II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Development of novel detection system for francium ions extracted fromonline surface ionizer
N. Ozawa,∗1 H. Nagahama,∗2 T. Hayamizu,∗3 K. Nakamura,∗2 M. Sato,∗3,∗4 S. Nagase,∗1 Y. Kotaka,∗2K. Kamakura,∗2 K. S. Tanaka,∗5 M. Otsuka,∗3 T. Aoki,∗3,∗4 Y. Ichikawa,∗3,∗6 A. Takamine,∗3 H. Haba,∗3H. Ueno,∗3 and Y. Sakemi∗2
One of the origins of the matter-antimatter asymmetry in the Universe is considered to be the violation of
the charge conjugation and parity symmetries.1) The existence of a non-zero permanent electric dipole moment(EDM) of an elementary particle is an indicator of symmetry violation.
In particular, electron EDM (eEDM) is enhanced inparamagnetic atoms,2) up to 103 in the case of francium(Fr), which is the heaviest alkali.3,4) We are currently
developing an eEDM measurement system using lasercooled Fr atoms.
Fr atoms are produced in a surface ionizer via the nuclear
fusion-evaporation reaction, 197 Au(18 O, xn)215−x Fr.5)
An 18 O primary beam is irradiated onto a solid Au target. A fraction of the produced Fr atoms reach the surface via thermal diﬀusion, where they are released as ionsowing to the surface ionization eﬀect on the Au surface.
Using electrodes placed next to the target, the ions areextracted at a 45-degree angle at 1 keV energy. The
Fr ion production rate depends on the Au target temperature, which is controlled using an infrared heater
installed on the opposite side of the beam irradiation
surface of the target.
The Fr yield is monitored by detecting the α particles
emitted during their decay using a Si solid state detector (SSD). In previous experiments, because the infrared
light from the heater acted as a noise source for the SSD,
the heater needed to be stopped for each measurement.
However, turning oﬀ the heater frequently caused theFig. 1. Developed Fr ion beam detector. Light from infrared
heater is shielded by shielding rings.
∗1
∗2
∗3
∗4
∗5
∗6

Department of Physics, University of Tokyo
Center for Nuclear Study, University of Tokyo
RIKEN Nishina CenterGraduate School of Arts and Sciences, University of TokyoCyclotron and Radioisotope Center, Tohoku University
Department of Physics, Kyushu University

Fig. 2. Typical energy spectrum of α particles emitted during
decay of ions captured on FC surface, obtained using theSSD. Labelled peaks within the range of 6.5–6.8 MeV are
identified as Fr.

Fr production to become ineﬃcient, owing to the rapid
drop in the Au target temperature.
To overcome this problem, we have developed a novel
Fr ion detection system that is insensitive to the infrared
light of the heater. The detection system, as shown in
Fig. 1, consists of an SSD, a Faraday cup (FC), and amoving rod attached to four shielding rings. The extracted Fr ions are ﬁrst irradiated onto the FC surface,where the beam intensity is observed as electronic current. Subsequently, the FC is moved to the front of theSSD, where the α particles are detected. Throughout
the operation, the shielding rings ﬁt to the cylindricalwall of the chamber with a negligible clearance; therefore the light hardly enters the SSD.
In September 2020, an experiment on Fr production was conducted. Figure 2 shows the energy spectrum of the α particles detected by the SSD. The spectrum analysis shows that a 210 Fr ion beam of 5 × 106 /s
was extracted. The Au target was heated to 960◦ C,
and an 18 O6+ beam of 1 particle µA accelerated to7 MeV/nucleon using an RIKEN AVF cyclotron wasused as the primary beam.
Throughout the experiment, the infrared heater hada negligible eﬀect on the SSD. Therefore, the newly developed Fr ion detection system enables beam detection
without stopping the infrared heater, which is advantageous for stable production of Fr ions.
References1) A. D. Sakharov, Sov. Phys. Usp. 34, 392 (1991).
2) M. Popeslov et al., Ann. Phys. 318, 119 (2005).
3) D. Mukherjee et al., J. Phys. Chem. A 113, 12549 (2009).
4) T. M. R. Byrnes et al., Phys. Rev. A 59, 3082 (1999).
5) G. Stancari et al., Nucl. Instrum. Methods Phys. Res. A557, 390 (2006).
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