Magtapes and magnetic fields (HELP!)

[Michael Steckner]

I am working at a Hospital with a Magnetic Resonance Imager. This
device uses a very strong magnet (15000 gauss, earth's magnetic field
is 0.5 gauss, approximately) which can easily wipe out any magnetic 
media which comes with 15 feet of the magnet. Our backup tapes 
(9 track, 6250 bpi tapes) are stored in a separate room where the 
magnetic field strength has been measured to be 7 gauss. We seem to
be finding that many of our tapes cannot be read yet our service
people claim that the tape drive is working properly. Does anyone
know how strong a magnetic field a tape can withstand and not suffer
data degradation? Any help on this question would be appreciated.

I realize this is an esoteric question and I really don't know which
group to post this to, so I have posted it to the following:

sci.physics
sci.med.physics
comp.unix.admin
comp.unix.wizards
alt.cd-rom