Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 52 (2019)

Production of Np isotopes in nuclear reactions
for standard material in accelerator mass spectrometry
Y. Hayakawa,∗1 A. Yokoyama,∗2 A. Sakaguchi,∗3 K. Yamamori,∗1 K. Sekiguchi,∗3 S. Yanou,∗4 Y. Komori,∗4T. Yokokita,∗4 and H. Haba∗4
The techniques for highly sensitive mass spectrometry, such as ICP-MS and AMS, are rapidly being developed currently. Several elements or nuclides thatwere not supposed to be applicable in those techniques
several decades ago are now quantitatively analyzed.
Especially, for long-lived actinides elements, the techniques are becoming more important as a promisingalternative to radioactivity measurements. The tracer
nuclide for target elements, which is non-existent innature and absent in target samples, is necessary toestablish a standard in the new techniques. Tracers
for several elements are available now but the tracer
for neptunium has not been developed yet. We aim todevise an eﬃcient method for the production of Np236 in the ground state of half life 1.54 × 105 y as acandidate for tracer nuclide.
In this study, the Np tracer production was implemented in the reactions of 232 Th + 7 Li and 238 U + p
to measure Np-237 in environmental samples.
For irradiation, target stacks of Th metal foils of two
tytpes of thickness were prepared: a thin target for theexcitation function measurement and a thick target for
tracer production of Np. A silver foil was irradiated at
the same time to monitor the beam intensity from theradioactivity of 111 In generated during the irradiation
with current integration using a beam course equippedwith Faraday cup.
We irradiated the targets with 42 MeV 7 Li ions fromthe RIKEN AVF cyclotron. Chemical procedures wereperformed to isolate Np atoms from the target. As
an example of the procedure, the target material wasdissolved in 3 M HNO3 with Np-239 tracer to checkchemical recoveries and the sample was dried by heating. The residue was then dissolved in conc. HCl,
repeatedly dried three times, and ﬁnally adjusted to3 M HNO3 solution of 4 ml. This solution was introduced into a TEVA resin column, subsequently treatedwith 3 M HNO3 and 10 M HCl for puriﬁcation, and ﬁnally treated with 0.1 M HCl for the elution of Np. To
remove Pa, another process with a TK 400 column wasperformed. To determine the yields of Np isotopes andby-products, γ-ray spectrometry was conducted for theeﬄuents with a Ge detector.
As an example of the result, a measured γ spec∗1∗2∗3∗4Graduate 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
RIKEN Nishina Center- 202 -Fig. 1. Example of γ spectrum of isolated Np. (Red: Np236m, Green: Np-234)

Fig. 2. Exciting functions of 236m Np in reactions of 238 U +p and 232 Th + 7 Li.

trum is shown in Fig. 1, where many photopeaks ofNp isotopes including Np-236m are observed. Figure 2
compares the excitation function of Np-236m obtained
from this study to that obtained from the 39 MeV p+ U experiment conducted in RCNP of Osaka University. Although the cross section of Th + Li is nearly
two orders of magnitude lower than that of U + p,
it is predicted that the isomeric ratio of Np-236g toNp-236m for the products of the former reaction islarger than that for the latter, which had been measured to be around 0.5.1) The analysis of the result isstill in progress and additional experiments including
mass spectrometry are in the planning stage at present.
Reference1) J. Aaltonen, M. Brenner, S. A. Egorov, A. M. Fridkin, V. B. Funschtein, E. A. Gromova, P. Manngård,V. A. Rubchen’a, Yu. A. Selitsky, V. A. Yakovlev,V. S. Zenkevich, Phys. Rev. C 41, 513 (1990).