magnet shielding summary
T. Pratum (pratum@u.washington.edu)
Tue, 25 Feb 1997 10:53:19 -0800 (PST)
I would like to thank all those who replied to my request for information
regarding magnet shielding. I found most responses very useful, although
there was some humor involved as well (humor is also useful). As I told
many of the respondents, I am hoping that we can convince the "powers that
be" to move the proposed study center elsewhere. Most respondents
provided some discussion of the elementary physics involved with trying to
shield a magnet. I can summarize that as follows: The magnetic shielding
is just trying to reduce the field lines in some regions by increasing
their density in the higher susceptibility material (the shield). The
higher the susceptibility of the shield material, the better (thus, mu
metal is recommended if possible- $$). Field lines cannot be broken, and
can only be routed away from unwanted areas. Such a shield is best done by
enclosing the whole magnet (this is done with many imaging magnets,
including some in our Radiology Department). If you are not going to
enclose the whole magnet, then you can expect at least 2 things to happen
at the edges of the shield:
(1) the field will increase sharply, and (2) large inhomogeneities will be
induced in the shielded magnet. Because of (2), the shield needs to be as
symmetric as possible about the magnet center. A couple of people did say
that they had success with using steel plates above the magnets (lower
field <10T) to shield the room above, but most were skeptical that such a
thing would work. Nonetheless, at least one person has implemented an
"above the magnet" shield on higher field magnets (Rolf Tschudin at NIH).
He provided the following description:
" We used M-45 magnet steel, 24 ga ( .63 mm) thick. We have 6 layers over
the 500's, 8 layers over the 600 and 16 layers over the 750. It was layed
in sheets of 3' by 8' alternating direction with each layer. The objective
was to keep the stray fields < 2Gs at desktop monitor level. To be fully
effective one would have to go about 7 m in all directions from the center
line of the magnet (750). We could only go 3 to 4 m. The result is about
4 Gs stray fields above the 750, < 2 Gs elsewhere. Monitors on desktops
at < 2Gs could be demagnetized. Monitors over the 750 were placed into
shielding boxes made from the same steel and demagnetized. "
As to the vibration question, there is at least one instance, at Bruker,
where steel above the magnet had its vibrations coupled into the magnet
itself. This apparently didn't happen at NIH, and I would guess it is a
function of the building itself. To be absolutely certain such a thing
wouldn't happen, one would have to vibrationally isolate the shield from
the rest of the building- which I would guess would mean suspending it
from the ceiling (in our case).
Hopefully this shared information will be of value in deciding what to do.
Tom Pratum
Dept of Chemistry
Box 351700
Univ of Washington
pratum@u.washington.edu
http://weber.u.washington.edu/~pratum