II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Commissioning study of SCRIT facility with 138 Ba nucleus
K. Tsukada,∗1,∗2 Y. Abe,∗2 A. Enokizono,∗2 T. Goke,∗3 Y. Honda,∗2,∗3 Y. Ito,∗1 K. Kurita,∗4 Y. Maehara,∗1T. Ohnishi,∗2 R. Ogawara,∗1,∗2 T. Suda,∗2,∗3 M. Watanabe,∗2 M. Wakasugi,∗1,∗2 and H. Wauke∗2,∗3The Self-Conﬁning RI Ion Target (SCRIT) electron scattering facility was constructed to realize electron scattering experiments for short-lived unstablenuclei.1) SCRIT uses a novel technique to achieve aluminosity of 1027 cm−2 s−1 , which is suﬃcient to perform elastic electron scattering experiments by trapping a small number of target ions, typically 108 particles/pulse, along the electron beam.
In 2021, we performed electron scattering from 138 Ba
for two electron beam energies, 150 and 300 MeV, asa commissioning experiment. 138 Ba is stable, and its
charge density distribution is known.2) This is the ﬁrst
time experiment with the gas target trapped in theSCRIT since the present SCRIT device was installed
in 2020, and a drift chamber of the electron spectrometer was replaced with a new one in 2018. The present
SCRIT device has a larger cross-section and less material around the trapping region, and it provides muchbetter uniformity of the electric potential compared tothe previous device. The present drift chamber in frontof the spectrometer magnet provides both vertical andhorizontal information of the trajectories of scatteredelectrons, whereas the previous one provided only horizontal information.
The data for 150 MeV is analyzed in this report.
The beam current was 250 mA at the beginning of
the data acquisition and dropped to 150 mA at theend. The achieved luminosity was approximately 1 ×
1027 cm−2 s−1 on average.
Figure 1 shows the reconstructed vertex-point distribution along the beam line after the spectrometer acceptance correction. It is found that the target length
in the SCRIT device is about 300 mm and the distribution is almost ﬂat. Compared to the previous data,3)
the apparent bump structure caused by the distortionof the electric potential disappears, which makes thedata easier to analyze and interpret.
Figure 2 shows the yield distribution after the acceptance correction and background subtraction. The
solid line represents a calculation using a phase-shift
calculation code, DREPHA,4) with the charge-density
distribution of 138 Ba.2) The distribution of our data
is well reproduced by the calculation, except for theforemost point, which corresponds to the end of theacceptance.
In summary, the target ions are trapped in thepresent SCRIT device almost uniformly as expected,∗1∗2∗3∗4Fig. 1. Reconstructed vertex-point distribution along thebeam line after acceptance correction.

Fig. 2. Yield distribution with varying eﬀective momentum
transfer after acceptance corrections. The solid line is aphase-shift calculation obtained using a charge-density
distribution of 138 Ba.2)

and the angular distribution is well reproduced by ourspectrometer. We are almost ready and will soon start
the world’s ﬁrst electron scattering experiment with an
unstable nuclear target.
References
1) M. Wakasugi et al., Nucl. Instrum. Methods Phys. Res.
317, 357 (2013).
2) H. de Vries et al., At. Data Nulc. Data Table 36, 495(1987).
3) K. Tsukada et al., Phys. Rev. Lett. 118, 262501 (2017).
4) B. Drepher, A phase-shift calculation code for elastic electron scattering (private communication withJ. Friedrich).

Institute for Chemical Research, Kyoto University
RIKEN Nishina Center
Research Center for Electron-Photon Science, Tohoku University
Department of Physics, Rikkyo University- 24 -1