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Center for Mathematical Science and Advanced Technology (MAT)

Seminar Schedule

[MAT Seminar]

Date:
2022/08/4 (Thursday) 13:00 - 14:00
Language:
English
Speaker:
Dr. Sebastian Baum (Stanford University)
Title:
Paleo Detectors: A new approach to use old minerals for Dark Matter and neutrino searches
Abstract:
Paleo-detectors are a proposed alternative approach for the direct detection of Dark Matter, the missing 85% of our Universe's matter content. In lieu of the conventional approach of using large target masses to search for Dark Matter induced nuclear recoils in real time, the idea behind paleo detectors is to use small detectors that could integrate signals from nuclear recoils over large timescales to achieve the necessary exposure for Dark Matter searches. Many natural minerals found on Earth are excellent solid state track detectors, i.e., they record damage tracks from nuclear recoils. Minerals commonly found on Earth are as old as a billion years, and modern microscopy techniques may allow one to reconstruct damage tracks with nanometer scale spatial resolution. Thus, paleo-detectors would constitute a technique to achieve keV recoil energy threshold with exposures comparable to a kiloton-scale conventional "real-time" detector. I will also discuss the possible application of paleo- detectors as tools to search for astrophysical neutrino sources such as supernovae or atmospheric neutrinos produced by cosmic rays interacting with Earth's atmosphere. The age of the minerals provides the ability to look back across gigayear-timescales, giving paleo detectors the unique ability to probe changes in the cosmic ray rate or the galactic supernova rate over such timescales.

[MAT Seminar]

Date:
2022/07/20 (Wednesday) 13:00 - 15:00
Language:
English
Speaker:
Taiyo Kobayashi(RIGC)
Title:
Improvement of CTD sensor to enhance salinity accuracy: by collaborative research with MAT
Abstract:
The salinity measured by SBE 41CP and SBE 61 CTD sensors on deep floats was evaluated by comparison with shipboard CTD casts at deployment. Both CTD sensors, unfortunately in their present states, did not meet the target accuracy of Deep Argo for salinity, because the salinity bias with a negative pressure dependency, expressed as ΔS = ΔS_offset + a_p ×pressure and a_p < 0, were identified statistically for almost all CTD sensors: the average a_p of -0.54 × 10-6 dbar-1 for SBE 61 and -1.80 × 10-6 dbar-1 for SBE 41CP. The fresher salinity (ΔS_offset: -0.012) was observed at sea surface for SBE 41CP; meanwhile ΔS_offset was slightly positive (+0.8 × 10-3) for SBE 61. For SBE 41CP, sensors with a fresher ΔS_offset tended to have a smaller a_p. The fresh salinity bias found for SBE 41CP changed toward saline over time, mainly because ΔS_offset became less fresh with a decreasing rate. Note that most Argo floats had the SBE 41/41CP CTD sensor, which was the original CTD model for deep floats; an analysis for 383 Argo floats did not identify a statistically significant pressure dependency for salinity.
Salinity is affected by conductivity measurements via changes in the CTD measuring cell geometry under pressure. To remove the effect, a canceling factor for pressure CPcor is prepared theoretically (-9.57 × 10-8 dbar-1) by the manufacturer considering the isotropic deformation of a single-cylinder measuring cell under pressure; the observed negative pressure dependency, however, requires an even smaller CPcor (-14.2 × 10-8 dbar-1 for SBE 41CP and -11.0 × 10-8 dbar-1 for SBE 61).
The causes of the inconsistency for salinity measurements were examined with a dual-cylinder cell model, considering that the actual CTD measuring cell is a glass cell covered by a polyurethane jacket. The dual-cylinder cell model clarifies that the CTD measuring cell deforms anisotropically to yield a smaller CPcor; mainly because the inner glass cell deforms radially to a greater extent than the single-cylinder model due to the radial stress from the outer cylinder (jacket) being stronger than the hydrographic pressure.
The observed fresh bias at the sea surface is attributed to a slight shrinkage of the outer jacket because of compression set of polyurethane in deep ocean. The shrunken polyurethane jacket is elongated over time due to a creep phenomenon, which causes less-freshening of the fresh bias found at the sea surface. The observed linear relationship between the smaller CPcor and the fresher bias at the sea surface is attributed to the variation of the wall thickness of the polyurethane jacket when it shrinks similarly.
The observed features suggest that both CTD sensors could almost achieve the target accuracy of Deep Argo by the following improvements: aging treatment with high pressure on the sensor, accurate calibration of pressure-aged sensors, and a suitable CPcor. These operations can greatly reduce the salinity biases derived from the pressure dependency, the offset at the sea surface, and their changes over time. Another idea for the improvement is suggested by the model study: a change of the jacket material into an elastomer. The improvement makes the present setting of CPcor be more valid and reliable, because the elastomer jacket hardly affects the change of the measuring cell geometry under pressure, which in turn removes the pressure effects on salinity measurements.
The dual-cylinder cell model could explain the observed features of the salinity biases well, which were, however, limited qualitatively. In order to enhance the accuracy of CTD measurements for salinity, there are many issues to be examined. I think that most of them will be resolved with a sophisticated model; this is the reason why I have proposed a collaborative study with MAT.

[MAT Seminar]

Date:
2022/05/17 (Tuesday) 13:00 - 15:00
Language:
English
Speaker:
Shigenobu Hirose
Title:
Development of Paleo-detectors
Abstract:
Old minerals are natural particle detectors, here called “paleo-detectors”. This is because in the mineral an atom recoiled by a particle such as a dark matter particle or a neutrino is stopped via nuclear collisions, leaving a crystal defect (i.e. event signal) that cannot be erased unless thermally annealed. While normal particle detectors have large exposure owing to their huge target mass, the paleo-detectors, in spite of their tiny mass, can have comparable exposure thanks to their very long exposure time over the geological time scale.

A big issue with paleo-detectors is how to read out the “signals” left inside the minerals. The crystal defects made by the nuclear stopping are expected to be weak and short (O(10-100) nm). This is contrasted with the fission tracks, which are made when daughter atoms are decelerated by the electronic stopping and thus are much clearer and longer (O(10) micron). Therefore, it is crucial for the paleo-detectors to develop methods for reading out efficiently the weak and short defects in the mineral crystals.

Paleo-detectors were firstly applied a few decades ago to search magnetic monopoles (Price and Salamon 1986) and weakly-interacting massive particles (WIMPs) (Snowden-ifft et al. 1995). Recently paleo-detectors have been focused again by possible improvements of read-out methods (Baum et al. 2020) and their new applications to search dark matter much heavier than WIMPs. Following these new works, we have started working on the paleo-detectors. In this paper, we will introduce our initial attempts to read-out the crystal defects inside mineral samples irradiated by low-energy (~keV/amu) heavy ions or fast neutrons (~O(1) MeV) to mimic the natural nuclear recoil events.

[MAT Seminar]

Date:
2022/04/27 (Wednesday) 13:00 - 15:00
Language:
English
Speaker:
Arakawa Sota
Title:
On the collisional growth and fragmentation of dust aggregates
Abstract:
Understanding the collisional behavior of dust aggregates consisting of submicron-sized grains is essential to unveiling how planetesimals formed in protoplanetary disks.
It is known that the collisional behavior of individual dust particles strongly depends on the strength of viscous dissipation force; however, impacts of viscous dissipation on the collisional behavior of dust aggregates have not been studied in detail, especially for the cases of oblique collisions.
Here we investigated the impacts of viscous dissipation on the collisional behavior of dust aggregates.
We performed three-dimensional numerical simulations of collisions between two equal-mass dust aggregates with various collision velocities and impact parameters.
We also changed the strength of viscous dissipation force systematically.
We found that the threshold collision velocity for the fragmentation of dust aggregates barely depends on the strength of viscous dissipation force when we consider oblique collisions.
In contrast, the size distribution of fragments changes significantly when the viscous dissipation force is considered.
We obtained the empirical fitting formulae for the size distribution of fragments for the case of strong dissipation, which would be useful to study the evolution of size and spatial distributions of dust aggregates in protoplanetary disks.

[MAT Seminar]

Date:
2022/04/20 (Wednesday) 13:00 - 15:00
Language:
English
Speaker:
Daniel Shigueo Morikawa
Title:
Landslide simulations with the SPH method using GPU programming
Abstract:
Landslides are a very challenging phenomena to numerically simulate. Summarizing a few of its challenging points: (a) its geometry is usually complex, especially because it may change its topology if comparing the initial configuration with the final one; (b) it may be triggered by various external conditions such as increased internal pore water pressure (b1), earthquakes (b2), and others; (c) the material behavior of a landslide is very complex, given that soil is, by definition, composed by several materials such as soil grains, water and air.
In this presentation I intend to show my previous research achievements using the SPH method, which is a type of numerical method that discretizes the space into particles. In summary, using topographical data and a computer code to translate into a particle mode to solve (a) and two SPH numerical models to solve (b1) and (b2). All methods were developed using GPU programming, which is a highly effective way to increase computational efficiency through parallel computation.
Lastly, I will present my current and future plans to improve landslide simulations with SPH. As for the current goal, I intend to increase the scale of simulations by developing a code that can utilize an arbitrary number of GPU cards in parallel. For the future, the objective is to tackle the problem of complex material behavior of the soil (c) using the SPH method to simulate the soil in microscale, then, using Neural Networks to bridge the gap between micro and macro scales in an efficient way.

[MAT Seminar]

Date:
2022/04/13 (Wednesday) 13:00 - 14:00
Language:
English
Speaker:
Tei Mika
Title:
Agritech imaging of underground plant root growth using a distributed fiber optic sensor
Abstract:
Recent studies have shown that root system architecture determines crop resilience and productivity. However, roots grow invisibly underground and are notoriously difficult to track. Root visualization requires digging, which is time-consuming and destructive. The lack of real-time non-invasive underground imaging methods has made it challenging to study this vital organ. Here, we report a method for imaging underground root system using the distributed fiber optic sensor. device named “Fiber-RADGET”. By formulating an optical fiber into spiral polytetrafluoroethylene film, the sensor device named Fiber-RADGET detects and monitors geophysical strain generated by root development. Agricultural technology is increasingly becoming automated with seamless feedback through Internet-of-Things remote sensors. The device highlighted here represents a significant addition to the repertoire of tools that next-generation agriculturalists can use for data-driven automation.