One Step Forward to Resolve the Contradiction in the Amount of Lithium Produced by the Big Bang (Cosmology)

Announcement points

  • Big bang nucleosynthesis(Note 1) in accordance 7 of Li estimators of observables 3 – 4 times greater was measured nuclear reaction cross sections relating to “Space lithium problem”, 7 possible estimate of Li is adjusted downward about 10% Showed sex.
  • Trojan horse method for reactions between unstable nuclei ( 7 Be + neutrons) that are difficult to measure directlyIt was measured using the indirect method (Note 2), and the contribution of thepreviously unknowntransition of 7 Li to the first excited state was quantitatively shown for the first time.
  • It is a big step forward to be able to propose a clear experimental reduction of the 7 Li estimator toward the solution of the space lithium problem . In addition, this experimental method is expected to be applied to other neutron + unstable nuclear reaction measurements in the future.

Presentation summary

The estimated production of hydrogen and helium isotopes by Big Bang Nucleosynthesis (BBN) is very consistent with the observations, which is one of the great evidences supporting the standard Big Bang theory. On the other hand, although the amount of lithium produced is much smaller, the “space lithium problem” has long been the problem that the estimated value is 3 to 4 times the observed value even if the indeterminacy of theory and observation is carefully considered. It remains unresolved.

The key to the amount of fragile 7 Li produced in BBN is that of the parent 7 Be. Although some experimental progress has been made in recent years, no results have been reported that impose new restrictions on BBN.

Specially Appointed Assistant Professor Hayakawa and Lecturer Yamaguchi of the Nuclear Science Research Center, Graduate School of Science, the University of Tokyo have measured the cross-sectional area of the 7 Be + neutron reaction, which works to reduce the amount of 7 Be produced. It was shown that the estimated amount of 7 Li may be revised downward by about 10% due to the contribution of the transition to the 7 Li first excited state . In this study, we applied an indirect method called the Trojan horse method to measure the reaction between neutrons and unstable nuclei, which are difficult to measure directly.

While there are various theoretical proposals for solving the space lithium problem, it can be said that the fact that we were able to propose a clear reduction in the 7 Li estimator by experiments is a step toward the solution. In addition, this experimental method is expected to be applied to other neutron + unstable nuclear reaction measurements in the future.

Announcement content

Background of the study The
standard Big Bang theory is now widely supported, and one of the definitive proofs is the Big Bang nucleosynthesis (BBN). However, as mentioned above, although the overall depiction of light nucleosynthesis can be made consistent with the observation, there is a “cosmic lithium problem” that cannot explain the difference between the theoretical estimate and the observation estimate of the amount of lithium produced. To solve the problem, the primitive 7 Li amount estimated from the observation of low metal stars in the early universe, the baryon / photon number ratio derived from the cosmic microwave background radiation observation, and the BBN model calculation based on the nuclear reaction data. Each must be verified for consistency (Fig. 1).

Figure 1: Baryon / photon ratio dependence of  7 Li production by BBN  (C. Pitrou et al. Phys. Rep. 754 (2018) 1 – 66).  The difference between the theoretically calculated  7 Li amount and the estimated value from the low metal amount star observation is the space lithium problem. The new  7 Li estimator from  this study is also shown (red line). © University of Tokyo

From a nuclear physics point of view, the 7 Li produced in BBN reacts with a large amount of protons and is immediately decomposed into two 4 He particles. On the other hand, the isobaric 7 Be survives until after the end of BBN, which lasts about 20 minutes, and is converted to 7 Li and stabilized by electron capture with a half-life of 53 days . In other words, it is rather the parent nucleus 7 Be that determines the amount of 7 Li produced in BBN , and the reaction cross section of the nuclear reaction involved in the increase or decrease in the amount of 7 Be produced is important (Fig. 2).

Figure 2: Important reaction processes in BBN and the role of the reactions measured in this study. © University of Tokyo

In recent years, several nuclear reaction experiments related to the amount of 7 Be produced have been reported one after another by the group of INFN-Bari in Italy and the group of Kyoto University. Although significant progress from the perspective of experimental nuclear physics, no results have been reported to impose new restrictions on BBN. Also, the most influential 7 Be + n → p + 7 Li reaction(Note 3) isthe subject of long-term discussion onlyfor the transition to the ground state (p 0 + 7 Li), and the transition to the first excited state (p 1 + 7 Li *).How much contribution (Note 4) has remained unmeasured.

One of the reasons why sufficient experimental data are not obtained by measuring the 7 Be + n reaction is that it is difficult to directly measure the reaction as a beam + target because both of them are unstable nuclei. In this study, the cross section of the 7 Be + n → p + 7 Li, 7 Be + n → 4 He + 4 He reaction, which greatly affects the amount of 7 Be produced, was measured.

Research content: Specially appointed assistant professor Hayakawa and lecturer Yamaguchi of the Nuclear Science Research Center, Graduate School of Science, The University of Tokyo, National Institute of Nuclear Physics, Italy-Southern National Institute (INFN-LNS), Seiseikan University, Korea The international collaborative research group, which consists of the above, measured the cross-sectional area of the 7 Be neutron capture reaction , which works to reduce the amount of 7 Be produced , and changed to the 7 Li first excited state , which had not been measured so far . It was shown that the contribution may lower the estimated amount of 7 Li by about 10%. This study is a Trojan horse methodThis is one of the first examples of the reaction measurement of neutrons and unstable nuclei, which is difficult to measure directly, by applying the indirect method (Note 2) .

The Trojan horse method uses a deuterium target instead of neutrons, and is a quasi-free reaction process of 7 Be + d → p + p + 7 Li, 7 Be + d → p + 4 He + 4 He reaction.By kinematically selecting the contribution of (Note 5) ,the cross sectionof the above 7 Be + n → p + 7 Li, 7 Be + n → 4 He + 4 He reaction can be derived (Fig. 3). ).

Figure 3: Experimental setup (left) and a conceptual diagram of the Trojan horse method (right). © University of Tokyo

The experiment was conducted by the Center-for-Nuclear-Study Radioactive Isotope Beam, a radioisotope beam generator at the Nuclear Science Research Center, Graduate School of Science, the University of Tokyo, which receives a beam from the AVF Cyclotron at the Nishina Accelerator Research Center of RIKEN. separator (CRIB, (Note 6) In ), a 7 Be beam was generated and separated. A deuterium target (polyethylene deuterated) was irradiated with a 7 Be beam,andpairs of 7 Li – p and 4 He – 4 Heemitted through a nuclear reaction weredetected by 6 pairs of silicon detectors (Fig. 3). .. The event of the quasi-free reaction process was extracted from the detected particle position and energy information, and thereaction cross section of7Be + n → p + 7 Li, 7 Be + n → 4 He + 4 He was derived (Fig. 4).

Figure 4: Comparison of cross-sectional area (× center of gravity energy 1/2 ) data measured in this experiment with past data. The fit by R matrix analysis is also depicted. The position of the vertical arrow indicates the main resonance level. Horizontal arrows indicate the key energy ranges for each reaction channel at typical BBN temperatures (0.7 GK). In this study, 7 p transition to the first excited state of the Li 1 + 7 the Li * channel contribution was first measured at BBN energy region at the BBN 7 was estimated effects on Li generation amount. © University of Tokyo

The cross-sectional area data obtained in the experiment, together with the data available in the past, is the resonance structure of 8 Be, which is a complex nucleus.R matrix analysis considering (Note 7)Fitted by (Note 8) , the cross-sectional area was continuously derived over a wide energy range of 10 meV – 1 MeV (Fig. 4). As a result of applying the thermal reaction rate obtained using this to the BBN calculation, the conventionally used 7 Be + n → p + 7 Li reaction rate was replaced with that of this study (Fig. 5), and lithium-hydrogen.It was found that thenumber ratio 7Li / Hdecreasedfrom5.63 +0.22 -0.24 × 10 −10 to 5.18 +0.22 -0.25 × 10 −10 (Fig. 1). Since the estimated value extrapolated from the observation of the amount of lithium in the early objects is 7 Li / H = 1.58 ± 0.3 × 10 −10, it is possible that other factors are needed to completely solve the space lithium problem. Obviously, thefact that we were able to experimentally show that the 7 Li / H value was reduced by about 10%without changing any other conditions of the BBNcould help guide problem solving in the right direction in the future. Are expected.

Figure 5: 7 Be + n → p + 7 Li Comparison of reaction rates. The response rate in this study is higher than the Cyburt 2004 response rate, which is widely used for BBN calculation, and the indeterminacy is greatly improved compared to the Damone 2018 response rate (INFN-Bari group) in the previous experiment. Understand. ©University of Tokyo

Social significance / future plans
Various solutions have been theoretically proposed for the space lithium problem, but no consensus has been obtained yet. The fact that we were able to propose a clear reduction in the 7 Li estimator from the experiment is a step forward toward solving the problem. In addition, this experimental method is expected to be applied to other neutron + unstable nuclear reaction measurements in the future. Although this study is motivated by purely scientific interests, revealing the origins of the elements used in industry is how precious and epic space journeys each have in our hands. I think it has social significance in the sense that it reminds me of that.

Publication magazine

  • Journal title: Astrophysical Journal Letters
  • Paper title: Constraining the Primordial Lithium Abundance: New Cross Section Measurement of the 7 Be + n Reactions Updates the Total 7 Be Destruction Rate .
  • Author: S. Hayakawa *, M. La Cognata, L. Lamia, H. Yamaguchi, D. Kahl, K. Abe, H. Shimizu, L. Yang, O. Beliuskina, S. Cha, KY Chae, S. Cherubini, P Figuera, Z. Ge, M. Gulino, J. Hu, A. Inoue, N. Iwasa, A. Kim, D. Kim, G. Kiss, S. Kubono, M. La Commara, M. Lattuada, E. Lee, JY Moon, S. Palmerini, C. Parascandolo, S. Park, VH Phong, D. Pierroutsakou, RG Pizzone, GG Rapisarda, S. Romano, C. Spitaleri, XD Tang, O. Trippella, A. Tumino, and N. Zhang.
  • DOI number: 10.3847 / 2041-8213 / ac061f
  • Abstract URL:


Note 1: Big Bang nucleosynthesis

With the expansion of the post-universe beginning of time, the nuclei gradually lowered temperature to the extent that binding (approximately 3 minutes after the Big Bang started, <10 9 K) deuterium is, nuclei of protons and neutrons are one by one bond A sufficient amount can be present, which will be a stepping stone for nucleosynthesis in the future. By the time the universe expands further and the material density in the universe becomes too small for nucleosynthesis (about 20 minutes after the start of the Big Bang), isotopes of light elements such as hydrogen, helium, and lithium are produced. 

Note 2: Trojan horse method

The trojan horse method is to select an appropriate alternative nucleus (deuteron target for proton or neutron capture reaction) and to measure the energy region important in space nuclear physics (typically <1 MeV, for nuclear experiments) It is an indirect method that can measure the reaction cross section (in the category of “low energy”). This is made possible by kinematically selecting a “quasi-free” virtual reaction with the nucleon in the deuteron. It has been developed mainly by collaborators, the INFN-LNS group in Catania, Italy. In analogy to Greek mythology, a trojan horse (deuteron) can send soldiers (protons or neutrons) and measure the reaction without being affected by the castle wall (Coulomb barrier), so it has historically been mainly applied to proton capture reactions. However, in principle, it can also be applied to neutron capture reactions without the Coulomb barrier, and this study is one of the first attempts to apply the trojan horse method to unstable nuclei + neutron reactions. .. 

Note 3:  7 Be + n → p + 7 Li reaction

At first glance, it seems that the amount of 7 Li is increased, but during BBN, the generated 7 Li is immediately destroyed by 7 Li + p → 4 He + 4 He, so this reaction process also decreases the amount of 7 Li. Work in the direction. (See Fig. 2.)   

Note 4: Transition to the first excited state (p 1 + 7 Li *)

7 Be + n produced by the reaction 7 Li is possible to take the lowest ground state of quantum mechanical energy, the state of being energetically excited, different cross sections in each case. The latter was measured for the first time in this study. 

Note 5: Semi-free reaction process

Here, it refers to a state in which the momentum distribution of protons in deuterons does not change before and after the reaction (that is, it is a “bystander” who is not involved in the reaction). This can actually be confirmed from the experimental data.  

Note 6: CRIB

CRIB (Center for Nuclear Study Radioactive Ion Beam separator) is a device for generating and separating low-energy unstable nuclear beams, and due to its energy characteristics (typically several MeV per nucleon), astronomical nuclear reactions and atomic nuclei It is mainly used to study the cluster structure of. 

Note 7:  8 Be resonance structure

Resonance in a nuclear reaction is a phenomenon in which the likelihood of a reaction (reaction cross section) increases sharply at a certain reaction energy, and the resonance of 7 Be + n is greatly affected by the structure of the excited level of the composite nucleus 8 Be. Receive. 

Note 8: R matrix analysis

The R queuing theory is a framework that describes the nuclear reaction quantum mechanically based on the boundary conditions inside and outside the complex nucleus and expresses the resonance scattering phenomenon. The analysis using this theory has the advantage that the excitation function of the reaction / scattering cross section obtained in the experiment can be directly calculated based on the parameters such as the energy eigenvalue and spin parity partial width that characterize the resonance. 

Provided by University of Tokyo

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