The impact of individual nuclear properties on r-process nucleosynthesis

Abstract

The astrophysical rapid neutron capture process or 'r process' of nucleosynthesis is believed to be responsible for the production of approximately half the heavy element abundances found in nature. This multifaceted problem remains one of the greatest open challenges in all of physics. Knowledge of nuclear physics properties such as masses, β-decay and neutron capture rates, as well as β-delayed neutron emission probabilities are critical inputs that go into calculations of r-process nucleosynthesis. While properties of nuclei near stability have been established, much still remains unknown regarding neutron-rich nuclei far from stability that may participate in the r process. Sensitivity studies gauge the astrophysical response of a change in nuclear physics input(s) which allows for the isolation of the most important nuclear properties that shape the final abundances observed in nature. This review summarizes the extent of recent sensitivity studies and highlights how these studies play a key role in facilitating new insight into the r process. The development of these tools promotes a focused effort for state-of-the-art measurements, motivates construction of new facilities and will ultimately move the community towards addressing the grand challenge of 'How were the elements from iron to uranium made?'. IntroductionOne of the major open questions in all of physics is the identification of the sites responsible for the production of the heaviest elements [1,2]. It has been understood since the 1950s that the solar system abundances of nuclei heavier than iron can be divided roughly in half based on the nucleosynthesis processes that create them. Slow neutron capture process, or s-process, nuclei lie along the middle of the valley of stability, and rapid neutron capture process, or r-process, nuclei are found on the neutron-rich side, with a third process, the p process, responsible for the significantly less abundant nuclei on the proton-rich side of stability [3,4]. Since that time much progress has been made, e.g. [5], and the basic mechanisms of and astrophysical sites for the creation of the s-process [6] and heavy p-process [7,8] nuclei are on a firm footing. The site or sites of the r process still evade definitive determination [9,10,11].The r-process pattern is extracted from the solar system abundances by subtracting the s-process and p-process contributions [12,13]. The residual pattern consists of three main abundance peaks at A ∼ 80, 130, and 195, associated with the N = 50, 82, and 126 closed shells. Building up to the heaviest r-process elements requires on the order of 100 neutrons per seed nucleus. Additional constraints come from meteoritic data, e.g. [14], and observations of r-process elements in old stars in the galactic halo, e.g. [15,16]. This data points to distinct origins for the light A < 120 ('weak') and heavy A > 120 ('main') r-process nuclei. The pattern of main r-process elements is remarkably similar among r-process enhanced halo stars and is a match to the solar residuals. Thi...

Systematic and Statistical Uncertainties in Simulated r-Process Abundances due to Uncertain Nuclear Masses

“…Fig. 10 in [3]. They are significantly broader, however, than those estimated by applying a range in mass models as in Fig.…”

“…For an r process that proceeds in equilibrium between neutron captures and photodissociations, masses determine the relative abundances along each isotopic chain. Masses are also crucial inputs in the calculations of all other nuclear physics properties important for the r process, including β-decay properties and neutron capture rates; see recent reviews [2,3] and references therein.…”

“…On the nuclear physics side, very little experimental information is available for the thousands of nuclei that participate in the r process. Different theoretical models applied to determine the remaining data can disagree by large factors [3]. Here we examine an aspect of the latter limitation and attempt to quantify uncertainties in r-process abundance patterns that result from uncertainties in the masses of nuclei out to the neutron drip line.…”

The Rare Earth Peak and the Astrophysical Location of the r Process

“…The uncertain nuclear inputs to the r process are of utmost importance in predicting the final abundances observed in nature [1]. One way to move forward in solving this difficult problem is to try to elucidate the uncertain nuclear physics far from stability which is responsible for key abundance features.…”

“…We discuss the influence of systematically slower or faster rare earth β-decay rates on the relative height of the REP and the impact on the predicted mass surface. 1 …”