Ⅱ-8. Accelerator
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Production of high intense Ca-ion beam for RILAC II acceleration
T. Nagatomo,∗1 Y. Higurashi,∗1 J. Ohnishi,∗1 and T. Nakagawa∗1
Calcium(Ca)-48-ion beam is indispensable for studying light nuclei with excess neutrons. The 48 Ca beam
was obtained from RILAC as an injector for RIBF accelerators before the upgrade of RILAC with superconducting (SC) acceleration cavities.1) The injection line
for the RIBF cyclotrons was removed owing to the construction required for the upgrade, and has not been restored at present. Currently, the only way is to use RILAC II, which is mainly used to inject heavy ion beams(krypton, xenon, uranium), as the injector for RIBF.
Despite the global shortage of 48 Ca (natural abundance= 0.187%) samples, it is estimated that low-level enriched samples (∼20%) will be available within 2 years.
However, the low enrichment of 20% implies that only
20% of the produced Ca beam can be used. Therefore,in accordance with the Ca beam acceleration scheme
using RILAC II, we conducted a test to extract ultraintense Ca11+ -beam above 100 µA from a SC-ECR ionsource (SC-ECRIS).
The experiment was conducted using an SC-ECRISfor RILAC (R28G-K),2,3) which exhibits the same performance as that of RILAC II. An ion-extraction voltage of 13.00 kV was applied to ﬁt to the acceleration
scheme. We selected a high-temperature oven (HTO),4)
which was used in the previous Ca16+ beam production
last year with good results.5) Compared to the previous
test, because the Ca11+ has a lower charge state, thestable plasma conditions, such as the electron temperature and ion conﬁnement time, diﬀer. The magnetic
mirror was set to lower ﬁeld strength than the Ca16+
case, similar to the Ref. 6), resulting in the decreased
electron temperature. Natural CaO samples were prepared using the same procedure as in a previous study.5)
Two evaporation methods to obtain the neutral Ca gas
were attempted: direct heating of the sample at temperatures up to 2000◦ C (DH), and use of the redox
reaction of 3CaO + 2Al → 3Ca + Al2 O3 to lower the Caevaporation temperature (RR). The DH method is thesame method as in the Ca16+ beam production, andRR is a common method as presented in Ref. 6). The
temperature was controlled by the applied power on
the HTO (PHTO ). Oxygen (O2 ) and helium (He) gases
were tested as support gases (SGs) to maintain a stable
plasma. The total microwave power with frequencies of
18 and 28 GHz, Ptot. , as well as the SG ﬂow rate wereadjusted to optimize the beam intensity and stability.
Table 1 summarizes the obtained intensities of 40 Caion beam extracted from R28G-K along with the experimental conditions. The data for the ionic charge state
Q = 16 are obtained from a previous study.5) Here,
Istart and Istop represent the beam intensities at thebeginning and end of the operation (duration time ∆t),
∗1

RIKEN Nishina Center- 67 -Table 1. 40 Ca ion beam intensities with the charge state Q
together with the some experimental conditions such as
the support gas (SG) and the Ca evaporation method(Meth.). Measurements were performed over a period of∆t. See text for other abbreviations.
QSG Meth.

1111
11
11
16
16
16

O2O2O2HeO2O2O2DHDHRRRRDHDHDH

Ptot.
kW
3.4
2.7
2.8
2.1
3.0
3.0
3.0

PHTO
W
900.
800.
190.
190.
720.
660.
600.

∆thours
9.3
14.3
17.9
7.7
16.3
39.7
37.1

Istart
µA
242.
101.
74.
105.
84.
35.
25.

Istop
µA
204.
59.
33.
90.
85.
70.
30.

respectively. As shown in Table 1, in all cases of Ca16+ ,the beam intensity gradually increased, and eventuallyreached to constant values. However, the Ca11+ beam
intensity tended to decrease with the elapsed time. This
trend is more pronounced in the O2 case than in theHe case. Note that, in the previous study,5) the Ca16+
beam intensity was also tested with He support gas;
however, it was found to be only half that of the O2
support gas. The DH method seems to cause a chemical reaction between the tungsten crucible and the CaO
sample at elevated temperature of ∼2000◦ C. From theM/Q analyses using the analyzing magnet, when thePHTO is 700 W, the heavy-ion beams, which appear
to be tungsten ions, are not noticeable; however, asthe PHTO is increased to 800 W and 900 W, they become more pronounced. Owing to the chemical reaction, the RR method, which can be used at lower temperature, is preferable to produce low-charge Ca ionbeams. Although the measurement time was not suﬃciently long, the consumption rate of Ca was estimatedto be ∼0.7 mg/h using the RR method using the He gas
to produce a Ca11+ beam of approximately 100 µA.
This study has only just begun, and the search forconditions that reduce the Ca consumption is ongoing.
References1) N. Sakamoto et al., Proc. SRF2019, Dresden, Germany (2019), p. WETEB1.
2) T. Nagatomo et al., Rev. Sci. Instrum. 91, 023318(2020).
3) G. D. Alton et al., Rev. Sci. Instrum. 65, 775 (1994).
4) J. Ohnishi et al., Proc. ECRIS2018, Catania, Italy(2018), p. 180.
5) T. Nagatomo et al., RIKEN Accel. Prog. Rep. 55, S18(2022).
6) J. Y. Benitez et al., Rev. Sci. Instrum. 85, 02A961(2014).