Ⅰ. HIGHLIGHTS OF THE YEAR
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Relative formation probabilities
for ﬂuoride and oxyﬂuoride anions of U, Np, Pu and Am inaccelerator mass spectrometry measurements at VERA†
A. Wiederin,∗1,∗2 M. Kern,∗1,∗2 K. Hain,∗1 M. Martschini,∗1 A. Sakaguchi,∗3,∗4 P. Steier,∗1 A. Yokoyama,∗3,∗5and R. Golser∗1Atomic anions of actinides are not formed eﬃciently
in a Cs sputter ion source, so they must be extracted
as molecular anions for AMS measurements.1) Fluoride
anions were introduced as an alternative to the oxideanions previously established for this purpose.2) Somepotential beneﬁts of ﬂuoride extraction of actinides
have already been reported: Fluorine is monoisotopic,
ﬂuoride anions have the potential for increased ionization yields in low cesium ﬂux ion sources,2) could reduce
the hydride background2) and show some degree of elemental selectivity.3) Furthermore, ﬂuoride and oxyﬂuoride molecular anions could also be of use in isobar
separation techniques such as Ion Laser InterActionMass Spectrometry (ILIAMS)4) or the Isobar Separator
for Anions (ISA).5) The eﬃciency with which an anion
species is formed in the sputter process is determined by
the electron aﬃnity of the corresponding neutral atomor molecule.1) The stability of an anion against photodetachment in systems like ILIAMS is in turn also
related to this electron aﬃnity.
Developing such isobar separation methods in theactinide range could open up new possibilities for thestudy of actinides in the environment, such as new potential isotopic spikes for normalization or an extended
multi-actinide analysis with simpliﬁed chemistry froma single AMS sample.
Based on estimates of the releases from nuclear tests
and reprocessing plants, 237 Np is thought to be thesecond most abundant anthropogenic actinide in theenvironment.6) 237 Np exhibits conservative behavior innatural waters as it mainly exists in the highly soluble7)
Np(V)O+and has a
2 form under oxidizing conditions
6
long half-life (T1/2 = 2.1 × 10 y). These two properties
could turn 237 Np into a valuable tool for tracking environmental processes such as water mass transport.8,9)
The main obstacle to establishing 237 Np as an environmental tracer is the lack of an isotopic spike materialfor a matrix independent and reliable normalization of237
Np measurements by mass spectrometric methods.
For this purpose, 236 Np was produced via the232
Th(7 Li, 3n)236 Np reaction at the RIKEN Nishina
Center. In addition to the desired 236g Np, isobaric contamination can arise either from 236 U produced by neu†
∗1
∗2
∗3
∗4
∗5Condensed from the article in Nucl. Instrum. Methods Phys.
Res. B 528, 40 (2022)
Faculty of Physics, Isotope Physics, University of Vienna
Vienna Doctoral School of Physics, University of Vienna
RIKEN Nishina Center
Faculty of Pure and Applied Sciences, University of Tsukuba
Institute of Science and Engineering, Kanazawa University

tron capture on 235 U impurities in the irradiation target, or from the decay of metastable 236m Np to 236 U or236Pu, so further analysis of the mass 236 fraction wasrequired.
The relative formation probabilities of AnF−
5,−−−
,AnF,AnFOandAnF
O
AnF4 O− , AnF3 O−32 2,24
with An representing the actinides U, Np, Pu or Am,−
have been investigated. The AnF−
4 /AnF5 ratios ofthese actinides have been shown to be element-speciﬁc
within each measurement, with an order of magni−
tude separating the AnF−
4 /AnF5 ratios from one actinide element to the next. The magnitude factor isrobust and reproducible. Based on these ﬁndings, the−AnF−
4 /AnF5 ratio serves as an elemental ﬁngerprint. A
signiﬁcant amount of isobaric interference would shift
this ratio relative to the reference ratio for the actinideof interest as determined on calibration samples in thesame measurement. The prospective neptunium spikematerial provided a ﬁrst application for this kind ofisobar analysis, and the tentative ﬁrst results are compatible with a successful production and chemical separation of 236 Np.
Choosing the tetraﬂuoride system molecular anions
for the extraction of Np and U suppresses the interfering isobar 236 UF−
4 by an order of magnitude compared
to 236 NpF−
intheion source. The large number of4(oxy) ﬂuoride anions of various actinides greatly enhances the possibility of ﬁnding suitable molecular ion
systems for isobar separation using methods such as
element-selective photodetachment in ILIAMS.
References
1) R. Middleton, Nucl. Instrum. Methods Phys. Res. 122,35 (1974).
2) X. L. Zhao et al., Nucl. Instrum. Methods Phys. Res. B
510, 10 (2022).
3) X. L. Zhao et al., Nucl. Instrum. Methods Phys. Res. B
294, 356 (2013).
4) M. Martschini et al., Nucl. Instrum. Methods Phys. Res.
B 456, 213 (2019).
5) J. Eliades et al., Nucl. Instrum. Methods Phys. Res. B
268, 839 (2010).
6) J. M. Kelley et al., Sci. Total Environ. 237-238, 483(1999).
7) W. L. Keeney-Kennicutt et al., Mar. Chem. 15, 133(1984).
8) P. Lindahl et al., J. Environ. Radioact. 82, 285 (2005).
9) M. Lopez-Lora et al., Sci. Total Environ. 708, 135222(2020).

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