Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 48 (2015)

Study of particle identification and RI beam separation for the Z~80 region
using projectile fragmentation of 238U
N. Inabe,*1 N. Fukuda,*1 H. Takeda,*1 H. Suzuki,*1 Y. Shimizu,*1 D. Murai,*2 D. S. Ahn,*1 H. Sato,*1 Y. Sato, *1K. Kusaka,*1 Y. Yanagisawa,*1 M. Ohtake,*1 K. Yoshida,*1 M. Sako,*1 N. Imai,*3 S. Kimura,*3 M. Mukai,*3 H. Miyatake,*3N. Iwasa,*4 and T. Kubo*1
An atomic number of Z~80 produced through a
projectile fragmentation of 238U is the next area for RI beam
production at the BigRIPS.1) In this area, charge-state
distributions are broad, which causes some problems for
production concerned with both particle identification (PID)
and isotope separation. To study this effect, we performed
a test experiment to produce RI beams of high Z.
The test experiment was carried out using a 345 MeV/u238
U beam. To study PID, RI beams were produced by the238
U beam in low counting rates of ~2 x105 pps so as tomake PID easy by sometimes mixing the 238U primary beamof the charge state 92+ at F1. The production target was
5.79-mm-thick Be and the magnetic rigidity (B) of the first
dipole D1 was 6.001 Tm. Degraders were not used at F1and F5. The F1 slits were set to produce a momentum
byte in the range between -3% and 0.1% when the primarybeam was not mixed and between -3% and 0.5 % whenmixing it. The PID was performed by determining Z andA/Q on an event-by-event basis using the E-TOF-B
method. E was measured by MUSIC at F7 and TOF wasmeasured using two plastic scintillators at F3 and F7. B
was determined by track reconstruction using positions andangles measured by PPACs located at F3, F5, and F7.
Total kinetic energy (TKE) was also measured using by NaIat F7 to determine A.
Figure 1a and b show the PID spectra of A/Q vs. Z for Z> 30 and Z > 85, respectively. We can see not only
projectile fragments but also fission fragments below Z~70in Fig. 1a. Fig. 1c shows the similar plot as Fig. 1b when
primary beams are mixed. We can clearly see four charge
states of 238U, which are produced by changing the charge
state from 92+ in the scintillator at F3. Resolutions of Z
are 0.58 % for Z~90 and 0.41 % for Z~50. The worse
resolution in the high-Z area might be attributed to charge
state straggling, which broadens energy loss spectra when
passing through a detector with some charge states.
We performed the PID of U isotopes. Fig. 2 shows anA/Q spectrum of U produced by cutting with Z= 92±0.3.
We identified the main peaks of A/Q > 2.55 but could not
assign those of low A/Q because of overlap with isotopes ofZ=91. The resolution of A/Q is 0.05%. We tried to examine
whether other small peaks exist in large peaks, for example,
233 89+U in the 236U90+ peak but we could not determine thisbecause of the quenching of the TKE counter.
In order to study isotope separation, we produced RI
beam of which the central orbit was 226Bi82+ with the7-mm-thick Be target. Bof the first dipole was 6.13 Tm.
The degrader at F1 of 0.3 mm and momentum acceptanceof ±3% were used to produce many types of isotopes atthe same time such as a search for new isotopes. Theisotopes were not well-separated in this experiment owing
to large unexpected contaminants of fission fragments with
Z~60 and high counting rates of ~100 kHz/pnA. Thesephenomena have not been predicted by the simulation.

Fig. 1aZ vs. A/Q plot (Z>30)

Fig. 1b Z vs. A/Q plot (Z>85)

Fig. 1c Z vs. A/Q plot on mixing with the primary beam.

Fig.2 A/Q spectrum of U isotopes
Reference1) T. Kubo: Nucl. Instr. Meth. B 204, 97 (2003).

RIKEN Nishina Center＊2
Department of Physics, Rikkyo University
＊3
KEK
＊4
Department of Physics, University of Tohoku＊1- 73 -完全版2014_本文.indd 73

15/10/16 17:44