Isotope separation with new ion-optics mode

H. Takeda,∗1 T. Kubo,∗1 N. Fukuda,∗1 H. Suzuki,∗1 N. Inabe,∗1 and H. Geissel∗2

Two-stage structure is an important feature of thein-flight fragment separator BigRIPS.1) The first stage,which consists of two dipoles (D1 and D2) and foursuperconducting triplet quadrupoles (STQs) (STQ1-4), is located between the production target F0 andachromatic focus F2 with the momentum dispersivefocus F1. The second stage, which consists of fourdipoles (D3-D6) and eight STQs (STQ7-14), is locatedbetween the achromatic foci F3 and F7 with the mo-mentum dispersive foci F4, F5, and F6. The stage be-tween F2 and F3, which consists of two STQs (STQ5-6), acts as a matching section. Rare-isotope (RI) beamis produced and separated in the first stage with anenergy-loss degrader at F1, and particle identificationand momentum analysis are performed in the secondstage.1,2) Another energy-loss degrader placed at F5 isoften effective when unwanted isotopes are transmittedas a result of the charge state change or the secondaryreaction in the first degrader at F1. The thicknessesand shapes of the degraders at F1 and F5 are chosenso as to optimize the RI beam according to the exper-imental requirements.

The two stages act independently and their isotopicseparation power can be added or subtracted, depend-ing on the experimental condition. When the separa-tion powers of the two stages are subtracted, the hor-izontal spatial distance of the separated isotopes be-comes small at the final focus F7. Adding the isotopicseparation powers of the two stages would increase thehorizontal spatial distance. Because widths also in-crease, improvement of the final resolving power willdepend on the experimental condition.

The ion-optical solution for adding the separationpower can be realized by having either one or threefoci in the matching section F2-F3, which have twofoci (F2 and F3) in the standard mode. At least onefocus in the standard mode is abandoned in the formercase. It is not desirable because both F2 and F3 areimportant for slits and diagnostics of the RI beam.Therefore, here we present the solution having threefoci.

Figure 1 shows the horizontal (X) and vertical (Y )tracks of the beam from F0 to F7 obtained from the so-lution of the first-order ion-optical calculation. In thissolution, ion-optical conditions of the two stages (F0-F2 and F3-F7) are the same as the ones in the standardmode. Only the matching section F2-F3 is modified tohave an additional focus in the X direction at the mid-point “F2.5” between F2 and F3, while the beam is notfocused in the Y direction at F2.5 to reduce excitation

∗1 RIKEN Nishina Center∗2 GSI Helmholtzzentrum fur Schwerionenforschung GmbH

Fig. 1. Horizontal (X) and vertical (Y ) tracks of the beam

in the new mode.

Fig. 2. Comparison of tin isotope distribution at F7 be-

tween the standard mode (left) and new mode (right)

simulated with LISE++.

currents of STQ5 and STQ6. Reversing the polarityof all quadrupoles of the two STQs is also necessary toreduce the currents. The maximum magnetic rigidity(Bρ) of 8.7 Tm is achieved in this solution. The valueis almost the same as the maximum Bρ of the secondstage (8.8 Tm). There is a trade-off between achievinghigh Bρ and large acceptance.

Figure 2 shows an example of horizontal distribu-tions of tin isotopes at F7, simulated by LISE++, in a238U + Be 4 mm reaction at 345 MeV/nucleon using3 mm- and 2.2-mm-thick aluminum degraders at F1and F5, respectively. The spectrometer is tuned for132Sn with D1 Bρ = 7.49 Tm. The left and right pan-els show the results in the standard and new modes,respectively. In the standard mode, all isotopes accu-mulate in the center (X = 0) because of the subtrac-tion of the separation power. In contrast, the isotopeseparation is improved in the new mode.

Note that the difference between the two modes issmall for separation of isotones because they often col-lect at approximately the same position at F2. Alter-ing the combination of energy and degrader thicknessis required for isotone separation.

A machine study for the new ion-optical mode pro-posed here had been scheduled in November 2014, butwas cancelled. We expect to perform this study in thenext year.

References1) T. Kubo: Nucl. Instr. Meth. B204, 97 (2003), T. Kubo

et al.: Prog. Theor. Exp. Phys. 2012, 03C003 (2012).2) N. Fukuda et al.: Nucl. Instr. Meth. B317, 323 (2013).

A NaI(Tl) detector array for measurements of γ rays from fastradioactive isotope beams †

S. Takeuchi,∗1 T. Motobayashi,∗1 Y. Togano,∗2 M. Matsushita,∗3 N. Aoi,∗4 K. Demichi,∗5 H. Hasegawa,∗5 andH. Murakami∗1

The recent development of fast radioactive ion (RI)beams requires a new type of γ-ray detector array forin-beam spectroscopic studies. Experiments are per-formed in inverse kinematics by using fast RI beamswith a high velocity of v/c ≃ 0.3 − 0.6 that causesa large Doppler shift for γ rays emitted. In order toextract the transition energy in the rest frame of theprojectile, precise measurements of emission angles ofγ rays as well as a good intrinsic energy resolutionare required. Another requirement for γ-ray detectionis high efficiency, because the secondary-beam inten-sity for nuclei far from stability is typically low. TheRIKEN RIBF provides the world’s highest intensityexotic beams to study unstable nuclei. To capitalizeon the performance of RIBF, we have constructed anew γ-ray detector array called DALI2 (Detector Ar-ray for Low Intensity radiation 2) for in-beam γ-rayspectroscopy experiments.The design of the DALI2 array follows a concept

similar to the original array DALI1,2), which was devel-oped for experiments at the old facility at RIKEN thatprovides light exotic beams with v/c ≃ 0.3. In exper-iments performed at the new RIBF facility providinghigher-velocity exotic beams with v/c ≃ 0.6, the per-formance of DALI is not optimized. Therefore, DALI2was designed to fulfill the required conditions for ex-periments performed at the RIBF facility by improvingthe angular resolution and the detection efficiency. Inorder to compromise on requirements such as intrinsicresolution, detection efficiency, and cost, we adoptedNaI(Tl) as the detector material. The DALI2 arrayconsists of a large number of detectors, 160-186, which,depending on the experimental conditions, are at vari-ous distances from the target. As shown in Fig. 1, thedetectors are arranged to form twelve layers that areset perpendicularly to the beam axis, and a detectormatrix covers the forward angles. Each layer consistsof 6-14 detectors and the forward matrix consists of 64detectors. In this standard configuration, DALI2 cancover a polar angle between 15◦ and 160◦.The performance of DALI2 was examined by us-

ing measurements with standard γ sources and byperforming the Monte Carlo simulations with theGEANT3 code3). Simulations reproduce measurements

† Condensed from the article in Nucl. Instrum. and Methodsin Phys. Res. Sect. A, 763, 596 (2014)

∗1 RIKEN Nishina Center∗2 Department of Physics, Tokyo Institute of Technology∗3 CNS, University of Tokyo∗4 RCNP, Osaka University∗5 Department of Physics, Rikkyo University

Fig. 1. Schematic view of DALI2 in its standard configu-

ration consisting of 186 NaI(Tl) crystals.

well, including results obtained by in-beam experi-ments. A typical full-energy-photopeak resolution of10% (FWHM) and 20% efficiency are achieved for 1-MeV γ rays emitted from moving nuclei with v/c ≃ 0.6without applying add-back analysis. This resolutionis satisfactory for spectroscopy of low-lying states ineven-even nuclei. The high efficiency enables γ-γ coin-cidence measurements even for beam intensity as lowas 1 Hz. The DALI2 array has been applied success-fully to a variety of experiments at the old RIKENfacility and more recently at the new RIBF facility.This will be used in many more experiments to studynuclear structures of exotic nuclei at RIBF. For fur-ther spectroscopic studies of heavy or odd-mass nuclei,the SHOGUN array with superior energy resolution isplanned4).

References1) T. Motobayashi et al.: Phys. Lett. B 346, 9 (1995).2) T. Nishio et al.: RIKEN Accel. Prog. Rep. 29, 184

(1996).3) GEANT3, CERN Program Library Long Writeup

W5013, 1993.4) P. Doornenbal et al.: RIKEN Accel. Prog. Rep. 42, 182

(2009).

- 208 - - 209 -

Ⅱ-9. Instrumentation RIKEN Accel. Prog. Rep. 48 (2015)RIKEN Accel. Prog. Rep. 48 (2015) Ⅱ-9. Instrumentation

完全版2014_本文.indd 209 15/10/16 17:54