RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEAR

Identification and separation of radioactive isotope beams by theBigRIPS separator at the RIKEN RI Beam Factory†N. Fukuda,∗1 T. Kubo,∗1 T. Ohnishi,∗1 N. Inabe,∗1 H. Takeda,∗1 D. Kameda,∗1 and H. Suzuki1
We have developed a method for achieving excellent
resolving power for in-flight particle identification ofradioactive isotope (RI) beams, which is routinely usedwith the BigRIPS fragment separator1) at the RIKEN
RI Beam Factory (RIBF)2) . In the BigRIPS separator, RI beams are identified by their atomic number Z
and mass-to-charge ratio A/Q, which are in turn deduced from the measurements of time of flight (TOF),
magnetic rigidity (Bρ), and energy loss (∆E). Suchin-flight particle identification is essential for delivering tagged RI beams, making it possible to perform
various types of experiments including secondary reaction experiments. Since the total kinetic energy is notmeasured in this scheme, and consequently A and Q
cannot be determined independently, the resolution ofA/Q must be adequately high to identify the charge
state Q of RI beams. This is achieved in the Z versus A/Q particle identification plot as demonstrated
in Fig. 1, where fully stripped and hydrogen-like peaks
are very closely located.
We achieved a high A/Q resolution by precisely determing the Bρ and TOF values. Precise Bρ was determined by the trajectory reconstruction method forwhich ion-optical transfer matrix elements were experimentally determined up to the third-order. The significant improvement in A/Q resolution by our trajectory reconstruction technique is clearly seen in Fig. 2,
where comparion of the A/Q resolution among threedifferent transfer matrix elements in the trajectory reconstruction is shown. Precise TOF was determinedby the slew correction method for TOF signals. We iteratively carried out the derivation of transfer matrix
elements and slew correction such that the A/Q resolution was best optimized. Furthermore we completelyremoved background events to enhance the reliabilityof particle identification.
The excellent particle identification thus achieved
allows us to supply tagged RI beams to a variety ofexperiments at RIBF. Furthermore it helps us to reliably identify new isotopes from a very small number ofevents. Such enhanced capability of the BigRIPS separator is significantly advancing the research on exoticnuclei at RIBF.

58

Z

Te51+
142
Te52+

105
104

52

103

50

102

48

1046
44
2.55 2.6 2.65 2.7 2.75 2.8

1A/Q

Fig. 1. Z versus A/Q particle identification plot for fission fragments produced in the 238 U + Pb reaction at345 MeV/nucleon. The experimental conditions andBigRIPS setting are given in the G3 Setting section inRef.3) .

108 (a) COSYσ A/Q = 0.064% for 135Sn50+
106
104
102
1108 (b) Experimental: 1st order 135 50+
σ A/Q = 0.050% for SnCounts

106104102

1order108 (c) Experimental: Up to 3rd135
σ A/Q = 0.039% for Sn50+

106

Sn49+
138
Sn50+

135

104
102
12.55

2.6

2.65

2.7

2.75

2.8

A/Q

References

∗1139

A-1 A

54

Fig. 2. Comparison of the A/Q resolution among threedifferent transfer matrix elements used in trajectoryreconstruction. The comparison is shown for Sn isotopes produced by in-flight fission of a 238 U beam at345 MeV/nucleon. The experimental conditions andBigRIPS settings are the same as those in Fig. 1.

1) T. Kubo: Nucl. Instr. Meth. B 204, 97 (2003).
2) Y. Yano: Nucl. Instr. Meth. B 261, 1009 (2007).
3) T. Ohnishi et al.: J. Phys. Soc. Jpn. 79, 073201 (2010).
†A+3

56

Condensed from the article in Nucl. Instr. Meth. B 317, 323(2013)RIKEN Nishina Center-v-