Cross Sections for Positrons Colliding with H, H 2, He Department of Physics National Tsing Hua University G.T. Chen 2007/6/12

Outline Introduction Introduction Cross sections Cross sections - H - H - H 2 - H 2 - He - He Discussion and Future Work Discussion and Future Work

Introduction Some problems for the 511 keV annihilation line had been reported. Some problems for the 511 keV annihilation line had been reported. In order to understand the evolution of positrons in our galactic center region, we first have to know cross sections of positrons colliding with H, H 2, He. In order to understand the evolution of positrons in our galactic center region, we first have to know cross sections of positrons colliding with H, H 2, He. N. Guessoum et al A&A “ The live and deaths of positrons in the interstellar medium ” “ The live and deaths of positrons in the interstellar medium ”

Introduction The varieties of cross sections: The varieties of cross sections:  Charge Exchange  Ionization  Excitation  Direct annihilation with free electrons  Radiative combination (with free electron)  Direct annihilation with bound electrons

Introduction Charge Exchange (CE): Charge Exchange (CE): P+P+ Positronium Atom or Molecule e+e+ Positron

Introduction Ionization (ION): Ionization (ION): Atom or Molecule e+e+ Positron e+e+ e-e- P+P+

Introduction Excitation (EX): Excitation (EX): Atom or Molecule e+e+ Positron e+e+ *

Introduction Threshold energy of each process Threshold energy of each process CE EX ION H H2H2 He

Cross Sections H H2H2 He

Cross sections (H-CE)

Cross sections (H-ION) 40eV

Cross sections (H-EX)

Cross sections (H)

Cross Sections H H2H2 He

Cross Sections (H 2 -CE)

Cross Sections (H 2 -ION)

Cross Sections (H 2 -EX)

Cross Sections (H 2 ) ？

Cross Sections HH2H2 He

Cross Sections (He-CE)

Cross Sections (He-ION) 150eV

Cross Sections (He-EX)

Cross Sections (He) ？

Discussion and Future Work Study other processes and energy loss processes Study other processes and energy loss processes Calculate the evolution of positrons in the galactic center region by diffusion equation Calculate the evolution of positrons in the galactic center region by diffusion equation Produce spectrum with 511 keV annihilation line Produce spectrum with 511 keV annihilation line

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Introduction Problems: The production of positrons The production of positrons The galactic map of the annihilation line The galactic map of the annihilation line The propagation/evolution of the positrons between their production sites and annihilation places The propagation/evolution of the positrons between their production sites and annihilation places

Introduction

Introduction After thermalization of positrons, some reactions occur: After thermalization of positrons, some reactions occur:  charge exchange with H, H 2, He  direct annihilation with free electrons  radiative combination (with free electron)  direct annihilation with bound electrons

Diffusion Equation