Mu Metal Shields
Earth's Magnetic Field
Effect of Earth's field on PMTs
Updated Study (July 24, 2006)
The images below indicated that there is a 25% reduction in the amplitude of a Burle 8575 PMT without mu-metal shield, in the horizontal plane (at 45 and 135 degrees with respect to North) due to Earth's magnetic field, as compared to the amplitude when the shield was present. There is no significant affect in the vertical direction, as expected. A thin mu-metal foil is therefore required, since Eric Scott has measured residual magnitization in the stell inserts surrounding the PMTs, as much as 3 Gauss, but more commonly around 1 Gauss. Regina will order foil in suitable rectangular sheets, which we can then roll and tape and insert around the PMT.
The following 6 images represent: 135 degrees with respect to North (horizontal plane) with and without shield, 45 degrees with and without, and vertical direction with and without.
Scope traces of a Burle 8575 PMT attached to a small Nai crystal with an embedded 241Am source, taken with and without its mu-metal shield. A small amplitude difference appears that will be investigated further.