RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-7. Astrophysics and Astro-Glaciology

Time-reversal measurement of the p-wave cross sections of the7Be(n, α)4 He reaction for the cosmological Li problem†∗1∗1 Y. Fujikawa,∗1∗1 T. Goto,∗1∗1 M. Ichikawa,∗2∗2Kawabata,
Furuno,Hashimoto,Itoh,Y. Fujikawa,∗2∗2T.T.Furuno,
T. Goto,∗2∗2T.T.Hashimoto,M. Ichikawa,∗2∗2M.M.Itoh,N.
T.T.Kawabata,
∗3,∗4
∗1
∗1,∗2
∗4
∗2
∗2
∗3,∗4
∗1
∗1,∗2
∗4
∗2
∗2
∗1∗1
Kanada-En’yo, A.A.Koshikawa,Koshikawa, S.S.Kubono,Kubono, E.E.Miyawaki,Miyawaki, E.E.Mizuno,Mizuno, K.K.Mizutani,Mizutani,
N.Iwasa,Iwasa,
Y.Y.Kanada-En’yo,
∗1,∗2
∗1
∗1,∗2
∗4
∗2
∗2
∗1,∗2
∗4
∗2
∗2
∗2
T. Morimoto,K. Murata, T.
S.
T. Nanamura,
Nanamura,
S. Nishimura,Nishimura, S.
S. Okamoto,
Okamoto, Y.
Y. Sakaguchi,
Sakaguchi, I.
I. Sakata,
Sakata,∗2∗1
∗2
∗2
∗2
∗1,∗2
∗2
∗1
∗2
∗2
∗2
∗1,∗2
∗2
A. Sakaue,
Sakaue, R.
R. Sawada,
Sawada, Y.
Y. Shikata,Shikata, Y.
Y. Takahashi,Takahashi, D.
D. Takechi,Takechi, T. Takeda,
C. Takimoto,A.
Takimoto,
M. Tsumura,∗1 K. Watanabe,∗1,∗2 and S. Yoshida∗2†∗1
∗2
∗3
∗4

Condensed from the article in Phys. Rev. Lett. 118, 052701(2017).
Department of Physics, Kyoto University
Faculty of Science, Kyoto University
Department of Physics, Tohoku University
RIKEN Nishina Center- 142 -low energies down to Ec.m. = 0.20 MeV was obtained,as shown in Fig. 1, for the ﬁrst time. The solid circles
and squares show the total cross sections of the (n, α)
reaction on the ground and ﬁrst excited states in 7 Be.
The shaded area presents the eﬀective-energy window
for the p-wave reaction at T9 = 0.6–0.8.
The cross sections evaluated by the indirect methods
are compared with the present results. The estimationfrom p + 7 Li scattering5) is plotted by the open triangles in Fig. 1, whereas the cross section from theevaluated nuclear data library ENDF/B-VII.16) basedon the R-matrix analysis of several indirect reactions
is shown by the dashed line. It was found that theseevaluated cross sections are very close to the present
data for the 7 Beg.s. (n,α)4 He reaction.
The cross section for the 7 Be(n,α)4 He reaction wasﬁrst estimated by Wagoner,7) as shown by the solidline in Fig. 1. Currently, this evaluation is widely usedin the BBN calculations. The present values of the7Be(n,α)4 He cross sections are much smaller than theWagoner’s calculation. Thus, we concluded that thepresent results suggest that the 7 Be(n,α)4 He reaction
does not solve the cosmological lithium problem.
10 3

Cross Section (mb)The primordial abundances of light elements produced in the Big Bang nucleosynthesis (BBN) provide
important insights into the early universe. Accurate
estimation of the primordial abundances is crucial for
testing cosmological theories by comparing the predicted values with the observations.
A comparison between the theoretical predictions
and observations is in good agreement with those forhelium and deuterium. However, there remains a serious problem: the 7 Li abundance does not agree withany theoretical BBN calculations. This discrepancy isknown as the cosmological lithium problem, and has
been of great interest in recent years.1) Several ideas
have been proposed to solve this problem. One idea is
to improve the current understanding of the stellar processes that exhaust lithium in metal-poor stars. Otherideas are to ﬁnd new physics beyond the standard BBN
model, e.g., a cosmological variation of the fundamental constants,2) decay of supersymmetric particles,3)and so on. However, there is no experimental evidence
to conﬁrm these models.
From the viewpoint of nuclear physics, nuclearreaction rates involved in the BBN theory should be examined. The main process of the 7 Li production in theBBN is the electron-capture decay of 7 Be, which is synthesized in the 3 He(4 He,γ)7 Be reaction. Direct measurements of the cross section for the 3 He(4 He,γ)7 Be
reaction were extensively carried out in the past, anduncertainties in this thermonuclear reaction rate are
now very small. There is no room to modify the 7 Be
production rate to solve the lithium problem.4)
It was pointed out that the 7 Li abundance would begreatly reduced in the BBN calculation if the destruction rate of 7 Be is enhanced. One of the candidate
channels for destroying 7 Be is the 7 Be(n,α)4 He reaction. Unfortunately, the 7 Be(n,α)4 He reaction at thecosmological energy has been scarcely examined.
Very recently, we have measured the total cross section for the 4 He(α, n)7 Be reaction, which is the time
reverse reaction of the 7 Be(n, α)4 He reaction. Using
the detailed balance principle, the total cross sectionof the 7 Be(n, α)4 He reaction for p-wave neutrons at74Be(n,α) HeWagoner [7]
ENDF/B-VII1 [6]

10 2

1074

Present Begs(n,α) He
74
Present Be*(n,α) He
74
Derived from Li(p,α) He [5]110-210-1

1Ec.m. (MeV)

Fig. 1. Measured total cross sections for the 7 Be(n,α)4 Hereaction compared with the previous evaluations.

References
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5) S. Q. Hou et al., Phys. Rev. C 91, 055802 (2015).
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(2011).
7) R. V. Wagoner, Astrophys. J. Suppl. 18, 247 (1969).