Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Status of SLOWRI beamline construction
1䠆1
䠆1 *1
1 I. Katayama,
*1 䠆
*1,*2
Takao M. Kojima,
Ichiro*Katayama,
Tetsu
Sonoda,andMichiharu
Wada䠆1,䠆2
T. M.
Kojima,T. Sonoda,
andM. Wada

To utilize rare RIs generated at BigRIPS as slow (~10keV) ion beams, we have been constructing a 60 m long
beamline in parallel to the development of slow RI beam
generators, such as PALIS (Parasitic RI-beam production byLaser Ion-Source) and other gas cell type ion stoppers. 1)
The beamline will transport singly charged RI ions, which
are separated and slowed by PALIS at the F2 chamber ofBigRIPS, from F2 to the experimental area E11. It consistsof four sector dipole magnets, two focal plane chambers, 62
electrostatic quadrupole singlets, and 11 electrostatic
quadrupole quartets. Fourteen 450 L/s turbo molecular
pumps evacuate the beamline, which is divided into nine
sections (two of them are focal plane chambers) delimited
by gate valves. The beamline was installed two years ago.
In contrast to the case of usual fast accelerator beams, itis very important to keep the beamline in a good vacuum
since the collisional electron capture cross sections of ionswith residual gas molecules are large for slow ions.2) In
some cases, the cross section reaches ൎ ͳ ൈ ͳͲିଵହ  ଶ atapproximately 10 keV. 3) Assuming this value for the crosssection, we estimated the beam fraction surviving after
60 m flight for some vacuum pressures at a temperature of25ႏ. The results showed that beam fractions of 48.2%,93.0%, and 99.3% survive through pressures of ͷ ൈ ͳͲିସ ,ͷ ൈ ͳͲିହ and ͷ ൈ ͳͲି଺ Pa, respectively. Therefore, avacuum of the order of ͳͲି଺ Pa should be sufficient. Inorder to realize such a good vacuum, the beamline wasbaked with wire heaters coiling the ducts at 120ႏfor about
10 months.
Figure 1 shows a typical vacuum change of the beamline
during the baking. Before baking, the vacuum pressure wasabout ͳ ൈ ͳͲିସ Pa. When the heaters were turned on in
the middle of October 2015, the pressure temporarily
increased up to ͵Ǥ͵ ൈ ͳͲିସ Pa and then decreased
gradually. Some dips of pressure appearing in Fig. 1
correspond to the days in which heaters were off because ofChristmas holidays, RIKEN Open Day, and so on. Many ofthe beamline vacuum ducts (and electrostatic quadrupoles
inside them) were previously used at other facilities and had
been stored for about 15 years. Therefore many molecules
were supposed to be stuck on their inside surfaces.
Especially, it was considered that the Kapton (polyimide)
insulators inside the ducts had adsorbed a considerable
amount of H2O molecules. It took almost 10 months toreach the range of ͳͲି଺ Pa. The heaters were turned off on
August 10, 2016, when the vacuum was about ͻ ൈ ͳͲି଺Pa, and then the vacuum became about ͷ ൈ ͳͲି଺ Pa. Somespikes (sudden rise of pressure) after baking were caused by
the stoppage of evacuation pumps due to power outage andmaintenance. Once water molecules stuck tight on the䠆1
䠆2

RIKEN Nishina Center
KEK Wako Nuclear Science CenterFig. 1. Typical vacuum change of the beamline during
baking. The dips correspond to the days in which
baking heaters were turned off (see text).

Fig. 2. Protector shell (zebra patterned) covering thebeamline ducts over the passage.
surface have been baked out, the vacuum can reach therange of 10-6 Pa easily even after ventilation of the beamlinewith dry nitrogen gas.
Since the baking period was long, for the safety ofexperimental workers and tour visitors who pass under thebeamline, the ducts over the passage were covered with aprotector shell (see Fig. 2). The protector is made ofaluminum and thermally insulated with thick glass wool
rings. The surface of the protector is covered with zebra
cushions to minimize physical damages if someone bumped
his/her head against it.
In the next year, we will install a conventional ion sourceand beam profile monitors, following which beamtransportation will be tested with some ions.
References
1) M. Wada et al., RIKEN Acc. Prog. Rep. 47, 203 (2014).
2) E. W. McDaniel, J. B. A. Mitchell, M. Eugene Rudd: AtomicCollisions: Heavy Particle Projectiles (John Wiley & Sons, NewYork, 1993).
3) See, for example, data in the following Web Data Base:
CMOL (http://dbshino.nifs.ac.jp/nifsdb/cmol/top)
CHART (http://dbshino.nifs.ac.jp/nifsdb/chart/top)- 195 -