User:BGOOD Collaboration/sandbox

BGOOD

Diagonal view of the experimental setup of the BGOOD experiment. The electron beam from ELSA enters from the right side.

The BGOOD experiment is a particle physics experiment located at the particle accelerator ELSA at the University of Bonn, Germany. It is a fixed-target meson photoproduction experiment designed for the investigation of hyperons at extreme forward angles, meaning at low-t values. This is achieved by combining a tagging system with a central detector with almost 4π acceptance and a large aperture forward magnetic spectrometer.

Etymology

‘BGOOD’ is an acronym consisting of the parts ‘BGO’ and ‘OD’, where ‘BGO’ refers to the scintillator material of the central calorimeter, Bismuth Germanate, and ‘OD’ refers to the 100 ton heavy Open Dipole magnet of the magnetic spectrometer.

ELSA

ELSA is a three-stage electron accelerator consisting of a linear accelerator, a booster synchrotron and a stretcher ring, delivering a maximum beam energy of 3.2GeV with a beam current of approximately 1nA. Both polarized or non-polarized electrons can be delivered. The beam can then be extracted towards BGOOD using betatron resonance.

BGOOD Hardware

BGOOD consist of three main parts: The tagging system, the central detector and the forward spectrometer.

Tagging System

The tagging system servers to purposes: On the one hand, it converts the electrons coming from ELSA to photons which can be used for the photoproduction experiment. On the other hand, it measures the energy of said photons without directly interfering with them, which is termed ‘tagging’. The electron beam stemming from ELSA impinges on a radiator. The specific radiator can be selected and oriented with extreme precision using a goniometer. For the creation of unpolarized bremsstrahlung photons, an amorphous radiator is selected, while for polarized bremsstrahlung photons, a diamond is selected. Once the photons are created, the post-bremsstrahlung electrons traverse a magnetic field of up to B=2T and are deflected by the Lorentz force. As they leave the magnetic field, their positions are measured in the tagging hodoscope. The positional information can be related to the radius of the trajectory within the magnetic field, therefore to the electron momentum, and therefore to the electron energy after the bremsstrahlung process. The energy before the bremsstrahlung process is known, as the electrons come from ELSA with a defined energy. The difference in the electron energy before and after the bremsstrahlung process is then assigned to the bremsstrahlung photon, ‘tagging’ it. An energy resolution of about 10% of the initial energy E₀ to 90 % of E₀ can be achieved. Using the scintillator fibre detector ARGUS in addition to the tagging system, the energy resolution can be improved to 0.08% E₀. The tagger offers a time resolution of about 180ps. The created bremsstrahlung photons are collimated and leave the tagging system in forward direction. Part of the tagging system is also the Flux Monitor (FluMo) and Gamma Intensity Monitor (GIM) located at the very end of the experiment.

Central detector

At the heart of the central detector sits the target. Target cells of 6cm and 11cm lengths can be installed and filled with either liquid hydrogen or liquid deuterium. The target cell is surrounded by a multi-wire proportional chamber (MWPC), which can be used for vertex reconstruction. The next layer is the scintillator barrel, which allows do differentiate between charged and uncharged particle tracks, followed by the outermost layer, which consists of 480 Bismuth Germanate (BGO) crystals as a calorimeter, covering almost 4π. In forward direction, an acceptance gap is closed by a scintillating ring (SciRi), which functions as a veto detector. The central detector allows for a time resolution of about 2ns and the typical decay π⁰ → γγ can be reconstructed with anergy resolution of about 10%.

Forward Spectrometer

Particles that leave the central detector in forward direction will enter the forward spectrometer. The positions of charged particles are measured in the scintillating fibre detectors SciFi and Momo. Their trajectories then curve in the magnetic field of the 100 ton heavy Open Dipole (OD) magnet with a field strength of B=0.5T. The field strength inside the magnet has been measured with fine granularity. Their positions are then measured in the driftchamber system located right behind the OD, which consists of 8 driftchambers which are rotated relative to one another for improved precision, filled with a mixture of argon and CO2. Knowing the particles' positions in front and behind the magnet together with the magnetic field strength allows to determine the momenta of the particles with a resolution of about 3%. Furthermore, the particles are then measured in time-of-flight walls (ToF), which are scintillating bars. Combining this time information with the time information from the tagging system allows to determine the particles' β with a resolution of about 2.4%. Knowing the particles' momenta and β allows to determine the particles' masses.

The yearly budget of the experiment amounts to approximately 200000€. Combining the hardware cost with the development costs amounts to a monetary value of approximately 20000000 € . A detailed description of the experimental setup can be found in the technical paper ‘The BGOOD experimental setup at ELSA‘, which has been published in EPJA.

External links

• Official website