Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 49 (2016)

Development of Particle identification method of high-intensity
secondary beams at BigRIPS
S. Ota,∗1 J. Zenihiro,∗2 H. Takeda,∗2 S. Michimasa,∗1 M. Dozono,∗1 M. Matsushita,∗1 K. Kisamori,∗1M. Kobayashi,∗1 Y.N. Watanabe,∗3 Y. Kiyokawa,∗1 Y. Yamaguchi,∗1 S. Masuoka,∗1 S. Shimoura,∗1 K. Yako,∗1N. Imai,∗1 H. Baba,∗2 T. Kubo,∗2 K. Yoshida,∗2 N. Inabe,∗2 N. Fukuda,∗2 H. Suzuki,∗2 D.S. Ahn,∗2 D. Murai,∗2H. Sato,∗2 Y. Sato,∗2 H. Otsu,∗2 and T. Uesaka∗2Recent development of accelerators enables us toutilize high-intensity primary beams and resultant
high-intensity secondary beams at RIBF. Available
intensity of the secondary beam becomes more than
105 cps at BigRIPS. However, when we apply the standard particle identiﬁcation method of TOF-Bρ-∆E forsuch high-intensity beams, in particular around Z=50
beams, two problems arise.
One serious problem is that the resolution of ∆E
becomes worse due to the pile up of the slow signal
from the ionization chamber, thereby reducing the resolution of atomic number. The other problem is theradiation damage of plastic scintillators that are used
to measure the TOF. In addition to these problems,
the PPAC had larger discharge probability with thehigh-intensity beams than that with the low-intensity
beams. In this paper, we report a new particle identiﬁcation method using detectors with high radiation
hardness and fast time response at BigRIPS, in orderto resolve this problem and identify the high-intensity
secondary beams.
We propose the new particle identiﬁcation methodof TOF-Bρ-Bρ, where ∆E is not measured directly.
An energy degrader is placed at the dispersive second
focus (F5) and the energy loss at the degrader is indirectly used for the extraction of atomic number. Assuming that mass number and charge do not change
at the degrader, the energy loss at F5 is expressed byBρ5735∆E = (γ35 −1)Amu −(γ57 −1)Amu = cZ( Bρβ35 − β57 ).
Combining this expression with the Bethe-Bloch for2
mula for the energy loss, ∆E ∼ d βZ2 , where d is the35thickness of the degrader, and assuming that the ion is
fully stripped, namely Q = Z, the atomic number canbe obtained as,
Z = C(2Bρ35Bρ57 β35.
−)
β35
β57dFig. 1.: Correlation between the atomic numbers deduced using the new method (Zdeg) and the standard
method (Zic).

(a) 104 cps.

Fig. 2.: Correlation between the atomic number andthe mass-to-charge ratio.

correlation between the atomic numbers deduced using the new method and the standard method, which
are shown by the horizontal and vertical axes, respectively. There is a clear correspondence of two values,
which validates that the new method extracts the correct atomic number. Figure 2(a) shows the plot forparticle identiﬁcation with a low-intensity beam (lessthan 103 cps). Vertical and horizontal axes show theatomic number and mass-to-charge ratio, respectively.
Figure 2(b) shows the same plot with the high-intensity
beams (about 106 cps). Eﬃciency seems worse for thehigh-intensity beams, which may be caused by wrong
position deduction of LP-MWDC, which will be recovered by further analysis.

(1)

In order to overcome the diﬃculty of radiation damage and discharge, plastic scintillators and PPACs are
replaced with diamond detectors1) and low-pressure
multi-wire drift chambers2) (LP-MWDC), respectively.
The test experiment was performed using a 132 Sn
beam at incident energies of 100 and 200 MeV/u.
The typical intensity was 1×106 cps and the maximum intensity was 3.4×106 cps. Figure 1 shows the∗1∗2∗3(b) 106 cps.

References
1) S. Michimasa et al., Nucl. Instrum. Meth. B317 (2013)710.
2) H. Miya et al., Nucl. Instrum. Meth. B317 (2013) 701.

Center for Nuclear Study, The University of Tokyo
RIKEN Nishina Center
Department of Physics, The University of Tokyo- 15 -