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Nuclear Mass Measurement for Very Short-Lived Nuclei and the Impact on

 Nucleosynthesis in Explosive Stars


In a recent paper [1] published in Physical Review Letters, scientists of the Institute of Modern Physics (IMP), Chinese Academy of Sciences, have reported their new results from the Cooler Storage Ring (CSR) at the Heavy Ion Research Facility in Lanzhou, China. This is one of the recently constructed major scientific facilities in China, which is only the second storage ring of this kind in the world, the first being at the Helmholzzentrum fuer Schwerionenforschung in Darmstadt, Germany. In this experiment, the exotic nuclei were produced by projectile fragmentation of 78Kr and injected into the experimental storage ring. In this way, the scientists at IMP were able to determine the nuclear masses of 63Ge, 65As, 67Se, and 71Kr, which have never been measured before. 

Weighing the masses of very short-lived nuclei is extremely difficult. For many years, only theoretical masses for these nuclei were listed in the nuclear mass table [2]. Data of a test experiment at IMP prior to the one discussed above had error bars which were too great [3], thus definite conclusions regarding masses could not be drawn[4]. But knowing the exact masses of short-lived nuclei is crucial for understanding the processes of energy release and element production in the universe. The origin of heavy elements (heavier than iron) has been one of the major unsolved puzzles in physics [5]. The point that Tu et al. emphasize [1] is that a small correction in the masses of the exotic nuclei can make a great impact on the question of the origin of the elements in the universe.

The claim is related to the astrophysical objects called x-ray bursts, a class of x-ray binary stars exhibiting periodic and rapid increases in luminosity peaked in the x-ray region of the electromagnetic spectrum. The associated systems are composed of an accreting neutron star and a companion. Material (rich in hydrogen and helium) in the companion star can transfer onto the neutron star surface. It is compressed and heated to extremely high temperatures so that thermonuclear runaways take place. The explosive stellar nucleosynthesis begins with the hot CNO cycle which quickly yields to what is called the rapid proton capture process (or rp-process) [6]. In the rp-process, protons fuse with seed nuclei, synthesizing isotopes near the proton drip-line ? the boundaries of nuclear existence beyond which nuclei simply cannot bind another proton. The so-produced short-lived nuclei in the path eventually decay and populate nuclei toward the region of stability. It is believed that many elements in the universe are created in this way.
Simulation of nucleosynthesis in the rp-process requires reliable nuclear physics input [6]; in particular, nuclear mass, or more specifically the proton separation energy (the energy it takes to remove one proton from a nucleus) is critically important. Theoretical mass values have to be used if there are no measurements. Thus discussions on rp-process nucleosynthesis rely heavily on the quality of mass predictions. However, as pointed out by Tu et al. [1], the predictions are not always reliable. In fact, detailed x-ray burst model calculations that use the new mass value of 65As has changed the traditional view of nucleosynthesis [7]. As pointed out in Ref [1], this leaves many possibilities for the later stages of element production in the x-ray burst environment, and could open up new research opportunities in astrophysics, nuclear physics, and astronomical observations.



[1] Tu X L, Xu H S, Wang M, et al. Direct mass measurements of the short-lived A = 2Z-1 nuclides 63Ge, 65As, 67Se and 71Kr and their impact on nucleosynthesis in the rp process, Phys. Rev. lett. 2011, 106:112501

[2] Audi G, A. Wapstra H, Thibault C. The AME 2003 atomic mass evaluation: (II). Tables, graphs and references, Nucl. Phys. A 2003, 729, 337-676.

[3] Xu H S, Tu X L, Yuan Y J, et al. First mass measurement of short-lived nuclides at HIRFL-CSR, Chinese Sci. Bull. 2009, 54: 4749-4752

[4] Sun Y. Nuclear masses near the proton drip-line and their impact on nucleosynthesis in explosive stars, Chinese Sci. Bull. 2009, 54: 4594-4595

[5] Haseltine E, et al. The 11 Greatest Unanswered Questions of Physics, Discovery (Feb. 1, 2002)

[6] Schatz H, Aprahamian A, Goerres J, et al. rp-process nucleosynthesis at extreme temperature and density conditions, Phys. Rep. 1998, 294: 167-263

[7] Sun Y. A small difference in nuclear mass could make a big impact in the cosmos, Chinese Sci. Bull. 2011, 56: 1637-1638

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