BGO ball

The central detector of the experimental setup is the high resolution and large solid angle (0.9 · 4π) BGO (Bi4 Ge3 O12) electromagnetic calorimeter of the former GRAAL experiment. The detector is ideally suited for the detection of photons with an excellent energy resolution of about 3% at Eγ = 1000 MeV. In addition, protons can be detected in a limited range. Up to a kinetic energy of Tp = 100 MeV their energy can be measured reliably, protons with Tp > 450 MeV escape the detector.


The calorimeter consists of 480 BGO crystals with a length of 24cm (> 21 radiation lengths). The carbon fibre support structure is segmented into 15 polar sectors (25° to 155°), called crowns, and 32 azimuthal sectors (0° to 360°). The support frame of the detector is separated into two halves which can be moved individually on a 4.5 m long rail system to allow access to the target and the detectors inside. The crystals are shaped like pyramidal sectors with trapezoidal basis and are arranged in such a way that a constant thickness for all particles emitted from the target (central) point can be achieved. Each crystal is coupled to a photomultiplier tube, which is shielded with μ-metal, which is sufficient for low magnetic fields. An additional shielding is necessary due to the significant fringe field of the Open Dipole magnet, which is typically 3 mT at the most downstream end of the BGO Ball. Therefore an iron cover is mounted on the downstream end of the support structure.

Each subset of 32 modules (crystal plus standard photomultiplier) is connected to a mixer, which is equipped with a programmable attenuator. In addition the sum of all signals, which is proportional to the total energy deposited in the calorimeter, enters after discrimination into the general trigger logic. The energy and time information of the BGO is measured for each channel with sampling ADCs AVM16 from Wiener11. The modules have a 160 MHz sampling frequency, 12 bit resolution and feature extraction. The time resolution is about 2 ns, while the energy resolution for 1 GeV photons, is about 3%.

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22Na energy spectrum in one BGO crystal. The lower energy peak at approximately 140 channels corresponds to a 511 keV photon from the annihilation of an e⁺ of the β⁺ decay of 22Na to 22Ne*. The higher energy peak at approximately 400 channels is the 1.27 MeV photon from the deexciation  of 22Ne* to 22Ne.


Three steps are made for an accurate energy calibration. The initial energy calibration of the crystals, which is needed for the energy sum in the experimental trigger, is obtained using the 511keV and 1.27 MeV photons from several 22Na sources, which are evenly installed around the target area inside the detector. The response of the 480 BGO modules is equalised by adjusting the high voltage of the photo tubes to set their gain. The precision of the equalisation (maximum deviation) is better than 1.5%.

The final calibration is performed by fitting to the π⁰ mass from the π⁰ → 2γ decay.  A run-by-run correction is applied to compensate for gain fluctuations mainly caused by temperature changes. Finally, a correction is applied for each crystal to ensure the measured π⁰ mass is in the right position. As each correction affects the spectra for the other crystals, this procedure is done iterativly.

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