her Isomeric Purity

Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Development of a high density 7 Be beam at CRIB
A. Inoue,∗1 A. Tamii,∗1 H. Yamaguchi,∗2 K. Abe,∗2 S. Adachi,∗1 S. Hayakawa,∗2 J. Isaak,∗1 N. Kobayashi,∗1H. Shimizu,∗2 and L. Yang∗2

The 7 Li problem is a discrepancy between the stanLaboratory Energy (MeV)
dard Big-Bang Nucleosynthesis (BBN) model and observations. Our research goal is to measure the crosssection of the 7 Be(d, p) reaction to solve this 7 Li problem. A recent theoretical BBN model predicts a primordial 7 Li abundance that is 3 times larger than the
recent precise observation.1) This diﬀerence is quite
large, while the theoretical calculation reproduces theabundance of the other light nuclei well. One possible scenario to solve the problem, which has not been
included yet in the BBN model, is that 7 Be was destroyed in the nuclear reaction after the Big Bang. The7Be(d, p)8 Be and the 7 Be(n, α)4 He reactions are two
promising processes for destroying 7 Be. We are focusing on the 7 Be(d, p)8 Be reaction since the contribution
from 7 Be(d, p)8 Be is suggested to be larger that from7Be(n, α)4 He.2,3)
We are developing an unstable 7 Be target for highresolution measurement of the 7 Be(d, p)8 Be reaction
in normal kinematics. This is a big technical challenge
since 7 Be is an unstable nucleus. We suggested to makethe 7 Be target by implantation in a host material. Thisis called the Implantation method. The development isongoing at CRIB, Center for Nuclear Study (CNS),
University of Tokyo. The ﬁrst experiment was performed in June 2016. The primary beam was 7 Li2+ at
5.6 MeV/nucleon. The secondary beam was producedby the 1 H(7 Li, 7 Be) reaction by employing a cryogenichydrogen gas target. The gas thickness is 8 cm andthe gas pressure was 760 Torr. The secondary beamenergy was 4.0 MeV/nucleon. A 10-µm thick gold foil
as a host target was irradiated with the 7 Be beam afteran energy degrader made of gold with a thickness of
15 µm and a collimator with a diameter of 3 mm. We
evaluated the amount of the implanted 7 Be by detecting 477 keV γ rays with a LaBr3 detector after the implantation. The γ ray is emitted through the electron
capture decay of 7 Be with a branching ratio of 10.4%.
We obtained 1.3×1011 (4.3×1010 /mm2 ) 7 Be particles
after 19 hours of irradiation. The average beam intensity was 6.3 ×105 /mm2 . However, the number ofthe 7 Be particles is almost 10 times smaller than ourestimation from the maximum intensity (～106 /mm2 )of CRIB’s previous performance. We suspect that thebeam spot size and the beam proﬁle at F2 were notfully optimized for the high-intensity 7 Be beam downstream of the collimator and not maintained at a ﬁxed
position during the long irradiation time since we did
not check the 7 Be beam proﬁle when the beam inten5678910∗1∗2sity was increased. This is because it was not possible
to count such a high intensity beam directly.
Based on the result of the experiment in 2016, weperformed a development experiment at CRIB to optimize the beam line optics and obtain an intense beamof 7 Be, in April 2017. Previously, the beam proﬁlewas checked with the PPAC detector at CRIB. However the PPAC detector is not a detector for such ahigh rate, so we could not count the high intensity
beam with the existing detector. To solve this issue,
we installed a metal mesh at F1 and a plastic scintillator at F2 to count the beam intensity. This wasa new trial at CRIB. The metal mesh was used to reduce the 7 Be beam intensity, and hence we could countthe 7 Be beam intensity directly by the plastic scintillator. In this experiment, we tuned the ion-opticalparameters and the steerer on the beam line for thebest-positioning and focusing of the secondary 7 Be,by counting the intensity with the plastic scintillator.
We achieved 6.8×106 /mm2 as the average beam intensity after the optimization of the settings of beam-lineoptics. We obtained 1.2×1012 (1.7×1011 /mm2 ) 7 Be
particles with 7-hours irradiation. As the next step,we plan to measure the 7 Be(d, p) reaction at Japan
Atomic Energy Agency, tandem facility. The 7 Be target will be produced at CRIB before the (d, p) reaction measurement, planned for 2018. About 8.2×1012(2.6×1012 /mm2 ) 7 Be ions will be implanted in 2 daysof irradiation.

11Fig. 1. Plane view of CRIB, where the Q1, Q2, Q3, and M3
magnets and the steerer were optimized in the presentwork. The installation of the mesh at F1 and the plastic
scintillator at F2 was a new trial at CRIB to count thehigh intensity beam. The 7 Be beam was counted after
the 3 mm diameter collimator.

References
1) R. H. Cyburt et al., J. Cosmol. Astropart. Phys. 11,012 (2008).
2) S. Q. Hou et al., Phys. Rev. C 91, 055802 (2015).
3) T. Kawabata et al., Phys. Rev. Lett. 118, 052701(2017).

Research Center for Nuclear Physics, Osaka University
Center for Nuclear Study, University of Tokyo- 77 -