Measurements of the magnetic field around the p-side enclosure

During the week of May 13, the TOF group measured the magnetic field around the enclosure on the P side of PHOBOS. The results of these measurements are summarized in the following figure (note: only the magnitudes of the components of the field are shown):

The magnetic field on the left-hand side of the enclosure (the side at the exit of the magnet) is around 1000 - 1250 Gauss. The magnetic field on the righ-hand side of the enclosure (the side closest to the ring 2 and the beam-pipe support) is around 10 Gauss. The magnetic field between these two extremes is between 50 - 100 Gauss. The PMTs used for the T0 counters have a "build-in" shield that saturates at 10 Gauss.

In order to study the possible locations of the T0 counters, we have explored shielding possibilities with magnetic shield corp. This company also produced the magnetic shields for the TOF array. This company proposed the following shield, which will procide a shielding factor of 1000 (the reduction in the magnetic field):

A dual layer shield, 30cm long, 2.50" I.D., with .25" stainless steel spacers between the layers. The exterior NETIC cylinder (.050") is painted black, the inner cylinder is .040" CO-NETIC AA.

The following issues should be noted:

• The stainless steel insert plays a critical role in the shield. Without the insert, the shielding efficiency would be reduced from 1000 to 30.
• The magnetic shield has a mass of about 4 kg. This results in a significant increase in the total mass of the array.
• The magnetic shield extends 10 cm in front of the face of the photomultiplier tube. This is required in order to deal with the significant z component of the magnetic field.