研究計画

I am going to take part in Nuclear Physics for investigate a method of proton spin polarization under high temperature and low magnetic field strength and others related research, such as the scattering of heavy isotopes on polarized proton target. The project is branch of Dynamic nuclear polarization, which is transferring spin polarization from electrons to nuclei (protons) by cross polarization.

The polarization of proton can be used in width range of applications. It is called Hyper-polarization, because the polarization beyond thermal equilibrium or the Boltzmann Distribution.  This can be applied to enhance the efficiency of Nuclear Magnetic Resonance (NMR) and improve the resolution and application range of Magnetic Resonance Imagining (MRI), which has greatly useful from medical, chemistry to quantum computing.

One important application of the polarized proton is investigate the nuclear structure. For example, polarization study of unstable isotopes by scattering it with highly polarized proton target.  Since the current technique is difficult and expensive to achieve low temperature (sub-Kevin degrees) and high magnetic field (several Teslas). The thermal movement of atoms will destroy the polarized electron spin by collision with other atoms. Therefore, lower temperature can get better polarization efficiency. According to the Curie's law, the magnetization, or the electron spin polarization, is proportional to the external magnetic field and inversely proportional to temperature. Therefore, low temperature and high magnetic field is essential for spin polarization. The study is hard to be deployed. 

Currently, Professor Tomohiro Uesaka's group had achieved the polarization at temperature 100 K and magnetic field strength 0.1 T. The resultant proton polarization rate is 20%. The group use doped crystal of naphthalene (C₁₀H₈) as the polarization target, because of the π-electrons play an important role in the “Inter-system crossing” among different energy levels. Therefore, the group use Laser as a mean for exciting the electrons and undergo energy level transition. The electrons will be distributed on a triplet state m = +1, 0, -1 with different population and then polarized. By cross polarization, the group is able to polarize the proton spin.

The main objectives for my doctoral research is to investigate and improve the condition for maximize the polarization rate. The spin of proton also contributes magnetic field. The polarized proton spin gives a much stronger magnetic field. Professor T. Uesaka's group uses total magnetization (Polarization ratio times Total volume of the crystal) per power of laser as a measurement of the polarization efficiency. The current efficiency is around 7 [% mm³ / mW]. At this point, if the efficiency keeps unchanged, higher laser power will give out higher polarization ratio. The bigger challenge will be a higher ration, bigger crystal volume and lower laser power. Another improvement is increasing the working temperature or lowering the magnetic field while keeping the polarization ratio the same. This is possible in theory, because the population of the triplet state is independent of temperature and magnetic field.

By studying the spin polarization, several possibilities will be opened, such as applying the spin dynamic into quantum computing or study unstable heavy nuclei, like O-22, (which has 16 neutrons), C-22; or He-8. The unstable nuclei has un-ordinary spin-orbit coupling that changed the fine structure in nuclear system. This is an important discovery in modern physic that explained why some isotopes are more stable than others. The fine structure in nuclear system is similar to atomic system. The strong nuclear force in the nuclear shell model plays a similar role as the electromagnetic force in the electron shell model. The shell model gives protons and neutrons orbiting in shells. Thus, spin-orbit coupling will change the energy level. In 1963, Wigner, Mayer and Jensen shared a Nobel Prize for it. The change of fine structure of an unstable nuclei can be largely different form a stable one. With the help of a strong magnetic field from a strongly polarized crystal, we can study of the spin-orbit coupling. Therefore, proton spin polarization is a foundation for future research and study.

I have a vision after I finished the course. I become both skillful in experimental technique and mastering theories. I believe this is basic requirement for research career. Besides knowledge, I wish through out studying oversea, I could able to learn how to work with others, who are from different countries and cultures. Also, I can live by myself and look after others. These are very crucial for doing research and personal enhancement as well. At last, I want to become a mature and professional scientist. That is my dream and also my parent's.