A hybrid radiation detector for simultaneous spatial and temporal dosimetry

In this feasibility study an organic plastic scintillator is calibrated against ionisation chamber measurements and then embedded in a polymer gel dosimeter to obtain a quasi-4D experimental measurement of a radiation field. This hybrid dosimeter was irradiated with a linear accelerator, with temporal measurements of the dose rate being acquired by the scintillator and spatial measurements acquired with the gel dosimeter. The detectors employed in this work are radiologically equivalent; and we show that neither detector perturbs the intensity of the radiation field of the other. By employing these detectors in concert, spatial and temporal variations in the radiation intensity can now be detected and gel dosimeters can be calibrated for absolute dose from a single irradiation.

Clinical use of diodes and micro-chambers to obtain accurate small field output factor measurements

There have been substantial advances in small field dosimetry techniques and technologies, over the last decade, which have dramatically improved the achievable accuracy of small field dose measurements. This educational note aims to help radiation oncology medical physicists to apply some of these advances in clinical practice. The evaluation of a set of small field output factors (total scatter factors) is used to exemplify a detailed measurement and simulation procedure and as a basis for discussing the possible effects of simplifying that procedure. Field output factors were measured with an unshielded diode and a micro-ionisation chamber, at the centre of a set of square fields defined by a micro-multileaf collimator. Nominal field sizes investigated ranged from 6��6 to 98��98 mm2. Diode measurements in fields smaller than 30 mm across were corrected using response factors calculated using Monte Carlo simulations of the full diode geometry and daisy-chained to match micro-chamber measurements at intermediate field sizes. Diode measurements in fields smaller than 15 mm across were repeated twelve times over three separate measurement sessions, to evaluate the to evaluate the reproducibility of the radiation field size and its correspondence with the nominal field size. The five readings that contributed to each measurement on each day varied by up to 0.26%, for the ���very small��� fields smaller than 15 mm, and 0.18% for the fields larger than 15 mm. The diode response factors calculated for the unshielded diode agreed with previously published results, within 1.6%. The measured dimensions of the very small fields differed by up to 0.3 mm, across the different measurement sessions, contributing an uncertainty of up to 1.2% to the very small field output factors. The overall uncertainties in the field output factors were 1.8% for the very small fields and 1.1% for the fields larger than 15 mm across. Recommended steps for acquiring small field output factor measurements for use in radiotherapy treatment planning system beam configuration data are provided.

Effect of radiation on the Peierls transition

The effect of radiation on the Peierls transition in a one-dimensional metal is investigated. It is pointed out that an external radiation field satisfying appropriate frequency conditions reduces the width of the Peierls gap.

The effect of radiation on the electron pairing in a multiband system

The possible occurrence of a generalized (1-wave) nonequilibrium superconducting state in a multiband system under certain conditions is studied. In the model the radiation field causes interband mixing, and phonons of an appropriate mode (branch) are involved in the interband scattering of electrons of two conduction bands of the system. The strength of the generalized 1-wave pairing interaction between quasiparticles belonging to new radiation admixed states depends on the density (n o/V) of quanta in the system. The coupling constant has the form Xl= AiB(n o/V)/[C + B(no/V)], where A1, B, and C are parameters. For C > B(n0/V), the transition temperature T1* increases with (no/V) in the initial stages. It levels off with higher power. With further increase of power, the transition temperature is expected to drop sharply due to heating effects which cause pair breaking. Estimates show that p-wave (triplet state) pairing may be possible under radiation-induced nonequilibrium situations in appropriate systems. Estimates for lifetimes of various processes quasiparticle, phonon, pair relaxation, and photon-induced mixing) show that the coherence required for the mixing and pairing effects will be maintained for the temperature range and photon density considered.

The modulation of radiation in an electron-positron plasma

The modulational instability of a large-amplitude, linearly polarized electromagnetic wave propagating in an electron-positron plasma is considered, including the combined effect of relativistic mass variation of the plasma particles, harmonic generation, and the non-resonant, finite-frequency electrostatic density perturbations, all caused by the large-amplitude radiation field. The radiation from many strong sources, such as AGN and pulsars, has been observed to vary over a host of time-scales. It is possible that the extremely rapid variations in the non-thermal continuum of AGN, as well as in the non-thermal radio radiation from pulsars, can be accounted for by the modulational instabilities to which radiation may be subjected during its propagation out of the emission region.

Graphene field-effect transistors as room-temperature terahertz detectors

The unique optoelectronic properties of graphene make it an ideal platform for a variety of photonic applications, including fast photodetectors, transparent electrodes in displays and photovoltaic modules, optical modulators, plasmonic devices, microcavities, and ultra-fast lasers. Owing to its high carrier mobility, gapless spectrum and frequency-independent absorption, graphene is a very promising material for the development of detectors and modulators operating in the terahertz region of the electromagnetic spectrum (wavelengths in the hundreds of micrometres), still severely lacking in terms of solid-state devices. Here we demonstrate terahertz detectors based on antenna-coupled graphene field-effect transistors. These exploit the nonlinear response to the oscillating radiation field at the gate electrode, with contributions of thermoelectric and photoconductive origin. We demonstrate room temperature operation at 0.3 THz, showing that our devices can already be used in realistic settings, enabling large-area, fast imaging of macroscopic samples. �� 2012 Macmillan Publishers Limited. All rights reserved.

Investigation on the N-eff reverse annealing effect using TSC/I-DLTS: Relationship between neutron induced microscopic defects and silicon detector electrical degradations

Neutron induced defect levels in high resistivity silicon detectors have been studied using a current-based macroscopic defect analysis system: thermally stimulated current (TSC) and current deep level transient spectroscopy (I-DLTS). These studies have been correlated to the traditional C-V, I-V, and transient current and charge techniques (TCT/TChT) after neutron radiation and subsequent thermal anneals. It has been found that the increases of the space charge density, N-eff, in irradiated detectors after thermal anneals (N-eff reverse anneal) correspond to the increases of deep levels in the silicon bandgap. In particular, increases of the double vacancy center (V-V and V-V-- -) and/or C-i-O-i level have good correlations with the N-eff reverse anneal. It has also been observed that the leakage current of highly irradiated (Phi(n) > 10(13) n/cm(2)) detectors increases after thermal anneals, which is different from the leakage current annealing behavior of slightly irradiated (Phi(n) < 10(13) n/cm(2)) detectors. It is apparent that V-V center and/or C-i-O-i level play important roles in both N-eff and leakage current degradations for highly irradiated high resistivity silicon detectors.

Cell survival responses after exposure to modulated radiation fields.

In the present study survival responses were determined in cells with differing radiosensitivity, specifically primary fibroblast (AG0-1522B), human breast cancer (MDA-MB-231), human prostate cancer (DU-145) and human glioma (T98G) cells, after exposure to modulated radiation fields delivered by shielding 50% of the tissue culture flask. A significant decrease (P &lt; 0.05) in cell survival was observed in the shielded area, outside the primary treatment field (out-of-field), that was lower than predicted when compared to uniform exposures fitted to the linear-quadratic model. Cellular radiosensitivity was demonstrated to be an important factor in the level of response for both the in- and out-of-field regions. These responses were shown to be dependent on secretion-mediated intercellular communication, because inhibition of cellular secreted factors between the in- and out-of-field regions abrogated the response. Out-of-field cell survival was shown to increase after pretreatment of cells with agents known to inhibit factors involved in mediating radiation-induced bystander signaling (aminoguanidine, DMSO or cPTIO). These data illustrate a significant decrease in survival out-of-field, dependent upon intercellular communication, in several cell lines with varying radiosensitivity after exposure to a modulated radiation field. This study provides further evidence for the importance of intercellular signaling in modulated exposures, where dose gradients are present, and may inform the refinement of established radiobiological models to facilitate the optimization of advanced radiotherapy treatment plans.

DNA damage responses following exposure to modulated radiation fields

During the delivery of advanced radiotherapy treatment techniques modulated beams are utilised to increase dose conformity across the target volume. Recent investigations have highlighted differential cellular responses to modulated radiation fields particularly in areas outside the primary treatment field that cannot be accounted for by scattered dose alone. In the present study, we determined the DNA damage response within the normal human fibroblast AG0-1522B and the prostate cancer cell line DU-145 utilising the DNA damage assay. Cells plated in slide flasks were exposed to 1 Gy uniform or modulated radiation fields. Modulated fields were delivered by shielding 25%, 50% or 75% of the flask during irradiation. The average number of 53BP1 or ?H2AX foci was measured in 2 mm intervals across the slide area. Following 30 minutes after modulated radiation field exposure an increase in the average number of foci out-of-field was observed when compared to non-irradiated controls. In-field, a non-uniform response was observed with a significant decrease in the average number of foci compared to uniformly irradiated cells. Following 24 hrs after exposure there is evidence for two populations of responding cells to bystander signals in-and out-of-field. There was no significant difference in DNA damage response between 25%, 50% or 75% modulated fields. The response was dependent on cellular secreted intercellular signalling as physical inhibition of intercellular communication abrogated the observed response. Elevated residual DNA damage observed within out-of-field regions decreased following addition of an inducible nitric oxide synthase inhibitor (Aminoguanidine). These data show, for the first time, differential DNA damage responses in-and out-of-field following modulated radiation field delivery. This study provides further evidence for a role of intercellular communication in mediating cellular radiobiological response to modulated radiation fields and may inform the refinement of existing radiobiological models for the optimization of advanced radiotherapy treatment plans. �� 2012 Trainor et al.

Monte Carlo radiation hydrodynamics:Methods, tests and application to Type Ia supernova ejecta

In astrophysical systems, radiation-matter interactions are important in transferring energy and momentum between the radiation field and the surrounding material. This coupling often makes it necessary to consider the role of radiation when modelling the dynamics of astrophysical fluids. During the last few years, there have been rapid developments in the use of Monte Carlo methods for numerical radiative transfer simulations. Here, we present an approach to radiation hydrodynamics that is based on coupling Monte Carlo radiative transfer techniques with finite-volume hydrodynamical methods in an operator-split manner. In particular, we adopt an indivisible packet formalism to discretize the radiation field into an ensemble of Monte Carlo packets and employ volume-based estimators to reconstruct the radiation field characteristics. In this paper the numerical tools of this method are presented and their accuracy is verified in a series of test calculations. Finally, as a practical example, we use our approach to study the influence of the radiation-matter coupling on the homologous expansion phase and the bolometric light curve of Type Ia supernova explosions. �� 2012 The Authors Monthly Notices of the Royal Astronomical Society �� 2012 RAS.

��tude des collisions proton-proton dans l���exp��rience ATLAS avec les d��tecteurs ATLAS-MPX

Les seize d��tecteurs MPX constituant le r��seau ATLAS-MPX ont ��t�� plac��s �� diff��rentes positions dans le d��tecteur ATLAS et sa averne au CERN dans le but de mesurer en emps r��el les champs de radiation produits ar des particules primaires (protons des faisceaux) et des particules secondaires (kaons, pions, g, protons) issues des collisions proton-proton. Des films de poly��thyl��ne (PE) et de fluorure de lithium (6LiF) recouvrent les d��tecteurs afin d���augmenter leur sensibilit�� aux neutrons produits par les particules primaires et secondaires interagissant avec les mat��riaux pr��sents dans l���environnement d���ATLAS. La reconnaissance des traces laiss��es par les particules dans un d��tecteur ATLAS-MPX se fait �� partir des algorithmes du logiciel MAFalda (���Medipix Analysis Framework���) bas�� sur les librairies et le logiciel d���analyse de donn��es ROOT. Une ��tude sur le taux d���identifications erron��es et le chevauchement d���amas a ��t�� faite en reconstruisant les activit��s des sources 106Ru et 137Cs. L���efficacit�� de d��tection des neutrons rapides a ��t�� mesur��e �� l���aide des sources 252Cf et 241AmBe (neutrons d�����nergie moyenne de 2.13 et 4.08 MeV respectivement). La moyenne des efficacit��s de d��tection mesur��es pour les neutrons produits par les sources 252C f et 241AmBe a ��t�� calcul��e pour les convertisseurs 6LiF et PE et donnent (0.8580 �� 0.1490)% et (0.0254 �� 0.0031)% pour LiF et (0.0510 �� 0.0061)% et (0.0591 �� 0.0063)% pour PE �� bas et �� haut seuil d�����nergie respectivement. Une simulation du calcul de l���efficacit�� de d��tection des neutrons dans le d��tecteur MPX a ��t�� r��alis��e avec le logiciel GEANT4. Des donn��es MPX correspondant aux collisions proton-proton �� 2.4 TeV et �� 7 TeV dans le centre de masse ont ��t�� analys��es. Les flux d��tect��s d�����lectrons et de photons sont particuli��rement ��lev��s dans les d��tecteurs MPX01 et MPX14 car ils sont plus pr��s du point de collision. Des flux de neutrons ont ��t�� estim��s en utilisant les efficacit��s de d��tection mesur��es. Une corr��lation avec la luminosit�� du LHC a ��t�� ��tablie et on pr��dit que pour les collisions �� 14 TeV dans le centre de masse et avec une luminosit�� de 10^34 cm-1*s-1 il y aura environ 5.1x10^8 �� 1.5x10^7 et 1.6x10^9 �� 6.3x10^7 particules d��tect��es par les d��tecteurs MPX01 et MPX14 respectivement.

Mesures et analyses de la luminosit�� d���ATLAS gr��ce aux d��tecteurs MPX

Medipix2 (MPX) sont des d��tecteurs semi-conducteurs au silicium mont��s sur 256x256 pixels. Chaque pixel a une aire de 55x55��m2. L���aire active d���un d��tecteur MPX est d���environ 2 cm2. Avec deux modes de d��tection, un seuil et un temps d���exposition ajustables, leur utilisation peut ��tre optimis��e pour une analyse sp��cifique. Seize de ces d��tecteurs sont pr��sentement install��s dans l���exp��rience ATLAS (A Toroidal LHC ApparatuS) au CERN (Organisation Europ��enne pour la Recherche Nucl��aire). Ils mesurent en temps r��el le champ de radiation d�� aux collisions proton-proton, au point d���interaction IP1 (Point d���Interaction 1) du LHC (Grand Collisionneur d���Hadrons). Ces mesures ont divers buts comme par exemple la mesure du champ de neutrons dans la caverne d���ATLAS. Le r��seau de d��tecteurs MPX est compl��tement ind��pendant du d��tecteur ATLAS. Le groupe ATLAS-Montr��al s���est int��ress�� �� l���analyse des donn��es r��colt��es par ces d��tecteurs pour calculer une valeur de la luminosit�� du LHC au point de collision des faisceaux, autour duquel est construit le d��tecteur ATLAS. Cette valeur est d��termin��e ind��pendamment de la luminosit�� mesur��e par les divers sous-d��tecteurs d���ATLAS d��di��s sp��cifiquement �� la mesure de la luminosit��. Avec l���augmentation de la luminosit�� du LHC les d��tecteurs MPX les plus proches du point d���interaction d��tectent un grand nombre de particules dont les traces sont impossibles �� distinguer sur les images ("frames") obtenues, �� cause de leur recouvrement. Les param��tres de mesure de certains de ces d��tecteurs ont ��t�� optimis��s pour des mesures de luminosit��. Une m��thode d���analyse des donn��es permet de filtrer les pixels bruyants et de convertir les donn��es des images, qui correspondent �� des temps d���exposition propres aux d��tecteurs MPX, en valeur de luminosit�� pour chaque LumiBlock. Un LumiBlock est un intervalle de temps de mesure propre au d��tecteur ATLAS. On a valid�� les mesures de luminosit�� premi��rement en comparant les r��sultats obtenus par diff��rents d��tecteurs MPX, et ensuite en comparant les valeurs de luminosit�� relev��es �� celles obtenues par les sous-d��tecteurs d���ATLAS d��di��s sp��cifiquement �� la mesure de la luminosit��.

Studies on the interaction of two level atoms with quantized electromagnetic field

The present study described about the interaction of a two level atom and squeezed field with time varying frequency. By applying a sinusoidal variation in the frequency of the field, the randomness in population inversion is reduced and the collapses and periodic revivals are regained. Quantum optics is an emerging field in physics which mainly deals with the interaction of atoms with quantised electromagnetic fields. Jaynes-Cummings Model (JCM) is a key model among them, which describes the interaction between a two level atom and a single mode radiation field. Here the study begins with a brief history of light, atom and their interactions. Also discussed the interaction between atoms and electromagnetic fields. The study suggest a method to manipulate the population inversion due to interaction and control the randomness in it, by applying a time dependence on the frequency of the interacting squeezed field.The change in behaviour of the population inversion due to the presence of a phase factor in the applied frequency variation is explained here.This study also describes the interaction between two level atom and electromagnetic field in nonlinear Kerr medium. It deals with atomic and field state evolution in a coupled cavity system. Our results suggest a new method to control and manipulate the population of states in two level atom radiation interaction,which is very essential for quantum information processing.We have also studied the variation of atomic population inversion with time, when a two level atom interacts with light field, where the light field has a sinusoidal frequency variation with a constant phase. In both coherent field and squeezed field cases, the population inversion variation is completely different from the phase zero frequency modulation case. It is observed that in the presence of a non zero phase ��, the population inversion oscillates sinusoidally.Also the collapses and revivals gradually disappears when �� increases from 0 to ��/2. When �� = ��/2 the evolution of population inversion is identical to the case when a two level atom interacts with a Fock state. Thus, by applying a phase shifted frequency modulation one can induce sinusoidal oscillations of atomic inversion in linear medium, those normally observed in Kerr medium. We noticed that the entanglement between the atom and field can be controlled by varying the period of the field frequency fluctuations. The system has been solved numerically and the behaviour of it for different initial conditions and different susceptibility values are analysed. It is observed that, for weak cavity coupling the effect of susceptibility is minimal. In cases of strong cavity coupling, susceptibility factor modifies the nature in which the probability oscillates with time. Effect of susceptibility on probability of states is closely related to the initial state of the system.

THE INTERSTELLAR MAGNETIC FIELD CLOSE TO THE SUN. II.

The magnetic field in the local interstellar medium (ISM) provides a key indicator of the galactic environment of the Sun and influences the shape of the heliosphere. We have studied the interstellar magnetic field (ISMF) in the solar vicinity using polarized starlight for stars within 40 pc of the Sun and 90 degrees of the heliosphere nose. In Frisch et al. (Paper I), we developed a method for determining the local ISMF direction by finding the best match to a group of interstellar polarization position angles obtained toward nearby stars, based on the assumption that the polarization is parallel to the ISMF. In this paper, we extend the analysis by utilizing weighted fits to the position angles and by including new observations acquired for this study. We find that the local ISMF is pointed toward the galactic coordinates l, b = 47 degrees +/- 20 degrees, 25 degrees +/- 20 degrees. This direction is close to the direction of the ISMF that shapes the heliosphere, l, b = 33 degrees +/- 4 degrees, 55 degrees +/- 4 degrees, as traced by the center of the "Ribbon" of energetic neutral atoms discovered by the Interstellar Boundary Explorer (IBEX) mission. Both the magnetic field direction and the kinematics of the local ISM are consistent with a scenario where the local ISM is a fragment of the Loop I superbubble. A nearby ordered component of the local ISMF has been identified in the region l approximate to 0 degrees -&gt; 80 degrees and b approximate to 0 degrees -&gt; 30 degrees, where PlanetPol data show a distance-dependent increase of polarization strength. The ordered component extends to within 8 pc of the Sun and implies a weak curvature in the nearby ISMF of +/- 0 degrees.25 pc(-1). This conclusion is conditioned on the small sample of stars available for defining this rotation. Variations from the ordered component suggest a turbulent component of +/- 23 degrees. The ordered component and standard relations between polarization, color excess, and H-o column density predict a reasonable increase of N(H) with distance in the local ISM. The similarity of the ISMF directions traced by the polarizations, the IBEX Ribbon, and pulsars inside the Local Bubble in the third galactic quadrant suggest that the ISMF is relatively uniform over spatial scales of 8-200 pc and is more similar to interarm than spiral-arm magnetic fields. The ISMF direction from the polarization data is also consistent with small-scale spatial asymmetries detected in GeV-TeV cosmic rays with a galactic origin. The peculiar geometrical relation found earlier between the cosmic microwave background dipole moment, the heliosphere nose, and the ISMF direction is supported by this study. The interstellar radiation field at +/- 975 angstrom does not appear to play a role in grain alignment for the low-density ISM studied here.