The mechanism by which stars shine in the universe and explosions such as supernovae occur can be explained by the action of small atomic nuclei inside atoms. "Astronuclear physics" is a research field that connects celestial bodies floating in the vast universe with tiny atomic nuclei. One of the major goals of astronuclear physics is to answer the fundamental question of where and how the elements that make up the Earth we live on and the bodies of living things were created. In this universe that began with the Big Bang, heavy elements have been created from light elements in many stars, but it is mainly the power of atomic nuclei that controls the fate of stars from their birth to their demise.

Big-bang Nucleosynthesis

The universe is thought to be created as an extremly high temperature and dense condition, which is called as Big Bang. Three minutes after the moment of Big Bang, the Big-Bang nucleosynthesis (BBN), which is the first nucleosynthesis in the universe, occurs, making the material of the stars that are created later. The elements synthesized at that time were mainly hydrogen, helium and small amount of lithium. In spite of its simplicity, BBN is still not understood completely to the details. In particular, the discrepancy of the observed and calculated abundance of lithium remains as a big problem. This problem is still bothering the reseachers in the world, but a solution could be given by a precise study of the nuclear reactions in BBN. The nuclear astrophysics group in CNS is making experimental studies on the reactions of Beryllium-7 with neutron or deutron at the BBN temperature. (Yamaguchi Lab.)

X-ray bursts and Reactions of Unstable Nuclei

X-ray burst is known as the most frequent thermonuclear explosion in the universe. Observations have been made for the amount of X-ray decaying in time (light curve) for various X-ray burst events, however, we still cannot reproduce the light curve completely by simulation. One missing factor is a precise nuclear reaction rate, which is not easy to measure for reactions with unstable nuclei, while they play an essential role in X-ray bursts, as the processes of proton-rich nuclei (rp-process, αp-process). By making use of CRIB, the RI beam separator of CNS, we have been studying reactions involving unstable nuclei. (Yamaguchi Lab.)

Galactic gamma-ray and Aluminum-26 Nucleus

By the advanced technology in the observation of cosmic gamma-rays, now we know a plenty of gamma-rays originating from the aluminum-26 (26Al) nucleus are coming to the earth from the galaxy. 26Al is a relatively long-lived unstable nuclide, and cannot be found much on the earth, but it exists in the stars. This observation is an evidence of an on-going nucleosynthesis. However, which kind of stars are producing how much 26Al is still unclear. What complicates the problem is that the 26Al nucleus has an “isomer” state, in additon to the normal (ground) state. At CNS, we succeeded in making an isomeric 26Al beam, and making a unique study on the reaction with 26Al. (Yamaguchi Lab.)

Origin of heavy elements in the universe, supernovae vs neutron-star merger

鉄よりも重い重元素の起源は、人類にとって未解決の物理学的課題の一つです。これらの重元素は、中性子捕獲反応とβ崩壊を通じて生成されます。金や白金のような元素を宇宙で合成するためには、爆発的な環境が必要であり、中性子密度が 10²⁰ cm⁻³を超え、温度が 10⁹ K以上であることが求められます。

このような極限状態では、わずか数秒のうちに水素からウランに至るまでの元素が合成されます。この元素合成過程は**r過程（急速中性子捕獲過程）**と呼ばれ、これまで超新星爆発がその主要な候補とされてきました。しかし、最新のシミュレーションでは、ウランのような重い元素まで合成することは困難であることが示されています。

近年では、中性子星同士の合体後にランタノイド元素が間接的に観測されたことで、中性子星連星の合体が新たな有力候補として注目を集めています。このような爆発的な環境下では、自然界には存在しない中性子過剰核が関与し、その核反応が反応過程全体の律速段階となります。

しかしながら、これらの中性子および中性子過剰核に関する反応率は、理論モデルによって一桁から二桁以上の差が生じることがあり、大きな不確実性を抱えています。

私たちは、放射性核廃棄物の消滅手法研究において開発した新しい実験的手法を応用し、これらの反応率を実験的に評価することで、重元素の起源解明に迫ります。(Imai Lab.)