Low Field Nuclear Magnetic Resonance Electromagnetic Field Shielding Problem

Because of the very weak echo signal generated by NMR, the manufacture anddesign of NMR spectrometer are very demanding. A large proportion of the totalNMR spectrometer improvement effort in recent years has been spent in improvingthe probe part of the NMR spectrometer instrument to reduce the noise in the signal.One practical technique is to place conductive shields around the solenoidal coil of theprobe.Firstly, the probe part of the NMR spectrometer is analyzed, and a cylindricalshield with an opening is determined to be the basic form. The shield is used to shieldthe instrument from outer space noise, and minimize the interference and influence ofthe alternating magnetic fields generated by the solenoidal coil upon the homogeneityof the static magnetic fields.Secondly, while the dimensions of the shield varying, the shielding effectivenessof the shield and some parameters of the coil are computed, and some related curvesare gained. These dimensions include the radius, the depth, the height, the openingradius, as well as the thickness in the three dimensions. The shield does harm to thesolenoidal coil as it practices its shielding function. The coil parameters include theinductance, quality factor, as well as the magnetic flux density.Finally, the shielding effectiveness of a spherical shell is calculated with Matlabaccording to the analytic formulae. The results of two types of shields are in goodagreement. Some instructive suggestion of designing the shield is given.