MOMO is a scintillating fiber vertex detector incorporating 672 channels. It consists of six identical modules arranged in three layers of 2.5 mm thick circular fibers. The planess contain 224 parallel fibers each and are rotated by 60° to each other, with readouts through 16-channel Hamamatsu R4760 phototubes. The sensitive area is circular with a diameter of 44 cm. In addition, a 4.5 cm wide central hole allows the beam to pass through.

This detector was originally built for the MOMO experiment at COSY and was successfully used there for the detection of pion and kaon pairs in proton and deuteron induced two meson production reactions [1] [2].

A new support structure has been built, which is hanging on two rails fixed on top of the magnet yoke. This allows the detector to be moved back and forth with respect to the magnet.

Originally the PMTs were shielded against magnetic fields by 1 mm thick mu-metal cylinders wrapped around the 16-channel tubes. However, this was not sufficient for the rather strong magnetic field (>0.1 T) present at its position at BGO-OD about 1 m from the magnet. So additional shielding was added. 1 mm thick Permenorm cylinders were placed around the tubes mounts. Permenorm is a shielding material, that has about twice the magnetic saturation density than mu-metal. Tests showed, that this combination of mu-metal and Permenorm is sufficient for the operation of the detector al maximum magnetic field.

The signals from the phototubes are read out by 16-channel leading edge discriminators. The threshold for each channel can be programmed individually via a serial bus. The discriminator outputs then enter multihit TDC modules via a standard LVDS signal format. The spectra look like espected and do not change when the the magnet is turned on.

In summary, MOMO is fully operational by now.


[1] F. Bellemann et al., Phys. Rev. C 60 (1999) 061002
[2] F. Bellemann et al., Phys. Rev. C 75 (2007) 015204