Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

Production of a high-intensity francium ion beam
for the measurement of the electron electric dipole moment
H. Nagahama,∗1 K. Nakamura,∗1 N. Ozawa,∗1 Y. Kotaka,∗1 H. Haba,∗2 T. Hayamizu,∗2 M. Ohtsuka,∗2,∗3K. S. Tanaka,∗4 H. Ueno,∗2 Y. Ichikawa,∗2 and Y. Sakemi∗1The striking imbalance between matter and antimatter observed in the universe is known to be one of
the biggest mysteries in modern physics. For decades,
physicists have been attempting to investigate CP (C:charge conjugation, P: parity transformation) violations for diﬀerent physical systems, which could lead tothe discovery of the fundamental reasons for this mystery. One of these intriguing systems is the electric
dipole moment of an electron (eEDM). The existenceof an absolute value of the eEDM implies that CP symmetry has been violated. At the same time, this would
also reveal the existence of a new theory beyond thestandard model of particle physics.
Since the eEDM is expected to be small,1) it is necessary to adopt a system that enhances the eEDM toa suﬃciently large value. For this purpose, we havechosen francium. Francium has an eEDM enhancement factor of approximately 103 , which is known tobe the largest among any ground-state atom.2) The
ideal environment for Ramsey spectroscopy to measure the eEDM using francium atom is one in which alarge amount of francium atoms are trapped in a tiny
volume where the interactions with other particles arehighly suppressed. To this end, we are planning todevelop a three-dimensional optical lattice to conﬁnelaser-cooled francium atoms.
In this study, we constructed a surface ionizer to create a high-intensity francium ion beam. The surface
ionizer consists of a vacuum chamber, a gold target, deﬂection electrodes, and an infrared radiation heater, as
shown in Fig. 1. An 18 O6+ beam (6.28 MeV/nucleon)
provided by the AVF cyclotron in RIKEN RIBF will beirradiated on the gold target to induce the following fusion reactions inside the gold: 197 Au(18 O, xn)215−x Fr.
By heating up the target using the infrared heater, thethermal diﬀusion process of the franciums will occur,and some fractions will reach the surface. Most of thefranciums at the surface will be ionized according to
the Saha-Langmuir equation3) and, subsequently, thermally released into the vacuum. Since high voltages
are applied to the target (∼1000 V) as well as to thedeﬂection electrodes (∼930 V), the thermal francium
ions will experience the electric-ﬁeld gradient and betransported as a secondary beam (∼1 keV), which will
be guided at an angle of 45◦ with respect to the 18 O6+beam. The francium ion beam generated will ﬁnally∗1∗2∗3∗42 EHDP 0&366'
FKDPEHU)UEHDP'HIOHFWLRQ
HOHFWURGHV 9,QIUDUHGUDGLDWLRQKHDWHU :*ROGWDUJHW
9Fig. 1. Schematic of the experimental apparatus to produce a high-intensity francium ion beam.

be monitored using a beam diagnostic system. The
beam diagnostic system consists of a micro-channel
plate (MCP) to monitor the beam’s characteristics anda silicon semiconducting detector (SSD) to observe αradiation caused by the α decay of the franciums. Anα radiation source (Am) is placed near the SSD to calibrate the energy of the α radiation.
In September 2019, we constructed this experimental apparatus and performed an oﬄine test. During theoﬄine test, we heated up the target to 400 ∼ 800◦ C using the infrared heater and thermally ionized particles
already existing on the surface. These ionized particles
were monitored by the MCP, which allowed us to optimise the voltages applied to the target and the deﬂection electrodes. We have conﬁrmed that the optimised
voltages are consistent with our previous simulation using SIMION 8.1. After the oﬄine test, we performed
online experiments on October 25, 2019 and November 22, 2019. Through data analysis, we veriﬁed that
we had succeeded in producing a 210 Fr+ beam with anintensity of 1.3 × 106 /s.
References

1) M. S. Safronova et al., Rev. Mod. Phys. 90, 025008(2018).
2) J. S. M. Geinges, V. V. Flambaum, Phys. Rep. 397, 63(2004).
3) M. J. Dresser, J. Appl. Phys. 39, 338 (1968).

Center for Nuclear Study, the University of Tokyo
RIKEN Nishina Center
Waseda University Honjo Senior High School
Cyclotron and Radioisotope Center, Tohoku University- 57 -