III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Measurement of the isotopic ratio of Np-236 to Np-237 inTh-232 + Li-7 reaction products by accelerator mass spectrometry
A. Nakajima„ ∗1,∗2 K. Teranishi,∗1,∗3 A. Yokoyama,∗1,∗4 A. Sakaguch,∗1,∗5 K. Hain,∗6 R. Morita,∗1,∗3A. Seto,∗1,∗4 A. Nagai,∗1,∗4 D. Mori,∗1 Y. Komori,∗1 T. Yokokita,∗1 Y. Wang,∗1 and H. Haba∗1Neptunium-236 can be a useful tracer in the determination of Np-237, an isotope of the minor actinide
Np, which exists in tiny quantities in the environment
owing to its release from nuclear facilities and nuclear
tests. Such determination is of practical use in various earth science ﬁelds such as surface material circulation and environmental pollution assessment.1) The
measurement of Np-237 is expected to be quantiﬁedby accelerator mass spectrometry (AMS), although aninternal standard method for Np needs an appropriate spike. Chemical yield tracers for several elements
are available now, but the spike for neptunium has
not been developed yet. We aim to devise an eﬃcientmethod for the production of Np-236 in the groundstate of half-life 1.54 × 105 y as a candidate for thespike nuclide, with minimal contamination by Np-237.
In this study, Np-236 tracer production was implemented in the reaction of Th-232 + Li-7 to measureNp-237. The resulting yields of Np-236m and Np-236g
were presented in the previous report.2) The yield inthe ground state (6− ) was found to be larger than thatin the excited state (1+ ) in the studied energy range.
This demonstrates that the reaction system adopted ispromising. In addition, we have to measure the isotopic ratios of Np-236g to Np-237 in the same nuclearsystem.
We irradiated Th foil targets with 42 MeV Li-7 ionsfrom the RIKEN AVF cyclotron while integrating thebeam current with a Faraday cup in the irradiation
course. Chemical procedures were performed to isolate
Np atoms from the target. Details of the procedures
were described previously.2)
For the Np-236g and Np-237 measurement, samples
for AMS and inductively coupled plasma mass spectrometry (ICP-MS) were prepared through puriﬁcation with UTEVA resin after waiting for Np-236m todecay out, and they were brought to the VERA facility
in the University of Vienna, with which this project isin collaboration.
Figure 1 shows the preliminary isotopic ratios of Np236g to Np-237 isotopes measured using AMS. The
yield of Np-236 was found to be approximately 10
∗1
∗2
∗3
∗4
∗5
∗6

RIKEN Nishina Center
Master’s Program in Chemistry, University of Tsukuba
Graduate School of Natural Science and Technology,Kanazawa University
Institute and College of Science and Engineering, Kanazawa
UniversityCenter for Research in Isotopes and Environmental Dynamics, University of Tsukuba
Faculty of Physics, Isotope Physics, University of Vienna

Fig. 1. Preliminary isotopic ratios of Np-236g to Np-237 in
the Th-232 + Li-7 reaction system compared with thetheoretical prediction. Closed circles represent the observation, while open circles indicate data from calculation.

times larger than that of Np-237 in the studied energy range. The ﬁgure also shows the ratios calculated under a simple assumption with the nuclear reaction model code empire23) comprising various nuclear models. A large discrepancy is observed between
the observation and prediction. It may be due to difﬁculty in the yield reproduction of used models at theenergy region beyond the maximum yield energy forthe products. The observation indicates that the reaction system adopted is promising thus far, although
the data may need to be modiﬁed because of contamination by uranium nuclides and other nuclides with amass number of 236.
The analysis of the result is in progress, and additional experiments including mass spectrometry are inplanning. We aim to conﬁrm the precision and reproducibility of the data in consideration of the contamination by isobars, which aﬀects the mass measurement
and is a major concern.
References

1) W. Rund, G. S. Goﬀ, Radionuclide in the Environment,edited by D. A. Atwood (Wiley, 2013).
2) Y. Hayakawa et al., RIKEN Accel. Prog. Rep. 53, 179(2020).
3) M. Herman et al., Nucl. Data Sheets 108, 2655 (2007).

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