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NMR, magnets

Magnetic shielding to reduce stray fields of NMR magnets.

Magnetic shielding against NMR stray fields is required to reduce the strong magnetic fields of NMR’s, mostly to keep the area around the magnet below 5 Gauss (500 µT). However, the application of the magnetic shield is not limited to influence only the 5 gauss line. With aid of the shielding system Systron PowerShield® STF-I, other goals of limiting the stray fields can also be targeted.

Description

NMR stray field shielding system, Systron PowerShield® STF-I

Simulation of the stray field of a magnet  
With the Systron PowerShield® STF-I stray field shielding system, stray fields of NMRs are being attenuated. The shielding system can be used for both, shielded and unshielded NMR’s.
 
The appropriate dimensions and thickness of the shield is calculated with a computer simulation. Only materials approved by the NMR-manufacturer are used for the shielding.
 
The Systron PowerShield® stray field shielding STF-I is applied as floor, wall or ceiling shielding. In this combination, entire rooms can be shielded.
Simulation of the shielding effect on the computer    

Example, protection of a lecture hall

Fields in a lecture hall above an NMR had to be limited to below 5 Gauss.

Lecture hall above NMR
 
Initial situation
The lecture hall of the university is directly above the NMR-room. The computer simulation showed that the 5 gauss line penetrates into the auditorium. Thus the stray fields into the auditorium had to be limited to below 5 Gauss.
Auditorium above NMR-room  
 
Software simulation of magnetic stray field of NMRStray field plot with shielding
 
Software simulation
The area and thickness of the shielding were determined with the FEM (Finite Element Modeling) software. The calculation showed that an area of 64 m2 had to be shielded with a shield thickness of 4 mm. For practical reasons it was decided to attach the shield to the ceiling of the NMR-room.
Installation of shielding plates on ceiling below lecture hall
Mounting of the shielding panels
 
Mounting the shield
The shield panels consisting of several layers of silicon iron were mounted on the 5 m high ceiling with aid of a lift. The panels were mounted in several layers, so that the joints of the lower layers are covered by the following layer.
 
The multi-layered elements were placed accurately with hydraulic lifters installed placed on the scissors lift and fixed tightly with heavy duty anchors.
NMR room with shielded ceiling
 
NMR Room
The fully furnished NMR-room below the lecture hall. The ceiling shield is installed and painted in white. Installations like air ducts, gas pipes and power lines could be directly fixed on the shield.
This was possible due to a construction plan for all ceiling mountings which had been developed in close collaboration with the construction engineers. Thus the shielding had not to be pierced for further installations.
Fully set up NMR room
 
 
Measurement of remaining magnetic fields in lecture hall
 
Control measurements
After the commissioning of the NMR, the attenuation of the shielding was measured in a 50 x 50 cm mesh grid on the floor of the auditorium. Field above  5 gauss were only measured on one side, however, this slightly elevated field is inside a built-in cabinet.
 
Conclusion: The measurements confirmed the FEM based simulation. The lecture hall could be released for public access.
Measured values on the floor of the auditorium
 
 
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