II-9. Instrumentation
RIKEN
Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Development of 216 Th and 220 Th beams at the BigRIPS separator
N. Fukuda,∗1 H. Suzuki,∗1 Y. Shimizu,∗1 H. Takeda,∗1 J. Tanaka,∗1 and K. Yoshida∗1We report the status of the development of 216 Th andTh beams that are to be used in the recently proposed
nuclear-reaction experiment at SAMURAI. The goal ofthis development is to determine the BigRIPS separator
settings that completely meet the requirements for thebeams used in the experiment.
The Th (216 Th or 220 Th) beam is produced by meansof the projectile fragmentation of a 345 MeV/nucleon
238
U beam and separated using the BigRIPS separator. The essential requirements for the beam are asfollows:
220(1) Th-beam rate ≥104 Hz(2) Total beam rate at BigRIPS-F7 ≤ 5 × 104 Hz(3) Beam energy ≥250 MeV/nucleon

In order to meet the requirments of (1) and (2) simultaneously, the purity of the Th beam must be 20%
or heigher. Meanwhile, to achieve a higher beam energy (requirement (3)), the thicknesses of the production target and degrader must be small, which could result in insuﬃcient isotope separation and thereby makeit diﬃcult to obtain high purity. Another concern isthe particle identiﬁcation (PID) of heavy fragments.
In BigRIPS, in-ﬂight PID based on the TOF-Bρ-∆Emethod1) has successfully been performed for heavy
fragments with Z = 82–90 and beam energies of approximately 200 MeV/nucleon.2) However, for fragments withhigher beam energy (≥250 MeV/nucleon in the presentcase), PID might be more diﬃcult because of the deterioration in Z resolution caused by possible energy-loss
straggling due to charge-state ﬂuctuations.3)
A test of the Th-beam production was conducted asa machine study (MS-EXP20-02) in November 2020. In
consideration of the limited beam time of 12 h, we focused on the evaluation of PID performance and theinvestigation of contaminants. The RI beams around220
Th were produced by the projectile fragmentation ofa 238 U beam impinging on a 1-mm-thick beryllium target. The setting of the BigRIPS separator was nearly
optimized for the production of a 220 Th beam, in which
the magnetic rigidity Bρ values at D1, D2, and the second stage (D3–D6) of BigRIPS were tuned for He-like,
H-like, and He-like 220 Th ions, respectively, to remove
huge contaminants from large ﬁssion-fragment yields.
A 1-mm-thick aluminum degrader was installed at F5,
while there was no suitable (suﬃciently thin) degrader
installed at F1. Instead, the parallel-plate avalanche(PPAC) detector at F1 served as a charge-exchange foil
to reduce the amount of contaminants.
Figure 1 shows the Z vs. A/Q PID plot for fragments
produced (a) without and (b) with the F5 degrader. The
A/Q values of the fragments were deduced under theassumption that the charge states of the ions did not∗1RIKEN Nishina Center

Fig. 1. Particle identification plot of Z vs. A/Q for fragments
produced in the 238 U+Be reaction a) without energy degraders and b) with an energy degrader (1-mm-thick Al)
was used at F5. The red solid circle indicates the expected location of 220 Th88+ (A/Q = 2.5, Z = 90).

change at F5. The relative A/Q resolution is evaluatedto be 0.08% in 1σ. No signiﬁcant amounts of contaminants were observed in either setting. In the presence ofthe F5 degrader, fragments in the region of Z = 85–95
were extracted as intended, showing that a thickness of
1 mm is suﬃcient for the aluminum degrader. The expected location of 220 Th88+ is indicated by the red solid
circle, in which no blobs of isotopes are found in each setting. This is probably because most of the ions transmitted to F7 changed their charge state at F5, and consequently, their A/Q values could not be deduced correctly
under the present assumption. Therefore, charge-state
identiﬁcation based on accurate Bρ analysis is required
to achieve correct PID. It should also be noted that therelative Z resolution is as poor as 0.69% (1σ). Elaborate
data analysis is currently in progress.
References

1) N. Fukuda et al., Nucl. Instrum. Methods Phys. Res. B
317, 323 (2013).
2) T. Sumikama et al., Nucl. Instrum. Methods Phys. Res.
B 463, 237 (2020).
3) H. Weick et al., Nucl. Instrum. Methods Phys. Res. B
168, 164 (2000).

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