The NOP Project[English|Japanese]
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.
|Neutrons interact with nuclei in matters.
The interaction can be described using a potential in low energy regions.
We define the "effective potential" as the volume average of nuclear potential
in a matter.
The neutron propagation in matters can be described using the effective potential
if the neutron wavelength is comparable or longer than the distance of atoms, which
is true in the energy regions of thermal neutrons and cold neutrons.|
The value of the effective potential is, in general, a complex number and depends on atomic and isotopic constitution of the sample. The imaginary part is usually negligibly smaller than the real part. For an example, the nickel of natural abundance has the effective potential of about 240 neV. Some elements have negative real parts.
|Neutrons interacts with the magnetic field through the mangetic dipole interaction. The difference in potential energies for the cases where neutron spin is parallel and antiparallel to a 1 tesla magnetic field is about 120 neV.|
|The earth's gravity is the only gravitational interaction which should be considered practically. Pushing up a neutron by 1 meter requires the energy of about 100 neV.|
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
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.