Non-destructive analysis of C-type asteroid Ryugu using negative Muon:
Determination of bulk chemistry of Ryugu samples*K. Terada1, T. Nakamura2, K. Ninomiya1, T. Osawa3, T. Takahashi4, Y. Miyake5, S. Takeshita5, I. Umegaki5, I. Chiu1, M.
K. Kubo6, A. Taniguchi7, T. Wada2, S. Watanabe8, H. Yurimoto9, T. Noguchi7, R. Okazaki10, H. Yabuta11, H. Naraoka10 ,K. Sakamoto8, S. Tachibana4, S. Watanabe12, Y. Tsuda8 and J-PARC Muon Team13
1Osaka University, 2Tohoku University, 3Japan Atomic Energy Agency, 4The University of Tokyo, 5High Energy Accelerator
Research Organization, 6International Christian University, 7Kyoto University, 8Japan Aerospace Exploration Agency,9Hokkaido University, 10Kyushu University, 11Hiroshima University, 12Nagoya University and 13Japan Proton Accelerator
Research Complex

We have been developing a non-destructive Muonic X-ray analysis for extraterrestrial materials [1, 2, 3, Figure-1(a)]. Sincemuon has a mass about 200 times greater than that of an electron, negative muon behaves as a heavy electron in the sample. In
the classical Bohr model, orbital radii of negative leptons (electron and/or muon) around an atomic nucleus are inversely
proportional to the lepton mass, and therefore a muon trapped by an atom has an orbit closer to the atomic nucleus than electrons
due to its ~200 times heavier mass (Figure-1(b)). As a result, muon-induced characteristic X-rays have about 200 times higher
energies than those associated with the orbital transition of electrons. For instance, electron-induced Kα–X-ray of C has anenergy of 0.3 keV, whereas muonic Kα–X-ray of C has an energy of 75 keV, which is large enough to penetrate a rock sampleof about 1 cm in size without significant self-absorption. Another advantage is that the position at which the characteristic Xrays are generated (i.e. the depth of penetration from the sample surface) can be controlled by controlling the momentum of theincident muons, making it possible to non-destructively analyse the chemical composition inside a material from light elements
to heavy elements.

Figure 1. (a) Muon beam line at J-PARC,(b) Schematic view of muon-induced characteristic X-ray.

Ten coarse Ryugu samples (A0026, A0064, A0067, A0094, C0002, C0025, C0033, C0061, C0076, and C0103), which wereallocated to the initial analysis STONE team, have been investigated at J-PARC MUSE (Japan Proton Accelerator ResearchComplex, MUon Science Establishment) [4]. Because of the muon beam size (more than 3 cm in diameter), we obtained a bulk
elemental abundance of total 122.86 mg (17.72 mg from chamber-A and 105.14 mg from chamber-C). For comparison, pelletsof the meteorites Murray (CM2; 306.5 mg) and Orgueil (CI; 195 mg) were also measured. In order to avoid the contaminationfrom atmosphere (N and O), during the analysis, samples were set in the chamber which was filled with helium [3]. Figure-3
shows the muonic X-ray spectra from Ryugu and Orgueil. Significant Muonic X-rays of C, N, O, Na, Mg, Si, S and Fe weredetected from Ryugu and Orgueil. Signals of Cu and Be, originated from the inside shield of vacuum chamber, the sample holder
and the detector windows, were also detected.

Figure 2. Muonic X-ray spectra of Ryugu and Orgueil (CI)We calculated elemental mass ratios X/Si (X=C, N, O, Na, Mg, S, and Fe) of Ryugu samples from relative intensity ratios ofmuonic X-rays using Murray with known X/Si ratios. The obtained mass ratios of C, N, O, Na, Mg, S and Fe relative to Si are
0.338±0.008, 0.019±0.009, 3.152±0.099, 0.039±0.006, 0.890±0.021, 0.510±0.019, and 1.620±0.040, respectively. Theseelemental ratios are in a good agreement with the reported values for CI chondrites [5] and the conventional solar abundance [6],except for the 25% lower O/Si for Ryugu relative to CI(Figure-3). This means that Ryugu is depleted in oxygen by
11.3 % by mass compared to CI, assuming similar Si
concentration for Ryugu to CI chondrites. The Ryugu samples
were prepared and analyzed in low oxygen conditions (<0.1%) and at low dew points (< -50°C), thus the very low
oxygen concentration is indigenous feature. This result isconsistent with [7], which was obtained from a chamber-A
grain (~1 mg). Lesser water content in Ryugu samples relative
to CI chondrites [7] is likely the main factor for the low oxygen
concentration.
Thus, we have succeeded in a non-destructive Muonic Xray analysis for Ryugu sample, including light elements such
C, N and O. This innovative technique will also be powerfulfor sample returns from the C-type asteroid Bennu and/orfrom the Martian satellite Phobos, which are expected to beenriched in organic matter.

References[1] K. Terada et al. (2014) Sci. Rep. 4, 5072
[2] K. Terada et al. (2017) Sci. Rep. 7, 15478[3] T. Osaka et al. (2022) Analytical Chemistry submitted[4] T. Nakamura et al. (2022) Science 10.1126/science.abn8671.
[5] K. Lodders (2021 )Space Science Reviews 217, 44[6] M. Asplund, A. M. Amarsi, N. Grevesse (2021) Astronomy & Astrophysics 653, A141.
[7] T. Yokoyama et al. (2022) Science 10.1126/science.abn7850.