II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Degrader optimization for ZeroDegree gas cellFig. 1. Sketch of the ZeroDegree gas cell and degrader system.
∗1
∗2
∗3
∗4
∗5∗6
∗7
∗8
∗9Department of Physics, The University of Hong Kong
Wako Nuclear Science Center (WNSC), IPNS, KEK
Institute of Modern Physics, Chinese Academy of Science
RIKEN Nishina Center
Department of Physics, Osaka University
Advanced Science Research Center, JAEA
Department of Physics, Kyushu University
Department of Chemistry and Biochemistry, New Mexico
State University
Institute of Mass Spectrometry and Atmospheric Environment, Jinan University

100LISEunstopped in He gas
stopped in He gas

50x5030x5405085GeZAs

8540As85

33Ge320.230312.55

2.6

2.65

A/Q

0.1

20EXP

10rate @ MRTOF (cps)

3485

0.3

pass-through fraction

Using a high-quality beam provided by BigRIPS andthe ZeroDegree spectrometer, a system combining a gas
cell and a multi-reﬂection time-of-ﬂight (MRTOF) mass
spectrograph was set up at F11 to study the massesof exotic nuclei.1) The ﬁrst online commissioning wasconducted in the winter of 2020 as parasitic experiments during the HiCARI campaign. To achieve themass measurement in the MRTOF device, the radioactive beams received from ZeroDegree must be stopped
in the gas cell, which is 500-mm long and ﬁlled withHe gas. The typical beam energy behind ZeroDegree is100–200 MeV/nucleon. To stop such high-energy beams
in the He gas, a degrader is essential, and its thickness
must be precisely adjusted. Therefore, a movable degrader system was introduced.
Figure 1 shows a sketch of the gas cell with the degrader system. The degrader system contains a rotational ﬂat degrader placed in front of the gas cell and adownstream silicon detector array inside the outer chamber. The ﬂat degrader was coupled to a step motor,
which can rotate the degrader plate from 0◦ to 55◦ insteps of 0.0072◦ ; therefore, it can ﬁnely adjust the effective thickness of the degrader. The downstream silicon detectors contain 15 Si PIN photodiods s3204-09,
arranged in a 5 × 3 array to cover the full range of thebeam spot. After losing most of its energy in the ﬂatdegrader, part of the beam is stopped in the He gas,
while the rest deposits energy in the Si detectors. The
signal from the Si detectors was acquired in coincidencewith the beam-line detectors of ZeroDegree for particle

rate (arb. units)

S. Chen,∗1,∗2 Y. Hirayama,∗2 D. Hou,∗3,∗2 S. Iimura,∗4,∗5 H. Ishiyama,∗4 Y. Ito,∗6 S. Kimura,∗4 J. Liu,∗3H. Miyatake,∗2 S. Nishimura,∗4 T. Niwase,∗7,∗4,∗2 M. Rosenbusch,∗2 P. Schury,∗2 A. Takamine,∗4 M. Wada,∗2Y. X. Watanabe,∗2 H. Wollnik,∗8 W. Xian,∗1,∗2 and S. Yan∗9,∗2x100

0304050

Degrader Angle (degree)

Fig. 2. Degrader optimization for 85 Ge (solid) and 85 As(dash). The black curve shows the ratio of the rate detected by an Si detector to the rate at F11. Only one Si
detector was used in the analysis. The red curve is therate of isotope identiﬁcation in the mass spectrum. The
inset shows the ZeroDegree particle identiﬁcation. The
upper panel displays the corresponding lise calculations
using the beam energy measured in ZeroDegree.

identiﬁcation; therefore, optimization can be performed
for speciﬁc isotopes. The stopping in He gas can be optimized according to the beam energy loss measured by
the Si detectors2) or simply using the beam rate fromthe Si detectors.
Figure 2 demonstrates the degrader optimization witha 2.5-mm-thick Al ﬂat degrader for 85 Ge and 85 As during the commissioning run. The fractions (black), calculated as the beam rate detected in the Si detectors
compared with the rate at F11, decrease with increasing degrader angle. The rates of isotopes observed in
the MRTOF mass spectrum (red) show peak structures,from which we can determine the optimized degrader
angle for each isotope. The lise calculations well reproduce the measured pattern; however, discrepancies
in the absolute angles are observed, which emphasize
the importance of such direct measurement in experiments. For low-intensity isotopes (rate in MRTOF as
low as 1 count/h), the direct measurement of the rate
curve in MRTOF is not feasible. In this case, the fraction curve from Si can still be measured. Combined withthe lise calculation, the optimized degrader angle can be
determined in future experiments.
References

1) M. Wada, RIKEN Proposal NP1912-RIBF176.
2) J. Liu, RIKEN Accel. Prog. Rep. 53, 133 (2020).

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