Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Charge-state determination for new isotopes near the proton drip-line
Y. Ohkoda,*1,*3 T. Kubo,*1 N. Fukuda,*1 N. Inabe,*1 H. Takeda,*1 D. Kameda,*1 H. Suzuki,*1 K. Yoshida,*1 K. Kusaka,*1K. Tanaka,*1 Y. Yanagisawa,*1 M. Ohtake,*1 H. Sato,*1 Y. Shimizu,*1 H. Baba,*1 M. Kurokawa,*1 K. Ieki,*1,*2 D. Murai,*1,*2N. Iwasa,*1,*3 A. Chiba,*1,*3 E. Ideguchi,*4 S. Go,*1,*5 R. Yokoyama,*5 T. Fujii,*5 D. Nishimura,*1,*6 H. Nishibata,*1,*7S. Momota,*1,*8 M. Lewitowicz,*9 G. DeFrance,*9 I. Celikovic,*9 K. Steiger,*10 O. B. Tarasov,*1,*11 D. Bazin,*1,*11
D. J. Morrissey,*1,*11 and B. M. Sherrill*1,*11
resolution by selecting the stropping range according to theB measurement or by using silicon detectors whose deadlayer thickness is significantly small.
We can select events of fully stripped isotopes from a Z
versus Z-Q plot, where Z-Q gives the number of electrons.
Figure 2 shows a Z versus A-2Q PID plot for the fully
stripped events (Z-Q = 0). Here, the Z value is obtained
from the E-TOF-B method, while A-2Q is calculatedusing the A and Q values obtained in the present work. Notethat for isotopes with A/Z ~ 2, the resolution of A-2Q iscomparable to the A/Q resolution achieved by theE-TOF-B method, because of the nature of error
propagation.
As shown in Fig. 2, the present TKE measurement
confirms the identification of four new isotopes, 81.82Mo and85,86
Ru, which we previously observed using theE-TOF-B method.2) This also confirms that the newisotopes are fully stripped.
0.3

Q resoluon(σ)

Particle identification (PID) based on the E-TOF-B
method, in which atomic number Z and mass-to-charge
ratio A/Q are calculated from measured energy loss (E),time of flight (TOF), and magnetic rigidity (B), does notwork well for isotopes whose A/Z value is close to an
integer number, such as 2 or 3. This is becausehydrogen-like and fully stripped events are very closely
located in a Z versus A/Q PID plot. In these cases,
measurement of total kinetic energy (TKE) is additionallyneeded to identify the charge state. We performed such TKE
measurement to calculate the charge state number Q for
medium heavy proton-rich isotopes with A/Z ~ 2.
The experiment was performed in December 2011 at
RIBF using a 124Xe beam at 345 MeV/nucleon. TheBigRIPS separator1) was used to separate and identify
produced isotopes, and was tuned for very proton-richisotopes with Z = 30-45. The PID based on the E-TOF-B
method was made at the second stage of the BigRIPSseparator.2) The TKE measurement was made using a stackof eleven 1-mm-thick silicon detectors, placed downstreamof the BigRIPS separator. The energy loss data from thesilicon detectors were added to calculate the TKE. We
calculated the A value from the TKE and TOF, and the Q
value from the A/Q value obtained by the E-TOF-B
method.
The relative resolution achieved in the TKE measurement
is 0.48% on average. The resulting Q resolution wascalculated to be  = 0.25 on average, which allows 4.0
separation for Q = 1 in charge-state identification plot. Weobserved the dependence of Q resolution on the stopping
range in the silicon stack detector. Figure 1 shows the Q
resolution as a function of the stopping range of the isotopes,
where some deterioration of the Q resolution is observed
around a certain value of stopping range. This can be
attributed to a thin dead layer on the surface of the silicondetectors. We expect that it is possible to improve the Q

81Zr
82Zr
83Zr
86Mo
85Mo
85Nb
84Nb
79Y
80Y
81Y
78Sr0.25
0.2
0.15
0.1
4,500

5,000

5,500

6,000

Stopping range in Si[μm]Fig.1 Charge state (Q) resolution as a function of the stopping range*1RIKEN Nishina Center
Department of Physics, Rikkyo University*3
Department of Physics, Tohoku University*4
Research Center for Nuclear Physics, Osaka University*5
Center for Nuclear Study, University of Tokyo*6
Department of Physics, Tokyo University of Science*7
Department of Physics. Osaka University*8
Kochi University of Technology*9Grand Accelerateur National d’Ions Lourds*10
Physik Department, Technische Universität München*11
National Superconducting Cyclotron Laboratory, MichiganState University*2Fig. 2 Z versus A-2Q particle identification plot for projectile fragments
produced in the reaction 124Xe+Be at 345 MeV/nucleon.

References

1) T. Kubo et al.: Nucl. Instr. and Meth. B 204, 97 (2003).
2) H. Suzuki et al.: Nucl. Instr. and Meth. B 317, 756 (2013).

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