In summary, it sounds like the original information is correct: it is
prudent to locate electron microscopes outside the 1-2 Gauss stray field
line of the magnet. While some of the responses suggested greater caution,
nobody who responded by email or phone (including microscope field
engineers) was able to cite a single "real world" example of unworkable
interference from an NMR magnet. If any further information is received, I
will pass it on.
Original Post:
>>>
Hello all:
We are in the process of moving our NMR lab to a new lab that is close to
our electron microscope. We are trying to establish the maximum stray
field strength that the SEM can handle, so that we can decide the exact
location in the rooms for our magnets.
The electron microscope vendor (JEOL) says that they can not tolerate
fields > 3 mG. But after talking to them, it seems like they mean AC
fields, not the (mostly) static fields that our magnets throw off. After
all, they seem to be able to work with the 500 mG stray field of the earth!
;->
The Varian Site Planning Guide suggests that the 1-2 Gauss line is an
appropriate boundary for electron microscopes. Do any of you have magnets
close to electron microscopes? If so, at what distance and/or stray field
strength? Has anybody encountered degradation in electron microscopy
performance due to NMR magnets?
Thanks in advance. I will post anonymized summary to group.
David Kinney
Lyondell Chemical Company
david_kinney@arcochem.com
New email (effective 4/5/99) david_kinney@lyondell.com
>>>
Responses:
>>>
Maybe I can clarify the issue, since JEOL
makes both electron microscopes and NMR spectrometers.
For static magnetic fields, JEOL SEM's can tolerate
at least 2G. The magnetic field will result in a beam shift
in the microscope which can be adjusted with the
normal SEM beam focus controls. During the magnet
energization (or a quench) the beam will shift, and the
SEM operator will need to adjust it accordingly.
At our USA headquarters, we keep the electron microscopes
at least 20-30 ft from the NMR magnets without incident.
Of course the Electron Optics Division is much more
concerned about time-varying magnetic fields from
motors and such. For fluctuating fields the site specification
is <3mG. The JEOL service department has the appropriate
equipment to survey the area for fluctuating fields.
If you have additional questions, please feel free
to contact me and I will be happy to put you in contact
with the correct SEM personnel.
J. Douglas Meinhart, Ph.D.
NMR Product Manager
JEOL USA, Inc.
11 Dearborn Road
Peabody, MA 01960
tel 978-536-2303 fax 978-536-2288
meinhart@jeol.com
>>>
There is an electron microscope across the hall from the NMR lab.
The 5G line is inside the wall of the hallway. The magnet closest
to the wall is a 250, so the field in the hall is small. There has
been no effect on the EM that we can tell.
>>>
We have a Jeol SEM in the next room to our unshielded 500 MHZ magnet.
The problem was easily solved as we bought our NMR from Jeol and
required in the contract that they had to make the two work together.
(Aside... Have you looked at the new Jeol NMR equipment? IMHO the
hardware at routine fields (<600 MHZ) rivals the other manufactures and
the software is superior to the Bruker and especially the Varian offering.
This was hard to swallow for this former Bruker user...)
The distance between the magnet and the SEM is approximately 25 feet
which makes the field (ignoring the attenuation of the wall) well outside
the 1 gauss line. Even wiith this separation, our SEM operator was
pretty uncomfortable about the placement of the magnet. However, we
have not heard any whining since the magnet was energized a few
months ago, so it appears we will have no problem. The approach we
used was to have Joel tune up the SEM before the magnet is energized
then after to check for any negative observed effects. In case we had
interaction problems with our SEM, we were prepared to provide
additional shielding for the SEM (which is commercially available) or build
a Faraday cage into the wall separating the instruments.
>>>
1-2 Gauss line is quite risky to the electron microscopic images. I guess
as long as the electron microscope is outside of the 0.5 gauss line, it
should be fine since the earth magnet is about 0.5 Gauss . Nevertheless,
we placed the SEM far outside of the 0.5 gauss line (~25 ft away from the
400MHz magnet) to make sure that we do not get any distortion on the images
due to the magnetic field.
>>>
We have just such a setup and all works fine. Our magnet is less than
10m from a 1980's vintage EM. When we took a magnet down, then brought
up a replacement in 1993 (both magnets were Oxford 500/51) there was no
change required in the EM focus. We do, however, have a magnetic "hot
spot" in an adjacent prep lab (about 5m from the magnet) which can be
detected by small changes in coloration on a badly shielded computer
monitor. Even this field strength shouldn't be a problem to the EM, as
the beam is enclose in a thick conductive shell.
>>>
Our NMR facility magnet (an 8.45 T Oxford) is located across the hall
from 2 electron microscopes. One is a JEOL TEM (I believe in the 150kv
range) located about 20 meters away and the other an older one (I
believe a Zeiss running at lower voltage) about 10 meters away.
When the JEOL was first installed, there were complaints about
de-focussing but this was eventually identified to be the elevator shaft
located directly behind the scope with its motor 2 floors up. We have
been living with the EM lab in peace and hormony for over 10 years. One
consideration: Even though the field from the NMR is DC, when a steel
cart, gas cylinder hand truck, or the like is moved between the magnet
and the scope, this field will be affected.