Download or read book L annihilation des positrons galactiques written by Vincent Lonjou and published by . This book was released on 2005 with total page 179 pages. Available in PDF, EPUB and Kindle. Book excerpt: L'origine des positrons galactiques reste un des sujets les plus controversés de l'Astrophysique des hautes énergies depuis la découverte de la raie d'annihilation électron-positron en provenance du centre galactique à la fin des années 1970. L'étude de la raie d'annihilation à 511 keV est actuellement un des objectifs majeurs de SPI : le spectromètre d'INTEGRAL. Cette thèse s'ouvre sur l'étude de la partie centrale de SPI: son plan de détection. La procédure de calibration en énergie ainsi que l'étude de la dégradation des détecteurs sont reportées. Par la suite, une étude approfondie du bruit de fond instrumental permet d'utiliser SPI dans les meilleures conditions. Le traitement des données aboutit alors à des cartes du ciel et des spectres avec une précision inégalée. Pour finir, les contributions des diverses sources potentielles de positrons galactiques sont confrontées aux résultats obtenus.
Download or read book L ANNIHILATION DES POSITRONS ET LE CENTRE GALACTIQUE written by PIERRE.. WALLYN and published by . This book was released on 1912 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: LE CENTRE GALACTIQUE EST L'OBJET D'ETUDES INTENSIVES DE LA PART DES ASTROPHYSICIENS QUI ETUDIENT LE RAYONNEMENT GAMMA DEPUIS LE DEBUT DES ANNEES 70, DATE A LAQUELLE LES CHERCHEURS DE L'UNIVERSITE DE RICE (USA) DETECTENT POUR LA PREMIERE FOIS, LA RAIE D'ANNIHILATION ELECTRON-POSITRON EN PROVENANCE DE CETTE REGION. CETTE RAIE D'ENERGIE 511 KEV EST LE RESULTAT DE DEUX PROCESSUS DISTINCTS: (1) L'ANNIHILATION DE POSITRONS EMIS PAR DES OBJETS ASTROPHYSIQUES COMME LES SUPERNOVAE, LES NOVAE, ETC... ET QUI S'ANNIHILENT DANS LE MILIEU INTERSTELLAIRE POUR PRODUIRE UNE COMPOSANTE DIFFUSE ET STATIONNAIRE; (2) L'ANNIHILATION DE BOUFFEES DE POSITRONS EMIS SPORADIQUEMENT, SUR DE COURTES PERIODES, PAR UN OU PLUSIEURS CANDIDATS TROUS NOIRS SITUES DANS LA DIRECTION DU CENTRE GALACTIQUE ET PRODUISANT UNE COMPOSANTE A 511 KEV, VARIABLE DANS LE TEMPS. LE PROFIL DE LA RAIE D'ANNIHILATION OBSERVE PAR DES EXPERIENCES A HAUTE RESOLUTION EN ENERGIE (DETECTEURS AU GERMANIUM) INDIQUE QUE LE MILIEU D'ANNIHILATION DES POSITRONS DE LA COMPOSANTE DIFFUSE EST UN GAZ CHAUD, EVENTUELLEMENT IONISE, A 8000 K ENVIRON. LA COMPOSANTE A 511 KEV, VARIABLE DANS LE TEMPS, SEMBLE CORRESPONDRE A L'ANNIHILATION DE POSITRONS DANS UN NUAGE FROID, AUTOUR DE 80 K COMME LE NUAGE G-0.86-0.08, SITUE A PROXIMITE DE LA SOURCE 1E1740.7-2942. L'EXPERIENCE BALLON HEXAGONE (MAI 1989) A MIS EN EVIDENCE UNE RAIE AUTOUR DE 170 KEV DANS LA DIRECTION DU CENTRE GALACTIQUE. NOUS PROPOSONS UN MODELE EXPLICATIF DU PHENOMENE EN TERME DE RETRODIFFUSION DE PHOTONS D'ANNIHILATION A LA SURFACE D'UNE BULLE DE MATIERE EJECTEE PAR LE DISQUE D'ACCRETION AUTOUR D'UN TROU NOIR, LORS D'UN ETAT SUPER-CRITIQUE DU SYSTEME. CE MODELE EST EGALEMENT UTILISE POUR EXPLIQUER LA SEQUENCE TEMPORELLE DES STRUCTURES AUTOUR DE 170 ET 102 KEV OBSERVEE PAR SIGMA SUR NOVA MUSCAE. L'ANNIHILATION DES POSITRONS DANS LA PHASE FROIDE DU MILIEU INTERSTELLAIRE (80 K) EST ETUDIEE EN SE BASANT SUR DES MESURES RECENTES DE SECTIONS EFFICACES D'INTERACTION DES POSITRONS AVEC L'HYDROGENE ATOMIQUE ET MOLECULAIRE. NOUS CALCULONS LA VALEUR DE LA FRACTION DE POSITRONIUM AVANT THERMALISATION POUR H ET H#2 ET PRECISONS LE TEMPS DE THERMALISATION DES POSITRONS EN FONCTION DE LA DENSITE DU MILIEU ET DE L'ENERGIE INITIALE DE LA PARTICULE. LA DUREE DU PHENOMENE D'ANNIHILATION EN VOL PAR ECHANGE DE CHARGE EST EGALEMENT CALCULEE. CECI NOUS PERMET DE DEFINIR DE NOUVELLES CONTRAINTES SUR L'OBSERVATION EVENTUELLE DES RAIES DE RECOMBINAISON DU POSITRONIUM
Download or read book Mod lisation de L mission D annihilation Des Positrons Galactiques written by William Gillard and published by . This book was released on 2008 with total page 226 pages. Available in PDF, EPUB and Kindle. Book excerpt: Positrons annihilate in the central region of our Galaxy. This has been known since the detection of a strong emission line centered on an energy of 511 keV in the direction of the Galactic center. This gamma-ray line is emitted during the annihilation of positrons with electrons from the interstellar medium. The spectrometer SPI, onboard the INTEGRAL observatory, performed spatial ans spectral analyses of the positron annihilation emission. This thesis presents a study of the Galactic positron annihilation emission based on models of the different interactions undergone by positrons in the interstellar medium. The models are relied on our present knowledge of the properties of the interstellar medium in the Galactic bulge, where most of the positrons annihilate, and of the physics of positrons (production, propagation and annihilation processes). To obtain constraints on the positrons sources and physical characteristics of the annihilation medium, we compared the results of the models to measurements provided by the SPI spectrometer.
Download or read book Origine et physique d annihilation des positrons dans la galaxie written by Anthony Alexis and published by . This book was released on 2014 with total page 193 pages. Available in PDF, EPUB and Kindle. Book excerpt: Une émission gamma à 511\kev\ est observée depuis le début des années 1970 dans la direction du centre Galactique. Cette émission est la signature de l'annihilation d'électrons avec des positrons qui sont les antiparticules des électrons. Malheureusement, l'origine de ces positrons galactiques reste à l'heure actuelle un mystère. De nombreuses sources de positrons ont été proposées mais elles présentent toutes des difficultés à expliquer cette émission d'annihilation galactique. Celle-ci possède une distribution spatiale particulière. Depuis 2002, le spectromètre SPI à bord de l'observatoire spatial INTEGRAL révèle une émission fortement concentrée dans le bulbe galactique et une faible émission en provenance du disque galactique. Cette distribution spatiale est totalement atypique car elle ne ressemble à aucune distribution galactique connue, que ce soit d'une population stellaire ou d'un gaz interstellaire. L'hypothèse selon laquelle les positrons s'annihilent à proximité de leur source (c.-à-d. que la distribution spatiale de l'émission d'annihilation est identique à la distribution spatiale des sources) a donc été remise en cause. Des études récentes semblent suggérer que les positrons pourraient se propager loin de leur source avant de s'annihiler. Ceci permettrait de résoudre éventuellement l'énigme sur l'origine des positrons galactiques. Cette thèse a été consacrée à modéliser la propagation puis l'annihilation des positrons dans la Galaxie, dans le but de comparer des modèles spatiaux de l'émission d'annihilation aux dernières données mesurées par SPI/INTEGRAL. Cette méthode nous permet en effet de poser des contraintes sur l'origine des positrons galactiques. Nous avons donc développé un code de simulation Monte Carlo de transport des positrons dans la Galaxie dans lequel nous avons implémenté toutes les connaissances théoriques et observationnelles de la physique des positrons (sources, modes de transport, pertes en énergie, modes d'annihilation) et du milieu interstellaire de la Galaxie (distributions du gaz interstellaire, champs magnétiques galactiques, structure des phases gazeuses). Dues aux incertitudes entourant de nombreux paramètres physiques (champs magnétique du halo galactique, structure des phases du milieu interstellaire, etc.), nous avons implémenté plusieurs modèles pouvant potentiellement rendre compte de ces paramètres. Ces paramétrages différents de la Galaxie nous ont ainsi permis d'estimer leur impact sur la propagation et l'émission d'annihilation des positrons. Nous avons appliqué ce code à l'étude des positrons émis par la décroissance \betaplus\ de l'\alu, du \titan\ et du \nickel\ qui sont continûment produits dans la Galaxie au coeur des étoiles massives et des explosions de supernovae. Nous avons étudié ces sources car l'idée que celles-ci pourraient expliquer l'émission d'annihilation galactique est largement répandue depuis des décennies. Nous avons montré que, peu importe le paramétrage de la Galaxie, ces positrons permettent d'expliquer l'émission du disque mais pas la totalité de l'émission du bulbe. La raison est simple : ces positrons ne se propagent pas très loin de leur source avant de s'annihiler. Dans ce cadre, une source supplémentaire de positrons dans le bulbe est nécessaire pour rendre compte de la totalité de l'émission. Nous avons montré qu'une source transitoire de positrons (d'énergie \simMeV) située au centre de la Galaxie pourrait expliquer l'émission du bulbe.
Download or read book Observations of Galactic Positron Annihilation Radiation written by Michael S. Briggs and published by . This book was released on 1991 with total page 508 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Positrons from Supernova and the Origin of the Galactic center Positron annihilation Radiation written by and published by . This book was released on 1983 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The emission of positrons from supernova ejecta is dicussed in terms of the galactic-center annihilation radiation. The positrons from the radioactive sequences 56Ni.-->.56Co.-->.56Fe are the most numerous source from supernova. Only type I supernova will allow a significant fraction to escape the expanding ejecta. For a neutron star model of a type I SN a fraction 4 x 10−3 of the escaped positron is enough to create the observed several year fluctuation of the annihilation radiation. The likelihood of this model is discussed in terms of other astrophysical evidence as well as the type I SN light curve.
Download or read book Positron Annihilation in Chemistry written by Ole E. Mogensen and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 279 pages. Available in PDF, EPUB and Kindle. Book excerpt: The only critical discussion available on the chemistry of the two "strange" light particles, the positron and positronium, with much space devoted to the excess electron. Positron annihilation allows the investigation of many unusual phenomena in the reaction kinetics of the positron, positronium, and excess electron, and in radiation chemistry and physics, while also providing important information on defects in solids.
Download or read book Monoenergetic Positronium Beams and Laboratory Simulations of Galactic Positron Annihilation written by Benjamin Lathrop Brown and published by . This book was released on 1987 with total page 229 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Positron Annihilation written by A.T. Steward and published by Elsevier. This book was released on 2012-12-02 with total page 455 pages. Available in PDF, EPUB and Kindle. Book excerpt: Positron Annihilation focuses on the process of positron annihilation in different environments. Partitioned into two parts with 42 chapters, the book contains the contributions of authors who have done research on the annihilation of positrons, which brought about valuable information on the properties of matter. The first part of the book deals with lengthy review articles, including a survey of the physics of positron annihilation; positron annihilation in metals and the theory involved in the process; and positron annihilation in alkali halides and ionic crystals. Positronium formation and interaction in gases, molecular substances, and ionic crystals are also given attention. Gaseous positronics and positron annihilation in condensed gases and liquids are also discussed. The second part of the book focuses on developments on positron annihilation and the direction of research on this field. The studies concentrate on positron annihilation in various crystals, metals, mercury, liquefied gases, helium, and metal oxides. Numerical representations and analyses are presented to support the processes involved. The book can best serve the interest of those who want to explore further the annihilation of positrons.
Download or read book Positron Distribution in the Inner Galactic Ridge written by and published by . This book was released on 1997 with total page 21 pages. Available in PDF, EPUB and Kindle. Book excerpt: OSSE was used to measure the longitude and latitude distribution of positron annihilation in the inner galactic ridge using differential measurements relative to intensities at latitudes of plus or minus 10 degrees. An improved understanding of the 50 keV to 10 MeV continua has recently permitted accurate determinations of the positronium continuum flux intensities; this provides an - 5 times greater signal than was available in previous analyses which used only the 511 keV annihilation line. Within the statistical precision of the measurements, the line and positronium continuum components show indistinguishable distributions in both latitude and longitude. A differential measurement of their latitude distributions near the center indicates a narrow Gaussian ridge of -4.4 degrees FWHM. The longitude distribution of the ridge intensity differs from other known galactic ridge radiation distributions in that it shows a narrowly peaked maximum around the center. It can be characterized by an -12 degrees FWHM Gaussian central bulge plus a broad disk-like component. The only physical model based on plausibly associated galactic ridge distributions which can fit the observations well in longitude is the predicted galactic nova spheroid distribution, but scaled down by -2/3 from the latitude-integrated model bulge longitude width. The best fit has no additional discrete source or broad-disk components, although small contributions are permitted by the data. Novae are thought to be progenitors of Type I supernovae, probable sources of positrons in the galaxy, so this correlation suggests that Type I supernovae are the primary source of the positrons producing the observed galactic ridge annihilation radiation.
Download or read book Positron Annihilation in the Galactic Center written by and published by . This book was released on 1986 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Two separate observations of the .gamma.-ray spectrum originating from the Galactic Center were made by HEAO-3 in the fall of 1979 and in the spring of 1980. The 2.gamma. 511 KeV annihilation line flux decreased by a factor of three over the corresponding six month period, whereas the excess .gamma.-ray continuum below the 511 KeV line, often interpreted as 3.gamma. decay of orthopositronium, barely changed. This apparent discrepancy in the temporal behavior makes it difficult to associate the bulk of the excess continuum with the 3.gamma. decay of positronium. We will show that Compton scattering of the line and high energy radiation provides a natural explanation for the surprisingly small change seen in the excess continuum. 11 refs., 2 figs.
Download or read book OSSE Observations of Positron Annihilation in the Galactic Plane written by and published by . This book was released on 1996 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: OSSE has measured the galactic longitude distribution of both the 511 keV annihilation line radiation and the three-photon positronium continuum within ~40 degrees of the center. They have similar shapes, with a composite longitude distribution well represented by an ~11 degrees FWHM Gaussian central bulge together with a possible broad disk component comprising as much as 15% of the integral flux between 40 degrees and 320 degrees longitude. Contributions from one or more discrete source(s) at or near the center with total annihilation radiation (line plus continuum) intensities of up to ~4 x 10(exp -4) photons cu cm sec(exp -1) (2 sigma upper limit) are compatible with the observed distribution. The best estimate of the positronium fraction is 0.97 plus or minus 0.03. No evidence for variation in the positronium fraction is observed.
Download or read book The Origin of the Galactic 511 KeV Positron Annihilation Emission Morphology written by and published by . This book was released on 2009 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The origin of the Galactic 511 keV positron annihilation emission morphology.
Download or read book Diffuse Galactic Positron Annihilation Radiation and the Underlying Continuum written by Jeffrey George Skibo and published by . This book was released on 1993 with total page 274 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book POSITRON ANNIHILATION written by INTERNATIONAL CONFERENCE ON POSITRON ANNIHILATION,7,1985,NEW DELHI. and published by . This book was released on 1985 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Etude de l annihilation lectron positon dans la r gion du centre galactique avec le spectrom tre INTEGRAL SPI written by Patrick Sizun and published by . This book was released on 2007 with total page 222 pages. Available in PDF, EPUB and Kindle. Book excerpt: The imaging and spectral capabilities of the SPI spectrometer onboard the INTEGRAL satellite enabled us to further study the source of the 511 keV line from electron-positron annihilation detected in the Galactic centre region. A better determination of the spatial extent of the source, the intrinsic width of the line and the fraction of positrons annihilating in-flight, directly or via the formation of ortho-Positronium atoms would improve our knowledge of both the annihilation medium and the initial source of positrons. The first part of this thesis deals with a key ingredient of the extraction of the annihilation spectrum: the optimization of the instrumental background model. New data screening and tracer selection procedures are presented. Classical multi-linear models are compared to neural and bayesian networks. Finally, three years of observation are used to constrain the width of the source and derive its spectrum. The second part of the thesis focuses on one of the possible scenarios explaining the high positron injection rate deduced from the flux of the 511 keV line: the annihilation of light dark matter particles into electron-positron pairs. The various radiation mechanisms involved are modeled and confronted to observations in order to set an upper limit on the mass of the light dark matter particle.
Download or read book Positron Annihilation ICPA 10 written by Yuan Jin He and published by Trans Tech Publications Ltd. This book was released on 1994-11-24 with total page 1104 pages. Available in PDF, EPUB and Kindle. Book excerpt: Positron Annihilation - ICPA-10 presents new results and ideas of researchers who seek more profound understanding of the nature of positron annihilation. All these scientific and technological thoughts are included in these two-volume proceedings, which contain 7 review talks, 203 contributed papers (among them, 20 are invited), and 3 summary talks. The volume is complete with keyword and author indices.
