BGO-Ball

The central detector of the experimental setup is the high resolution and large solid angle (0.9 · 4π) BGO10 electromagnetic calorimeter of the former GRAAL experiment [25, 27]. The calorimeter is combined with two multi-wire proportional chambers (MWPC) for inner tracking and a plastic scintillator barrel for particle identification through the measurement of dE/dx.

Ergebnisse_G_omega_4PhD.png
Figure 3.6: Experimental area: BGO detector in front of the forward spectrometer. The photon beam enters from right.Picture from December 2011.

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°), so-called crowns, and 32 azimuthal sectors (0° to 360°). The crystals are shaped like pyramidal sectors with trapezoidal basis and arranged in such a way that a constant thickness for all particles emitted from the target (central) point can be achieved. The support frame of the detector is separated into two halves which can be moved individually on a new, 4.5 m long rail system. Each subset of 32 modules (crystal plus standard photomultiplier) is connected to a so-called 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 new sampling ADCs AVM16 from Wiener11. The modules have a 160 MHz sampling frequency, 12 bit resolution and feature extraction. The time resolution is about 4-5 ns. The full setup of the ADC system was set into operation in March 2012. The final commissioning of the electronic chain will be finished in Autumn 2012.

The absolute energy calibration of the crystals, which is needed for the energy sum in the experimental trigger, is obtained using the 1.27 MeV photons from three 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%. Further details on calibration and detector monitoring can be found in [27, 28].

InvMass.png
Figure 3.7: Two photon invariant mass spectrum taken with the BGO calorimeter in March 2012.

The detector is ideally suited for the detection of photons with an excellent energy resolution of about 3% at Eγ = 1000 MeV [29]. 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, but protons with Tp > 450 MeV escape the detector [30]. Studies of the neutron detection efficiency were made with the GRAAL setup and show promising results (εn ≈ 40%) [31]. Figure 3.7 shows a first two γ invariant mass spectrum from the commissioning beam time in March 2012. After a rough energy calibration but an incomplete setup (no charge information, no tagged photon beam), peaks at the pion and eta mass with a reasonable width are clearly visible.

The scintillator barrel inside the BGO is located between the crystals and the inner tracking detector (MWPC). It consists of 32 scintillators made from bicron BC-448 (length ≈ 430 mm, thickness ≈ 5 mm) which are coupled to Hamamatsu H3164-10 sel PMTs. The standard readout is based on two 16 channel Multievent QDC (CAEN V792N) and a 128 channel Multihit TDC (CAEN V1190A). The detector is set into operation in June 2012 sucessfully.

The tracking resolution of the calorimeter will be improved by the use of two multi wire proportional chambers. New chambers were build by the collaborators of the I.N.F.N. Sez. Pavia and currently undergo a conditioning procedure. More details and perspectives about the MWPC are described on the MWPC page.

Footnotes:

10) Bi4 Ge3 O12
11) Wiener, Plein & Baus GmbH

References:

[25] O. Bartalini et al., Eur. Phys. J. A 26 (2005) 399
[27] F. Ghio et al., Nucl. Instr. Meth A 404 (1998) 71
[28] A. Zucchiatti et al., Nucl. Instr. Meth A 403 (1998) 22
[29] P Levi Sandri et al., Nucl. Instr. and Meth. A 370 (1996) 396
[30] A. Zucchiatti et al., Nucl. Instr. and Meth. A 321 (1992) 219
[31] O. Bartalini et al., Nucl. Instr. and Meth. A 562 (2006) 85