From nuclear clusters to neutron stars

doi:10.1126/science.abf2427

GSTDTAP > 气候变化

DOI	10.1126/science.abf2427
	From nuclear clusters to neutron stars
	Or Hen
	2021-01-15
发表期刊	Science
出版年	2021
英文摘要	When neutron stars are formed, their massive gravitational pressure “crushes” most of their protons and electrons into neutrons. Understanding the interaction between the remaining protons (∼5%) and neutrons in the star's core is required to model the neutron star equation of state, which relates its pressure and density and determines many of its macroscopical properties. These proton-neutron interactions are governed by the extremely complex strong nuclear force that challenges direct EOS calculations. This problem is traditionally overcome by using effective mean-field models. Although overall instructive, the important impact of nucleon-nucleon correlations and nuclear clusters are usually not explicitly included in these calculations. On page 260 of this issue, Tanaka et al. ([ 1 ][1]) now report precise measurements of α cluster (helium-4 nuclei) formation in the outer neutron “skin” of a wide range of neutron-rich tin isotopes that should help constrain models. A neutron star is born when a massive star runs out of nuclear fuel. After all of the matter at the very center of the star has been transformed into iron or similar elements through nuclear fusion, it collapses under its own gravitational pressure and leaves an extremely compact remnant ([ 2 ][2]). A typical neutron star is about as big as Manhattan (∼10 km radius) but has about 1.4 times the mass of the Sun (∼106 times the mass of Earth). The density inside their core ranges from about two to five times that of atomic nuclei. In the mean-field models often used to study nuclei and neutron stars, individual interactions between particles (neutrons and protons) are averaged and replaced by the average potential experienced by each particle in the system. Such models contain tunable parameters whose values are chosen to reproduce known properties of atomic nuclei. However, these parameters could depend on the neutron-proton asymmetry and system density, both of which are far higher in the star's core than in normal atomic nuclei. Neutron-proton asymmetry effects are described by the nuclear symmetry energy, E sym(ρ), that describes the difference in the energy between symmetric nuclear matter (in which the number of protons equals the number of neutrons) and pure neutron matter at density ρ. The density dependence of the symmetry energy is then described by its slope, L (ρ) = 3ρ dE sym(ρ)/ d ρ. The symmetry energy is well constrained at the density of terrestrial atomic nuclei (ρ), but its density dependence L (ρ) is not ([ 3 ][3], [ 4 ][4]). Determining L would greatly improve modeling of the matter inside the cores of neutron stars. Usually, the method used to constrain L is to measure nuclear neutron skins, which are the differences between the radii of the neutron and proton distributions in neutron-rich nuclei ([ 2 ][2], [ 5 ][5]). This method is based on the density decrease of atomic nuclei near their surface; neutrons are pushed out to a larger radius depending on L . This structural model is based on mean-field calculations, which show a strong correlation between calculated neutron skins and the density dependence of the symmetry energy ([ 5 ][5]) but usually neglect the effects of nucleon-nucleon correlations and nuclear clusters. ![Figure][6] Probing neutron skins The neutron skin, the region where neutron density exceeds proton densities in nuclei, is affected by a cluster formation in the outer low-density regions (as measured by Tanaka et al. ) and short-ranged clusters at higher-density regions. GRAPHIC: N. DESAI/SCIENCE Because protons are electrically charged, the proton distribution radius of many nuclei has been measured with electromagnetic probes ([ 6 ][7], [ 7 ][8]). By contrast, neutrons are electrically neutral, and measuring their distribution radii has proven to be an extremely challenging task. The experiment of Tanaka et al. used a 392 MeV proton beam to knock out α clusters from the nuclear surface. The measured cross sections, which decreased with increasing mass number, are directly sensitive to the abundance of α clusters. Their dependence on the proton-neutron asymmetry of the measured isotopes showed excellent agreement with the mean-field models that include cluster effects ([ 8 ][9], [ 9 ][10]). The same calculations also show that cluster formation affects the correlation between the neutron skin and the density dependence of the symmetry energy. This finding is supported by the recent chiral dynamics study of Miller et al. ([ 10 ][11]), which found that a different type of clustering—two-nucleon short-range correlations ([ 11 ][12], [ 12 ][13])—can also potentially affect the thickness of the neutron skin. The impact of such cluster correlations was previously considered but deemed to be negligible compared with the large experimental uncertainties in the determination of the neutron skin. This assumption is now being challenged by a new generation of high-precision experiments. Most notably, the PREX (208Pb Radius Experiment) collaboration ([ 13 ][14]) at the U.S. Jefferson Lab accelerator uses parity-violating elastic electron scattering to directly measure the neutron radius of lead and recently reported achieving 1% accuracy. Along with precision measurements made with pion photoproduction reactions ([ 14 ][15]) by the Crystal Ball collaboration at the German Mainz Microtron (MAMI) accelerator, future measurements may reach 0.5% accuracy. The results of Tanaka et al. call for new precision theoretical calculations that directly account for nuclear correlations and clusters. Such theoretical advances are in progress ([ 15 ][16]) and will enable a precise extraction of the density dependence of the symmetry energy from the neutron skin measurements. 1. [↵][17]1. J. Tanaka et al ., Science 371, 260 (2021). [OpenUrl][18][Abstract/FREE Full Text][19] 2. [↵][20]1. J. M. Lattimer, 2. M. Prakash , Science 304, 536 (2004). [OpenUrl][21][Abstract/FREE Full Text][22] 3. [↵][23]1. J. M. Lattimer, 2. Y. Lim , Astrophys. J. 771, 51 (2013). [OpenUrl][24][CrossRef][25] 4. [↵][26]1. B. A. Li, 2. X. Han , Phys. Lett. B 727, 276 (2013). [OpenUrl][27] 5. [↵][28]1. X. Roca-Maza et al ., Phys. Rev. Lett. 106, 252501 (2011). [OpenUrl][29][CrossRef][30][PubMed][31] 6. [↵][32]1. I. Angeli, 2. K. P. Marinova , At. Data Nucl. Data Tables 99, 69 (2013). [OpenUrl][33][CrossRef][34] 7. [↵][35]1. R. F. Garcia Ruiz et al ., Nat. Phys. 12, 594 (2016). [OpenUrl][36] 8. [↵][37]1. S. Typel, 2. G. Ropke, 3. T. Klahn, 4. D. Blaschke, 5. H. H. Wolter , Phys. Rev. C Nucl. 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领域	气候变化 ; 资源环境
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引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/311473
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Or Hen. From nuclear clusters to neutron stars[J]. Science,2021.
APA	Or Hen.(2021).From nuclear clusters to neutron stars.Science.
MLA	Or Hen."From nuclear clusters to neutron stars".Science (2021).