Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

First result of elastic electron scattering from 132 Xeat the SCRIT facility†
K. Tsukada,∗1,∗2 A. Enokizono,∗2 T. Ohnishi,∗2 K. Adachi,∗3 T. Fujita,∗3 M. Hara,∗2 M. Hori,∗3 T. Hori,∗2S. Ichikawa,∗2 K. Kurita,∗3 K. Matsuda,∗1,∗2 T. Suda,∗1,∗2 T. Tamae,∗1,∗2 M. Togasaki,∗3 N. Uchida,∗3M. Wakasugi,∗2 M. Watanabe,∗2 and K. Yamada∗3100

10 210-1
10-2100(c)0-200 -100 0 100 200Vertex position on the beam [mm]Fig. 1. Reconstructed vertex distributions of scattered electrons. Panels (a) and (c) show the vertex point distributions with and without the target ions. Panel (b)shows the scatter plot of the vertex point distribution
with the target ions.

∗2
∗3

0.8

1.0

1.2

1.4

q eff [fm -1]

This work demonstrates that the SCRIT techniqueenables us to perform electron scattering experimentfor unstable nuclear targets. The RI production forexperiments on unstable nuclei has already started.2)
Electron scattering oﬀ short-lived unstable nuclei will
be realized in near future.

Figure 2 shows the diﬀerential cross sections multiplied by luminosity for elastic electron scattering. Bychanging the electron beam energies, a wide range ofmomentum transfer can be covered. The lines in Fig. 2
represent the elastic scattering cross sections calculated by a phase shift calculation code DREPHA,4)
assuming nuclear charge density distributions. In thepresent analysis, a two-parameter Fermi distribution,
ρ(r) = ρ0 /(1 + exp (4 ln 3(r − c)/t)), is assumed to∗10.6

Fig. 2. Diﬀerential cross sections multiplied by luminosity, versus eﬀective momentum transfer. The lines represent the results of phase shift calculations assuming nuclear charge density distributions obtained by
the two-parameter Fermi distribution (black solid line),
the Hartree-Fock + phenomenological calculation (reddashed line),6) and the beyond-relativistic-mean-ﬁeldtheory (blue dotted line).7) The parameters of the twoparameter Fermi distribution are the best values evaluated in this analysis.

-20†Xe target

10 010-30.4

101(b)
20Exp. ( Ee = 151 MeV)
Exp. ( Ee = 201 MeV)
Exp. ( Ee = 301 MeV)
2-param. Ferm i
LapikasMei

132

10 1(a)10
Vertical position [mm]extract the nuclear charge distribution. The luminosity is also considered as a ﬁtting parameter, because the study of the luminosity monitor (LMon)5)
to determine the absolute value of luminosity is under way. The achieved luminosity is evaluated tobe around 1×1027 cm−2 s−1 on average. From thisanalysis, the most probable values (c = 5.42+0.11−0.08 fm,⟨ 2 ⟩1/2+0.29+0.12= 4.79−0.10 fm) are obt = 2.71−0.38 fm, and rtained.
L d  [(cm -2s-1)(cm 2sr -1)]dFor the ﬁrst time, nuclear charge density distribution was extracted from only 108 nuclei target (132 Xe)at the SCRIT (Self-Conﬁning Radioactive Isotope Target) electron scattering facility,1,2) which has been constructed at RIKEN to realize electron scattering oﬀ
unstable nuclei. The data were taken in 2016, and reported in the previous report.3)
Figures 1(a)–(c) show the reconstructed vertex distributions of scattered electrons along the beam andat vertical positions. The background contributions
from residual gases are shown in Figs. 1(a) and (c)as blue histograms. It was found that the target ionswere clearly trapped in the SCRIT by the transverse
focusing force given by the electron beam itself andthe electrostatic potential well provided by both endsof the electrodes.

References
1) M. Wakasugi et al., Nucl. Instrum. Methods B 317, 668(2013).
2) T. Ohnishi et al., Physca Scripta, T166 014071 (2015).
3) K. Tsukada et al., RIKEN Accel. Prog. Rep. 50, 11(2016).
4) B. Drepher et al., a phase-shift calculation code for elastic electron scattering, communicated by J. Friedrich.
5) T. Fujita et al., RIKEN Accel. Prog. Rep. 50, 188(2016).
6) L. Lapikas, HERMES Internal Report No. 04–41.
7) H. Mei, K. Hagino, private communication.

Condensed from the article in Phys. Rev. Lett. 118, 262501(2017)RIKEN Nishina Center
Research Center for Electron-Photon Science, Tohoku University
Department of Physics, Rikkyo University- 65 -