Neutron Capture 36Cl in Ryugu Samples
Kunihiko Nishiizumi1, Marc W. Caffee2, Jozef Masarik3, Keisuke Nagao4, Ryuji Okazaki5, Hisayoshi Yurimoto6, Tomoki
Nakamura7 Takaaki Noguchi8, Hiroshi Naraoka5, Hikaru Yabuta9, Kanako Sakamoto10, Shogo Tachibana11, 12, Sei-ichiro
Watanabe10, 12, Yuichi Tsuda10
1Space Sci. Lab., Univ. of California, Berkeley, CA 94720-7450, USA., 2Dept. of Phys. & Astro., Purdue Univ., West Lafayette,IN 47907-2036, USA., 3Comenius Univ., Batislava, Slovakia, 4KOPRI, Incheon 21990, Korea,5Kyushu Univ., Fukuoka 8190395, Japan, 6Hokkaido Univ., 7Tohoku Univ.,8Kyoto Univ.,9Hiroshima Univ., 10ISAS/JAXA, 11Univ. of Tokyo, 12Nagoya Univ.
Hayabusa2 arrived at the C-type asteroid 162173 Ryugu in Jun. 2018, and successfully collected surface samples from two
sampling sites, returning ~5.4 g of samples to Earth on Dec. 6, 2020. The surface samples stored in Chamber A were collectedby the 1st touchdown (TD) on Ryugu’s surface on Feb. 21, 2019. A crater (diameter of ~14 m) on Ryugu’s surface was madeusing a collision device - denoted “Small Carry-on Impactor (SCI)” - on Apr. 5, 2019 [1]. The samples in Chamber C werecollected proximal to this artificial crater and are possibly ejecta from the north side of the crater by the 2nd TD on Jul. 11, 2019[2].
Our studies are based on the measurement of those nuclides produced in asteroidal surface materials by cosmic rays - bothsolar (SCR) and galactic cosmic rays (GCR). Cosmic-ray-produced (cosmogenic) nuclides are used to determine the durationand nature of the exposure of materials to energetic particles. Our goals are to understand both the fundamental processes on
the asteroidal surface and the evolutionary history of its surface materials. With this information we hope to better understandasteroid-meteoroid evolutionary dynamics. For Hayabusa2 samples, there are several specific questions we aim to address: (1)are the Chamber C samples, collected during the 2nd TD ejecta deposits from the artificial crater, (2) if so, what is the original
depth of each recovered sample in the Ryugu regolith, and (3) what is the surface exposure time, mixing rate, and erosion/escaperate of Ryugu’s surface? To answer these questions, we were allocated and received 2 particles from Chamber A (A0105-19and -20) and 6 particles from Chamber C (C0106-09, -10, -11, -12, C0002-V01, and -V02) for measurements of cosmogenic
radionuclides and noble gases. Each sample is several hundred µm in size.
As a part of initial analysis of Ryugu we have measured cosmogenic 10Be (t1/2 = 1.36 x 106 yr) [3], 26Al (7.05 x 105 yr) [4],and stable noble gases [5] in above samples. The Table 1 summarizes 26Al, 10Be, and cosmogenic 21Ne concentrations in eachsample. Based on those measurements, we found that sampling depth of A0105-19 was 10-15 g/cm2 and A0105-20 was ~5g/cm2. The exposure ages of both A0105 samples were 6.4-7.5 Myr. The chamber A samples were exposed to cosmic rays
at Ryugu’s near surface (~10 cm), as expected, and had similar exposure ages of ~7 Myr. On the other hand, the four C0106
particles were exposed at depth of ~50, 110, 130, and 145 g/cm2 respectively and had exposure ages of ~1.6, 3.2, 4.5, and 5.8
Myr respectively. Our results indicate that Chamber C samples are mixture of particles ejected from various depths on Ryugu
by the SCI impact. It is noteworthy that all four particles had different exposure ages on Ryugu. Exposure depths of eachsample are also shown in Table 1.
To further study of the exposure condition of Ryugu surface materials, we analyzed cosmogenic 36Cl (3.01 x 105 yr). Afterdissolution of each sample with a few drops of HF-HNO3 mixture, Cl was separated as AgCl prior to Be and Al separation [3,4]. After chemically purified AgCl, the concentration of 36Cl was measured by accelerator mass spectrometry (AMS) at Purdue
University [6], using a 36Cl AMS standard [7] for normalization. The concentrations of 36Cl (dpm/kg) in each Ryugu sample
are shown in Table 1 along with that of Nogoya CM2 chondrite for validation. The 36Cl concentrations in all Ryugu samples
are more than an order of magnitude higher than that of Nogoya. The dominant production pathway for 10Be and 26Al is byhigh-energy neutron spallation reactions. Although 36Cl can be produced by both thermal neutron capture reaction, 35Cl (n,g)
36Cl, and high-energy neutron spallation on K, Ca, and Fe, the thermal neutron production of 36Cl is dominant owing to the highH and Cl concentrations in Ryugu. Using the MCNP Code System [8], we calculated GCR production rate of 36Cl by spallation
reactions for a body having a 2π geometry with Ryugu’s chemical compositions [9]. The obtained production rates are 5.4 36Cl
atom/min/kg at surface to 3.3 at ~150 g/cm2. The thermal neutron production of 36Cl is calculated by subtracting spallation
contribution from measured value and normalized to Cl concentration in Ryugu and shown in Table 1 as 36Clth. We adopted aCl concentration of 780 ppm [9]. The measured 36Cl concentrations in Ryugu are extraordinarily high. Among carbonaceouschondrites, only the CI chondrite Orgueil is higher, having 160 ± 1 dpm/kg (unpublished), assuming exposure in a 4π geometry.
This corresponds to 220 ± 3 dpm 36Cl /g Cl assuming 700 ppm Cl in Orgueil.
Assuming a chemical compositions for each particle the same as the bulk analysis of Ryugu [9], the thermal neutron produced
36Cl in Ryugu is maximum at slightly below 100 g/cm2 or between depth of C0106-10 and -11.
The depth of maximum
production for the low-energy neutron capture reaction on Ryugu is shifted toward surface compared to that of the Moon (~150g/cm2). This occurs because Ryugu contains high H (0.94 %) and C (4.6 %) [9] which are effective moderators of neutrons.
Since we don’t have exact Cl concentration in each particle, it is hard to compare observed 36Cl concentration to model
calculation such as MCNP code system at present. Although we are planning measurements of thermal neutron capture 41Ca
(t1/2 = 0.10 Myr) in Ryugu, it requires larger sample size than this work because low thermal neutron capture cross section on
40Ca compared to that of 35Cl (0.43 b vs. 43.6 b) and lower AMS sensitivity of 41Ca measurements.
Table 1. Cosmogenic nuclide 36Cl, 26Al, 10Be, and 21Ne concentrations in Ryugu samples and Nogoya CM2 chondrite.
36Clb36Cl c26Ald
10Bee
21Ne fSampleMass
Depthathc
2-9(µg)(g/cm )
(dpm/kg)(dpm/g Cl)(dpm/kg)(dpm/kg)(10 cm3STP/g)
A0105-19242.9
531.1±2.333±327.1±1.1
12.76±0.37
7.55A0105-20
206.1
10-15
28.9±1.930±233.3±1.8
12.75±0.29
7.75
C0106-09122.8
5072.0±6.286±823.3±1.4
7.10±0.30
1.79C0106-10154.3
110116.5±6.9
144±925.7±1.3
7.48±0.26
3.58
C0106-11189.8
13023.4±2.526±325.5±1.2
7.21±0.43
5.07C0106-12959.8
14524.2±0.927±123.8±0.77.36±0.33
6.54
C0002-V0145.3
12530.1±4.3
34±6
24.9±1.98.29±0.95
C0002-V0211.1
12511±1321.9±5.1
7.87±1.80
Nogoya CM2459.4
2.2±0.5
2.09±0.13
Nogoya CM2343.7
2.2±0.6
7.7±0.5
2.12±0.09
Nogoya CM2
204.9
0.6±1.0
9.1±0.6
2.00±0.13
aEstimated depth based on 10Be and 26Al [4]; bThis work; cAfter subtraction of spallation component (see text); d[4]; e[3]; f[5].
Acknowledgments: We thank Hayabusa2 project and initial analysis team, especially Hayabusa2 curation members. Z. Nettand K. C. Welten helped a part of laboratory works. Nogoya was obtained from Field Museum of Natural History. This work
was supported by NASA’s LARS program.
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2021. Hayabusa Symposium 2021 S3-1. [4] Nishiizumi K. et al. 2022. Lunar Planet. Sci. 53, #1777. [5] Nagao K. et al. 2022.
Meteor. Planet. Sci. 57, A337 #6320. [6] Sharma P. et al. 2000. Nucl. Instrum. Methods Phys. Res. B 172, 112-123. [7] Sharma
P. et al. 1990. Nucl. Instrum. Methods Phys. Res. B52, 410-415. [8] Masarik J. and Reedy R. C. 1994. Geochim. Cosmochim.
Acta 58, 5307-5317. [9] Yokoyama T. et al. 2022. Science 10.1126/science.abn7850.