The aluminum shielded room has been an important part of ultra-low-field magnetic resonance imaging （ULF MRI） based on the superconducting quantum interference device （SQUID）. The shielded room is effective to att...The aluminum shielded room has been an important part of ultra-low-field magnetic resonance imaging （ULF MRI） based on the superconducting quantum interference device （SQUID）. The shielded room is effective to attenuate the external radio-frequency field and keep the extremely sensitive detector, SQUID, working properly. A high-performance shielded room can increase the signal-to-noise ratio （SNR） and improve image quality. In this study, a circular coil with a diameter of 50 cm and a square coil with a side length of 2.0 m was used to simulate the magnetic fields from the nearby electric apparatuses and the distant environmental noise sources. The shielding effectivenesses （SE） of the shielded room with different thicknesses of aluminum sheets were calculated and simulated. A room using 6-mm-thick aluminum plates with a dimension of 1.5 m x 1.5 m x 2.0 m was then constructed. The SE was experimentally measured by using three-axis SQUID magnetometers, with tranisent magnetic field induced in the aluminum plates by the strong pre-polarization pulses. The results of the measured SE agreed with that from the simulation. In addition, the introduction of a 0.5-mm gap caused the obvious reduction of SE indicating the importance of door design. The nuclear magnetic resonance （NMR） signals of water at 5.9 kHz were measured in free space and in a shielded room, and the SNR was improved from 3 to 15. The simulation and experimental results will help us design an aluminum shielded room which satisfies the requirements for future ULF human brain imaging. Finally, the cancellation technique of the transient eddy current was tried, the simulation of the cancellation technique will lead us to finding an appropriate way to suppress the eddy current fields.展开更多
This paper mainly reports an improvement of frozen-image model which can qualitatively describe the influence of lateral moving speed on vertical force in a HTS levitation system under lateral movement with field-cool...This paper mainly reports an improvement of frozen-image model which can qualitatively describe the influence of lateral moving speed on vertical force in a HTS levitation system under lateral movement with field-cooling condition.The model is improved by introducing a dipole which represents the influence of lateral moving speed and modifying the rule of diamagnetic dipole based on frozen-image model.The vertical and lateral forces that are obtained by this improved model agree with the previous measurements qualitatively.This model can also describe the effect of finite scale of superconductor sample in a levitation system.展开更多
From a quantum mechanical analysis of the melting points of the transition elements the authors put forward a hypothesis that the valence electrons are moving between the occupied orbitals and the empty orbitals as we...From a quantum mechanical analysis of the melting points of the transition elements the authors put forward a hypothesis that the valence electrons are moving between the occupied orbitals and the empty orbitals as well as they are playing a role as "free electrons" moving in the crystal space. Both of these processes constitute a mixed type of metallic bonding. By means of this hypothesis a relation (2/1)m_p/(n_en_o+n_er_a) has been recognised and these values must be crowded in a narrow numerical field. By calculation they are found to be 1.006士0.103 in average.Calculations have also been made on m_p/(n_en_o+n_er_e) and it is found that the ratios of the average values of the three transition periods are 1:1.155:1.322.These are in lmost exact accord with the ratios of the momentum of these periods,i.e.(n+(2/1l(l+1))).By assuming that "free electrons" are responsible essentially for the electro-and thermo-conductions, the average values of (2/1)m_pQ’(or Q") re found to be 1.66士0.26 and 1.65士0.29 for these conductions respectively. Howev-er, there exist regularities in their deviations,implying that the two metallic constitutions must play a coop-erative role in these transfers, and under the ordinary temperatures (between ambient and m. p.)"free elec-trons" play an essential role. At last, the authors have inferred the probable mechanism of superconduc-tivity.展开更多
基金Project supported in part by the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(Grant No.XDB04020200)in part by the National Natural Science Foundation of China(Grant No.11204339)
文摘The aluminum shielded room has been an important part of ultra-low-field magnetic resonance imaging （ULF MRI） based on the superconducting quantum interference device （SQUID）. The shielded room is effective to attenuate the external radio-frequency field and keep the extremely sensitive detector, SQUID, working properly. A high-performance shielded room can increase the signal-to-noise ratio （SNR） and improve image quality. In this study, a circular coil with a diameter of 50 cm and a square coil with a side length of 2.0 m was used to simulate the magnetic fields from the nearby electric apparatuses and the distant environmental noise sources. The shielding effectivenesses （SE） of the shielded room with different thicknesses of aluminum sheets were calculated and simulated. A room using 6-mm-thick aluminum plates with a dimension of 1.5 m x 1.5 m x 2.0 m was then constructed. The SE was experimentally measured by using three-axis SQUID magnetometers, with tranisent magnetic field induced in the aluminum plates by the strong pre-polarization pulses. The results of the measured SE agreed with that from the simulation. In addition, the introduction of a 0.5-mm gap caused the obvious reduction of SE indicating the importance of door design. The nuclear magnetic resonance （NMR） signals of water at 5.9 kHz were measured in free space and in a shielded room, and the SNR was improved from 3 to 15. The simulation and experimental results will help us design an aluminum shielded room which satisfies the requirements for future ULF human brain imaging. Finally, the cancellation technique of the transient eddy current was tried, the simulation of the cancellation technique will lead us to finding an appropriate way to suppress the eddy current fields.
基金supported by the Key Fund of National Natural Science Foundation of China(11032006)the Natural Science Foundation of Gansu Province(096RJZA050)Fundamental Research Funds for the Central Universities and Specialized Research Fund for the Doctoral Program of Higher Education(20090211120010)
文摘This paper mainly reports an improvement of frozen-image model which can qualitatively describe the influence of lateral moving speed on vertical force in a HTS levitation system under lateral movement with field-cooling condition.The model is improved by introducing a dipole which represents the influence of lateral moving speed and modifying the rule of diamagnetic dipole based on frozen-image model.The vertical and lateral forces that are obtained by this improved model agree with the previous measurements qualitatively.This model can also describe the effect of finite scale of superconductor sample in a levitation system.
文摘From a quantum mechanical analysis of the melting points of the transition elements the authors put forward a hypothesis that the valence electrons are moving between the occupied orbitals and the empty orbitals as well as they are playing a role as "free electrons" moving in the crystal space. Both of these processes constitute a mixed type of metallic bonding. By means of this hypothesis a relation (2/1)m_p/(n_en_o+n_er_a) has been recognised and these values must be crowded in a narrow numerical field. By calculation they are found to be 1.006士0.103 in average.Calculations have also been made on m_p/(n_en_o+n_er_e) and it is found that the ratios of the average values of the three transition periods are 1:1.155:1.322.These are in lmost exact accord with the ratios of the momentum of these periods,i.e.(n+(2/1l(l+1))).By assuming that "free electrons" are responsible essentially for the electro-and thermo-conductions, the average values of (2/1)m_pQ’(or Q") re found to be 1.66士0.26 and 1.65士0.29 for these conductions respectively. Howev-er, there exist regularities in their deviations,implying that the two metallic constitutions must play a coop-erative role in these transfers, and under the ordinary temperatures (between ambient and m. p.)"free elec-trons" play an essential role. At last, the authors have inferred the probable mechanism of superconduc-tivity.
