Development of Precise Shimming Technique With Materials Having Low Saturation Magnetization

Magnetic field shimming using low saturation magnetization material was tested to realize spatial magnetic field homogeneity of less than 1 ppm. The magnetic moment per unit mass were measured for three candidates, that is, Nickel, magnetic fluid and magnetic putty, and it was verified that those have low magnetic moments suitable for precise shimming below 1 ppm. The magnetic shimming test was performed using MRI magnet for MUSEUM experiment at 1.2 T with iron plates, Ni thin plates and magnetic putty. The magnetic shimming with iron plates was conducted employing the two step shimming scheme,and the homogeneity could be reached to 0.29 ppm peak-to-peak (ppm(pp)) after six shimming trials. Then, the precise shimming was carried out with Ni thin plates and magnetic putty, and the homogeneity could be reached to 0.16 ppm peak-to-peak with Nickel and 0.17 ppm(pp) with magnetic putty. No deformation of putty in the magnetic field was observed even after two months. The magnetic field monitoring for two months showed that magnetic field strength and homogeneity largely depended on the temperature, and the temperature of the magnet room has to be controlled within +/- 0.35 degC under the circumference condition of the shimming test in order to keep the magnetic field change and the homogeneity within +/- 0.1 ppm. In addition, it is showed that the refilling of LHe in the MuSEUM magnet system should he performed once per 29 days The practical implementation method for magnetic putty was suggested for the existing magnet system by using plastic spacers with holes.