Nuclear ordering; nuclear orientation

Japanese: 核整列 - かくせいれつ（英語表記）nuclear ordering; nuclear orientation

The state of the nuclear spins of a substance is in a certain order state. It is also called nuclear polarization. When a substance is cooled, the thermal disturbance becomes smaller and the substance's inherent ground state is realized, which is said to be in an order state. For example, ferromagnetism is a type of order state in the electron spin system. The magnetic moment of nuclear spins is about 1/2000 of that of electron spins, and they are localized, so the interaction between nuclear spins is very small. Therefore, the nuclear spin system is in a disordered state even at extremely low temperatures. To induce nuclear alignment, the interaction between the nuclear spins and an external magnetic field is used, and a magnetic field of about 10 T is applied at a low temperature of about 10 mK. Nuclear spins in ferromagnetic materials are also aligned by a strong internal magnetic field. The temperature at which nuclear alignment occurs due to the interaction between nuclear spins alone without an external magnetic field is very low, about 1 μK in the case of dipole interactions. However, in the case of solid helium-3, which has a large quantum effect, alignment occurs at about 1 mK due to exchange interactions. Dynamic polarization is a method in which electron spins are aligned under an external magnetic field, and then microwaves are applied to disturb the electron spins, so that when the electron spin system tries to return to an ordered state, the nuclear spin system is also aligned at the same time due to the magnetic interaction between the electron spins and nuclear spins. This method is used for polarized targets in nuclear physics experiments.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

物質の核スピンの状態が，ある種の秩序状態にあること。核偏極ともいう。物質を冷却してゆくと，熱的な攪乱が小さくなって物質固有の基底状態が実現し，これを秩序状態にあるという。たとえば，強磁性は電子スピン系の秩序状態の1種である。核スピンは，磁気モーメントが電子スピンの磁気モーメントと比べて約 2000分の1程度に小さく，また局在しているので，核スピン間の相互作用は非常に小さい。そのため核スピン系は極低温においても無秩序な状態にある。核整列を起させるには，核スピンと外部磁場の相互作用を用い，10mK程度の低温度で 10T程度の磁場を加える。強磁性体中の核スピンは，強い内部磁場により同様に核整列する。外部磁場なしに核スピン間の相互作用だけで核整列する温度は非常に低く，双極子相互作用の場合は 1μK 程度である。しかし，量子効果が大きい固体ヘリウム3の場合には交換相互作用により約 1mKで整列する。動的偏極法は，外部磁場のもとで電子スピンを整列させておき，マイクロ波を加えて電子スピンを攪乱させて電子スピン系が秩序状態に戻ろうとするときに電子スピンと核スピンの磁気的相互作用で核スピン系も同時に整列させる方法である。核物理実験の偏極ターゲットなどに利用される。

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