II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Improved position resolution of the beam diagnostics detector for theRare-RI Ring
G. Hudson-Chang,∗1 S. Naimi,∗1 T. Moriguchi,∗2 S. Suzuki,∗2,∗3 M. Hayashi,∗2 N. Kaname,∗2 K. Tomita,∗2A. Yano,∗2 A. Ozawa,∗2 Y. Abe,∗4 D. Nagae,∗5 and the Rare-RI Ring collaboration
To measure masses of rare isotopes at RIBF using theRare-RI Ring, it is essential to increase the eﬃciency bybetter monitoring the beam condition at the injection
into the ring. Therefore, we are developing a large area
position-sensitive detector with a thin foil. The principleof the detector operation is based on secondary electrons(SEs) emitted from a thin foil when the beam passes
through. The SEs are ﬁrst accelerated by the acceleration grid and then reﬂected by the mirror grids potential(see Fig. 1). The beam position is inferred from SEs position measurement with the delay line MCP placed 90◦
from the foil. To achieve high position resolution the SEs
velocity spread should be minimized inside the detector
by simply increasing the acceleration voltage. However,increasing the voltage led to undesirable eﬀects such as
bending of the grid’s wires and electrical discharges. We
report here on the improvements introduced to minimize
these eﬀects. We tested diﬀerent versions of the detector during four experiments conducted at HIMAC witha 200 MeV/nucleon 84 Kr beam. The details of the experimental setup can be found eslewhere.1) The position
resolution as well as the accuracy of the detector for eachexperiment are shown in Fig. 1. In Table 1 are summarized the improvements introduced for each version of
the detector as well as the acceleration potential that
could be applied. The new wiring method for the grids
is based on the method developed in this Ref. 2). With
this method, the grids’ wires were tighter and higher
voltage could be applied resulting in improved resolution. By reducing the thickness of the spacer between
the mirrors and acceleration grids, higher voltage up to
7 kV could be applied. This could be understood in thecontext of the Paschen Curve that related the productof the gas pressure and distance between the grids tothe discharge voltage. By a systematic study of diﬀerent spacer sizes we deduced that reducing the spacer size
was necessary for the same vacuum condition. The reso-lution achieved in this condition was the best resolution
ever achieved for large area detector of this type. We
also tested the detector with a larger pitch size (2 mm)
for a better transparency of the detector. However, it
showed the worst performance. To improve further theposition resolution we are planning to test a compact
version of the same detector.
Acceleration spacer

Mirror grids

Thin foilYXSEsZBeam
Mirror spacer

Accelerationgrid
MCP plates(Not to scale)

Delay Line

Table 1. Condition for each experiment (see text)

New wiring method
Pitch of girds [mm]Mirr Spacer [mm]Acc Spacer [mm]Acc. Potential [kV]
∗1
∗2
∗3
∗4
∗5Jul.
19X
18103Nov.
19O
28104Oct.
20(1)O
18104Oct.
20(2)
O
1687

RIKEN Nishina Center
Department of Physics, University of Tsukuba
IMP, CAS
National Institute of Radiological Sciences (NIRS), QST
Faculty of Sciences, Kyushu UniversityFig. 1. (top) Schematic side-view of the position-sensitive
detector. (bottom) Detector resolution (A&B) and accuracy (C&D) in the horizontal X and vertical Y position from four HIMAC experiments. The accuracy is deﬁned as deviation from expected position. For Oct20(2),the accuracy is not plotted for the Y because a diﬀerent
method was used to optimize the voltage and accuracy
could not be estimated.

References

1) R. Crane et al., RIKEN Accel. Prog. Rep. 53, 117 (2019).
2) O. K. Yoon et al., J. Am. Soc. Mass Spectrom. 18, 1901
(2007).
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