DC Magnetic Shield for Thomas Jefferson National Accelerator Facility
VitaTech was contracted by the Department of Energy to remove an
existing DC shield that was improperly designed in an area where a
NASA cyclotron was once located. Over a 20 year period steel rebar
and I-beams in the building and adjacent concrete radiation blocks
were magnetized. VitaTech designed, installed, tested and certified
a 2,500 sq. ft. DC magnetic shield with a 0.5 G (500 mG) shielding
performance guarantee similar to the earth's geomagnetic field as required
by the researchers.
Inside the existing shield the mean (average) static DC magnetic fields
were 1.8 Gauss (G) on the floor with a peak of 4.7 G near the south
wall. Levels jumped to 3.1 G (mean) and 6.2 G (peak) when the existing
3/8-inch thick low-carbon steel plate shield was removed. The calculated
mean (average) shielding factor (SF) was about 5 dB for the existing
shield.
The final performance test after VitaTech's DC shield was installed
revealed a substantial attenuation of the DC magnetic fields with a
mean of 0.175 G achieving our 0.5 G guarantee, which is a 25 dB shielding
factor (increase of 20 dB). VitaTech also designed and installed an
automated degaussing system (see wires on the shielded walls) to restore
shielding performace every four to eight years as the DC shield naturally
magnetizes over time.
VitaTech left the front of the DC shield open providing researchers
easy access to the shielded test area. Although elevated levels up
to 1.6 G were recorded at the front entrance, emissions rapidly decayed
to 0.3 Gauss 1-m inside the shield. Several areas at 1-meter above
the floor approach 0 mG: remarkable shielding performance without
using highly permeable & expensive mumetal (nickel-iron alloy)
materials.
Field Data - Before and After DC Magnetic Shieding Installed
