiplexer with micro-ring resonator filters, which could separate optical frequency combs with the wavelength of 1555[Formula: see text]nm, 1557[Formula: see text]nm, 1559[Formula: see text]nm, and 1561[Formula: see text]nm due to the selection of suitable values for the radius of the ring resonators. The mean transmission efficiency, spectral bandwidth, and quality factor were obtained to be 99.51%, 0.275[Formula: see text]nm, and 5675, respectively. The channel spacing in the proposed demultiplexer was 2[Formula: see text]nm and the minimum and maximum crosstalk were −36[Formula: see text]dB and −11.3[Formula: see text]dB.

Microdosimetry-based investigation of biological effectiveness of 252Cf brachytherapy source: TOPAS Monte Carlo study

Objective. To investigate biological effectiveness of 252Cf brachytherapy source using Monte Carlo-calculated microdosimetric distributions. Approach. 252Cf source capsule was placed at the center of the spherical water phantom and phase-space data were scored as a function of radial distance in water (R = 1–5 cm) using TOPAS Monte Carlo code. The phase-space data were used to calculate microdosimetric distributions at 1 μm site size. Using these distributions, Relative Biological Effectiveness (RBE), mean quality factor () and Oxygen Enhancement Ratio (OER) were calculated as a function of R. Main results. The overall shapes of the microdosimetric distributions are comparable at all the radial distances in water. However, slight variation in the bin-wise yield is observed with R. RBE, and OER are insensitive to R over the range 1–5 cm. Microdosimetric kinetic model based RBE values are about 2.3 and 2.8 for HSG tumour cells and V79 cells, respectively, whereas biological weighting function-based RBE is about 2.8. ICRP 60 and ICRU 40 recommendation-based values are about 14.5 and 16, respectively. Theory of dual radiation action based RBE is 11.4. The calculated value of OER is 1.6. Significance. This study demonstrates the relative insensitivity of RBE, and OER radially away from the 252Cf source along the distances of 1–5 cm in water.

Metre-scale damage zone characterization usingS-coda waves from active ultrasonic transmission measurements in the STIMTEC project, URL Reiche Zeche, Germany

SUMMARYStudies of controlled hydraulic stimulation experiments with active and passive seismic monitoring conducted in Underground Research Laboratories (URLs) benefit from specific knowledge of hydraulic parameters, close by microseismic monitoring revealing structural details of the rock mass, and detailed evolution of seismicity in response to injection operations. Microseismic monitoring is commonly used to characterize a stimulated reservoir volume, for example, in terms of damage evolution of the rock mass. Since seismic attenuation is affected by damage of the rock volume, active seismic sources covering sizes from the centimetre to decimetre scale may help us to investigate space–time varying attenuation properties in a reservoir. This may allow us to monitor damage evolution of the stimulated rock volume in more detail, also since active seismic sources produce stronger signals leading to a broader frequency range that can be analysed compared to passive seismic signals. Within the STIMTEC project in the URL Reiche Zeche (URL-RZ) in Freiberg (Germany), more than 300 active Ultrasonic Transmission (UT) measurements were performed before and after hydraulic stimulations in two boreholes in the targeted rock volume, an anisotropic metamorphic gneiss. The signal-frequency content ranges between 1 and 60 kHz. Assuming scattering attenuation to dominate over intrinsic attenuation, we here apply the single isotropic scattering model. S-coda waves of 88 spatially representative UT measurements are used to estimate the coda quality factor (QC). We obtain stable QC estimates for centre frequencies of octave-width frequency bands between 3 and 21 kHz. We group neighbouring UT measurements to stabilize the observations and form eight UT groups in total, covering different depth intervals in three boreholes and four different time periods to investigate scattering attenuation changes in a spatiotemporal manner. Our final mean QC ($\overline {{Q}_C} $) estimates show characteristic frequency-dependence as observed at the field scale in geological reservoirs. We find temporal variations of QC are strongly connected to hydraulic stimulation, and these variations are more significant than those resolved from velocity changes. $\overline {{Q}_C} $ estimates at frequencies above 15 kHz indicate healing of injection-induced small-scale fractures during a two-months post-stimulation phase. Larger fractures, mostly sampled by lower frequencies (<15 kHz), seem to be more persistent with time (over 15 months). We observe spatial differences of $\overline {{Q}_C} $ values near the mine galleries (driftway and vein drift) and relate these observations to different extents and characteristics of the galleries’ excavation damage zones. Our results further support previous assumptions based on borehole televiewer logs and mapped structures of an existing fault with larger damage zone that crosses the stimulated rock volume NW-SE between the galleries. We conclude that the coda analysis of active UT measurements complements established imaging methods used during experiments in URLs. In particular, coda analysis is a powerful tool for the detection of damage zones and for monitoring local fracture networks with immediate application for imaging georeservoirs considered for exploitation or underground storage of gases and liquids.

Microdosimetry-based quality factors for ISO reference photon and beta sources: TOPAS Monte Carlo study

In radiological protection, personnel dosimeters are calibrated in terms of operational quantities such as H p (10) and H p (0.07) against ISO reference photon and beta sources, respectively. H p (10) and H p (0.07) are defined at 1 cm and 0.07 μm depths in the ICRU-4 element tissue phantom. In present study, mean quality factor Q̅ is calculated at these depths using the TOPAS-based microdosimetric distributions at 1 μm site size and FLUKA-based Linear Energy Transfer (LET) dependent Q̅. The sources simulated are ISO reference narrow series X-rays, 137Cs and 147Pm, 85Kr, 90Sr/90Y and 106Ru/106Rh beta sources. Using the calculated microdosimetric distributions, Q̅ is calculated based on ICRP60 and ICRU40 recommendations, Keller-Hahn (KH) approximation, Microdosimetric Kinetic Model (MKM), Clonogenic Survival and Theory of Dual Radiation Action (TDRA). The study includes FLUKA-based Q̅ values calculated using Q(L) vs L relationship as per ICRP Report 60. The MKM, Clonogenic Survival-based RBE, ICRU40, TDRA and KH-based Q values of all the investigated sources are about unity. Depending on the source, microdosimetry-based Q̅ values using ICRP60 recommendations show significant difference from that calculated using other models. Q̅ values of 137Cs are unity, independent of the models.

Comparison of 126 MeV antiproton and proton—a FLUKA-based microdosimetric approach

Objective. This study aims at comparing dosimetric parameters of 126 MeV antiprotons and protons using microdosimetric approach. Approach. Microdosimetric distributions of 126 MeV proton and antiproton beams at 1 μm site size are calculated using the Monte Carlo-based FLUKA code. The distributions are calculated at various depths along the central axis in water phantom as well as at different off-axis locations. The study also includes calculations of secondary radiations produced by antiprotons and protons. Mean quality factor, is calculated using the ICRP 60 and ICRU 40 recommendations. The Relative Biological Effectiveness (RBE) of HSG tumour cell at 10% survival level is calculated based on Microdosimetric Kinetic Model. Main results. and RBE for antiprotons are higher by a factor of about 3.60, 3.41 and 1.24, respectively, at Bragg-peak and higher by a factor of about 1.41, 1.76 and 1.05, respectively, at plateau region of depth-dose profile when compared to protons. At 15 cm depth along the central axis, and RBE for protons are higher by a factor of about 1.42, 1.66 and 1.26, respectively, when compared to antiprotons. At the off-axis distance (Ld ) of 6 cm (at 11.5 cm depth in water), and of protons are higher than that of antiproton whereas the trend is opposite at off-axis distance of 4 cm. At Ld = 4 cm (at 11.5 cm depth in water), RBE of antiprotons is higher by about 4% than protons whereas at Ld = 6 cm, RBE of protons is higher by about 13% than antiprotons. Significance. The study shows that antiproton radiotherapy is advantageous as compared to protons considering enhancements in the absorbed dose and RBE-weighed dose values at the Bragg-peak.

Thick shielding against galactic cosmic radiation: A Monte Carlo study with focus on the role of secondary neutrons

The exposure to galactic cosmic radiation (GCR) is a major health concern for astronauts. Crewed missions with durations of several years are foreseen in future space exploration projects such as permanent habitats on the Moon and flights to Mars. This aim requires elaborate space radiation shielding concepts and a proper understanding of the underlying radiation physics and radiobiology as well as their interplay.In the present work, Monte Carlo simulations to assess the performance of different materials (polyethylene, aluminum, Moon regolith) as thick shields (up to 400 g/cm2) against GCR were conducted using the FLUKA code. Absorbed dose, dose equivalent and the mean quality factor at 1 cm depth in the ICRU sphere as a function of shielding thickness were calculated in a spherical shell configuration for both solar minimum and solar maximum GCR conditions. Large differences were observed in the performance of the studied materials as thick GCR shields. Special attention was paid to the build-up and moderation of secondary neutrons. A method to reduce the neutron contributions to ambient dose equivalent by means of a two-layer shielding combination is proposed. The present study can be useful for considerations on thick shielding of Moon or Mars habitats built from local regolith.

TISSUE-EQUIVALENCE OF H2 GAS FOR MICRODOSIMETRY IN NEUTRON FIELDS: A GEANT4 MONTE CARLO STUDY.

Hydrogen (H2) as filling gas in Tissue-equivalent proportional counter (TEPC) for measurements of microdosimetric distributions in neutron fields is investigated using the Monte Carlo-based Geant4 toolkit. The neutron fields considered are monoenergetic neutrons (1keV-14MeV) and ISO reference neutron sources 241Am-Be, 241Am-B, 252Cf and 252Cf+D2O. Based on the energy deposited in the gas cavities (tissue-equivalent (TE) propane and H2) of the TEPC, density scaling correction, ${f}_{\rho }$, to be applied on H2 gas to achieve tissue-equivalence is calculated for the above sources at 1μm site size. The calculated value of ${f}_{\rho }$ for the ISO-neutron sources is 0.40, which suggests that energy deposition in the gas cavities is predominantly due to crossers. In the case of monoenergetic neutrons, depending upon the energy, ${f}_{\rho }$ is in the range of 0.11-0.45. The study shows that the TE propane and H2-based microdosimetric distributions are comparable when the density of H2 is scaled appropriately. Mean quality factor $\overline{Q}$ calculated based on microdosimetric distribution increases initially with neutron energy up to 100keV and thereafter decreases with energy. Depending upon the neutron energy, TE propane and H2-based $\overline{Q}$ values are comparable to within 3-15%, and for the ISO sources, the comparison is within 5-8%. For the ISO neutron sources, $\overline{Q}$ values are in the range of ~10-15.

High-performance tunable multi-channel graphene-based square ring resonator demultiplexer

Here, a novel graphene-based square ring resonator is proposed to design 2-channel and 4-channel demultiplexer. Compared to typical ones, graphene demultiplexers are a better performer and recently considered as a highly promising candidate for low footprint and minimum crosstalk purposes. By introducing such a unique gradient, the plasmon resonances of the demultiplexer are directly excited and resonant wavelength tuning is achieved. To validate the performance of the proposed demultiplexer, coupled-mode theory is used to examine the structure theoretically. We demonstrate through simulations that the simulation results are in good agreement to the theoretical ones. The minimum crosstalk -48.12 dB and -34.13 dB with an average -38.44 dB and -27.12 dB are achieved and the mean spectral bandwidth, quality factor and power transmission efficiency 303.5 nm and 308.75 nm, 30.04 and 28.62, 60.5% and 47% are computed, respectively for 2-channel and 4-channel demultiplexers. These values are very suitable for photonic integrated systems in the mid-IR regime.

Performance of zero cross correlation resultant weight spectral amplitude codes in lower Earth orbit‐based optical wireless channel system

SummaryA consolidated performance investigation and design of newly constructed zero cross correlation resultant weight (ZCCRW) code is presented without mapping over optical wireless channel (OWC) in lower Earth orbit (LEO). Multiple access interference (MAI) is suppressed by incorporating proposed 1‐D code at 10 Gbps with an algorithm. A further state of the art comparison of diverse optical code division multiple access (OCDMA) codes such as multi diagonal codes and diagonal double weight codes is accomplished with proposed code in terms of root mean square (RMS) jitter, extinction ratio, MAI, quality factor (QF), and bit error rate (BER) at different linewidths, chip sizes, link lengths, and active users. It is perceived that for ZCCRW code, QF obtained is 16.5 for chip size (0.1 ns), and at 4000 km, BER 10‐9 is achieved using the forward error correction (FEC) technique. OWC system in LEO with lasers in spectral amplitude code (SAC) OCDMA is proposed for the first time as per the author's best knowledge.

A new proposal for a high-performance 4-channel demultiplexer based on 2D photonic crystal using three cascaded ring resonators for applications in advanced optical systems

In this paper, a high-performance 4-channel demultiplexer (DEMUX) based on 2D photonic crystal using three cascaded ring resonators is proposed. The proposed DEMUX is composed of square lattice of silicon dielectric rods with a refractive index of 3.46 suspended in air. For the proposed device, the lattice constant a = 590 nm and the filling factor r/a = 0.2. In this study, the plane wave expansion and finite-difference time-domain methods are used respectively to characterize the photonic band gap and to investigate the optical behaviour of the structure. It is demonstrated that by choosing appropriate values for the radius of the inner rods, scattering rods, coupling rods and the coupling length between the bus and the drop waveguides, the desired wavelengths can effectively be separated. The resonant wavelengths of the proposed DEMUX are 1528.7 nm, 1535.3 nm, 1531 nm, and 1533 nm. The mean quality factor, spectral bandwidth and the transmission efficiency of our device are computed to be respectively 7358.5, 0.217 nm and 99.25%. The proposed DEMUX has a mean channel spacing, minimum and maximum values of crosstalk of about 2.2 nm, − 46.68 dB and − 9.79 dB, respectively. Furthermore, the footprint of the device is 294.4 μm2, which is suitable for photonic integrated circuits.

Microdosimetric spectra simulated with MCNP6.1 with INCL4/ABLA model for kerma and mean quality factor assessment, for neutrons between 100 keV to 19 MeV

From the application of the microdosimetric approach in dosimetry, the standard TEPC, i.e., Rossi-type detector, was found to be a well-suited device to measure “first collision” neutron kerma and develop lineal energy spectra for incident neutrons. Subsequently, several attempts were carried out to assess the ambient dose equivalent for neutrons, with a mixed outcome, through the evaluation of the mean quality factor. The ICRU report 63, released in 2000, has officially recognized a good agreement between theoretical Kerma factor and value measured with standard TEPCs for neutron energies ranging over 5–66 MeV. As a follow-up, benchmark with simulated results for Kerma calculation and lineal energy spectra concluded to a satisfactory agreement for Monte-Carlo transport codes such as FLUKA or GEANT4. Nevertheless, previous studies have highlighted major discrepancies with MCNP results for energy above the capture threshold, mainly stemming from the weakness of the alpha release. This paper emphasizes the improvement of the kerma factor calculation as well as the development of lineal energy spectra from a standard A150 plastic-wall TEPC with MCNP6.1. The shortcomings of the alpha release are overcome using INCL4/ABLA implementation instead of the default model for the Intra-nuclear cascade. This new approach lead to lineal energy spectra overlapping those from experiments. In addition, the neutron kerma is evaluated with a mean relative deviation not exceeding 5% of the ICRU standard values, for mono-energies from 100 keV to 19 MeV. This new approach is also relevant for usual neutron sources as AmBe and 252Cf with a maximum deviation of roughly 6%. Nevertheless, major discrepancies remain for evaluation of the quality factor at the depth of 10 mm inside de ICRU sphere, with a mean relative deviation of 17%. However, this finding is consistent with discrepancies in literature. As regards the prospects, satisfactory results on lineal energy spectra and kerma evaluation henceforth offers the possibility to simulate microdosimetric spectra and determine neutron kerma for energies higher than a hundred keV up to twenty MeV.

Physical and Radiobiological Evaluation of Accelerated Intensity Modulated Radiotherapy for Locally Advanced Head and Neck Cancer and Comparison with Short-Term Clinical Outcomes.

Objective:The present study aims to evaluate the accelerated intensity modulated radiotherapy (IMRT) of head and neck (HandN) treatments using physical indices and radiobiological models with its clinical correlation using histogram analysis in radiation therapy (HART). The radiobiological evaluation in terms of tumor control probability (TCP) and normal tissue complication probability (NTCP) indices were compared with acute toxicity. Materials and Methods:A total of twenty patients with stage III and IV of HandN cases treated with accelerated IMRT using 6MV photons were chosen for the study. Using HART software, physical indices of the IMRT plans have been defined by universal plan indices (UPI’s) which summarize the various recognized plan indices. The overall quality factor (QF) of a plan was determined by a linear combination of all indices in UPI set. The clinical outcomes in terms of the acute toxicity like dysphagia and xerostomia were compared with NTCP values of the OAR calculated from HART software. Results:The mean QF and the mean Poisson TCP index was found to be 0.993±0.02 and 0.86 ±0.02 respectively. The mean JT Lyman NTCP index for bilateral parotid, constrictors, and larynx were found to be 0.23±0.14, 0.30±0.17 and 0.22±0.15 respectively. The acute toxicities in terms of severity of xerostomia and dysphagia have shown a moderate correlation with NTCP values of bilateral parotids, constrictors, and larynx, respectively. Conclusion:The mean QF based on UPI was found to be close to unity, which correlates with being a better IMRT plan. The present study suggested the existence of a moderate correlation between the calculated NTCP values and their respective severities of the organ at risk (OAR’s). Accelerated IMRT with chemotherapy is a clinically feasible option in the treatment of locally advanced head and neck squamous cell carcinoma (HNSCC) with encouraging initial tumor response and acceptable acute toxicities.

Low-loss high-Q silicon-rich silicon nitride microresonators for Kerr nonlinear optics.

Silicon nitride is a dielectric material widely used for applications in linear and nonlinear optics. It has an ultra-broad transparency window, low intrinsic loss, and a refractive index that allows for moderate optical field confinement in waveguides. The chemical composition of this material can be precisely set during the fabrication process, leading to an extra degree of freedom for tailoring the optical and mechanical properties of photonic chips. Silicon-rich silicon nitride waveguides are appealing for nonlinear optics, because they have a higher nonlinear Kerr coefficient and refractive index than what is possible with stoichiometric silicon nitride. This is a direct consequence of the increased silicon content. However, silicon-rich silicon nitride waveguides typically display higher absorption losses. In this Letter, we report low-loss (∼0.4 dB/cm) silicon-rich silicon nitride waveguides. The structures feature high optical confinement and can be engineered with low anomalous dispersion. We find an optimum silicon composition that, through an annealing process, overcomes optical losses associated to N-H bonds in the telecom band. Based on this technology, we successfully fabricate microresonators with mean quality factors (Q) ∼0.8×106 in the C and L bands. Broadband coherent microresonator frequency combs are generated in this platform, indicating its potential for efficient Kerr nonlinear optics.

Design of low cross-talk and high-quality-factor 2-channel and 4-channel optical demultiplexers based on photonic crystal nano-ring resonator

In this paper, 2-channel and 4-channel optical demultiplexers have been designed based on photonic crystal nano-ring resonator. The structure of both demultiplexers is based on the use of a photonic crystal filter. The mean quality factor of 2828, mean transfer factor of 95%, mean cross-talk of − 25.07 dB, and an inter-channel distance of 7.4 nm are among the structural advantages of the 2-channel demultiplexer. The advantages of the 4-channel demultiplexer include the mean quality factor of 4525, mean channel power transfer factor of 95%, and maximum and minimum channel cross-talk of − 19.6 and − 40.4, respectively. Also, the inter-channel difference is 7.8 nm and mean channel width is 0.375 nm. Moreover, the simplicity of the structure and the small size are among the characteristics of these designs. The proposed 2-channel and 4-channel demultiplexers can be used in WDM systems, especially DWDM systems.

Despeckling of Carotid Artery Ultrasound Images with a Calculus Approach.

Carotid artery images indicate any presence of plaque content, which may lead to atherosclerosis and stroke. Early identification of the disease is possible by taking B-mode ultrasound images in the carotid artery. Speckle is the inherent noise content in the ultrasound images, which essentially needs to be minimized. The objective of the proposed method is to convert the multiplicative speckle noise into additive, after which the frequency transformations can be applied. The method uses simple differentiation and integral calculus and is named variable gradient summation. It differs from the conventional homomorphic filter, by preserving the edge features to a great extent and better denoising. The additive image is subjected to wavelet decomposition and further speckle filtering with three different filters Non Local Means (NLM), Vectorial Total Variation (VTV) and Block Matching and 3D filtering (BM3D) algorithms. By this approach, the components dependent on the image are identified and the unwanted noise content existing in the high frequency portion of the image is removed. Experiments conducted on a set of 300 B-mode ultrasound carotid artery images and the simulation results prove that the proposed method of denoising gives enhanced results as compared to the conventional process in terms of the performance evaluation methods like peak signal to noise ratio, mean square error, mean absolute error, root mean square error, structural similarity, quality factor, correlation and image enhancement factor.

Numerical simulation of all optical demultiplexer based on pillar photonic crystal ring resonators

AbstractWe present the numerical simulation of an all‐optical demultiplexer using 2D photonic crystal ring resonators for optical communications systems. The structure consists of three ring resonators, one bus waveguide, and three drop waveguides. The photonic crystal of the demultiplexer has a square‐shaped structure made up of silicon rods with lattice constant a = 548 nm, filling factor(r/a) of 0.18, where r is the radius of the rods, and linear refractive index of n = 3.4. The structure can filter three standard wavelengths with 4.2 nm channel spacing between the wavelengths. From the simulations, the mean quality factor and transmission efficiency of our structure are computed to be respectively 4936 and 96.5%, at wavelengths very close to 1.55 μm. The minimum and maximum values of crosstalk are −18.6 dB and −34.6 dB, respectively. The footprint of the device is 293.7 μm2, which is suitable for photonic integrated circuits. The optical characteristics and the behavior of the proposed demultiplexer have been achieved by means of finite‐difference time‐domain (FDTD) method.

Simulated response of a multi-element thick gas electron multiplier-based microdosimeter to high energy neutrons

The response of a microdosimeter for neutrons above 14 MeV is investigated. The mean quality factors and dose-equivalents are determined using lineal energy distributions calculated by Monte Carlo simulations (Geant4 toolkit). From 14 MeV to 5 GeV, the mean quality factors were found to vary between 6.00 and 9.30 and the dose-equivalents were in agreement with the true ambient dose-equivalent at the depth of 10 mm inside the ICRU sphere, H*(10). An energy-independent dose-equivalent response around a median value of 0.86 within 22% uncertainty was obtained. Therefore, the microdosimeter is appropriate for dose-equivalent measurement of high-energy neutrons.

Analysis and mitigation of interface losses in trenched superconducting coplanar waveguide resonators

Improving the performance of superconducting qubits and resonators generally results from a combination of materials and fabrication process improvements and design modifications that reduce device sensitivity to residual losses. One instance of this approach is to use trenching into the device substrate in combination with superconductors and dielectrics with low intrinsic losses to improve quality factors and coherence times. Here, we demonstrate titanium nitride coplanar waveguide resonators with mean quality factors exceeding two million and controlled trenching reaching 2.2 μm in the silicon substrate. Additionally, we measure sets of resonators with a range of sizes and trench depths and compare these results with finite-element simulations to demonstrate quantitative agreement with a model of interface dielectric loss. We then apply this analysis to determine the extent to which trenching can improve resonator performance.

COMPARISON STUDY OF VARIOUS PLASTICS AS THE WALL MATERIAL OF THGEM-BASED MICRODOSEMETERS FOR FAST NEUTRON MEASUREMENTS.

To find appropriate substitutions for the expensive plastics of A-150 and rexolite used in the construction of thick gas electron multiplier (THGEM)-based tissue-equivalent proportional counters, in the present work, the responses of a THGEM-based microdosimetric detector made of A-150 and rexolite and three others composed of plexiglas (PMMA), polyethylene and polystyrene plastics as the wall materials have been compared. Lineal energy distribution, frequency-averaged lineal energy, dose-averaged lineal energy, mean quality factor and dose-equivalent for 0.1, 1 and 10 MeV neutrons and also for 241Am-Be neutrons are calculated using Geant4 simulation toolkit. Frequency-averaged lineal energy, dose-averaged lineal energy, mean quality factor and dose-equivalent values for all plastics are found similar. In addition, the response of an indigenously constructed microdosemeter with PMMA walls is also measured for 241Am-Be neutrons. The experimental results are in good agreement with the simulation predictions. Conclusively, it was found that the three considered plastics can be used as good candidates instead of A-150 and rexolite plastics in fast neutron microdosimetry.

Simulation and experimental study of an indigenously designed and constructed THGEM-based microdosimeter for dose-equivalent measurement

Most of the GEM/THGEM-based microdosimetric detectors presented in the literature simulate 2 μm of tissue which results in a flat neutron dose-equivalent response in the MeV region. The objective of this work was to introduce a neutron microdosimeter with a more extended flat response. In this regard, a THGEM-based microdosimeter with plexiglas walls, simulating 1 μm of tissue was designed and constructed. Its performance was investigated by both simulation and experimentation to determine the microdosimetric quantity of “lineal energy”.In the simulation study, lineal energy distribution, mean quality factor and dose-equivalent response of the microdosimeter for eleven neutron energies from 10 keV to 14 MeV, along with the energy spectrum of 241Am-Be neutrons, were calculated by the Geant4 simulation toolkit. Obtained lineal energy distributions were compatible with the distributions determined by a Rossi counter. Also, the mean quality factors agreed well with the values reported by the ICRU report 40 which confirmed tissue equivalent behavior of the microdosimeter. They were different from the effective quality factor values within 15% between 20 keV and 14 MeV. This led to a flat dose-equivalent response with 20% difference from a median value of 0.82 in the above energy range which was an improvement compared with other THGEM-based detectors, simulating 2 μm of tissue. In spite of the satisfactory determination of the dose-equivalent, the microdosimeter had low detection sensitivity.In the experimental study, the measured lineal energy distribution of 241Am-Be neutrons was in agreement with the simulated distribution. Further, the measured mean quality factor and dose-equivalent differed by 1.5% and 3.5%, respectively, from the calculated values. Finally, it could be concluded that the investigated microdosimeter reliably determined the desired dose-equivalent value of each neutron field with every energy spectrum lying between 20 keV and 14 MeV.