[en] The development of MOSFET dosimetry is presented with an emphasis on the development of a scanning MOSFET dosimetry system for modern radiation oncology modalities. Fundamental aspects of MOSFETs in relation to their use as dosemeters are briefly discussed. The performance of MOSFET dosemeters in conformal radiotherapy, hadron therapy, intensity-modulated radiotherapy and microbeam radiation therapy is compared with other dosimetric techniques. In particular the application of MOSFET dosemeters in the characterisation and quality assurance of the steep dose gradients associated with the penumbra of some modern radiation oncology modalities is investigated. A new in vivo, on-line, scanning MOSFET read out system is also presented. The system has the ability to read out multiple MOSFET dosemeters with excellent spatial resolution and temperature stability and minimal slow border trapping effects. (author)

[en] A model which describes the type conductivity inversion and the Fermi level stabilisation in high purity n-silicon under the fast neutron irradiation has been considered. This model takes into account shallow impurities, oxygen, A- and E-centers and deep amphoteric centers (the vacancy complexes). The role of divacancies is dominating in the Fermi level stabilisation effect. The neutron fluence when the type inversion is observed has been calculated for various initial n-silicon. The results obtained describe peculiarities of the behaviour of neutron integral dosemeters. Characteristics of p-i-n diodes used as fast neutron sensors in the wide dose range and the influence both the carrier life time and the silicon resistivity changes on the sensor sensitivity are discussed. Using long base diodes fabricated from high resistivity silicon (>10 kO cm) allows the sensitivity to reach 5 V.Gy^-1 (tissue). (author)

[en] A review of solid state microdosimetry is presented with an emphasis on silicon-based devices. The historical foundations and basics of microdosimetry are briefly provided. Various methods of experimental regional microdosimetry are discussed to facilitate a comparison with the more recent development of silicon microdosimetry. In particular, the performance characteristics of a proportional gas counter and a silicon microdosimeter are compared. Recent improvements in silicon microdosimetry address the issues of requirement specification, non-spherical shape, tissue equivalence, sensitive volume definition (charge collection complexity) and low noise requirements which have previously impeded the implementation of silicon-based microdosimetry. A prototype based on silicon-on-insulator technology is described along with some example results from clinical high LET radiotherapy facilities. A brief summary of the applications of microdosimetry is included

[en] Full text: Microdosimetry and nanodosimetry can provide unique information for prediction of radiobiological properties of radiation, which is important in radiation therapy for accurate dose planning and in radiation protection for cancer induction risk assessment. This demand measurements of the pattern of energies deposited by ionizing radiation on cellular scale and DNA levels.Silicon microelectronics technology is offering a unique opportunity for replacing gas proportional counters (TEPC) with miniature detectors for regional microdosimetry. Silicon on Insulator (SOI) technology has been used for the development of arrays of micron size sensitive volumes for modelling energy deposited in biological cells. The challenge in silicon microdosimetry is the development of well defined sensitive volume (SV) and full charge collection deposited by ionizing radiation in the SV. First generation SOI microdosimeters were developed at CMRP and investigated in a wide range of radiation fields for proton and neutron therapies and recently on isotopic neutron sources and heavy ions with energy up to lGeV/jj,m which are typical for deep space radiation environment. Microdosimetric spectra were obtained in a phantom that are well matched to TEPC and Monte Carlo simulations. Evidence that radiations with the same LET exhibit different biological effects demand development of new sensors sensitive to the track structure of ions or the type of particle for prediction of radiobiological effect of radiation using radiobiological models. New monolithic Si AE-E telescope of cellular size for simultaneous regional microdosimetry and particle identification will be presented and results will be discussed. The new design of the SOI microdosimeter is based on ^3D micron and submicron size of Si SVs. This approach allows improvement in the accuracy of the Si microdosimetry because of full charge collection and the ability to measure low LET as low as 0.01 keV/jjm, which is similar to TEPC. Microdosimetric and nanodosimetric measurements of 250 MeV proton radiation fields at the proton accelerator of Loma Linda University Medical Center (LLUMC) using SOI microdosimeter and gas nanodosimeter will be presented. Good agreement between GEANT Monte Carlo simulations of ionization cluster and pattern of deposited energies measured by nanodosimeter and microdosimeter have been achieved. Replacement of a gas nanodosimeter with 10 nm SV volume of a silicon detector is a challenge. However with the development of Si nanotechnology it is feasible and track structure sensitive array of ^3D submicron size Si detectors will be presented. Challenges in the conversion of Si microdosimetric and nanodosimetric spectra to tissue equivalent will be discussed. This project is a large scale collaboration with ANSTO and U NSW in Australia and LLUMC, USNA, Johns Hopkins Uni and MSKCC in the USA

[en] Microdosimetry is used to predict the biological effects of the densely ionizing radiation environments of hadron therapy and space. The creation of a solid state microdosimeter to replace the conventional Tissue Equivalent Proportional Counter (TEPC) is a topic of ongoing research. The Centre for Medical Radiation Physics has been investigating a technique using microscopic arrays of reverse biased PN junctions. A prototype silicon-on-insulator (SOI) microdosimeter was developed and preliminary measurements have been conducted at several hadron therapy facilities. Several factors impede the application of silicon microdosimeters to hadron therapy. One of the major limitations is that of tissue equivalence, ideally the silicon microdosimeter should provide a microdosimetry distribution identical to that of a microscopic volume of tissue. For microdosimetry in neutron fields, such as Fast Neutron Therapy, it is important that products resulting from neutron interactions in the non tissue equivalent sensitive volume do not contribute significantly to the spectrum. Experimental measurements have been conducted at the Gershenson Radiation Oncology Center, Harper Hospital, Detroit by Bradley et al. The aim was to provide a comparison with measurements performed with a TEPC under identical experimental conditions. Monte Carlo based calculations of these measurements were made using the GEANT4 Monte Carlo toolkit. Agreement between experimental and theoretical results was observed. The model illustrated the importance of neutron interactions in the non tissue equivalent sensitive volume and showed this effect to decrease with sensitive volume size as expected. Simulations were also performed for 1 micron cubic silicon sensitive volumes embedded in tissue equivalent material to predict the best case scenario for silicon microdosimetry in Fast Neutron Therapy

[en] Full text: This work is a first comprehensive investigation of the issues confronting silicon microdosimetry and its application for radiotherapy. New dosimeters based on silicon-on-insulator (SOI) device will be studied. Four main problems requiring investigation and comparison with spherical gas proportional microdosimeter are identified and addressed including shape of sensitive volume, tissue equivalence, noise minimization and sensitive volume definition. Experiments have been carried out with low noise prototypes of SOI microdosimeter at fast neutron therapy, on reactor and accelerator based BNCT and proton therapy facilities in the USA and Japan demonstrated great potential of new device

[en] Effect of gamma and proton irradiation on threshold voltage shift Δ V_Τ for n-channel MOSFET has been studied. For MOSFET with zero gate bias during gamma irradiation the shift Δ V_Τ is linear with dose D_γ (up to doses of 10 Gy) and the sensitivity Δ V_Τ/D_γ is approximately 120 mV.Gy^-1. The sensitivity of MOSFETS with positive gate bias V_g during irradiation varies as V_g^2/3 and no saturation is observed up to breakdown voltage. n-MOSFETs with bias of 100 V have the sensitivity of approximately 5 V.Gy^-1. When n-MOSFETs are irradiated with 50 MeV protons the shift Δ V_Τ varies as D_p^0.67 (for proton doses D_p ranged from 0.2 to 900 Gy). The positive charge storage in oxide is shown to contribute mainly to the radiation sensitivity of n-MOSFETs investigated. (author)

[en] 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 SiO₂ and displacement damage in terms of 1 MeV(Si) equivalent neutron fluence. The system uses MOSFETs and PIN dosimetric diodes

[en] The effect of fast neutron irradiation on the properties of wide-base p-i-n diodes has been investigated both theoretically and experimentally. Dependence of the base voltage on irradiation dose was studied for diodes at intermediate level injection. The change in resistivity of silicon under neutron irradiation should be taken into account as well as change in the carrier lifetime. The main contribution to the I-V curve shift of the wide-base p-i-n diode arises from the resistivity change for doses larger than 20-30 Gy. Such diodes have a high radiation sensitivity. Using p-i-n diodes at intermediate level injection one can significantly widen the measurement range of fast neutron doses. (author)

[en] A variety of high purity silicon grown on the basis of different manufacturing technologies were exposed to gamma irradiation (up to a dose of 10⁸ rad(Si)) and to neutron irradiation (up to a fluence of 10¹⁵ n/cm²). Observation was made of the conduction type and carrier concentration as a function of dose. The conversion point (n-Si to p-Si) of gamma irradiated silicon was found to vary over 2 orders of magnitude of gamma dose for different manufacturers of high purity silicon independent of the initial carrier concentration. A systematic study of the radiation hardness of high purity silicon allows the development of silicon detectors working under harsh radiation environments operating over a wide range of dose. Another important aspect of this research is the development of neutron dosimeters with a wider range of response in terms of 1 MeV(Si) equivalent neutron fluence for calibration of neutron test facilities with unknown neutron energy spectrums. High purity silicon PIN diodes were calibrated using an epithermal neutron beam to determine whether response in terms of 1 MeV(Si) neutrons was independent of the calibration spectrum used