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 75% reduction in the gain of a Burle 8575 PMT without mu-metal shield, in the horizontal plane (at 45 and 135 degrees with respect to North) from Earth's magnetic field. 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 gain difference appears that will be investigated further.
