Speaker
Description
In this poster, we present the results of our previous experimental study and discuss a future theoretical approach. The aim of our research is to achieve a comprehensive understanding of the Big Bang Nucleosynthesis (BBN) process and to potentially resolve the Cosmological Lithium Problem (CLP). The CLP is a well-known issue in astrophysics, referring to the overestimation of the primordial $^7\mathrm{Li}$ abundance in standard BBN models compared to astronomical observations.
The experimental approach focuses on measuring the cross section of the $^7\mathrm{Be}(d,p)^8\mathrm{Be}$ reaction, motivated by theoretical suggestions of its important role in the destruction of $^7\mathrm{Be}$ during BBN [1]. The majority of $^7\mathrm{Li}$ nuclei are produced through the electron capture decay of $^7\mathrm{Be}$ ($T_{1/2} = 53.22$ days $= 4.6 \times 10^6$ s). Since $^7\mathrm{Be}$ nuclei are produced within several hundred seconds during BBN, there is a timescale difference of over $10^4$ between the production of $^7\mathrm{Be}$ and the formation of $^7\mathrm{Li}$. This suggests that enhanced destruction of $^7\mathrm{Be}$ during BBN could reduce the final $^7\mathrm{Li}$ abundance, potentially resolving the discrepancy. The measurement of the absolute cross section in the Big Bang energy region ($E_{\mathrm{c.m.}} = 0.1$--$0.4$ MeV) is crucial for understanding nuclear reactions in the primordial universe. We produced a radioactive $^7\mathrm{Be}$ target and measured the $^7\mathrm{Be}(d,p)^8\mathrm{Be}$ reaction cross section at the tandem facility of Kobe University in Japan. A 2.36 MeV proton beam irradiated a natural Li target with a thickness of $30~\mu\mathrm{m}$, producing $^7\mathrm{Be}$ nuclei via the $^7\mathrm{Li}(p,n)^7\mathrm{Be}$ reaction. A total of $2.80 \times 10^{13}$ $^7\mathrm{Be}$ nuclei were produced in the Li host target over two days of irradiation. After target production, a deuteron beam was accelerated to energies of 1.6 and 0.6 MeV to measure the $^7\mathrm{Be}(d,p)^8\mathrm{Be}$ reaction cross section. The outgoing protons were detected using two sets of four-layer silicon detectors placed at scattering angles of 45$^\circ$ and 30$^\circ$. The thick-target analysis method was applied to determine the cross sections. The cross section at the lowest energy of $E_{\mathrm{c.m.}} = 0.12$ MeV was obtained with the highest sensitivity compared to previous data [2,3,4]. The measured $^7\mathrm{Be}(d,p)^8\mathrm{Be}$ cross sections indicate a limited impact on the understanding of the CLP.
In addition, a future theoretical study is outlined, focusing on the exploration of potential resonant contributions in key nuclear reactions. In this approach, nuclear reaction cross sections are parametrized using an external functional form, enabling a flexible inclusion of resonant contributions within the BBN reaction network. This allows us to investigate whether such resonant effects could modify reaction rates relevant to BBN and thereby provide further insight into the CLP.
References
[1] S. Q. Hou et al., Phys. Rev. C 91, 055802 (2015).
[2] R. Kavanagh, Nucl. Phys. 18, 492--501 (1960).
[3] C. Angulo et al., ApJ 630, L105 (2005).
[4] N. Rijal et al., Phys. Rev. Lett. 122, 182701 (2019).