快速中子捕獲過程中的中子捕獲反應平衡

Abstract

快速中子捕獲過程為雙中子星合併的噴射物提供放射性動力，而這些噴射物被人們觀測並命名為千新星。同時，快速中子捕獲過程中的核能釋放也會影響噴射物動力學，例如吸積盤的外流質量，或者也可能影響為後期伽馬射線放射提供動力的被吸積物質的吸積率。因此，在千新星噴射物的流體模擬中加入核合成計算核、考慮核能釋放的熱量是有意義的。先前的研究表明，快速中子捕獲過程中的主要反應—中子捕獲和光解離，可能在某些環境條件下達到平衡。在假設平衡的條件下，同位素豐度可以由一個簡單的公式決定，這可用於簡化並加速核合成的計算，使得在流體模擬中加入核合成計算的想法可以更實際。在這篇論文中，我們研究了在什麼條件下中子捕獲和光解離之間會達到平衡，並測試在假設平衡條件下的核合成計算可以加速多快，以及評估用此方法計算出的核能釋放熱量會有多少誤差。

R-process nucleosynthesis in material ejected from neutron star mergers radioactively power the ejecta, observed as kilonovae. Meanwhile, the nuclear energy release during the r-process can also affect ejecta dynamics, such as the amount of the outflow mass from the accretion disc, or the accretion rate of fall-back material that may also power late-time gamma-ray emission. Thus, it is meaningful to consider the nucleosynthesis heating in the hydrodynamic simulation of kilonovae ejecta. Previous studies show that the dominant reactions in the r-process, neutron captures and photodissociation, may reach equilibrium for certain environmental conditions. Given the equilibrium, the isotopic abundances can be determined by a simple formula, which may be used to speed up the calculation of nucleosynthesis, and make it more practical to couple the hydro simulation and r-process nucleosynthesis. In this thesis, we study under what condition the equilibrium between neutron captures and photodissociation would be attained; how fast the calculation may be accelerated, and how robust the nuclear energy generation can be modeled with the assumption of equilibrium.