Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 49 (2016)

Ion-optical study of additive and subtractive modes of BigRIPS
H. Takeda,∗1 T. Kubo,∗1 N. Fukuda,∗1 H. Suzuki,∗1 D. S. Ahn,∗1 Y. Shimuzu,∗1 T. Sumikama,∗1 D. Murai,∗1N. Inabe,∗1 K. Yoshida,∗1 K. Kusaka,∗1 Y. Yanagisawa,∗1 M. Ohtake,∗1 H. Sato,∗1 T. Komatsubara,∗1 andH. Geissel∗2
One of the important features of the in-ﬂight fragment separator BigRIPS1) is a two-stage separation
scheme. Rare-isotope (RI) beam is produced and separated in the ﬁrst stage (F0-F2) with an energy-loss degrader at the momentum dispersive focus F1. Particle
identiﬁcation and momentum analysis are performed
event-by-event in the second stage (F3-F7) with themomentum dispersive foci F4, F5, and F6.1,2) Anotherenergy-loss degrader placed at F5 is often used to separate unwanted isotopes produced in the ﬁrst degrader
at F1 as a result of the charge state change or the secondary reaction.
The two stages act independently and their isotopic
separation power can be added or subtracted, depending on the experimental condition. When the separation powers of the two stages are added, the horizontal
spatial distance at the ﬁnal focus F7 would increase.
Because widths also increase, improvement of the ﬁnal resolving power will depend on the experimentalcondition.
The additive and subtractive modes can be switchedby changing the sign of the magniﬁcation of a matching
section F2-F3 in between the two stages. In a previous
report,3) we have introduced a new ion-optical system
for the additive mode. The F2-F3 magniﬁcation isreversed compared to the standard (subtractive) modeby adding one focus in the horizontal direction at themid-point between F2 and F3. We performed machinestudies to examine the two modes in this year.
In April 2015, we compared the additive and subtractive modes under the same conditions for the ﬁrst
and second stages. Only the matching section F2-F3
was changed. Isotopes around 132 Sn produced in a
238 86+
U+ Be 5 mm reaction at 345 MeV/nucleon wereused to measure the optical properties. 5-mm- and3-mm-thick wedge-shaped aluminum degraders wereplaced at F1 and F5, respectively. Magnetic rigidity of
the ﬁrst dipole (D1) was 7.1 Tm and the other dipoles(D2-D6) were tuned so that the 132 Sn isotope came at
the center at each focus.
Table 1 shows the measured mass dispersions at F2,
F3, and F7 together with the LISE++ simulation. We
can see that the mass dispersion became large at F7
in the additive mode as expected. However, there aresome discrepancies in the additive mode between themeasured and calculated values compared to those of
the subtractive mode. These discrepancies may comefrom the discrepancies in the F2-F3 transfer matrix∗1∗2

RIKEN Nishina Center
GSI Helmholtzzentrum für Schwerionenforschung GmbH- 155 -Table 1. Mass dispersion in mm/ΔA at F2, F3, and F7.
See text for the RI beam setting in this measurement.

F2
F3
F7

subtractive
measured LISE++
5.82
6.05
-6.33
-6.41
0.23
-0.71

additivemeasured LISE++
5.54
6.05
4.72
6.11
12.1
18.5

elements. For example, the measured and calculated
magniﬁcations were 0.78 and 1.01, respectively, whilethey agreed in a few % in the subtractive mode. The
reason for the diﬀerence is under consideration. We
also found that the telescopic condition was not fulﬁlled due to the very large (a|x) value in the additivemode. The measured and calculated values were 1.89
and 2.14 mrad/mm, respectively. According to this(a|x) term, isotopes that come oﬀ-center at F2 havelarge angles at F3 and the track reconstruction in thesecond stage would become worse. The transmissionis also aﬀected. The 132 Sn yield in the additive mode
was about 30% less than that in the subtractive mode.
Events with a large angle caused by the large (a|x)
term may be lost after F2. Reduction of the (a|x)
term is under consideration.
Purities of the 132 Sn isotope are almost the same inboth additive and subtractive modes, being 8% and 6%
for the F1 slit at ±2 mm and ±64.2 mm, respectively,
when the F2 and F7 slits are optimized for 132 Sn. The
purities are dominated by the contamination from thesame isotones, which occupy almost the same position
at F2 in our case. The additive mode gives the sameseparation for such isotones as the subtractive mode.
Therefore, the purities were not improved in the additive mode as we had expected. Altering the combination of energy and degrader thickness is required forisotone separation.
In November 2015, we performed another machine
study for the two-stage separation. We applied thesame conditions for the ﬁrst stage as in April. Thickness of the aluminum degrader at F5 was changed(0 mm, 2 mm, 3 mm, and 5 mm) to investigate systematically the isotope separation in the subtractive mode.
The measured mass dispersions at F7 varied from −7
to 11 mm/ΔA as a function of the F5 degrader thickness and were well reproduced with LISE++ simulations. Detailed analysis, including transmission, purity, and so on, is in progress.
References
1) T. Kubo: Nucl. Instr. Meth. B204, 97 (2003).
2) N. Fukuda et al.: Nucl. Instr. Meth. B317, 323 (2013).
3) H. Takeda et al.: RIKEN Accel. Prog. Rep. 48, 208(2015).