Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 53

Development of ZeroDegree Large Acceptance Dispersive mode
H. Takeda,∗1 N. Fukuda,∗1 H. Suzuki,∗1 Y. Shimizu,∗1 D. S. Ahn,∗1 T. Komatsubara,∗1 N. Inabe,∗1 K. Yoshida,∗1S. Chen,∗2,∗3 M. Wada,∗3 A. Takamine,∗1 P. Doornenbal,∗1 and R. Tsunoda∗4
The ZeroDegree spectrometer1) is an RI-beam delivery
line following the BigRIPS separator.2) Four operating
modes are provided: two achromatic and two dispersivemodes with diﬀerent momentum resolutions and acceptances. Among them, the Large Acceptance Achromatic(LAA) mode is the standard one, and most experiments
have been conducted in this mode so far.
A parasitic experiment is proposed using ZeroDegree,where the online commissioning of a SLOWRI gas
catcher and a multi-reﬂection time-of-ﬂight mass spectrograph (MRTOF-MS) located at the ﬁnal focal plane
F11 will be performed symbiotically with other ZeroDegree experiments. For this purpose, mono-energetic
beams are desired to improve the stopping eﬃciency because the material thickness of a gas cell is very thin.
In order to realize such beams, a straight-forward solution is to employ a mono-energetic wedge degrader at thedispersive focal plane (F9 or F10) in the standard LAA
mode. However, transmission loss due to the degrader
may be a problem. Another option is to use a dispersive mode of ZeroDegree with a mono-energetic degrader
located immediately before the gas cell to minimize thetransmission loss. Both the previously prepared dispersive modes (MRD and HRD1) ) are undesirable because
they have a smaller acceptance compared to the LAA
mode as a result of aiming at a high momentum resolution. Thus, a new dispersive mode with a larger acceptance, named the Large Acceptance Dispersive (LAD)
mode, was developed. Figure 1 shows the ﬁrst-order ion
optics in the LAD mode of ZeroDegree. The horizontal acceptance and momentum resolution p/∆p are ±35
mrad and 2100, respectively, while they are ±20 mradand 4200 for the MRD mode and ±10 mrad and 6500
for the HRD mode, respectively.
A machine study with a 80 Zn beam produced by thein-ﬂight ﬁssion of a 238 U beam at 345 MeV/nucleon witha 4 mm-thick Be target was conducted to examine theFig. 1. First-order ion optics in the LAD mode of ZeroDegreein the horizontal (X) plane. Red, green, and blue curves
correspond to the momentum deviation δ = −1.5, 0, and1.5%, respectively.
∗1
∗2
∗3
∗4

RIKEN Nishina Center
Department of Physics, The University of Hong Kong
KEK Wako Nuclear Science Center
Center for Nuclear Study, the University of Tokyo

Table 1. Measured (Exp.) and designed (COSY) F8-F11
first-order transfer matrix elements in the LAD mode.
The units are mm, mrad, and % in x, a, and δ, respectively.
Exp.
COSY

(x|x)
1.02
1.0

(x|a)
0.27
0.0

(a|x)
−0.35
0.0

(a|a)
0.89
1.0

(x|δ)
20.3
21.0

(a|δ)
−5.49
0.0

Fig. 2. Z versus A/Q plot measured in the ZeroDegree LAD
mode. The 80 Zn beam is clearly separated.

LAD mode. The LAA mode with a mono-energetic degrader (LAAmono mode) was also tested3) in the samemachine study. We report the results for the LAD modehere.
The RI beam was identiﬁed and tagged4) in the BigRIPS separator and delivered to the ZeroDegree spectrometer. We ﬁrst measured the transfer matrix elements in the LAD mode. The results are summarizedin Table 1 and compared with the designed values calculated by COSY INFINITY. The main features are well
reproduced.
By using these matrix elements, particle identiﬁcation
was performed independently in the ZeroDegree spectrometer. The measured Z versus A/Q particle identiﬁcation plot is shown in Fig. 2. The A/Q resolution wasmeasured at narrow (±0.1%) and wide (±3%) momentum slit settings. The results were 0.15% and 0.17%
in σ, respectively. Higher-order aberration correction
is required to reach an expected value of 0.05% estimated from detector resolutions and ion-optical parameters. Analysis is in progress.
The transmission eﬃciency was also measured by comparing the number of 80 Zn with BigRIPS and ZeroDegree. The results were 85.5% (narrow) and 70.2%(wide). The transmission losses mainly occur at F10,
where the beam spreads for a large momentum deviation (δ) (see Fig. 1). The readjustment of ion optics isnecessary to reduce the beam spread.
References1) T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003 (2012).
2) T. Kubo, Nucl. Instrum. Methods Phys. Res. B 204, 97(2003).
3) S. Chen et al., in this report.
4) N. Fukuda et al., Nucl. Instrum. Methods Phys. Res. B
317, 323 (2013).

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