Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 53

Production of Np-236 in the 232 Th + 7 Li reaction for standard materialin accelerator mass spectrometry
Y. Hayakawa,∗1,∗2 A. Yokoyama,∗1,∗3 A. Sakaguchi,∗1,∗4 K. Teranishi,∗1,∗2 R. Morita,∗1,∗2 N. Matsumura,∗1,∗3A. Nakajima,∗1,∗4 D. Mori,∗1 Y. Komori,∗1 T. Yokokita,∗1 Y. Wang,∗1 and H. Haba∗1
Neptunium-236 can be a useful tracer in the determination of 237 Np, an isotope of minor-actinide Np, which
exists in tiny quantities in the environment owing to
its release from nuclear facilities. Such determination
is of practical use in various earth science ﬁelds such as
surface material circulation and environmental pollution
assessment.1) The measurement of 237 Np is expected tobe quantiﬁed by accelerator mass spectrometry (AMS),
although an internal standard method for Np needs anappropriate spike. Tracers for several elements are available now, but the spike for neptunium has not been developed yet. We aim to devise an eﬃcient method forthe production of 236 Np in the ground state of half-life
1.54 × 105 y as a candidate for the spike nuclide.
In this study, 236 Np tracer production was implemented in the reaction of 232 Th + 7 Li to measure 237 Np.
In the beginning of the project, the excitation functionsof Np isotopes and by-products were measured using target stacks of Th metal foils. We irradiated the targets
with 42 MeV 7 Li ions from the RIKEN AVF cyclotron
while integrating the beam current with a Faraday cup
in the irradiation course. Chemical procedures were performed to isolate Np atoms from the target. The target material was dissolved in a mixed acidic solution of0.027 M HF and 3 M HNO3 , and the sample was driedby heating. Then, the residue was adjusted to 4 mL of3 M HNO3 solution, reduced with ascorbic acid and Fe

Fig. 1. Excitation functions of the production of Np-236mand Np-236g isomers.
∗1
∗2
∗3
∗4

RIKEN Nishina Center
Graduate School of Natural Science and Technology,
Kanazawa University
Institute and College of Science and Engineering, Kanazawa
University
Center for Research in Isotopes and Environmental Dynamics,University of Tsukuba

Fig. 2. Isomeric ratios of 236 Np and angular momenta transferred into targets.

ions and introduced into a TEVA resin column; subsequently, it was treated with 3 M HNO3 and 10 M HClfor puriﬁcation and ﬁnally with 0.1 M HCl for the elution of Np. To remove Pa, another process with a TK
400 column was performed. To determine the yields of
Np isotopes and by-products, γ-ray spectrometry wasconducted for the eﬄuents with a Ge detector. For theNp-236g measurement, samples for AMS and ICP-MS
were prepared through puriﬁcation with UTEVA resin
after waiting for Np-236m to decay out, and they werebrought to the VERA facility in the University of Vienna, with which this project is in collaboration.
Figure 1 shows the excitation functions of the production of Np-236m and Np-236g. The yield in the groundstate (6−) was found to be larger than that in the excited state (1+) in the studied energy range, although
the former data are preliminarily taken.
Isomeric yields of the product are thought to be correlated with angular momentum in the composite system
if their spins substantially diﬀer from each other. Themaximum orbital angular momenta transferred to thenuclear system were estimated using the ALICE code2)
and compared to the isomeric ratios of 236 Np in Fig. 2.
The ﬁgure shows the correlation of observation to angular momenta. The analysis of the result is in progress,and additional experiments including mass spectrometry are in planning to conﬁrm the precision and reproducibility of the data.
References

1) W. Rund, G. S. Goﬀ, in Radionuclides in the Environment, edited by D. A. Atwood (Wiley, 2013).
2) M. Blann, “CODE ALICE/85/300,” UCID-20169 (September 1984).

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