Extending the Nuclear Chart,		 	

Expanding our Wisdom		

The BigRIPS separator is a new-generation superconducting in-flightfragment separator at the Radioactive Isotope Beam Factory (RIBF).
Since 2007, the BigRIPS has produced a wide range of radioactive isotope(RI) beams at unprecedented intensities. It is characterized by large ionoptical acceptances, a two-stage structure, and high resolving power forparticle identification (PID).
The large acceptances (±40 and ±50 mrad in the horizontal and verticaldirections, respectively, and ±3% in momentum) allow efficient productionof RI beams using not only projectile fragmentation, but in-flight fission ofa 238U beam with wide kinematical distribution as well.
The two-stage structure enables flexible operations, e.g. separatorspectrometer and separator-separator modes, and reduces demands fortagging detectors in the second stage. The latter point results in higher
particle identification capability when combined with a high resolving
power of the ion-optical system (see page v for details).

I. PREFACE-i-RIKEN Accel. Prog. Rep. 47 (2014)

GRAVURE OF THE YEARRI Beams & New Isotopes for ScienceSince 2007, 317 RI beams have been supplied to nuclear physics experiments at the RIBF. The
isotopes are produced from primary beams of 238U, 124Xe, 70Zn, 48Ca, and 18O, as shown with red
circles in Fig. 1 (see page iv for details).
At the same time, yields and production cross sections have been measured for a total of 877 rareisotopes (light blue squares). Among them 47 (blue squares) and 4 (green squares, proton-rich side)
new isotopes produced from the 238U and 124Xe beams are included. The total number of newisotopes produced at RIKEN, since Nishina’s discovery of 237U, is 87. This number is still increasing
and preliminary results of 69 new isotopes will be reported soon.

New Isomers~ Indications of new aspects of nuclear structure ~We have discovered 18 and 25 new isomers in the newisotope search experiments performed in 2008 and 2011,
respectively (see D. Kameda et al., Phys. Rev. C 86, 054319(2012) for details). The existence and properties of new
isomers provide us with information on the structure of therelevant nuclei that is otherwise unavailable.
The newly discovered isomers are of practical importancetoo. The characteristic γ-rays from the isomers enableunambiguous particle identification (PID) and can serve as an
irreplaceable calibration standard for TOF-Bρ-ΔE PID. A good
example is 117Ru shown in the figure on the right. The isomericstates discovered in 2008 have already been used for PID in RI
beam production in the EURICA experiments.
- ii -Reprinted from D. Kameda et al., Phys. Rev. C
86. 054319 (2012). Copyright (2012) by theAmerican Physical Society.

RIKEN Accel. Prog. Rep. 47 (2014)

GRAVURE OF THE YEAR

Cross Section and Production Yields of Rare Isotopes~Baseline of RI-beam science~	  Cross section and production yields of rare isotopes are critical in designing RI-beamexperiments. The measured production yields and cross sections are useful to improve ourunderstanding of the reaction mechanism of RI production and refine the theoretical models.
(see pages vi—viii for details)
In figure (a) on the right, the measured
production rates from the 238U+Be
fission are compared with the LISE++
calculations (version 8.4.1) which
employs the abrasion fission (AF)model for the nuclear fission.
The comparison clearly shows a large
disagreement in the region of Z > 50.

The RI production cross sections by thefragmentation of the 124Xe, 70Zn, 48Ca, and 18O
primary beams are compared with predictions of
the EPAX3.01 and EPAX2.15 models.
EPAX2.15 is found to give better predictions forthe 48Ca beam, as shown in the figure (b) on theright, while EPAX3.01 produces successful
predictions for the 124Xe primary beam.
These yield and cross section data are available
ath"p://www.nishina.riken.jp/RIBF/BigRIPS/intensity.html.

Future work
Primary-beam intensities at the RIBF are increasing every year. At the same time, the demand for
RI beams with higher Z and/or with higher intensity is increasing.
Recently, several trials were successful in increasing the limits of the counting rate for thedetectors and improving A/Q resolution for RI beams with high Z where many charge states aremixed; introduction of Ag electrodes to the parallel plate avalanche counters resulted in better rate
duration, i.e. stable operation up to 70 kcps even for RI beams with Z > 60. A new mechanism toshift plastic-scintillator positions, with respect to the beam position, helps us to avoid deteriorationof time resolution due to radiation damage by heavy-ion irradiation. New ion-optics is found to be
useful in improving the A/Q resolution, where Bρ resolution at the second stage is designed to be
doubled compared with the previously used optics.
The BigRIPS team continues to make efforts to provide RI beams with higher intensities andhigher qualities.
Hiroshi Suzuki for the BigRIPS team- iii -