Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Interaction cross section measurement of neutron-rich nuclei 17,19 B
A. T. Saito,∗1,∗2 T. Nakamura,∗1,∗2 Y. Kondo,∗1,∗2 Y. Togano,∗1,∗2 J. Gibelin,∗3 N. A. Orr,∗3 R. Minakata,∗1,∗2S. Ogoshi,∗1,∗2 and SAMURAI DayOne Collaborationhigh to identify the charged particle unambiguously.
The interaction cross sections are then obtained by thetransmission method. Further analysis to obtain theﬁnal interaction cross sections for 17,19 B is in progress.

We report the interaction cross section measurements of 2n-halo nuclei 17 B and 19 B on a carbon target at 270 and 220 MeV/nucleon, respectively, using the SAMURAI spectrometer at RIBF. The neutron halo is one of the most notable features found inlight nuclei located in the vicinity of the neutron dripline. The drip-line nucleus 19 B has attracted much attention because of its low two-neutron separation energy (S2n =0.14(39) MeV1) ) and the high matter radius (r̃m =3.11(11) fm) determined from the interaction cross section (σI =1219(81) mb) at an incident energy of approximately 800 MeV/nucleon2) . These results suggest the neutron halo structure in the groundstate of 19 B, and its microscopic structure is not wellunderstood owing to the large uncertainties of measured S2n and σI . Further experimental studies arethus called for to clarify the detailed nuclear structure, such as the valence neutron conﬁguration, coreexcitation, possible 4n halo/skin2) , and He-Li cluster
conﬁguration.3) The goal of the present measurement
is to obtain the interaction cross section of 19 B with
an improved accuracy together with that of 17 B at approximately 250 MeV/nucleon.
The details of the detector setup of SAMURAI
are described in Ref. 4). A cocktail beam of 17,19 B
was produced via the fragmentation reaction of a48
Ca beam on a 30-mm-thick beryllium target at345 MeV/nucleon. The 17,19 B beams impinged on acarbon secondary reaction target ( thickness1.8 g/cm2) installed at the SAMURAI target area. The particle
identiﬁcation (PID) before the carbon target was performed using BigRIPS with the TOF-∆E-Bρ method.
Figure 1 shows a PID plot of incoming beams reconstructed by TOF measured using plastic scintillators,
Bρ obtained from the horizontal position at F5, and∆E measured using an ionization chamber (ICB) at
F13. The beam proﬁle on the target was obtained using two drift chambers (BDC1 and BDC2). The PID
plot of the outgoing charged particles from the 19 B + C
reaction, obtained using SAMURAI, is shown in Fig. 2,
where the incident 19 B beam is selected. TOF and∆E were obtained using the plastic scintillator hodoscope (HODF). The Bρ value was reconstructedfrom the positions and angles measured by two drift
chambers (FDC1 and FDC2). The resolution of massto-atomic-number ratio (A/Z) and atomic number (Z)for the outgoing 19 B are 0.04 and 0.19 (FWHM), respectively. The achieved resolutions are suﬃciently∗1∗2∗398103

7
Z

62019C

51719B

C

102

B

4103
2
2.8

3

3.2

3.4
A/Z

3.6

3.8

4

1Fig. 1. Particle identification for the beam particles extracted from the data obtained using the standard BigRIPS detectors.

98102

7615Z

B

517B

19B104322.6 2.8133.2 3.4 3.6 3.8A/Z

44.2Fig. 2. Particle identification for the outgoing charged particles after the carbon target extracted from the dataobtained using the SAMURAI detectors after selecting
19
B from Fig. 1.

References
1) L. Gaudefroy et al., Phys. Rev. Lett. 109 202503 (2012).
2) T. Suzuki et al., Nucl. Phys. A 658 313 (1999).
3) Y. Kanada-Enyo et al., Phys. Rev. C 52, 647 (1995).
4) T. Kobayashi et al., Nucl. Instr. Meth. B 317, 294 (2013).

Department of Physics, Tokyo Institute of Technology
RIKEN Nishina Center
LPC-Caen, ENSICAEN, Université de Caen, CNRS/IN2P3- 47 -