Solid-state micro- and nano-dosimetry: theory and applications

Methods of determining the biological effects of radiation based on the concepts of linear energy transfer as a physical descriptor of the radiation field and absorbed dose are flawed due to limitations in these quantities. For example, radiation with the same linear energy transfer and absorbed dose may have very different biological effects due to the potential for very different track structures and energy deposition spectra. Methods of estimating biological effects of radiation which are independent of linear energy transfer and absorbed dose, such as track structure theory and microdosimetry are described. Track structure theory was applied to heavy charged particle relative thermoluminescence efficiencies in LiF:Mg,Ti and LiF:Mg,Cu,P. The Unified Interaction Model of dose response was applied to the experimental dose response of 100 keV synchrotron x-rays. The Unified Interaction Model has had success in explaining the thermoluminescence mechanisms responsible for the dose response of the various peaks in the glow curve, particularly the supralinearity at dose levels above ~1 Gy. Track structure theory requires matching of the dose response function with the spectra of secondary particles liberated by heavy charged particles slowing down in material. The energies of such particles are typically of the order of a few keV. Measurements at these energies have proven difficult and so the dose response at ultra-low electron energies has been estimated from the Unified Interaction Model by extrapolation of the maximum dose response values from available data. Calculations of relative proton and α-particle thermoluminescence efficiencies from track structure theory in LiF:Mg,Ti and LiF:Mg,Cu,P differ significantly (by factors up to ~30 for 4.95 MeV α-particles and ~10 for 1.43 MeV protons) from experimentally measured values. However, uncertainties in the experimental measurements, uncertainties arising from the estimation of the dose response function and possible uncertainties in previous calculations of radial dose distributions are significant. More sophisticated calculations and experimental measurements at ultra-low photon/electron energies are indicated for future studies. Microdosimetry was applied in heavy ion fields relevant to heavy ion therapy and space radiation fields using two generations of silicon-on-insulator microdosimeter arrays. Radiation protection for these applications requires the ability to measure the rapidly changing lineal energy of the particle with high precision. The high spatial resolution of both microdosimeters due to their 10 μm thickness was demonstrated by measurements of 4He and/or 12C ion beams at the HIMAC (Japan) and HIT (Germany) heavy ion therapy facilities. Contributions from secondary particles, particularly neutrons were observed, demonstrating the ability of microdosimetry to measure the lineal energy of components of the unknown spectrum of secondary radiation. Differences observed in the lineal energy spectra measured by first and second generation microdosimeters were attributed to slight differences in the mean chord lengths and chord length distributions due to the different sensitive volume geometries. Microdosimeters were also able to accurately reproduce intricate dose plans. Out-of-field measurements with the microdosimeters positioned lateral to the heavy ion field showed that the majority of secondary radiation originates inside the treatment volume upstream of the beam. A third generation microdosimeter, developed using n-type silicon-on-insulator (nSOI) and epitaxial technologies, was developed with the aim of increasing the sensitive surface area and yield of sensitive volumes. Charge collection studies indicated a 100% yield which is a significant improvement over the previous generations. However, both nSOI and epitaxial devices were found to suffer from charge sharing between sensitive volume and guard ring structures, as well as an enhanced energy response to heavy ions. In addition, the epitaxial microdosimeters were found to be highly susceptible to radiation damage. A coincidence analysis of ion beam induced charge collection designed to investigate the anomalous response of nSOI microdosimeters confirmed the occurrence of charge sharing between the sensitive volume and guard ring. The guard ring was applied as a veto electrode to discriminate shared charge, which was shown to improve the charge collection geometry and the measured energy deposition spectrum. The effective sensitive surface area of a single cell was reduced from ~20 μm to ~8 μm, which is closer to the 10 μm diameter of the nominal sensitive volume. However, as the geometry of the sensitive volume is still not properly understood this technique is useful for characterisation but not for experimental microdosimetry due to the requirement of a well-defined sensitive volume. The anomalous energy response was investigated using ion beam induced charge collection and spectroscopy with 12C, 4He and H ions of various energies and linear energy transfers. No correlation between particle LET and the energy over-response was found. The enhanced energy response was hypothesised to be a result of a displacement current induced in the active SOI layer by charge carriers induced in the substrate due to the parasitic capacitance of the SiO2. This hypothesis was investigated using the response of the device to 148Gd α-particles, whose range is less than the thickness of the active SOI layer. The enhanced energy response was not observed, indicating, although not confirming, that the enhanced energy response is a result of a displacement current. A second hypothesis for the cause of the enhanced energy response was that the thickness of the active SOI layer is greater than the value of 10 _m provided by the device manufacturer. A scanning electron microscopy study coupled with energy dispersive x-ray spectroscopy on an nSOI microdosimeter provided no evidence of the SiO2 insulating layer which limits the thickness of the SOI active layer to 10 μm. To confirm the viability of this technique for observing the SiO2 layer, the same investigation was performed on a second generation SOI microdosimeter. The SiO2 layer was clearly observed at a depth of 9.6±0.2 μm with a thickness of 1.9±0.2 μm, in agreement with the device specifications. This finding explains the enhanced energy observed, however, the question as to why full energy deposition is not observed remains unanswered

Experimental optimisation of the X-ray energy in microbeam radiation therapy

International audienceMicrobeam radiation therapy has demonstrated superior normal tissue sparing properties compared to broad-beam radiation fields. The ratio of the microbeam peak dose to the valley dose (PVDR), which is dependent on the X-ray energy/spectrum and geometry, should be maximised for an optimal therapeutic ratio. Simulation studies in the literature report the optimal energy for MRT based on the PVDR. However, most of these studies have considered different microbeam geometries to that at the Imaging and Medical Beamline (50 μm beam width with a spacing of 400 μm). We present the first fully experimental investigation of the energy dependence of PVDR and microbeam penumbra. Using monochromatic X-ray energies in the range 40–120 keV the PVDR was shown to increase with increasing energy up to 100 keV before plateauing. PVDRs measured for pink beams were consistently higher than those for monochromatic energies similar or equivalent to the average energy of the spectrum. The highest PVDR was found for a pink beam average energy of 124 keV. Conversely, the mi-crobeam penumbra decreased with increasing energy before plateauing for energies above 90 keV. The effect of bone on the PVDR was investigated at energies 60, 95 and 120 keV. At depths greater than 20 mm beyond the bone/water interface there was almost no effect on the PVDR. In conclusion, the optimal energy range for MRT at IMBL is 90–120 keV, however when considering the IMBL flux at different energies, a spectrum with 95 keV weighted average energy was found to be the best compromise

DOSIMETRIE EXPERIMENTALE POUR LA RADIOTHERAPIE PAR MICROFAISCEAUX SYNCHROTRON

International audienceLa principale problématique en radiothérapie anticancéreuse est de délivrer une dose maximale (énergie absorbée par unité de masse) de rayonnements ionisants à la cible, tout en épargnant les tissus sains avoisinants. Une option prometteuse, permettant d’atteindre cet effet différentiel significativement plus élevé dans le cadre du traitement de petites lésions cérébrales isolées, est l’utilisation de matrices de microfaisceaux de rayons x à très forts débits de dose [1]. La radiothérapie par micro-faisceaux (MRT) est une technique de radiothérapie innovante, basée sur le fractionnement spatial de la dose en utilisant des matrices de micro-faisceaux de rayons-x parallèles, d’une largeur comparable à celle d’un cheveu (~ 50 micromètres) et séparés par des zones qui ne voient que le rayonnement diffusé. De nombreuses études précliniques ont démontré que la MRT augmente significativement la dose administrée à la tumeur par rapport à la radiothérapie conventionnelle tout en permettant de mieux préserver les tissus sains sur le trajet des faisceaux d’irradiation [2]. Bien que la MRT soit actuellement limitée à la recherche dans les établissements de rayonnement synchrotron en raison du débit de dose très élevé requis pour sa mise en contrôle (5 000 fois plus élevé que la radiothérapie clinique), les cliniciens et les chercheurs collaborent étroitement avec les scientifiques du synchrotron pour transférer cette technique vers les essais cliniques.Le principal verrou d’un point de vue des problématiques de physique médicale associées à cette technique concerne la dosimétrie expérimentale (méthodes physiques permettant de mesurer quantitativement l’énergie absorbée par unité de masse dans les tissus irradiés, ou dans un mileu équivalent). En effet au regard de la taille des faisceaux et des débits de dose associés, Nous sommes très proches limites théoriques des méthodes existantes. Par exemple, le transfert clinique de la MRT nécessite la mise en oeuvre de codes de pratique cliniques pour la mesure de la dose qui, bien qu’ils soient établis dans la radiothérapie clinique conventionnelle, ils ne sont pas applicables à la MRT en raison du spectre des rayons X, des débit de dose élevés et des limitations des détecteurs [3].L’objectif de cette présentation est de faire un état des lieux des solutions dosimétriques existantes pour la MRT et de proposer quelques pistes permettant un contrôle encore plus précis de la dose

Mysteries of LiF TLD response following high ionisation density irradiation: nanodosimetry and track structure theory, dose response and glow curve shapes

Three outstanding effects of ionisation density on the thermoluminescence (TL) mechanisms giving rise to the glow peaks of LiF:Mg,Ti (TLD-100) are currently under investigation: (1) the dependence of the heavy charged particle (HCP) relative efficiency with increasing ionisation density and the effectiveness of its modelling by track structure theory (TST), (2) the behaviour of the TL efficiency, f(D), as a function of photon energy and dose. These studies are intended to promote the development of a firm theoretical basis for the evaluation of relative TL efficiencies to assist in their application in mixed radiation fields. And (3) the shape of composite peak 5 in the glow curve for various HCP types and energies and following high-dose electron irradiation, i.e. the ratio of the intensity of peak 5a to peak 5. Peak 5a is a low-temperature satellite of peak 5 arising from electron-hole capture in a spatially correlated trapping centre/luminescent centre (TC/LC) complex that has been suggested to possess a potential as a solid-state nanodosemeter due to the preferential electron/hole population of the TC/LC at high ionisation density. It is concluded that (1) the predictions of TST are very strongly dependent on the choice of photon energy used in the determination of f(D); (2) modified TST employing calculated values of f(D) at 2 keV is in agreement with 5-MeV alpha particle experimental results for composite peak 5 but underestimates the 1.5-MeV proton relative efficiencies. Both the proton and alpha particle relative TL efficiencies of the high-temperature TL (HTTL) peaks 7 and 8 are underestimated by an order of magnitude suggesting that the HTTL efficiencies are affected by other factors in addition to radial electron dose; (3) the dose–response supralinearity of peaks 7 and 8 change rapidly with photon energy: this behaviour is explained in the framework of the unified interaction model as due to a very strong dependence on photon energy of the relative intensity of localised recombination and (4) the increased width and decrease in Tmax of composite peak 5 as a function of ionisation density is due to the greater relative intensity of peak 5a (a low-temperature component of peak 5 arising from two-energy transfer events, which leads to localised recombination)

Mysteries of LiF TLD response following high ionization density irradiation: Glow curve shapes, dose response, the unified interaction model and modified track structure theory

Three outstanding effects of ionization density on the thermoluminescence (TL) mechanisms giving rise to the glow peaks of LiF:Mg,Ti (TLD-100) are currently under investigation: (i) the dependence of the heavy charged particle (HCP) relative efficiency on ionization density and the effectiveness of its modeling by track structure theory (TST) (ii) the behavior of the TL efficiency, f(D), as a function of photon energy and dose and (iii) the shape of composite peak 5 in the glow curve for various HCP types and energies and following high dose electron irradiation. It is concluded that (i) The predictions of TST are very strongly dependent on the choice of photon energy used in the determination of f(D), (ii) Modified TST employing calculated values of f(D) at 2 keV is in agreement with 5 MeV alpha particle experimental results for composite peak 5 but underestimates the 1.5 MeV proton relative efficiencies. Both the proton and alpha particle relative TL efficiencies of the high temperature TL (HTTL) peaks 7 and 8 are underestimated by an order of magnitude suggesting that the HTTL efficiencies are affected by other factors in addition to radial electron dose. (iii) The dose response supralinearity of peaks 7 and 8 change rapidly with photon energy: this behavior is explained in the framework of the Unified Interaction Model as due to a very strong dependence on photon energy of the relative intensity of localized recombination, (iv) The increased width and decrease in Tmax of composite peak 5 as a function of ionization density is due to the greater relative intensity of peak 5a (a low temperature component of peak 5 arising from two-energy-transfer events which leads to localized recombination)

Experimental investigation of the 100 keV x-ray dose response of the high-temperature thermoluminescence in Lif: Mg, Ti (TLD-100): theoretical interpretation using the unified interaction model

The dose response of LiF:Mg,Ti (TLD-100) chips was measured from 1 to 50 000 Gy using 100 keV X rays at the European Synchroton Radiation Facility. Glow curves were deconvoluted into component glow peaks using a computerised glow curve deconvolution (CGCD) code based on first-order kinetics. The normalised dose response, f(D), of glow peaks 4 and 5 and 5b (the major components of composite peak 5), as well as peaks 7 and 8 (two of the major components of the high-temperature thermoluminescence (HTTL) at high levels of dose) was separately determined and theoretically interpreted using the unified interaction model (UNIM). The UNIM is a nine-parameter model encompassing both the irradiation/absorption stage and the thermally induced relaxation/recombination stage with an admixture of both localised and delocalised recombination mechanisms. The effects of radiation damage are included in the present modelling via the exponential removal of luminescent centres (LCs) at high dose levels. The main features of the experimentally measured dose response are: (i) increase in f(D)max with glow peak temperature, (ii) increase in Dmax (the dose level at which f(D)max occurs) with increasing glow peak temperature, and (iii) decreased effects of radiation damage with increasing glow peak temperature. The UNIM interpretation of this behaviour requires both strongly decreasing values of ks (the relative contribution of localised recombination) as a function of glow peak temperature and, as well, significantly different values of the dose-filling constants of the trapping centre (TC) and LC for peaks 7 and 8 than those used for peaks 4 and 5. This suggests that different TC/LC configurations are responsible for HTTL. The relative intensity of peak 5a (a low-temperature satellite of peak 5 arising from localised recombination) was found to significantly increase at higher dose levels due to preferential electron and hole population of the trapping/recombination complex giving rise to composite glow peak 5. It is also demonstrated that possible changes in the trapping cross section of the LC and the competitive centres due to increasing sample/glow peak temperature do not significantly influence these observations/conclusions

Energy dependence of the supralinearity (f(D)max) of peaks 7 and 8 in the high temperature thermoluminescence of LiF:Mg,Ti (TLD-100): Interpretation using the unified interaction model

It is demonstrated that the supralinearity of the dose response of glow peaks 7, 8 in the glow curve ofLiF:Mg,Ti (TLD-100) are very strongly dependent on photon/electron energy. Previously published data onf(D)max at photon energies of 1.25 MeV, 100 keV and 8.1 keV effective energy coupled with new dataat w 540 keV using 90Sr/90Y beta rays reveals that the maximum supralinearity f(D)max decreases fromvalues ofw200 andw30 at 1.25MeV, through intermediate values at 540 keV and 100 keV, to values ofw30andw3 at 8.1 keV effective energy. The normalized dose response f(D) for all energies is modeled using theUnified Interaction Model and the dependence of f(D)max on energy is interpreted as arising from strongdependence of the relative intensity of localized recombination on particle energy (ionization density)

Large area silicon microdosimeter for dosimetry in high LET space radiation fields: Charge collection study

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

DOSIMETRY, DIAMOND DETECTOR, SYNCHROTRON RADIATION

National audienceA significant proportion of cancer patients benefit from radiotherapy. Besides conventional x-ray radiation, synchrotron has proven to offer significant advantages in radiotherapy by using high dose rate coherent x-rays beams. Indeed, High coherence allowing to produce micrometric fields to explore limits of a concept called dose-volume effect. The other important characteristics of synchrotron radiation (high dose rate) permits to take advantage of the so-called flash effect. The first phase I/II clinical study of synchrotron radiotherapy at the European Synchrotron Radiation Facility (ESRF) demonstrated the feasibility and safety of this technique. this method requires some development. One of them, in-vivo dosimetry (the real time dose delivered during the treatment), is particularly challenging, because of the high dose rate and low energy flux. A new approach based on pixelised diamond detectors, already validated for one point dosimetry in synchrotron radiation[1], will be developed. Before the full conception of one dimension dosimeter, first step is to characterize diamond detectors responses in synchrotron radiation (incident photons energy between 30 and 150 keV and an high dose rate which can reach 10 000 Gy/s) and show the project's viability. For this reason, some preliminary tests were performed on different diamond detectors (two mono-crystalline and one polycrystalline), to show their response for different energy and dose rate, already developed at LPSC (Laboratoire de Physique Subatomique et Cosmologie) and this presentation will focus on these tests