II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Development of a forward detector for the measurement of the meansquare radius of the neutron distribution of unstable nuclei by electronscattering
H. Wauke,∗1,∗2 A. Enokizono,∗1 T. Suda,∗1,∗2 K. Tsukada,∗1,∗3 T. Ohnishi,∗1 M. Wakasugi,∗1,∗3 M. Watanabe∗1
In nuclear physics, the nucleon density distribution isan essential physical quantity because it directly reﬂects
the wave function of nucleons. Electron scattering provides detailed information of the nucleus’s charge density distribution, dominated by the proton distribution.
However, it is diﬃcult to determine the neutron distribution by electron scattering because its contribution to
the charge density distribution is less than a few percent.
It was recently suggested that the mean square radius (msr) of the neutron distribution could be accessedby measuring the fourth–order moment of the charge
density distribution.1,2) The charge form factor deduced
from elastic electron scattering at the low momentum
2transfer can be Taylor-expanded as F (q) ∼ 1− <r6>c q 2 +<r 4 >c 4<r 6 >c 6
120 q − 5040 q · · · , where F is the form factor, q is
the momentum transfer, and < rn >c is the nth-ordermoments of the charge density distribution. Figure 1
shows the q 2 dependence of the 132 Xe form factor andthe contribution of each term of the above equation. The
msr of the neutron distribution measurement of unstable
nuclei by electron scattering will be obtained in a region
that covers the forward scattering angle (12–25◦ for theelectron beam energy Ee = 150 MeV). In the low momentum transfer region (less than 0.09 fm−2 in the case of Xe
isotopes), the form factor is sensitive almost to only thesecond- and fourth-order moments of the charge density
distribution.
The study of the msr of the neutron distribution of Xe

Fig. 1. q 2 dependence of the 132 Xe form factor obtained by
the past experiment.4) The black line shows the form factor for which all moments are calculated. The red andblue lines show the form factor up to the second- andfourth-order moment of the charge density distribution,respectively.
∗1∗2∗3RIKEN Nishina Center
ELPH, Tohoku University
Institute for Chemical Research, Kyoto University

SCRIT device(500 mm)

Vertex in the beam direction [mm]Fig. 2. Reconstructed vertex distribution of electron scattering events from the target region (blue line). The red line
shows the events where the scattered electrons have kinetic energies higher than 100 MeV. The events at −450
to −200 mm are from the material upstream of the target
region. The blue line shows events containing low energy.

isotopes (stable : 124–136 Xe, unstable : 138, 140 Xe) by electron scattering is under consideration at the SCRIT facility.3) A forward detector for measuring the fourth-ordermoment of the charge density distribution is under discussion. A series of background studies were performed
using an alternative detector to investigate the number ofbackground events at the new setup for the measurementin the low momentum transfer region. This is because it
may not be possible for the forward detector to distinguish between the elastic scattered electrons signals fromthe target nuclei and the background signal if there are
more or similar amounts of background events than thetrue one in the target region (length is 500 mm). The
background study was performed with a calorimeter array of seven CsI crystals and two drift chambers, which
covered a scattering angle of 18 ± 1◦ from the center of
the SCRIT device and measured the scattered electronsenergy Ee′ by summing energy deposits of CsIs.
Figure 2 shows the reconstructed vertex distributionof electron scattering events from the target region. The
red line in Fig. 2 denotes those of Ee′ > 100 MeV and is
identiﬁed as the elastic events. These events at −450 to−200 mm are considered as the elastic scattering eventsof the electron beam halo from the material upstream ofthe target region. The forward detector cannot separate
these events from the vertex distribution of the target
nuclei. It needs to consider approaches to reduce background events, for example, by reducing the material
around the SCRIT device.
References

1) H. Kurasawa, T. Suzuki, Prog. Theor. Exp. Phys. 2019,
113D01 (2019).
2) H. Kurasawa, T. Suda, T. Suzuki, in this report.
3) M. Wakasugi et al., Nucl. Instrum. Methods Phys. Res. B
317, 668 (2013).
4) K. Tsukada et al., Phys. Rev. Lett. 118, 262501 (2017).

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