Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 53

Large-acceptance achromatic mode with mono-energetic wedge
degrader for ZeroDegree spectrometer
S. Chen,∗1,∗2 H. Takeda,∗3 M. Wada,∗2 P. Doornenbal,∗3 D. S. Ahn,∗3 N. Fukuda,∗3 S. Iimura,∗3 H. Ishiyama,∗3J. Liu,∗1,∗2 M. Rosenbusch,∗2 P. Schury,∗2 Y. Shimizu,∗3 H. Suzuki,∗3 A. Takamine,∗3 and K. Yoshida∗3









Fig. 1. (a) ZeroDegree setup for the LAAmono mode. (b)Energy distributions of 80 Zn before (left) and after (right)
the F10 wedge degrader.
∗1∗2∗3Department of Physics, The University of Hong Kong
KEK Wako Nuclear Science CenterRIKEN Nishina Center

LAA mode333332102

Z

3130

102930102928272.55

LAAmono mode

3210231

ZAn experiment was proposed to achieve the online
commissioning of the SLOWRI gas catcher and multireﬂection time-of-ﬂight (MRTOF) mass measurements
symbiotically with in-ﬂight experiments using the ZeroDegree spectrometer.1) A 50-cm-long cryogenic He gas
cell will be placed downstream of the F11 chamber toprovide low-energy radioactive isotopes from the energetic radioactive isotope (RI) beams of ZeroDegree. For
ZeroDegree, the typically used large-acceptance achromatic (LAA) mode provides beams with a large energy
spread at the F11 focal plane, resulting in a relatively
low stopping eﬃciency (∼1%) in the gas cell. In order to improve the stopping eﬃciency, mono-energetic
beams are prefered. Two ion optic settings were proposed to make such beams: the large-acceptance dispersive (LAD) mode2) and the LAA mode with a monoenergetic wedge degrader (LAAmono). The transmission eﬃciencies and particle identiﬁcation resolutions
were investigated for both modes. Herein, we report
the results of the LAAmono mode.
A test was performed with a typical experimental con-

1

2.6

2.65

A/Q

2.7

2.75

2.8

28
27
2.55

1

2.6

2.65

2.7

2.75

2.8

A/Q

Fig. 2. Particle identification plots from the LAA mode (left)and LAAmono mode (right). The 80 Zn isotopes are indicated by the red ellipse in both plots.

dition for an in-beam gamma-ray experiment. A combination of a 345 MeV/nucleon 238 U primary beam anda 4-mm-thick 9 Be target was used to produce radioactive beams around 79 Cu at ∼270 MeV/nucleon. BothBigRIPS and ZeroDegree were tuned for the 80 Zn isotopes. The LAA and LAAmono mode were applied inturns for the ZeroDegree spectrometer. In the LAAmono mode, an Al wedge degrader with an angle of
20.8 mrad and a center thickness of 2.85 mm was employed at the momentum-dispersive focal plane (F10) of
the standard LAA mode, as shown in Fig. 1(a). Theenergy distribution of 80 Zn isotopes after the wedge degrader [Fig. 1(b) right] shows a good mono-energetic
property. A stopping eﬃciency of ∼5% in the gas cell
was estimated from this distribution.
The particle identiﬁcation (PID) for both modes was
performed using the time of ﬂight from F8 to F11, reconstructed Bρ between F8 and F10, Bρ between F10
and F11, and ∆E measured using the F11 ionizationchamber. The PID plots from both modes are shownin Fig. 2. The one from the LAAmono mode exhibitsa resolution slightly worse than the one from the LAAmode, but the resolution was suﬃcient to separate all
the diﬀerent isotopes well. The transmission eﬃcienciesof ZeroDegree were studied by comparing the statisticsof the 80 Zn isotopes in BigRIPS and ZeroDegree PID
plots. The LAAmono mode keeps 95% eﬃciency in comparison with the standard LAA mode in this test experiment. This eﬃciency is consistent with the value estimated from LISE++ calculations, which suggests thatthe main loss originates from the reaction losses in thewedge degrader.
References

1) M. Wada, RIKEN Proposal NP1912-RIBF176.
2) H. Takeda, in this report.

- 100 -