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Kitazato, Kohei*; Milliken, R. E.*; Iwata, Takahiro*; Abe, Masanao*; Otake, Makiko*; Matsuura, Shuji*; Takagi, Yasuhiko*; Nakamura, Tomoki*; Hiroi, Takahiro*; Matsuoka, Moe*; et al.
Nature Astronomy (Internet), 5(3), p.246 - 250, 2021/03
Times Cited Count：43 Percentile：96.93(Astronomy & Astrophysics)Here we report observations of Ryugu's subsurface material by the Near-Infrared Spectrometer (NIRS3) on the Hayabusa2 spacecraft. Reflectance spectra of excavated material exhibit a hydroxyl (OH) absorption feature that is slightly stronger and peak-shifted compared with that observed for the surface, indicating that space weathering and/or radiative heating have caused subtle spectral changes in the uppermost surface. However, the strength and shape of the OH feature still suggests that the subsurface material experienced heating above 300 
C, similar to the surface. In contrast, thermophysical modeling indicates that radiative heating does not increase the temperature above 200 C at the estimated excavation depth of 1 m, even if the semimajor axis is reduced to 0.344 au. This supports the hypothesis that primary thermal alteration occurred due to radiogenic and/or impact heating on Ryugu's parent body.
Es targetAsai, Masato; Tsukada, Kazuaki; Hirose, Kentaro; Toyoshima, Atsushi; Tomitsuka, Tomohiro; Chiera, N. M.; Ito, Yuta; Makii, Hiroyuki; Nagame, Yuichiro; Nishio, Katsuhisa; et al.
no journal, ,
Spontaneous fissions of 
Fm, 
Fm, and 
Lr have been measured using an Es target at the JAEA Tandem Accelerator Facility. These three nuclei show different fission properties: the asymmetric fission, the high total-kinetic-energy (TKE) symmetric fission, and the low TKE symmetric fission, and the mechanisms of these three fissions should be very different. It is of great interest to elucidate how these differences occur. We have produced 
Fm in multinucleon-transfer reactions with a Es target and 
O projectiles, and produced Lr in the 
Cm(
N,4n) fusion-evaporation reaction. The reaction products were mass-separated with a gas-jet-coupled on-line isotope separator, and were delivered into a rotating wheel alpha- and fission-fragment detection system. By comparing the mass and TKE distributions deduced from the present experimental data, we will discuss the differences in fission mechanism among Fm, Fm, and Lr.
Es targetAsai, Masato; Tsukada, Kazuaki; Hirose, Kentaro; Toyoshima, Atsushi; Tomitsuka, Tomohiro; Chiera, N. M.; Ito, Yuta; Makii, Hiroyuki; Nagame, Yuichiro; Nishio, Katsuhisa; et al.
no journal, ,
Spontaneous fissions of neutron-rich heavy actinide isotopes Fm, Fm, and Lr have been measured at the JAEA Tandem accelerator facility. Fm and Fm were produced in a multinucleon transfer reaction with an O beam and an extremely rare Es target with a half-life of 276 days. Reaction products were mass-separated with an on-line isotope separator (ISOL). A transition between the typical asymmetric mass distribution and a very sharp symmetric one was clearly observed between Fm and Fm, while the spontaneous fission of Lr was found to exhibit mixture of three different fission modes.
Asai, Masato; Tsukada, Kazuaki; Hirose, Kentaro; Toyoshima, Atsushi*; Tomitsuka, Tomohiro*; Chiera, N. M.*; Ito, Yuta; Makii, Hiroyuki; Nagame, Yuichiro*; Nishio, Katsuhisa; et al.
no journal, ,
We have studied spontaneous fission of neutron-rich Fm and Lr isotopes. The neutron-rich Fm isotopes were produced with the Es target and the O beam delivered by the JAEA tandem accelerator. The neutron-rich Lr isotope were produced with the Cm target and the N beam. Reaction products were mass-separated with an on-line isotope separator (ISOL), which enabled us to obtain non-contaminated samples and to measure fission fragments precisely. From the deduced fission-fragment mass and total kinetic energy distributions, we have obtained a new insight into the competition between the symmetric and asymmetric fission which is characteristic in this neutron-rich region.
Es targetAsai, Masato; Tsukada, Kazuaki; Hirose, Kentaro; Toyoshima, Atsushi*; Tomitsuka, Tomohiro*; Chiera, N. M.*; Ito, Yuta; Makii, Hiroyuki; Nagame, Yuichiro*; Nishio, Katsuhisa; et al.
no journal, ,
Neutron-rich fermium isotopes Fm and Fm and lawrencium isotope Lr were produced in the multinucleon transfer reaction on Es target with heavy-ion beam and in the heavy-ion fusion reaction on Cm target with N beam, respectively. Reaction products were mass-separated with an on-line isotope separator (ISOL), and spontaneous fission (SF) of these isotopes were measured. It was found that the mass distribution of the SF of Fm was asymmetric, while that of Fm was sharp symmetric, which confirmed the previous experimental results. On the other hand, the SF of Lr seemed to contain both symmetric and asymmetric mass distributions. On the basis of the present results of the mass and total kinetic energy distributions, we discussed the mechanism of fission for these isotopes.
