The NOP Project

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Introduction


Neutron is a neutral baryon with the spin of 1/2 and the magnetic dipole moment antiparallel to its spin. A free neutron decays into a proton, an electron and a neutrino with the mean lifetime of about 15 minutes. The 15 minute lifetime is extraordinarily longer than other unstable elementary particles. Thus neutrons can fly a macroscopic distance even if they are decelerated down to low energies comparable with the thermal motion below the room temperature. They have a strong penetrability through matters and are not very much deflected flying through the air.

Neutron mass is almost the same as proton mass. The conversion among the kinetic energy (E), the corresponding temperature (T) and the wavelength (l) is given below. Neutrons in the energy regions of thermal neutrons, cold neutrons and very cold neutrons are mainly used in material researches.

X-rays are mainly used in material researches. However, the importance of the use of neutrons has become more recognized because of its high sensitivity to light elements. X-rays interact with electrons in matters through the electromagnetic interaction and the interaction strength has a systematic dependence on atomic number (Figure). On the other hand, neutrons interact with nuclei in matters through the nuclear interaction. The strength of the nuclear interaction depends on nuclear structures and does not have a systematic dependence on the atomic number (Figure). Consequently, neutrons have relatively high sensitivity to light elements.

The figure below is an example to visualize the neutron sensitivity to light elements. In the figure, the geometrical cross section of the atom is proportional to the scattering cross section. In the X-ray image, the contribution of barium atoms is dominant and the contributions of hydrogen and oxygen are quite small. Hydrogen and oxygen, on the contrary, contribute dominantly in the neutron image. The hydrogen contribution can be further enhanced by deuterizing the sample. The method can be applied in other elements and is referred to as the isotope contrast. (In the case of hydrogen, a further contrast is possible by comparing the images with neutron spin is parallel or antiparallel to hydrogen nuclear spin.) The neutron scattering technique is expected to provide a unique and high sensitivity to hydrogen atoms or hydration water molecules in biomolecules or hydrogen and light element atoms in metals.

However, the neutron scattering is not always the leading experimental technique. Large facilities of nuclear reactors or high intensity accelerators to provide neutron beams since neutrons are produced in nuclear reactions, which implies that the neutron beam is expensive. We can roughly say that the inconvenience limits the neutron application. Additionally, the neutron beam intensity is quite low compared with X-rays and the measurement precision is limited by the low intensity. But a large amount of budget and development is necessary to constract a stronger neutron source. Therefore, efficient techniques to transport neutrons to samples and extract physical quantities should be developed. Neutron optics is one of the most important techniques to control neutron beam quality and to extract physical quantities. In this project, we aim to systematically develop neutron optical devices and their applications.

The following is the list of the interactions to control the neutron motion.

Neutron optic devices are designed on the basis of these interactions. We do not apply the gravity at this moment, since it is practically impossible to change the strength and the direction of the gravitational force.
  • Effective Potential
    Neutrons feel the effective potential on the boundary of matters. If the neutron is approching to the boundary sufficiently slowly, the neutron is totally reflected. The device to use the total reflection is referred to as the "neutron mirror".
    If the neutron is not totally reflected, the neutron enters into the matter. The neutron velocity is changed and refracted on passing through the boundary according to the effective potential. The refraction can be used analogous to ordinary optics. The optic devices with the neutron refraction on the boundary is referred to as the "compound refractive optic" device.
  • Magnetic Interaction
    The neutron potential energy changes if the magnetic field is changed. If the magnetic field is not uniformly distributed, the neutron is accelerated along the field gradient. A neutron optics can be designed using the acceleration along the magnetic field gradient. We refer to this type of optic devices as the "magnetic optic" device. We also use the terms such as "magnetic lens" and "magnetic prism". The magnetic optics has a neutron spin-selectivity.
  • Magnetic Mirror
    If a matter contains magnetic field and the field is sufficiently strong, neutrons are selectivly reflected on the boundary. This type of device is referred to as the "magnetic mirror". The magnetic mirrors have been developed and applied as neutron spin polarizers and analyzers. The principle of the magnetic mirror can be considered as a combination of the compound refractive optics and the magnetic optics.

In this project, we systematically develop neutron optic devices and their application aiming to open new possilibities of neutron scattering experiments. One of the straightforward applications is the "downsizing" of neutron experiments in which the capability to analyze tiny samples or tiny region of samples by realizing a "good beam" with the beam focus, beam divergence suppression and neutron spin polarization, by realizing a "sophisticated neutron analysis" with the direct kinetic energy determination and neutron spin analysis and by realizing a "instantaneous fine image" with a time-resolved neutron imaging detection. The downsizing of sample size will drastically expand the application fields. The downsized sample enables to shrink the distance between the sample and the detector to achieve the same angular resolution and to downsize the dimensiton of neutron spectrometers and reduces the cost and period of the development. The downsizing of neutron spectrometer requires better position resolution for the neutron detector. The improved neutron detector will increase the angular resolution if used in neutron spectrometers with the conventional dimension. Such the chain reaction of R&D would accelerate the innovation and open up a new field of neutron scattering experiments.