Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

SCRIT electron spectrometer (II)T. Suda,∗1,∗2 A. Enokizono ,∗1,∗3 S. Matsuo ,∗1,∗3 T. Miyamoto ,∗1,∗2 S. Wang,∗1,∗4 S. Yoneyama,∗1,∗2T. Tsuru,∗1,∗2 and T. Tamae∗1,∗2The SCRIT electron spectrometer is being constructed at the SCRIT electron scattering facility1,2) .
In this report, the current status of the construction
is described. The spectrometer, shown in Fig. 1, will
be used to measure the cross section for elastic electron scattering oﬀ short-lived nuclei to determine theircharge-density distributions3) . It consists of a dipole
magnet, two drift chambers, and a pair of plastic scintillators. The drift chambers sandwich the magnet
for trajectory measurements, and the plastic scintillators trigger data acquisition. Knowing the detailed
magnetic-ﬁeld distribution, the momenta, scattering
angles, and scattering positions of the electrons are determined. The spectrometer should have large acceptance and a good momentum resolution of the order of
10−3 to identify the elastic scattering events.
12Detectors

A large drift chamber, which is placed in the rearside of the magnet, was constructed in 2013. The small
drift chamber used in the previous SCRIT studies will
be used as the front drift chamber. A new readout system, RINEI RP 1212N, which digitizes the drift times
on board is currently being tested5) . A pair of large
plastic scintillators of dimensions 220 cm (length) ×30 cm (width) × 2 cm (thickness) are used for detecting the scattered electrons, and their coincidence triggers data acquisition. To reduce non-negligible false
triggers due to cosmic rays, veto detectors will be arranged. The drift chambers and the plastic scintillators
will be ready for use in the ﬁrst half of 2014.
3

Spectrometer commissioning

Magnet

The window-frame magnet, the gap region of which
is 170 cm (width) × 140 cm (length) × 29 cm (height),
is employed. The ﬁeld cramps were carefully designed
to reduce the fringing ﬁeld down to a few gauss at theelectron beam position. Magnetic-ﬁeld measurements
have been performed, and the detailed ﬁeld distribution was obtained4) . The magnet is palced on a movable platform, as shown in Fig. 2, such that the magnet can be moved away from the beam line by 1.5 m.
The re-positioning accuracy was conﬁrmed to be better than 50 µm. The magnet system will be ready for
operation immediately after the power line and coolingwater are set up; this is expected to be completed in
the ﬁrst half of 2014.

The SCRIT electron spectrometer will be commissioned using the W-wire target installed inside theSCRIT chamber in the second half of 2014. Using
the scattered electrons from the W wire target, one
can determine the track-reconstruction eﬃciency, momentum resolution, etc. After commissioning, we willimmediately conduct experiments on electron scattering oﬀ a short-lived nucleus, which will be 132 Sn . Theworld ʼs ﬁrst observation of electrons scattered oﬀ an
exotic nucleus is expected to take place in the ﬁscal
year 2014.

Fig. 2. The magnet with the movable platform.

References
Fig. 1. The SCRIT electron spectrometer.
∗1
∗2
∗3
∗4

RIKEN Nishina Center
Research Center for Electron Photon Science, Tohoku University
Department of Physics, Rikkyo University
School of Space Science and Physics, Shandong University,
China1) T. Adachi et al.: Accel. Prog. Rep. 45, 144 (2012).
2) T. Suda et al.: Accel. Prog. Rep. 46, 144 (2013).
3) T. Suda et al.: Prog. Theor. Exp. Phys. 03C, 008(2012).
4) T. Tamae et al.: In this report.
5) S. Matsuo et al.: In this report.

- 195 -