Nuclear magnetic resonance (NMR) measurement can be implemented by measuring response signals from atomic spins that constitute substances. The NMR measurement is an ultimate measuring technique that makes it possible to obtain microscopic information about substances. The basic principle behind the NMR measurement is as follows: a high-frequency magnetic field is applied to a sample placed in a uniform magnetic field. The response signals from spins thereby excited are received and analyzed. As a result, the properties of various substances can be measured. Conventionally, the NMR measurement has been used for studying solid state properties.
Recently, magnets that generate uniform high magnetic fields have been used for the enhancement of the resolution of NMR measurement. As a result, it has been made possible to clarify protein structures that are very difficult to measure with other measuring techniques. In general, a superconducting magnet is used to obtain a magnetic field of 10 tesla (T) or above. Presently, NMR apparatuses of 21.6 T (920 MHz) have been fabricated and in operation for the purpose of analyzing protein structures.
To accomplish high-resolution NMR measurement, the uniformity of magnetic field is one of the critical factors. In the above-mentioned protein structure analysis, in general, a uniformity of 10−9 or below is required in a space in which a sample as a measuring object exists.
In the protein structure analysis, response signals (free induction decay signals, FID signals) from a sample are extremely weak. To efficiently receive such signals, a high-sensitivity reception system is required. Especially, the enhancement of the sensitivity of probe coils (antenna elements) that receive signals is a technical challenge the solving of which is indispensable to NMR measuring apparatuses. Application of a probe coil using a superconducting material whose resistance to high-frequency currents is extremely low is effective in solving this challenge. Superconducting materials are lower in resistance to high-frequency currents by two orders of magnitude or more than ordinary normal-conducting materials (e.g. metallic materials such as copper and gold). Therefore, resistive loss in a reception coil can be remarkably reduced, and the enhancement of sensitivity is accomplished.
To accomplish the enhancement of sensitivity using superconducting material, a probe coil formed of superconductor must be placed in a low-temperature environment. To realize a low-temperature environment, the apparatuses must be provided with a cooling mechanism. If this cooling mechanism is used to cool a semiconductor amplifier connected to the stage subsequent to the probe coil, the sensitivity of the entire reception system can be further enhanced.
From the above-mentioned viewpoints, the following are important to realize an NMR measuring apparatus characterized in its high resolution and high sensitivity: ensuring a uniform magnetic field in a sample space, and adopting a probe coil formed of superconducting material. An NMR measuring apparatus provided with a cooling mechanism and a superconducting probe coil is described in Patent Document 1. Application of a superconducting coil is described in Patent Document 2 and Patent Document 3.
In the above-mentioned conventional technologies, superconducting solenoid coils are set (installed) to ensure the above-mentioned uniformity of high magnetic field. For this reason, bird cage or saddle is adopted for the shape of a probe coil. This is for the purpose of suppressing the uniformity of magnetic field from being disturbed by the perfect diamagnetism of superconductor. When a saddle or bird cage probe coil is formed of superconductor, especially, high-temperature superconductor to reduce the noise in the probe coil, the following problem arises: a superconducting film formed on a flat oxide single crystal substrate is utilized; therefore, the shape cannot be freely selected, and it is difficult to efficiently cover a sample. Though the noise in the coil caused by resistance can be reduced, as a result, the following problem arises: reduction in the filling factor related to shape lowers the efficiency of application of high-frequency signals.
Adoption of solenoid probe coil is a promising measure to further enhance the sensitivity of NMR measurement. Solenoid probe coils make it possible to further enhance the filling factor than conventional bird cage-shaped coils. As a result, the enhancement of sensitivity can be accomplished. A solenoid probe coil must be installed so that the direction of its central axis is orthogonal to a static magnetic field. Therefore, if a solenoid probe coil is inserted into a conventionally used integral-type superconducting solenoid coil that generates a uniform magnetic field, a space in which a measurement sample is to be placed cannot be ensured. Consequently, to use a solenoid probe coil, such a constitution that a uniform superconducting magnet that generates a magnetic field is split must be adopted.
[Patent Document 1] Specification of U.S. Pat. No. 5,247,256
[Patent Document 2] Specification of U.S. Pat. No. 5,585,723
[Patent Document 3] Japanese Patent Laid-Open No. H 11(1999)-133127