page_title: Mainz Microtron
section_title: Photoabsorption experiments
hierarchical_section_title: Mainz Microtron / Experimental facilities / Photoabsorption experiments
caption_attribution_description: English: MAMI crystal-ball
context_page_description: The Mainz Microtron, abbreviated MAMI,
  is a microtron which provides a continuous wave, high intensity, polarized electron beam with an energy up to 1.6 GeV. MAMI is the core of an experimental facility for particle, nuclear and X-ray radiation physics at the Johannes Gutenberg University in Mainz. It is one of the largest campus-based accelerator facilities for basic research in Europe. The experiments at MAMI are performed by about 200 physicists of many countries organized in international collaborations.
context_section_description: The A2 Collaboration studies reactions induced by high-energy photons incident
  on nucleons or nuclei. A beam of photons with known energy and flux 
  is produced via bremsstrahlung using a dedicated 
  tagging spectrometer provided by the University of Glasgow. 
  A polarized electron beam produces circularly polarized photons. Linearly 
  polarized photons can be obtained from coherent bremsstrahlung in an oriented crystal radiator.
  The central part of the detector system is a hermetic calorimeter consisting
  of the Crystal Ball detector (672 NaI crystals) in combination with the TAPS detector (384 BaF₂ crystals)
  in forward direction. For charged-particle tracking and identification 
  two layers of coaxial multi-wire proportional chambers and a barrel of 24 scintillation 
  counters surrounding the target are installed inside the cavity of the Crystal Ball sphere. 
  A frozen-spin target for polarized protons and deuterons is of particular importance for 
  studying spin degrees of freedom.
  The main physics goals are:
  Protons and neutrons are excited when they absorb a photon. If the photon energy is sufficiently high, mesons are emitted. The probabilities for such  meson production reactions as well as their angular and spin dependence incorporate indispensable information about nucleon excited states and meson-nucleon dynamics.
  Electric and magnetic polarizabilities are well known concepts in classical physics, which describe the influence of static electric and magnetic fields on composite systems. In case of protons and neutrons scalar and spin-dependent polarizabilities can be measured in low energy Compton scattering.
  At MAMI  and  mesons are photoproduced with high rate. With the Crystal Ball detector, decay modes of these mesons can be studied in an almost background-free environment.
  The charge distributions inside nuclei has been measured with high accuracy in electron scattering experiments. Information about the matter distribution can be obtained from the coherent photoinduced production of pions from nuclei, where photon and pion interact coherently with all protons and neutrons inside a nucleus.
  A publication database can be found here.
caption: The Crystal Ball/TAPS calorimeter
exif: {}