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Despite the low contribution of heavy ions to the total fluence in the space radiation environment, their radiobiological effect on the human body is extremely high. In this article, we investigated the radiation field which resulted from the interaction of galactic cosmic rays (GCRs), specifically some heavy ions and energies, typically encountered in space with a realistic multilayer sample of the International Space Station (ISS) Columbus module’s shielding wall. The quality factor, Q, and the normalized dose equivalent, H, derived from microdosimetric measurements for C, Ne, and Si ions behind different spacecraft wall configurations and materials are presented in this article. Particularly, carbon fiber, polyoxymethylene, and perspex with same areal density compared to currently used aluminum were investigated.

Microdosimetry provides measurements of stochastic lineal energy deposition in a micrometric sensitive volume (SV) that are comparable to human cell dimensions, typically in the order of 10–15 $\mu \text{m}$ in diameter. Silicon-based microdosimeters have been fabricated using “3-D technology,” providing true cell-like SVs that are fully encapsulated by a 3-D through substrate electrode. The unique geometry has been made feasible using modern microelectromechanical systems (MEMS). The combination of MEMS techniques and radiation detector fabrication brings forth challenges in achieving reliability, consistency, and high yield. This article reviews the technological challenges encountered in several prototype runs completed at SINTEF. The technological review and prototype iterations aim to deliver a future technology plan at the manufacture level with a high yield throughput.

The metal-oxide-semiconductor field-effect transistor (MOSFET) can be used as a dosimeter. It is robust, lightweight, cost-effective, is able to operate in real time with and without external bias, and also has a very small sensitive volume. Therefore, it is particularly suitable for in vivo dosimetry in modern radiation therapy and also as a patient dosimeter in diagnostic radiology. MOSFET sensitivity to ionizing radiation can be tailored to a specific application or a dose range. This is done by fabricating the radiation-sensitive volume of different thicknesses, or alternatively, by adjusting the external bias applied on the gate during irradiation. However, increased sensitivity comes at the cost of reduction in the MOSFET’s lifespan. This article investigated a way to maximize sensitivity while minimizing the reduction in lifespan. The MOSFETs we considered had a sensitive volume of thickness 0.68 or 1.0 $\mu \text{m}$ and different boron implantations under the gate. We then irradiated the MOSFETs by applying different positive biases on the gate. We assessed linearity of the dose-response relationship and sensitivity in photon beams that were produced using a megavoltage medical linear accelerator (linac) and an orthovoltage X-ray tube.

This article presents a study of the response of the silicon on insulator (SOI) microdosimeter with 3-D sensitive volumes (SVs) to 400 MeV/u 16O and 500 MeV/u 56Fe ions mimicking galactic cosmic rays outside and inside the International Space Station (ISS). An average quality factor ( Q¯ ) and the dose equivalent (H) of the radiation field were obtained experimentally, and the results were compared with GEANT4 simulations. Index Terms— Galactic cosmic rays (GCR), heavy ions, International Space Station (ISS), silicon on insulator (SOI) microdosimeter

Modern stereotactic radiation therapy modalities utilize small beams and large dose gradients to deliver radiation in few fractions, reducing the possibility to correct for mistakes during the treatment process. Therefore, in order to ensure best possible treatment for the patient, quality assurance for such treatments necessitates a stable, linear, and sensitive radiation detector with high spatial resolution and radiation hardness. In this work, two silicon detector arrays with high spatial resolution have been characterized by 6 MV and 18 MV medical LINAC irradiation, and 5.5 MeV He2+ heavy ion microprobe. A maximum discrepancy of 0.6 mm in field size has been found when comparing to two-dimensional radiochromic film dose profile, and charge collection efficiency obtained by means of ion beam induced charge collection (IBICC) is 66% when operating the array in photovoltaic mode. Radiation damage study by photons and photoneutrons is presented.

The choice of pinhole geometry is a critical factor in the performance of pinhole-collimator-based source tracking systems for brachytherapy QA. In this work, an analytical model describing the penetrative sensitivity of a single-cone pinhole collimator to photons emitted from a point source is derived. Using existing models for single-cone resolution and double-cone sensitivity and resolution, the theoretical sensitivity and resolution of the single-cone collimator are quantitatively compared with those of a double-cone collimator with an equivalent field of view. Monte Carlo simulations of the single and double-cone pinhole collimators using an accurate 3D model of a commercial high dose rate brachytherapy source are performed to evaluate the relative performance of each geometry for a novel real-time HDR brachytherapy QA system, H DR BrachyView. The theoretical penetrative sensitivity of the single-cone pinhole is shown to be higher than the double-cone pinhole, which is in agreement with the results from the Monte Carlo simulations. The wider pinhole response function of the single-cone collimator results in a larger total error between the projected center of the source and the estimated center of mass of the source projection for the single-cone collimator, with the greatest error (at the maximum FoV angle) being 0.54 mm for the double-cone pinhole and 1.37 mm for the single-cone at $\theta ={ 60^ \circ }$ . The double-cone pinhole geometry is determined to be the most appropriate choice for the pinhole collimator in the H DR BrachyView probe.

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.

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.

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 ${\rm He}^{2 +}$ and 2 MeV ${\rm H}^ {+}$ ions. Measurement of the microdosimetric characteristics allowed for the determination of the Relative Biological Effectiveness of the $^{12}{\rm C}$ 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.

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 252 Cf, 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 SV's 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.

Due to the ever-increasing complexity of treatment modalities in radiation therapy, there has been a greater need for detectors to perform quality assurance to ensure patients are treated correctly and safely. Modern radiation therapy techniques involve small field sizes, high dose gradients, and varying intensity of energy and rate. The ideal dosimeter for this treatment should display high spatial resolution, high linearity, accuracy, and radiation hardness. Silicon detectors have been widely used for radiotherapy measurements and have many attractive qualities as a dosimeter; weaknesses of silicon detectors are, however, decreases in sensitivity with accumulated dose. The Centre for Medical Radiation Physics has developed a new technology with an unusual charge collection efficiency variation with accumulated dose which stabilizes the response of the detector within ±5% after 120 kGy photon irradiation. The sensor has been also characterized by irradiation by an 18 MV medical LINAC with sensitivity to a photoneutron-induced damage of less than 0.5%/100 Gy. The radiation damage mechanism has been validated by TCAD simulations which confirmed the mechanism behind the CCE increase as a function of the accumulated dose.

An n-SOI microdosimeter which has been proposed as a device for predicting the occurrence of single event effects in semiconductor electronics in the high-energy, mixed heavy ion space radiation environment has been investigated to better understand the charge collection geometry and charge collection mechanisms. Ion beam induced charge collection studies using 20 MeV 12C ions, 5.5 MeV 4He ions, and 2 MeV H ions were carried out, and the effects of different bias conditions, angles of ion incidence, and coincidence analysis were observed to understand the sensitive volume geometry. The energy response of the n-SOI microdosimeter has been observed to exhibit an over-response of 56%, 113%, and 23% for the above ions compared to expected energy depositions calculated using SRIM 2008. No relationship between particle LET AU: Please provide spelling for “LET” and the enhance energy response was apparent. A comparison of experimentally measured and simulated spectra suggest a cylindrical charge collection geometry despite the physical rectangular parallelepiped geometry of the p-i-n diode. This was supported by observing the response of the microdosimeter to ions at oblique ion incidence. A simplified model of diffusion charge collection found that diffusion charge collection contributes to the low-energy tail observed in experimental spectra, but does not account for the observed enhanced energy response. This supports the current theory that the enhanced energy response is a result of a displacement current produced when charge carriers in the substrate induce charge in the SOI layer due to the parasitic capacitance of the buried SiO2 insulating layer.

The dosimetry of X-ray microbeams using MOSFETs results in an asymmetrical beam profile due to a lack of lateral charged particle equilibrium. Monte Carlo simulations were carried out using PENELOPE and GEANT4 codes to study this effect and a MOSFET on a micropositioner was scanned in the microbeam. Based on the simulations a new method of microbeam dosimetry is proposed. The proposed edge-on face-to-face (EOFF) MOSFET detector, a die arrangement proposed here for the first time, should alleviate the asymmetry. Further improvement is possible by thinning the silicon body of the MOSFET.

This paper is dedicated to the characterization of a novel diamond microdosimeter prototype with 3D sensitive volumes produced by high energy boron implantation. Diamond has been chosen in order to further improve solid state based microdosimeter in terms of radiation hardness and tissue equivalency. IBIC measurements were undertaken to determine the charge collection efficiency map of the device. It was demonstrated that the proposed ion implantation technology allows for the formation of an array of well defined 3D SVs. A Geant4 application was developed to explain the effect of Al electrode thickness on observed anomaly in deposited energy. Specifics of the results and an update on the current status of the project is presented.

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.

Electrical characteristics and neutron dosimetry properties of silicon based p-i-n diodes are presented in support of the applications in the sensors for beam monitoring and medical physics. Both the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of silicon planar p-i-n diode sensors with cylindrical geometry have been theoretically modeled and experimentally measured. The shifts of the forward and reverse diode characteristics of the sensors versus the neutron dose have been obtained. It is shown that the neutron irradiation caused shift of the forward voltage of the p-i-n diodes is proportional to the current at which it is measured in the case of the low level injection or to the square root of the current in the case of the high level injection. The C-V characteristics and the full depletion voltages of the diodes have been estimated and experimentally verified. It is shown that the sensitivity of planar cylindrical structures as neutron sensors can be optimized by the selection of the device geometry and the current at which the measurement is performed.

This paper presents initial results from electrical, spectroscopic and ion beam induced charge (IBIC) characterisation of a novel silicon PIN detector, featuring an on-chip n-channel JFET and matched feedback capacitor integrated on its p-side (frontside). This structure reduces electronic noise by minimising stray capacitance and enables highly efficient optical coupling between the detector back-side and scintillator, providing a fill factor of close to 100%. The detector is specifically designed for use in high resolution gamma cameras, where a pixellated scintillator crystal is directly coupled to an array of silicon photodetectors. The on-chip JFET is matched with the photodiode capacitance and forms the input stage of an external charge sensitive preamplifier (CSA). The integrated monolithic feedback capacitor eliminates the need for an external feedback capacitor in the external electronic readout circuit, improving the system performance by eliminating uncontrolled parasitic capacitances. An optimised noise figure of 152 electrons RMS was obtained with a shaping time of 2 μs and a total detector capacitance of 2pF. The energy resolution obtained at room temperature (21 ° C) at 27 keV (direct interaction of I-125 gamma rays) was 5.09%, measured at full width at half maximum (FWHM). The effectiveness of the guard ring in minimising the detector leakage current and its influence on the total charge collection volume is clearly demonstrated by the IBIC images. © 2006 IEEE.

This paper presents in phantom testing of a recently developed intraoperative minidosimetry system, designed to measure the dose along the urethra during low dose rate prostate brachytherapy. This system is based on a silicon minidetector and uses spectroscopy to calculate the localized dose from the treatment radiation. The minidosimetry system was demonstrated to be operational at body temperature, with a near isotropic response to radiation at all angles. Phantom measurements have shown the minidosimetry system to measure the dose from multiple seeds to within 5% of planning system calculated doses. This system is an ideal complement to ultrasound guided seed placement in providing online direct dosimetry during seed implantation, as well as providing dose planning system verification through post implant dosimetry.

This paper reports on the development of an interactive, intraoperative dose planning system for seed implant brachytherapy in cancer treatment. This system involves in vivo dosimetry and the ability to determine implanted seed positions. The first stage of this project is the development of a urethral alarm probe to measure the dose along the urethra during a prostate brachytherapy treatment procedure. Ultimately, the system will be used to advise the physicians upon reaching a preset dose rate or dose after total seed decay in the urethra during the seed placement. The second stage is the development of a method and instrumentation for in vivo measurements of the location of implanted seeds in the same frame as for dose planning and the use of these in intraoperative treatment planning. We have developed a silicon mini-detector, miniature front-end, and shaping amplifier with discriminator, connected to the mini-silicon detector at the end of a cable placed in a urological catheter, to satisfy the spectroscopic requirements of the urethral probe. This technique will avoid complications related to overdosing the urethra and the rectum.

New nonionizing energy losses (NIEL) sensors based on silicon planar p-i-n diodes of different geometry have been investigated and their response to fast neutron field compared with bulk diodes. The possibility of obtaining a wide range of sensitivities in these NIEL sensors simultaneously with measurements of IEL has been demonstrated.

In this paper, we report on spectral response data and gamma ray spectroscopy measurements using two newly developed silicon photodetectors that are designed to have enhanced sensitivity in the blue spectral region. The enhanced sensitivity is a result of our newly developed ion implantation profile used to create the active area of the photodetector. The quantum efficiency of the new photodetectors (without any optimized antireflective coating) has been measured to be /spl sim/40% at a wavelength of 420 nm. Gamma ray spectroscopy experiments have been performed using a thallium doped cesium iodide, [CsI(Tl)], and a cerium doped lutetium oxy-orthosilicate, (LSO) crystal excited by a /sup 137/Cs or /sup 22/Na source and read out by the new photodetectors. We have measured an energy resolution of 7.7% and 22.7% full-width at half-maximum (FWHM) for the 662-keV gamma rays from a /sup 137/Cs for the CsI(Tl) and LSO scintillator crystal respectively. We intend to use the photodetectors, in the form of a detector array optically coupled to CsI(Tl) or LSO, in the development of a new scintillator detector module for use in positron emission tomography (PET).

An automatic radiation damage monitoring system has been developed and tested. The system is based on two passive sensors for the measurement of integral ionizing and non-ionizing energy losses in silicon devices. Ionizing dose is measured in terms of dose in SiO/sub 2/ and displacement damage in terms of 1 MeV(Si) equivalent neutron fluence. The system uses MOSFETs and PIN dosimetric diodes.

A metal-oxide-semiconductor field effect transistor (MOSFET) has been used as a radiation monitor to map the profile of a variety of X-ray radiation microbeams. The present work studies the role of the topology of the MOSFET gate oxide when mapping radiation microbeams. The "edge-on" MOSFET has been introduced for microbeam mapping and the spatial resolution of the "edge-on" MOSFET is investigated. Comparison is made with other mapping techniques including, Gafchromic/sup TM/ film, radiographic film, ionization chamber. The results clearly demonstrate the superiority of the "edge-on" MOSFET when applied to mapping of narrow radiation beams. The spatial resolution of the "edge-on" MOSFET is estimated to be 1 /spl mu/m and appears to be limited by the width of the gate oxide thickness.