The 83rd JSAP Autumn Meeting 2022 Highlighted Presentations
October 24, 2022

Development of neutron spectrum measurement and imaging with ultra-fine-grained nuclear emulsion capableof 3D submicron tracking for recoil proton track

Innovative neutron imaging technology developed amid
the ultimate physical search for the elucidation of dark
matterToho University( 1), Nagoya University( 2), The University of Naples Federico II ( 3), Chiba Univ. RI Research Center( 4)Saya Akamatsu(1), Takuya Shiraishi(1), Tatsuhiro Naka(1), Takashi Asada(3), Ryuta Kobayashi(2), Ken'ichi Kuge (4)Summary• Development of neutron spectroscopy and imaging using !Nano-Imaging Tracker" (NIT)
devised amid the search for dark matter• The Nano-Imaging Tracker (NIT) is a unique high-precision detector with high directional
sensitivity, positional resolution, and energy resolution offering three-dimensional
tracking with a spatial resolution of 100 nm (1/10,000th of a millimeter).
• This research has great potential for imaging techniques such as neutron radiography,
nuclear fusion exploration, and medical applications.

A research group consisting of Saya Akamatsu, Takuya Shiraishi, and Tatsuhiro Naka from the Faculty ofScience at Toho University; Takashi Asada from the University of Naples Federico II; Ryuta Kobayashi fromthe Faculty of Science at Nagoya University; and Ken'ichi Kuge, an expert in photographic science, fromChiba University RI Research Center is collaborating to develop a detector, the “ Nano-Imaging Tracker(NIT)," for the elucidation of dark matter, which is one of the most important issues in modern physics andastronomy. NIT is a high-precision detector with high directional sensitivity, positional resolution, and energy
resolution offering three-dimensional tracking with spatial resolution of 100 nm (1/10,000th of a millimeter).
The research group is exploring the utilization of NIT technology in innovative imaging techniques such as
neutron radiography, nuclear fusion as a clean and safe energy source, and medical applications such as
neutron capture therapy and heavy particle therapy.

Detail

Detectors for the elucidation of dark matter, the most important issue in modern physics
Dark matter is arguably the most important issue in modern physics and astronomy. It has mass
but undergoes limited interaction with matter. It emits no light or radio waves of its own; therefore, itcannot be detected optically. However, it remains a mysterious substance believed to account for
more than a quarter of the energy of the universe.
Modern physicists are eager to understand dark matter. While its existence is confirmed fromvarious independent cosmological observations, it cannot be explained by the "Standard Model,"the framework of modern particle physics, regarded as the perfect theory to explain almost allphenomena in global-scale experiments (up to 10 teraelectron-volts); the existence of dark mattersuggests the existence of new physics beyond the Standard Model, and its unmasking is a toppriority in both particle physics and astrophysics.
"The search for dark matter is the 'ultimate physics' in both astrophysics and particle physics. The
pursuit of ultimate physics will lead to the development of ultrahigh-precision measurement
techniques. We believe that those technologies would be applicable to various fields and that is
why we are researching them," says Takuya Shiraishi. Shiraishi is working with a research teamfrom the Faculty of Science in Toho University and elsewhere to develop a "detector" for directly
observing dark matter.
"In the course of dark matter search, it is necessary to capture the extremely weak energy received
from dark matter that causes atoms to recoil*. Capturing the direction of the recoil is an important
evidence for the existence of dark matter, but it has been difficult to capture the direction in thisenergy band with conventional detectors. The !Nano-Imaging Tracker" (NIT), which we have beendeveloping, exhibits high directional sensitivity, positional resolution, and energy resolution, offering
three-dimensional tracking at a spatial resolution of 100 nm (1/10,000th of a millimeter), making it
possible to detect these very small energy trails and obtain detailed information such as direction,"says Shiraishi.
Shiraishi and his research group, in an international joint experiment at the Gran Sasso National
Laboratory (LNGS) in Italy, have conducted detailed measurements of neutrons generated as
background events (A background event supports the event to be measured) in dark mattersearches. The detector developed for this experiment is a Nano-Imaging Tracker (NIT), in which
AgBr crystals are microparticulated to less than 100 nm by controlling the addition rate andtemperature of a solution of silver nitrate and sodium bromide. The crystals are densely dispersed,
with more than 1,000 crystals per µm3, and each crystal acts as a charged-particle sensor,achieving a spatial resolution of 100 nm. When a recoil proton penetrates the crystals, thetrajectory of the proton is detected as if it were a photograph. Detailed information about theincident charged particles can be obtained by analyzing these trails with an automatic track readerdeveloped using our own optical microscope.
"Elastic scattering with various nuclei occurs in the NIT. Imagine a collision of billiard balls. Thetarget balls represent nuclei of various masses, and the incident neutron is the lightest ball. It is aball of comparable mass that is scattered the most strongly by the neutron, which is the hydrogen
nucleus or proton, which makes up to 1.8% of the NIT. The NIT can capture the trajectory of theserecoil protons with an unprecedented high spatial resolution, enabling us to obtain detailed
information on neutron energy, position, and direction," says Shiraishi.
Neutron imaging technology has a wide range of applications, from fluoroscopy to nuclearfusion

Shiraishi et al. suggested the development of new neutron imaging techniques by utilizing the highspatial resolution and directional sensitivity of NITs. One such example is neutron radiography.
Neutrons penetrate materials and their transmittance differs depending on the properties of thematerial. Neutron radiography is an imaging technique that uses differences in permeability.
"X-rays and Gamma rays, which are commonly used in fluoroscopy, are attenuated owing to thedensity of the material. Neutrons, however, are attenuated more strongly when the material
contains a large amount of hydrogen, which has a comparable mass. Using this feature, it ispossible to see through plastic material in a lead case, for example, which is impossible withconventional X-rays," says Shiraishi.
Shiraishi et al. are currently conducting a series of neutron radiography tests. He also says that
NIT technology can also be used in research on nuclear fusion, which is currently attracting
attention as a clean and safe energy source. Nuclear fusion refers to the fusion of light atomic
nuclei such as hydrogen into heavier atomic nuclei such as helium. High-energy particle beams
are emitted when this occurs. For example, during the fusion of deuterium and tritium, neutrons are
emitted along with helium.
"The fusion reaction is difficult to control and has not yet been put in practical use. The neutrons
produced by the fusion reaction provide valuable information. With NIT, we can figure out in detailwhere the neutrons come from, thereby obtaining information on the location and distribution ofreactions." says Shiraishi.
Other applications include neutron capture therapy, heavy-particle therapy, and other medical
fields. Shiraishi's goal is to pioneer the cutting edge of both particle physics in the search for dark
matter and social technology.
We have developed the ability to look at very short tracks in the search for dark matter," Shiraishi
said. "We are now thinking about applying this to longer trajectories, which could lead to a varietyof imaging techniques. I would like to explore such applications that will move the society forwardwith the technology that we have cultivated in the process of searching for the ultimate physics."
annotations"Recoiled atom: #An atom that is elastically scattered by a neutral particle (such as a neutron or dark matter) and anucleus. Owing to the law of conservation of momentum, the recoiled atoms provide information on the angle and kinetic
energy of the incoming neutral particles.

figuresFig1. Optical microscope image of a recoil proton track in an NIT irradiated with 880 keV monoenergetic neutrons
emitted from a T(p n)3He fusion reaction. The proton energy (Ep) is approximately 490 keV at a proton range of 5 µm. By
making full use of laboratory developed optical, microscope track reading system, stage movement control, imaging withthe microscope, and image analysis are all performed automatically.

Fig2. (a) Scattering angle (cosθ) and recoil energy of protons (Ep) detected in the sample shown in Figure 1. The
analysis targets recoil proton tracks larger than 2 µm. The events are distributed along the kinematic ideal curve$Ep = En cos 2 θ%, and the outliers are due to scattered neutrons. (b) Distribution of neutron energy calculated fromthe proton recoil energy and scattering angle. The measured neutron energy peak is in very good agreement with theirradiation energy, with an accuracy of 31% at 880 keV at full width of half maximum. This is an evidence that the proton
recoil energy and scattering angle can be measured with high accuracy even for sub-MeV neutrons.

Fig3. Detection methods for each neutron of various energies using NIT. In addition to the proton elastic scattering
method, which detects neutrons directly, high-energy neutron detection using inelastic scattering with nuclei andmethods using nuclear reactions by mixing 10B and 6Li, which have high reaction probability for low-energy neutrons(thermal neutrons), are used.