Ⅱ-9. Instrumentation
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Charge-exchange reaction of francium ions using an yttrium
neutralizer
H. Nagahama,∗1 K. Nakamura,∗1 N. Ozawa,∗2 M. Sato,∗3,∗4 T. Nakashita,∗3,∗4 S. Nagase,∗2 M. Fukase,∗2D. Uehara,∗2 Y. Kotaka,∗1 K. Kamakura,∗1 T. Aoki,∗4 H. Haba,∗3 A. Takamine,∗3 H. Ueno,∗3 and Y. Sakemi∗1∗1∗2∗3∗4Center for Nuclear Study, University of Tokyo
Department of Physics, University of Tokyo
RIKEN Nishina Center
Graduate School of Arts and Sciences, University of Tokyo- 107 -for 1.5–2 hours to clean its surface.
Figure 1 shows an example of count rate of alphaparticles emitted via the alpha decay of 208–211 Fr detected by an SSD, as a function of time. By evaluating
all the data collected during the beam time, we derived
a desorption eﬃciency of ϵdes = 29 ± 15%, when thefoil was heated to 750◦ C for approximately 3 seconds.
Considering an experiment to measure the desorptionenergy of rubidium (Rb) on a Y surface,4) almost all
the Fr on the surface should be released into the vacuum by heating the foil up to 350◦ C for several seconds. Note that Rb is an alkali element akin to Fr andhas chemical properties similar to those of Fr. This
discrepancy implies that the cleaning procedure of theY surface before the measurement was insuﬃcient torealize an ideal surface, which does not have any impurities nor passive layers.
5000
SSD count rate

The large imbalance in the amounts of matter andantimatter currently observed in the universe demands
physics beyond the standard model, which violates fundamental symmetries.1) Recently, the search for a nonzero permanent electric dipole moment (EDM) of an elementary particle has taken place actively world-wide,
because the existence of EDM violates charge conjugation parity symmetry.
It is predicted that the EDM of an electron (eEDM)
is enhanced in paramagnetic atoms.2) Francium (Fr),being the heaviest alkali element, is known to possess the largest eEDM enhancement factor amongst
any ground-state atom.3) Therefore, we aim to search
for the eEDM with unprecedented precision using Fratoms trapped in an optical lattice. To achieve thisgoal, it is necessary to prepare a large number of Fratoms trapped in a magneto-optical trap (MOT) before transporting them to an optical lattice. The number of trapped atoms in a MOT strongly depends onthe desorption eﬃciency of Fr atoms from a neutralizer, where the charge-exchange reaction of Fr ionstakes place. Here, we report the characterization of
the eﬃciency of Fr atoms desorbed from an yttrium(Y) neutralizer during beam time in 2022.
An 18 O6+ beam (7 MeV/nucleon) provided by theAVF cyclotron in RIKEN RIBF is irradiated on thegold target to induce the following fusion reactions:
197
Au(18 O, xn)215−x Fr. Heating up the target using
an infrared radiation heater results in thermal diﬀusionof the produced Fr, and some fractions will reach thesurface. Subsequently, most of the Fr on the surface
will be thermally ionized and extracted as a secondary
beam (100 eV, typically 5 × 106 s−1 for 210 Fr). The
Fr ions are transported through the electrostatic beamline and irradiated on a Y foil for 3 minutes. The Yfoil is then moved mechanically to the so-called silicon
semiconductor detector (SSD) chamber to characterize the number of Fr atoms deposited on the surfaceof the foil. After the characterization, the foil is transported to the so-called neutralization chamber to heat
the foil up to a maximum of 750◦ C and release the Fr as
neutral atoms into a vacuum. Finally, the foil is transported back to the SSD chamber to characterize thenumber of Fr atoms remaining on the surface. Before
conducting this experiment, the Y foil is bombarded byan argon (Ar) ion beam (10−2 A m−2 , 500 eV, 300 K)

heatingprocedure

100050010050
10050100150200Time (s)Fig. 1. Count rate of alpha particles emitted via alpha decay of 208–211 Fr detected by a silicon semiconductor detector (SSD), as a function of time.

In 2023, we are planning to realize an ideal Y surface
by optimizing the sputtering procedure using the Ar
ion beam and by annealing the Y foil to reconstruct
the surface structure. Consequently, this will allow themore eﬃcient release of Fr on the surface at a minimum
heating temperature, which will enable us to maximize
the MOT eﬃciency of the Fr atoms.
References
1) A. D. Sakharov, Sov. Phys. Usp. 34, 392 (1991).
2) M. Popeslov et al., Annu. Phys. 318, 119 (2005).
3) N. Shitara et al., J. High Energy Phys. JHEP02, 124(2021).
4) M. P. Cox et al., Surf. Sci. 129, 375 (1983).