Dale A. Prokopovich

A Cylindrical Silicon-on-Insulator Microdosimeter: Charge Collection Characteristics

A novel silicon-on-insulator microdosimeter for estimating the radiobiological effectiveness (RBE) of a mixed radiation field is presented. An ion beam induced charge collection study has confirmed the microdosimeter possesses well defined micron sized 3D cylindrical sensitive volumes. An array of these SVs has the capability of studying the track structure of high energy heavy ions typical of a deep space environment.

3D-Mesa “Bridge” Silicon Microdosimeter: Charge Collection Study and Application to RBE Studies in

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 He2+ and 2 MeV H+ ions. Measurement of the microdosimetric characteristics allowed for the determination of the Relative Biological Effectiveness of the 12C 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.

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The tissue equivalence of solid state silicon detectors in proton radiation fields was determined to improve the radiation protection applications of silicon detectors in aviation and space missions. The study was performed by means of Geant4 simulations. Results are presented showing that a simple c( ~ 0.56) of linear dimensions is adequate to convert experimentally obtained microdosimetric energy deposition spectra in silicon to equivalent microdosimetric energy deposition spectra in water.

Radiation Therapy

Silicon microdosimeters for the characterisation of mixed radiation fields relevant to the space radiation environment have been under continual development at the Centre for Medical Radiation Physics for over a decade. These devices are useful for the prediction of single event upsets in microelectronics and for radiation protection of spacecraft crew. The latest development in silicon microdosimetry is a family of large-area n-SOI microdosimeters for real-time dosimetry in space radiation environments. The response of n-SOI microdosimeters to 2 MeV H and 5.5 MeV He ions has been studied to investigate their charge collection characteristics. The studies have confirmed 100% yield of functioning cells, but have also revealed a charge sharing effect due to diffusion of charge from events occurring outside the sensitive volume and an enhanced energy response due to the collection of charge created beneath the insulating layer. The use of a veto electrode aims to reduce collection of diffused charge. The effectiveness of the veto electrode has been studied via a coincidence analysis using IBIC. It has been shown that suppression of the shared events allows results in a better defined sensitive volume corresponding to the region under the core electrode where the electric field is strongest.

Tissue Equivalence Correction in Silicon Microdosimetry for Protons Characteristic of the LEO Space Environment

The CERN-EU high-energy reference field facility is used for testing and calibrating both active and passive radiation dosemeters for radiation protection applications in space and aviation. Through a combination of a primary particle beam, target and a suitable designed shielding configuration, the facility is able to reproduce the neutron component of the high altitude radiation field relevant to the jet aviation industry. Simulations of the facility using the GEANT4 (GEometry ANd Tracking) toolkit provide an improved understanding of the neutron particle fluence as well as the particle fluence of other radiation components present. The secondary particle fluence as a function of the primary particle fluence incident on the target and the associated dose equivalent rates were determined at the 20 designated irradiation positions available at the facility. Comparisons of the simulated results with previously published simulations obtained using the FLUKA Monte Carlo code, as well as with experimental results of the neutron fluence obtained with a Bonner sphere spectrometer, are made.

Large Area Silicon Microdosimeter for Dosimetry in High LET Space Radiation Fields: Charge Collection Study

Abstract Purpose: Lethal cell damage by ionising radiation is generally initiated by the formation of complex strand breaks, resulting from ionisation clusters in the DNA molecule. A better understanding of the effect of the distribution of ionisation clusters within the cell and particularly in regard to DNA segments could be beneficial to radiation therapy treatment planning. Low energy X-rays generate an abundance of low energy electrons similar to that associated with MeV protons. The study and comparison of the track structure of photon and proton beams could permit the substitution of photon microbeams for single cell ion irradiations at proton facilities used to predict the relative biological effectiveness (RBE) of charged particle fields. Materials and methods: The track structure of X-ray photons is compared with proton pencil beams in voxels of approximate DNA strand size (2 × 2 × 5 nm). The Very Low Energy extension models of the Monte Carlo simulation toolkit GEometry ANd Tracking 4 (Geant4) is used. Simulations were performed in a water phantom for an X-ray and proton beam of energies 100 keV and 20 MeV, respectively. Results: The track structure of the photon and proton beams are evaluated using the ionisation cluster size distribution as well as the radial dose deposition of the beam. Conclusions: A comparative analysis of the ionisation cluster distribution and radial dose deposition obtained is presented, which suggest that low energy X-rays could produce similar ionisation cluster distributions to MeV protons on the DNA scale of size at depths greater than ∼10 μm and at distances greater than ∼1 μm from the beam centre. Here the ionisation cluster size for each beam is less than ∼100. The radial dose deposition is also approximately equal at large depths and at distances greater than 10 μm from the beam centre.

Geant4 simulation of the CERN-EU high-energy reference field (CERF) facility

This work presents a new generation silicon-on- insulator (SOI)microdosimeter device . This new device was designed and fabricated using planar processing techniques to produce a micrometer sized cylindrical shaped, well defined microdosimeter sensitive volume. Cylindrical structures were employed to allow for a better definition of the average chord length of the sensitive volume compared to the previous elongated parallelepiped solid state designs. The structures were manufactured on individual mesas situated on top of a buried oxide insulating layer. These mesa designs eliminated lateral charge diffusion. Two kinds of test structures were designed with sensitive region widths of 2 mum and 10 mum. ISE TCAD modeling of the electrostatic potential and electric field profile of the cylindrical microdosimeter were carried out to obtain 3D potential and electric field profiles. The modeling found that as expected a radial electric field was established across the cylindrical shaped sensitive volume. The modeling also found it was possible to increase the reverse bias voltage to a point where internal amplification of event signals was obtained. This is important finding for the measurements of low linear energy transfer (LET) events.

A comparison of X-ray and proton beam low energy secondary electron track structures using the low energy models of Geant4.

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 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.

Cylindrical Silicon-on-Insulator Microdosimeter: Design, Fabrication and TCAD Modeling

Energy resolution and detection efficiency were compared between new scintillators, lanthanum bromide (LaBr₃:Ce) and lanthanum chloride (LaCl₃:Ce) with conventional detectors, sodium iodide NaI(Tl) and cadmium zinc telluride (CdZnTe or CZT). The study has shown that LaBr₃:Ce and LaCl₃:Ce scintillator detectors provided by Saint-Gobain offer better resolution than NaI(Tl) detector. LaBr₃:Ce and LaCl₃:Ce detectors could resolve some closely spaced peaks from ¹³³Ba and ¹⁵²Eu, which Na(I) could not. LaBr₃:Ce has slightly better resolution and higher efficiency than LaCl₃:Ce. An overall advantage of LaBr₃:Ce and LaCl₃:Ce detectors over NaI(Tl) and CZT has been discussed. The intrinsic activity of LaBr₃:Ce is also demonstrated in this study.

3D Silicon Microdosimetry and RBE Study Using

A 4th generation silicon microdosimeter has been designed by the Centre for Medical Radiation Physics (CMRP) at the University of Wollongong using three dimensional (3D) Sensitive Volumes (SVs). This new microdosimeter design has the advantage of well-defined 3D SVs as well as the elimination of lateral charge diffusion by removal of silicon laterally adjacent to the 3D SVs. The gaps between the sensitive volumes are to be backfilled with PolyMethyl MethAcrylate (PMMA) to produce a surrounding tissue equivalent medium. The advantage of this design avoids the generation of secondary particles from inactive silicon lateral to SVs. The response of the microdosimeter to the neutron field from 252Cf, Pu-Be sources and an avionic radiation environment were simulated using the Geant4 Monte Carlo toolkit for design optimisation. The simulated energy deposition in the SVs from the neutron fields and microdosimetric spectra is presented. The simulation study shows a significant reduction in silicon nuclear recoil contribution to the energy deposition for the novel microdosimeter design. The reduction of silicon recoil events from outside of the SVs will consequently reduce the uncertainty in the calculated dose equivalent. The simulations have demonstrated that a 3D silicon microdosimeter surrounded by PMMA can produce microdosimetric spectra similar to those of a tissue equivalent microdosimeter.