I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Investigation of a new production and separation technique for RI
beams at BigRIPS, “In-separator two-step method”
H. Suzuki,∗1 N. Fukuda,∗1 H. Takeda,∗1 Y. Shimizu,∗1 M. Yoshimoto,∗1 K. Yoshida,∗1 Y. Yanagisawa,∗1 andH. Sato∗1A new production and separation method of neutronrich medium-heavy radioactive-isotope (RI) beams “inseparator two-step method” was proposed, and its usefulness was investigated. The beam intensities of such
RIs produced by this new method are expected to behigher than those produced by the standard method, inﬂight ﬁssion of a 238 U beam, when its intensity reaches
the goal value of 2000 particle nA in the RIBF upgrade
project and RI-beam total rates are limited at 1 kHz bydata acquisition (DAQ).
In the standard method, as shown in Fig. 1(a), an
objective RI beam (e.g., 128 Pd) is produced by in-ﬂight
ﬁssion at the F0 Be target. When the magnetic rigidity(Bρ) of the ﬁrst dipole is set at the momentum peak of128
Pd to maximize its yield, the total rate of the secondary beams becomes too high for DAQ or detectors
even with the current 238 U-beam intensity of 80 particle
nA. To reduce the total rate below 1 kHz, the Bρ setting
should be higher than the momentum peak. The 128 Pdyield decreases with such adjustment; however, the total rates can be decreased more eﬃciently. When theprimary-beam intensity increases in the future, this Bρsetting shift should be larger. Thus, the 128 Pd yield will
increase only by a factor of 9, although the 238 U-beam
intensity will be 25 times larger than the current.
To produce the RI beams more eﬃciently within thetotal-rate limit, the in-separator two-step method was
proposed, as shown in Fig. 1(b). The Be target is divided
into two at F0 and F2 for the ﬁrst and second reactions,respectively. In the ﬁrst step, less-exotic RIs (e.g., 133 Sn
and neighboring RIs) relative to the objective RI are
produced and roughly separated. In the second step,
the objective RI is produced at the secondary target atF2 and separated.

Through a simulation, the 128 Pd yield with the newmethod is expected to be ∼3 times larger than that obtained using the standard method with a 2000-particlenA 238 U beam and 1-kHz total-rate limit.
A test experiment was performed at BigRIPS. The
yield of 128 Pd with the new method was measured with
2- and 3-mm Be targets located at F0 and F2, respectively, and 2- and 4-mm degraders at F1 and F5, respectively. The RI beams along the central trajectories
in the ﬁrst and second stages were 133 Sn and 128 Pd, respectively. The measured 128 P (total) rate was 0.19 Hz(1 kHz) per 2000 particle nA. For comparison, the 128 Pdyield obtained using the standard method was simulatedwith the LISE++ code.1) A 5-mm F0 Be target, 5-mm F1degrader, and 1-mm F5 degrader were used. Here, thesums of thicknesses of the targets and degraders werethe same in both the methods. The 128 Pd rate was estimated to be 0.074 Hz per 2000 particle nA. From theseresults, an improved setting with the new method wasobtained to produce 128 Pd, under the conditions of the2000-particle nA 238 U beam and the 1-kHz total-rate
limit.
The region in the nuclear chart where the new method
is favorable will be searched using the LISE++ simulation. However, we have found that some problems should
be solved to perform reliable simulations. First, the momentum distribution and yields of ﬁssion products from238
U are not reproduced well. Second, the RI production
cross-sections from RI beams are not well known, except
for a 132 Sn-beam case.2) For further investigation, suchbasic data should be measured experimentally.
References
1) O. B. Tarasov, D. Bazin, LISE++ site, https://lise.
nscl.msu.edu/lise.html .
2) H. Suzuki et al., Phys. Rev. C 102, 064615 (2020).

Fig. 1. Schematics of RI-beam productions at BigRIPS. (a) Standard method. An objective RI beam is
produced from a 238 U beam in one step. (b) New method, “In-separator two-step method.” An objective RI
beam is produced via less-exotic RIs with two targets in two steps.
∗1RIKEN Nishina Center- S7 -1