Silicon Microdosimeter Research Articles

Solid state microdosimetry of a 148 MeV proton spread-out Bragg peak with a pixelated silicon telescope

A constant value of the Relative Biological Effectiveness (RBE), equal to 1.1, to weight the physical dose of proton therapy treatment planning collides with the experimental evidence of an increase of effectiveness along the depth dose profile, especially at the end of the particle range. In this context, it is desirable to develop new optimized treatment planning systems that account for a variable RBE when weighting the physical dose. In particular, due to the increasing interest on microdosimetry as a possible methodology for measuring physical quantities correlated with the biological effectiveness of the therapeutic beam, the development of new Tissue-Equivalent Proportional Counters (TEPCs) specifically designed for the clinical environment are in progress.In this framework, the silicon technology allows to produce solid state detectors of real micrometric dimensions. This is a valid alternative to the TEPC from a practical point of view, being simple, easy-of-use and more versatile. The feasibility of a solid state microdosimeter based on a monolithic double stage silicon telescope has been previously proposed and deeply investigated by comparing its response to the one obtained by reference TEPCs in various radiation fields. The device is constituted by a matrix of cylindrical elements, 2 μm in thickness and 9 μm in diameter, coupled to a single E stage, 500 μm in thickness. Each segmented ΔE stage acts as a solid state microdosimeter, while the E stage gives information on the energy of the impinging proton up to about 8 MeV.This work is dedicated to the description of the microdosimetric characterization of the 148 MeV energy-modulated proton beam at the radiobiological research line of the Trento Proton Therapy Centre by means of a pixelated silicon microdosimeter. All measurements were carried out at different positions across the spread-out Bragg peak (SOBP) and the corresponding microdosimetric distributions were derived by applying a novel extrapolation algorithm. Finally, microdosimetric assessment of Relative Biological Effectiveness was carried out by weighting the dose distribution of the lineal energy with the Loncol's biological weighting function. Benefits and possible limitations of this approach are discussed.

On the measurement uncertainty of microdosimetric quantities using diamond and silicon microdosimeters in carbon-ion beams.

The purpose of this paper is to compare the response of two different types of solid-state microdosimeters, that is, silicon and diamond, and their uncertainties. A study of the conversion of silicon microdosimetric spectra to the diamond equivalent for microdosimeters with different geometry of the sensitive volumes is performed, including the use of different stopping power databases. Diamond and silicon microdosimeters were irradiated under the same conditions, aligned at the same depth in a carbon-ion beam at the MedAustron ion therapy center. In order to estimate the microdosimetric quantities, the readout electronic linearity was investigated with three different methods, that is, the first being a single linear regression, the second consisting of a double linear regression with a channel transition and last a multiple linear regression by splitting the data into odd and even groups. The uncertainty related to each of these methods was estimated as well. The edge calibration was performed using the intercept with the horizontal axis of the tangent through the inflection point of the Fermi function approximation multi-channel analyzer spectrum. It was assumed that this point corresponds to the maximum energy difference of particle traversing the sensitive volume (SV) for which the residual range difference in the continuous slowing down approximation is equal to the thickness of the SV of the microdosimeter. Four material conversion methods were explored, the edge method, the density method, the maximum-deposition energy method and the bin-by-bin transformation method. The uncertainties of the microdosimetric quantities resulting from the linearization, the edge calibration and the detectors thickness were also estimated. It was found that the double linear regression had the lowest uncertainty for both microdosimeters. The propagated standard (k = 1) uncertainties on the frequency-mean lineal energy and the dose-mean lineal energy values from the marker point, in the spectra, in the plateau were 0.1% and 0.2%, respectively, for the diamond microdosimeter, whilst for the silicon microdosimeter data converted to diamond, the uncertainty was estimated to be 0.1%. In the range corresponding to the 90% of the amplitude of the Bragg Peak at the distal part of the Bragg curve (R90 ) the uncertainty was found to be 0.1%. The uncertainty propagation from the stopping power tables was estimated to be between 5% and 7% depending on the method. The uncertainty on the and coming from the thickness of the detectors varied between 0.3% and 0.5%. This article demonstrate that the linearity of the readout electronics affects the microdosimetric spectra with a difference in values between the different linearization methods of up to 17.5%. The combined uncertainty was dominated by the uncertainty of stopping power on the edge.

Silicon 3D Microdosimeters for Advanced Quality Assurance in Particle Therapy

The Centre for Medical Radiation Physics introduced the concept of Silicon On Insulator (SOI) microdosimeters with 3-Dimensional (3D) cylindrical sensitive volumes (SVs) mimicking the dimensions of cells in an array. Several designs of high-definition 3D SVs fabricated using 3D MEMS technology were implemented. 3D SVs were fabricated in different sizes and configurations with diameters between 18 and 30 µm, thicknesses of 2–50 µm and at a pitch of 50 µm in matrices with volumes of 20 × 20 and 50 × 50. SVs were segmented into sub-arrays to reduce capacitance and avoid pile up in high-dose rate pencil beam scanning applications. Detailed TCAD simulations and charge collection studies in individual SVs have been performed. The microdosimetry probe (MicroPlus) is composed of the silicon microdosimeter and low-noise front–end readout electronics housed in a PMMA waterproof sheath that allows measurements of lineal energies as low as 0.4 keV/µm in water or PMMA. Microdosimetric quantities measured with SOI microdosimeters and the MicroPlus probe were used to evaluate the relative biological effectiveness (RBE) of heavy ions and protons delivered by pencil-beam scanning and passive scattering systems in different particle therapy centres. The 3D detectors and MicroPlus probe developed for microdosimetry have the potential to provide confidence in the delivery of RBE optimized particle therapy when introduced into routine clinical practice.

Response of SOI microdosimeter in fast neutron beams: experiment and Monte Carlo simulations.

In this study, Monte Carlo codes, Geant4 and MCNP6, were used to characterize the fast neutron therapeutic beam produced at iThemba LABS in South Africa. Experimental and simulation results were compared using the latest generation of Silicon on Insulator (SOI) microdosimeters from the Centre for Medical Radiation Physics (CMRP). Geant4 and MCNP6 were able to successfully model the neutron gantry and simulate the expected neutron energy spectrum produced from the reaction by protons bombarding a 9Be target. The neutron beam was simulated in a water phantom and its characteristics recorded by the silicon microdosimeters; bare and covered by a 10B enriched boron carbide converter, at different positions. The microdosimetric quantities calculated using Geant4 and MCNP6 are in agreement with experimental measurements. The thermal neutron sensitivity and production of 10B capture products in the p+ boron-implanted dopant regions of the Bridge microdosimeter is investigated. The obtained results are useful for the future development of dedicated SOI microdosimeters for Boron Neutron Capture Therapy (BNCT). This paper provides a benchmark comparison of Geant4 and MCNP6 capabilities in the context of further applications of these codes for neutron microdosimetry.

Uncertainty budget assessment for the calibration of a silicon microdosimeter using the proton edge technique

The MicroPlus Bridge V2 silicon microdosimeter was exposed to collimated protons in a clinical radiotherapy beam, a mixed photon–neutron radiation field from a sealed 252Cf source and γ-rays from a 137Cs source in order to investigate the accuracy and the uncertainty budget associated with the calibration of this detector by means of the proton-edge technique. At first, the energy values associated with the proton- and electron-edges were assessed for the detector under study by performing radiation transport simulations using the Monte Carlo code PHITS. After calibrating the detector in pulse amplitude using a pulse generator and in energy imparted using the PHITS-determined proton-edge, the accuracy of the calibration was tested by comparing the position of the electron-edge in the experimental microdosimetric spectra with the theoretical value obtained using PHITS. A study on the determination of which marker point (inflection point, maximum of the second derivative, intercept of the tangent through the inflection point) is the most accurate and least affected by the arbitrary choice of the fitting range is included in the article, proving that the detector can be successfully calibrated using the proton-edge technique with a combined uncertainty of 4%.

Microdosimetry with a 3D silicon on insulator (SOI) detector in a low energy proton beamline

IntroductionAn accurate description of the radiation quality of proton beams is a precondition to increase our understanding of radiobiological mechanisms and to develop accurate biological response models for radiotherapy. However, there are few detectors capable of measuring microdosimetric quantities with high spatial resolution along the entire Bragg curve due to the rapid increase in stopping power at the Bragg peak (BP) and distal dose fall-off (DDF). The aim of this work was to measure the microdosimetric spectra along the Bragg curve in a low energy proton beamline used for radiobiological experiments with a novel 3D silicon-on-insulator (SOI) “mushroom” microdosimeter. MethodA silicon microdosimeter with an array of 3D structured diodes, creating well-defined sensitive volumes (SV) with excellent spatial resolution was used for microdosimetry. The microdosimeter was used to measure microdosimetric spectra and the relative dose throughout the Bragg curve of a 15 MeV proton beam by sequential insertion of 16 μm thick polyamide absorption films in front of the microdosimeter. The results were tissue corrected with a novel correction function and compared to Monte Carlo (MC) simulations performed in GATE. ResultsThe measured dose-mean lineal energy (yD‾) increased from 8 keV/μm at the entrance to 24 keV/μm at the BP, rising to a maximum of 35 keV/μm at the DDF. The measured yD‾ showed an overall good agreement with the MC simulated values, with deviation of less than 2% at the BP and DDF, while the largest deviation (12%) was found at the entrance. Clear changes in microdosimetric spectra were seen for each 16 μm step at the BP and DDF. ConclusionThe SOI microdosimeter with its well-defined 3D sensitive volumes is an excellent tool for characterizing low energy beamlines that demands very high spatial resolution. The good overall agreement between experimental and simulated results indicated that the detector is capable of accurate microdosimetric measurements.

Characterization of a pixelated silicon microdosimeter in micro-beams of light ions

The monolithic silicon telescope technology allows to produce solid state microdosimeters. A detector constituted by a matrix of pixels (2 μm in thickness) coupled with a deeper stage (about 500 μm in thickness) was designed, developed and characterized by comparing its response against Tissue Equivalent Proportional Counters (TEPCs) in different irradiation fields, showing promising results. Each pixel of the ΔE stage is a thin diode of 9 μm in diameter delimited by a guard ring of 14 μm in diameter.The aim of this work is the study of the charge collection efficiency of the single pixel of the silicon microdosimeter by irradiating a prototype at the micro-beam facility of the Ion Beam Center - IBC of the University of Surrey. Different light ions were exploited (protons, lithium ions and carbon ions) and scanned on different positions on and around a single pixel of the detector.The results show that the charge generated by events on the sensitive region of the ΔE stage is completely collected, while a charge collection efficiency lower than one characterizes the region between the central pixel and its guard ring. The guard ring delimits properly the sensitive region: no signals arise from events outside the 14 μm in diameter ring.Nevertheless, the microdosimetric distributions of the different charged particles highlight a minor distortion in terms of the absorbed dose. This latter result justifies the good agreement with TEPCs in terms of microdosimetric distributions in different hadrons beams described in previous works.

Validation of Geant4 for silicon microdosimetry in heavy ion therapy

Microdosimetry is a particularly powerful method to estimate the relative biological effectiveness (RBE) of any mixed radiation field. This is particularly convenient for therapeutic heavy ion therapy (HIT) beams, referring to ions larger than protons, where the RBE of the beam can vary significantly along the Bragg curve. Additionally, due to the sharp dose gradients at the end of the Bragg peak (BP), or spread out BP, to make accurate measurements and estimations of the biological properties of a beam a high spatial resolution is required, less than a millimetre. This requirement makes silicon microdosimetry particularly attractive due to the thicknesses of the sensitive volumes commonly being ∼10 m or less. Monte Carlo (MC) codes are widely used to study the complex mixed HIT radiation field as well as to model the response of novel microdosimeter detectors when irradiated with HIT beams. Therefore it is essential to validate MC codes against experimental measurements.This work compares measurements performed with a silicon microdosimeter in mono-energetic , and ion beams of therapeutic energies, against simulation results calculated with the Geant4 toolkit. Experimental and simulation results were compared in terms of microdosimetric spectra (dose lineal energy, ), the dose mean lineal energy, y D and the RBE10, as estimated by the microdosimetric kinetic model (MKM). Overall Geant4 showed reasonable agreement with experimental measurements. Before the distal edge of the BP, simulation and experiment agreed within ∼10% for y D and ∼2% for RBE10. Downstream of the BP less agreement was observed between simulation and experiment, particularly for the and beams. Simulation results downstream of the BP had lower values of y D and RBE10 compared to the experiment due to a higher contribution from lighter fragments compared to heavier fragments.

Microdosimetry with a sealed mini-TEPC and a silicon telescope at a clinical proton SOBP of CATANA

A sealed miniaturized Tissue Equivalent Proportional Counter (mini-TEPC) able to work in gas-steady modality was developed at the Legnaro National Laboratories of the Italian National Institute of Nuclear Physics (LNL – INFN, Legnaro, Italy).The aim of the present work is to compare the response of this mini-TEPC with that of a silicon microdosimeter based on a monolithic telescope.Pairwise measurements were performed at the 62 MeV proton clinical Spread Out Bragg Peak (SOBP) of CATANA at the Southern National Laboratories of INFN (LNS – INFN, Catania, Italy). The dose mean lineal energy values were derived from the spectra measured with the two detectors and compared with the total dose-averaged LET calculated by means of Geant4 Monte Carlo simulations. Finally, the possibility to apply the Microdosimetric Kinetic Model (MKM) to reproduce RBE variations with depth along the Spread Out Bragg Peak was investigated.

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.

The relative biological effectiveness for carbon, nitrogen, and oxygen ion beams using passive and scanning techniques evaluated with fully 3D silicon microdosimeters.

The aim of this study was to measure the microdosimetric distributions of a carbon pencil beam scanning (PBS) and passive scattering system as well as to evaluate the relative biological effectiveness (RBE) of different ions, namely 12 C, 14 N, and 16 O, using a silicon-on-insulator (SOI) microdosimeter with well-defined 3D-sensitive volumes (SV). Geant4 simulations were performed with the same experimental setup and results were compared to the experimental results for benchmarking. Two different silicon microdosimeters with rectangular parallelepiped and cylindrical shaped SVs, both 10 μm in thickness were used in this study. The microdosimeters were connected to low noise electronics which allowed for the detection of lineal energies as low as 0.15 keV/μm in tissue. The silicon microdosimeters provide extremely high spatial resolution and can be used for in-field and out-of-field measurements in both passive scattering and PBS deliveries. The response of the microdosimeters was studied in 290 MeV/u 12 C, 180 MeV/u 14 N, 400 MeV/u 16 O passive ion beams, and 290 MeV/u 12 C scanning carbon therapy beam at heavy ion medical accelerator in Chiba (HIMAC) and Gunma University Heavy Ion Medical Center (GHMC), Japan, respectively. The microdosimeters were placed at various depths in a water phantom along the central axis of the ion beam, and at the distal part of the Spread Out Bragg Peak (SOBP) in 0.5 mm increments. The RBE values of the pristine Bragg peak (BP) and SOBP were derived using the microdosimetric lineal energy spectra and the modified microdosimetric kinetic model (MKM), using MKM input parameters corresponding to human salivary gland (HSG) tumor cells. Geant4 simulations were performed in order to verify the calculated depth-dose distribution from the treatment planning system (TPS) and to compare the simulated dose-mean lineal energy to the experimental results. For a 180 MeV/u 14 N pristine BP, the dose-mean lineal energy yD¯ obtained with two types of silicon microdosimeters started from approximately 29 keV/μm at the entrance to 92 keV/μm at the BP, with a maximum value in the range of 412 to 438 keV/μm at the distal edge. For 400 MeV/u 16 O ions, the dose-mean lineal energy yD¯ started from about 24 keV/μm at the entrance to 106 keV/μm at the BP, with a maximum value of approximately 381 keV/μm at the distal edge. The maximum derived RBE10 values for 14 N and 16 O ions were found to be 3.10 ± 0.47 and 2.93 ± 0.45, respectively. Silicon microdosimetry measurements using pencilbeam scanning 12 C ions were also compared to the passive scattering beam. These SOI microdosimeters with well-defined three-dimensional (3D) SVs have applicability in characterizing heavy ion radiation fields and measuring lineal energy deposition with sub-millimeter spatial resolution. It has been shown that the dose-mean lineal energy increased significantly at the distal part of the BP and SOBP due to very high LET particles. Good agreement was observed for the experimental and simulation results obtained with silicon microdosimeters in 14 N and 16 O ion beams, confirming the potential application of SOI microdosimeter with 3D SV for quality assurance in charged particle therapy.

Thin Silicon Microdosimeter Utilizing 3-D MEMS Fabrication Technology: Charge Collection Study and Its Application in Mixed Radiation Fields

New 10- $\mu \text{m}$ -thick silicon microdosimeters utilizing 3-D technology have been developed and investigated in this paper. The TCAD simulations were carried out to understand the electrical properties of the microdosimeters’ design. A charge collection study of the devices was performed using 5.5-MeV He2+ ions which were raster scanned over the surface of the detectors and the charge collection median energy maps were obtained and the detection yield was also evaluated. The devices were tested in a 290 MeV/u carbon ion beam at the Heavy Ion Medical Accelerator in Chiba (HIMAC) in Japan. Based on the microdosimetric measurements, the quality factor and dose equivalent out of field were obtained in a mixed radiation field mimicking the radiation environment for spacecraft in deep space.

Characterization of proton pencil beam scanning and passive beam using a high spatial resolution solid-state microdosimeter.

This work aims to characterize a proton pencil beam scanning (PBS) and passive double scattering (DS) systems as well as to measure parameters relevant to the relative biological effectiveness (RBE) of the beam using a silicon on insulator (SOI) microdosimeter with well-defined 3D sensitive volumes (SV). The dose equivalent downstream and laterally outside of a clinical PBS treatment field was assessed and compared to that of a DS beam. A novel silicon microdosimeter with well-defined 3D SVs was used in this study. It was connected to low noise electronics, allowing for detection of lineal energies as low as 0.15keV/μm. The microdosimeter was placed at various depths in a water phantom along the central axis of the proton beam, and at the distal part of the spread-out Bragg peak (SOBP) in 0.5mm increments. The RBE values of the pristine Bragg peak (BP) and SOBP were derived using the measured microdosimetric lineal energy spectra as inputs to the modified microdosimetric kinetic model (MKM). Geant4 simulations were performed in order to verify the calculated depth-dose distribution from the treatment planning system (TPS) and to compare the simulated dose-mean lineal energy to the experimental results. For a 131MeV PBS spot (124.6mm R90 range in water), the measured dose-mean lineal energy yD¯ increased from 2keV/μm at the entrance to 8keV/μm in the BP, with a maximum value of 10keV/μm at the distal edge. The derived RBE distribution for the PBS beam slowly increased from 0.97±0.14 at the entrance to 1.04±0.09 proximal to the BP, then to 1.1±0.08 in the BP, and steeply rose to 1.57±0.19 at the distal part of the BP. The RBE distribution for the DS SOBP beam was approximately 0.96±0.16 to 1.01±0.16 at shallow depths, and 1.01±0.16 to 1.28±0.17 within the SOBP. The RBE significantly increased from 1.29±0.17 to 1.43±0.18 at the distal edge of the SOBP. The SOI microdosimeter with its well-defined 3D SV has applicability in characterizing proton radiation fields and can measure relevant physical parameters to model the RBE with submillimeter spatial resolution. It has been shown that for a physical dose of 1.82Gy at the BP, the derived RBE based on the MKM model increased from 1.14 to 1.6 in the BP and its distal part. Good agreement was observed between the experimental and simulation results, confirming the potential application of SOI microdosimeter with 3D SV for quality assurance in proton therapy.

3D silicon microdosimetry and RBE study using 12C ion of different energies

This paper presents a new version of the 3D mesa “bridge” microdosimeter comprised of an array of 4248 silicon cells fabricated on 10 µm thick silicon-on-insulator substrate. This microdosimeter has been designed to overcome limitations existing in previous generation silicon microdosimeters and it provides well-defined sensitive volumes and high spatial resolution. The charge collection characteristics of the new 3D mesa microdosimeter were investigated using the ANSTO heavy ion microprobe, utilizing 5.5 MeV He2+ ions. Measurement of microdosimetric quantities allowed for the determination of the Relative Biological Effectiveness of 290 MeV/u and 350 MeV/u 12C heavy ion therapy beams at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. The microdosimetric RBE obtained showed good agreement with the tissue-equivalent proportional counter. Utilizing the high spatial resolution of the SOI microdosimeter, the LET spectra for 70 MeV 12C+6 ions, like those present at the distal edge of 290 and 350 MeV/u beams, were obtained as the ions passed through thin layers of polyethylene film. This microdosimeter can provide useful information about the lineal energy transfer (LET) spectra downstream of the protective layers used for shielding of electronic devices for single event upset prediction.

New silicon microdosimetry probes for RBE and biological dose studies using stationary and movable targets in 12C ion therapy

Due to the high LET and dense ionisation tracks associated with ions, microdosimetric approaches have been used in carbon ion therapy to assess field quality and calculate radiobiological quantities for a variety of cell lines. There is however a lack of instrumentation for simple and routine use in a clinical environment, important for determination of RBE which provides accurate treatment planning and delivery in hadron therapy. In this study, a 10 μm thick silicon microdosimeter with 3D sensitive volumes has been used to investigate the effect of motion on the RBE and field quality of a typical 12C ion therapy beam. For a passively scattered 290 MeV/u 12C beam with 6 cm spread-out Bragg peak (SOBP), variations in biological dose along the SOBP were observed, as well as a significant changes to particle LET when incident on a moving target.

Characterization of a Large Area Thinned Silicon Microdosimeter for Space and Particle Therapy

The tissue-equivalent proportional counter is the gold standard detector in microdosimetry. The energy deposited in its volume is equal to that of a single cell through the use of low pressure tissue-equivalent gas. To overcome its bulky gas-flow ensemble, high operating voltage and poor spatial resolution, a 20 μm-thick silicon microdosimeter with 9600 micron-sized sensitive volumes has been developed. Presented are the results from its initial characterization including electrical properties and charge collection characteristics obtained using induced charge collection techniques at ANSTO, Australia, and the ESRF, France. In addition, the results from a 290 MeV/u C-12 irradiation at HIMAC, NIRS, Japan, are presented.

3D Silicon Microdosimetry and RBE Study Using <formula formulatype="inline"><tex Notation="TeX">$^{12}{\rm C}$</tex></formula> Ion of Different Energies

This paper presents a new version of the 3D mesa Bridge microdosimeter comprised of an array of 4248 silicon cells fabricated on 10 μm thick n-type silicon-on-insulator substrate. This microdosimeter has been designed to overcome limitations existing in previous generation silicon microdosimeters and it provides well-defined sensitive volumes and high spatial resolution. The charge collection characteristics of the new 3D mesa microdosimeter were investigated using the ANSTO heavy ion microprobe, utilizing 5.5 MeV He 2 + ions. Measurement of microdosimetric quantities allowed for the determination of the relative biological effectiveness of 290 MeV/u and 350 MeV/u 12 C heavy ion therapy beams at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. The microdosimetric RBE obtained showed good agreement with the tissue-equivalent proportional counter. Utilizing the high spatial resolution of the SOI microdosimeter, the LET spectra for 70 MeV 12 C +6 ions, like those present at the distal edge of 290 and 350 MeV/u beams, were obtained as the ions passed through thin layers of polyethylene film. This microdosimeter can provide useful information about the lineal energy transfer (LET) spectra downstream of the protective layers used for shielding of electronic devices for single event upset prediction.

3D cylindrical silicon microdosimeters: fabrication, simulation and charge collection study

This paper reports on the fabrication, simulation, and charge collection characteristics of a new generation of cylindrical silicon microdosimeters fabricated on SOI wafers. The devices consist of an array of p+ electrodes surrounded by trench n+ electrodes creating well defined, cylindrical sensitive volumes. A first batch of microsensors with 5.4 μm active thickness has been successfully fabricated. The devices are fully functional with good diode behavior and a depletion voltage of only 3 V. Their charge collection characteristics have been investigated using the IBIC technique with protons and alpha particles. The IBIC maps show a 100% yield of active cells in a microdosimeter array and full charge collection efficiency in the active area of the unit microsensors. These devices constitute an step forward in the current status of microdosimeters based on silicon technologies.

Novel detectors for silicon based microdosimetry, their concepts and applications

This paper presents an overview of the development of semiconductor microdosimetry and the most current (state-of-the-art) Silicon on Insulator (SOI) detectors for microdosimetry based mainly on research and development carried out at the Centre for Medical Radiation Physics (CMRP) at the University of Wollongong with collaborators over the last 18 years. In this paper every generation of CMRP SOI microdosimeters, including their fabrication, design, and electrical and charge collection characterisation are presented. A study of SOI microdosimeters in various radiation fields has demonstrated that under appropriate geometrical scaling, the response of SOI detectors with the well-known geometry of microscopically sensitive volumes will record the energy deposition spectra representative of tissue cells of an equivalent shape. This development of SOI detectors for microdosimetry with increased complexity has improved the definition of microscopic sensitive volume (SV), which is modelling the deposition of ionising energy in a biological cell, that are led from planar to 3D SOI detectors with an array of segmented microscopic 3D SVs. The monolithic ΔE−E silicon telescope, which is an alternative to the SOI silicon microdosimeter, is presented, and as an example, applications of SOI detectors and ΔE−E monolithic telescope for microdosimetery in proton therapy field and equivalent neutron dose measurements out of field are also presented. An SOI microdosimeter “bridge” with 3D SVs can derive the relative biological effectiveness (RBE) in 12C ion radiation therapy that matches the tissue equivalent proportional counter (TEPC) quite well, but 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, and the possibility of integrating the system onto a single device with other types of detectors.

3D-Mesa “Bridge” Silicon Microdosimeter: Charge Collection Study and Application to RBE Studies in $^{12}{\rm C}$ Radiation Therapy

Microdosimetry is an extremely useful technique, used for dosimetry in unknown mixed radiation fields typical of space and aviation, as well as in hadron therapy. A new silicon microdosimeter with 3D sensitive volumes has been proposed to overcome the shortcomings of the conventional Tissue Equivalent Proportional Counter. In this article, the charge collection characteristics of a new 3D mesa microdosimeter were investigated using the ANSTO heavy ion microprobe utilizing 5.5 MeV He <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> and 2 MeV H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ions. Measurement of the microdosimetric characteristics allowed for the determination of the Relative Biological Effectiveness of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> C heavy ion therapy beam at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. Well-defined sensitive volumes of the 3D mesa microdosimeter have been observed and the microdosimetric RBE obtained showed good agreement with the TEPC. The new 3D mesa “bridge” microdosimeter is a step forward towards a microdosimeter with fully free-standing 3D sensitive volumes.