Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Charge population investigation for the 248 Cm ﬁssion products
extracted from a helium gas catcher
A. Takamine,∗1 M. Rosenbusch,∗2 J. M. Yap,∗1,∗3 H. Ishiyama,∗1 M. Wada,∗2 S. Chen,∗3,∗4 T. Gao,∗1,∗3D. Hou,∗5,∗6 S. Iimura,∗1,∗7 S. Kimura,∗1 T. M. Kojima,∗1 S. Nishimura,∗1 T. Niwase,∗3 P. Schury,∗2 T. Sonoda,∗1H. Ueno,∗1 P. Vi,∗1 Y. X. Watanabe,∗2 and W. Xian∗3∗1∗2∗3∗4∗5∗6∗7

RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), IPNS, KEK
Department of Physics, University of Hong Kong
Faculty of Science, University of York
Institute of Modern Physics, Chinese Academy of Sciences
School of Nuclear Science and Technology, Lanzhou University
Department of Physics, College of Science, Rikkyo University

Ti degrader (2 μm)

Outer vessel (vacuum)

Cryogenic gas cell (RTE 200 mbar He)

248

Cm source

gate valve

Ti window (50 μm)

Mylar window (6 μm)

35
30

10.8

He 1st IP 24.6 eV

25
200.6

15100.2

500100Y100Zr103Zr
100N
b103N
b104N
b103M
o104M
o104T
c0.4

Ionization potential [eV]Fig. 1. Schematic of the gas catcher cell of the ZD-MRTOF(top view). The ions are transported upwards, out ofthe page. The ﬁssion source holder attached with the Ti
degrader is movable and rotatable.

Fraction of charge population

We are developing a helium gas catcher using the RFion guide technique and a multi-reﬂection time-of-ﬂight
mass spectrograph downstream of the ZeroDegree spectrometer, referred to as ZD MRTOF-MS.1) We aim toconvert BigRIPS beams to ultra low energy beams andprecisely measure the atomic masses of unstable nuclei.
After highly charged radioactive ion beams enter thegas cell, their energy deposition results in the ionization of helium atoms and even impurities, and they
undergo charge exchange with the helium and impurities. The charge state that will be dominant depends
on the ionization potentials of the elements, as discussed
in Ref. 2). Although it is naively expected that charge
exchange with helium would stop when the ionization potential falls below the ﬁrst ionization potential of helium,
the mechanism for determining the charge population iscomplex because it also depends on the level of impurities in the helium gas. For eﬃcient online experiments, apriori knowledge of the element dependent charge population is essential. Recently, a 248 Cm ﬁssion source witha 2-µm-thick Ti degrader was installed outside the cryogenic gas cell (Fig. 1). The 248 Cm source has an activityof 10 kBq with branching ratios of 91.6% and 8.4% foralpha and spontaneous ﬁssion decays, respectively. Theemitted alpha particles and the spontaneous ﬁssion products enter the gas cell through a 6-µm-thick Mylar window. This setup facilitates tests with a consistent andknowable intensity of ions under a fair approximation ofonline conditions.
We performed the charge population measurements for100
Y, 100, 103 Zr, 100, 103, 104 Nb, 103, 104 Mo, and 104 Tc ionsproduced by spontaneous ﬁssion decay from the 248 Cm
source oﬄine. The cryogenic gas cell temperature wasapproximately 80 K, and the gas density was equivalent to 200 mbar at room temperature (293.15 K). Theircount rates for the singly, doubly, and triply charged
states were measured with the MRTOF mass spectrograph detector. Their fractional charge population isshown in Fig. 2 with their second and third ionization potentials. It is reasonable that the doubly charged states
were predominantly extracted from the helium gas. The
fraction of triply charged ions decreases consistently with
the increase in the third ionization potential. In contrast, Zr and Y exhibited smaller triple charge fractions

1+2+3+

3rd IP

2nd IPFig. 2. Charge population of 100 Y, 100, 103 Zr, 100, 103, 104 Nb,103, 104
Mo, and 104 Tc extracted from a helium gas cell.
The 2nd and 3rd ionization potentials for Y, Zr, Nb, andMo are plotted. The red line shows the ﬁrst ionizaton
potential of helium.

than that of Nb despite the third ionization potentials
of Y and Zr being less than Nb. This result could imply a signiﬁcant molecularization probability leading to
the loss of atomic Zr and Y ions. The charge populations are expected to depend signiﬁcantly on the gas cellconditions such as temperature, cleanliness, etc. Further
investigations with various conditions and elements andthe searching for the molecularized ﬁssion products arein progress.

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RIKEN Accel. Prog. Rep. 56 (2023)

References
1) M. Rosenbusch et al., Nucl. Instrum. Methods Phys. Res.
A 1047, 167824 (2023).
2) P. Schury et al., Nucl. Instrum. Methods Phys. Res. B
407, 160 (2017).

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