Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 49 (2016)

Energyupgrade
upgrade forfor biological
Energybiological applications
applications
N. Fukunishi,＊1 M. Fujimaki,＊1 M. Komiyama,＊1 K. Kumagai,＊1 T. Maie,＊1 T. Hirano,＊1,＊2 Y. Watanabe,＊1 and T. Abe＊1
Energetic heavy ions have been used as an effective toolfor inducing mutations of flowers, crops and microbes at
RIKEN Nishina Center. Typical beams used in theseapplications are 135-MeV/nucleon C, N and Ne beams, a95-MeV/nucleon Ar beam, and a 90-MeV/nucleon Fe beam.
All of them are accelerated by RRC.1) However, the beamenergies of Ar and Fe ions are insufficient to obtain nearlyflat distributions of Linear Energy Transfer (LET) withinthick (or in-water) irradiated samples. It sometimes causes adifficulty such that the dependence of the effectiveness ofmutagenesis on LET becomes unclear. Hence an upgradefor beam energies available at the existing in-air irradiation
port for biological samples (E5H) is urgently required.
Our solution to this problem is the usage of IRC.2) IRC isoriginally designed to accelerate ions up to 127
MeV/nucleon using RILAC, RRC, and IRC in series or up
to 114 MeV/nucleon using RILAC2, RRC, fRC, and IRC inseries. However, the maximum beam energy obtained by
IRC can be increased up to 160 MeV/nucleon by employing
a new acceleration scheme as follows. The injector is AVF
cyclotron. It accelerates Ar ions up to 3.7 MeV/nucleon.
The ions extracted from AVF are charge-stripped there,
injected into RRC, and accelerated up to 66 MeV/nucleon.
After RRC, additional charge stripping and energy
degradation to 62 MeV/nucleon should be performed byusing a thick carbon disk, which is inevitable to compensate
for an energy mismatch inherent to this non-orthodox
acceleration scheme.
160-MeV/nucleon Ar ions extracted from IRC aretransported to the existing E5H irradiation port byextending the RIBF beam transport system as shown in Fig.
1. A beam extracted from IRC is deflected by DAKR dipole
magnet in order to separate the beam from the existing
SRC-injection line. The section from IRC extraction toDMR2 makes a dispersive IRC-extracted beam doubly
achromatic. The beam is bended up by DMR3 and bended
down by DMR4 to shift the beam vertically by 3 m tocompensate for the existing floor level difference. Here, thedoubly achromatic condition is also fulfilled in the verticaldirection. The section from DMR5 to DMR6 forms anachromatic bending system of 90 deg. The section fromDMR7 and DMR8 is also doubly achromatic. After that, thebeam line is joined to the existing beam delivery fishbone atDMA1.	 The section immediately after DMR2 to DMR6 isnot newly constructed but utilizes the existing
IRC-bypassing3) beam line in the reverse direction. Notethat the IRC-bypassing beam line was constructed to injecta beam accelerated by RRC directly to SRC in order to＊1

RIKEN Nishina Center＊2

Faculty of Agriculture, University of Miyazaki- 135 -

perform light-ion experiments. The maximum magnetic
rigidity of the new branch beam line is 4.4 Tm, which
covers not only 160-MeV/nucleon 40Ar18+ ions but alsoheavier ions such as 56Fe26+ ions.
Beam commissioning was made in January 2015. We
extracted a 160-MeV/nucleon Ar beam in the proposed
non-orthodox acceleration scheme. We found no sizable
beam loss along the whole new beam transport system andconfirmed that our optical design of the new branch beamline worked well. In addition, we successfully produced auniform irradiation field of 10 cm in diameter and obtained
a depth-dose curve of Ar ions as expected from numericalsimulations. Details of the beam commissioning are givenin Ref. 4.
One problem observed in the beam commissioning wassizable (~ 40%) beam loss during acceleration at IRC. The
cause of this abnormal behavior was fixed in the additional
beam test performed in October 2015. The observed beamloss was induced by a half integer resonance in the verticalbetatron motion. The vertical tune at beam extraction is 0.6,
which is not so close to 0.5 but strong harmonic magnetic
fields induced at the beam extraction region of IRC createdproblems. These strong harmonic fields are effective forouter most ions accelerated in the standard acceleration
schemes to secure a sufficient clearance from a baffle slit
attached to the IRC chamber wall. In October, we reduced
harmonic magnetic fields by rebalancing excitation currentsof outer trim coils and obtained a transmission efficiency ofup to nearly 100%.

Fig. 1. Layout of RIBF beam transport system.
References
1)Y. Yano : Proc. 12th Int. Conf. Cyclotrons and their
Applications, Berlin, Germany (1989) p. 13.
2) J. Ohnishi et al. : Proc. 17th Int. Conf. on Cyclotrons and TheirApplications, Tokyo, Japan (2004) p. 197.
3) Y. Watanabe et al. : Accel. Prog. Rep. 42, 111-112 (2009).
4) N. Fukunishi et al. : Proc. HIAT2015, MOPA04 (2015).