A strong neutron burst in jet-like supernovae of spinstars

Context. Some metal-poor stars have abundance patterns, which are midway between the slow (s) and rapid (r) neutron capture processes. Aims. We show that the helium shell of a fast rotating massive star experiencing a jet-like explosion undergoes two efficient neutron capture processes: one during stellar evolution and one during the explosion. It eventually provides a material whose chemical composition is midway between the s- and r-process. Methods. A low metallicity 40 M⊙ model with an initial rotational velocity of ∼700 km s−1 was computed from birth to pre-supernova with an extended nuclear network following the slow neutron capture process. A two-dimensional hydrodynamic relativistic code was used to model a E = 1052 erg relativistic jet-like explosion hitting the stellar mantle. The jet-induced nucleosynthesis was calculated in post-processing with an optimised network of 1812 nuclei. Results. During the star’s life, heavy elements from 30 ≲ Z ≲ 82 are produced thanks to an efficient s-process, which is boosted by rotation. At the end of evolution, the helium shell is largely enriched in trans-iron elements and in (unburnt) 22Ne, whose abundance is ∼20 times higher than in a non-rotating model. During the explosion, the jet heats the helium shell up to ∼1.5 GK. It efficiently activates (α, n) reactions, such as 22Ne(α, n), and leads to a strong n-process with neutron densities of ∼1019 − 1020 cm−3 during 0.1 s. This has the effect of shifting the s-process pattern, which was built during stellar evolution, towards heavier elements (e.g. Eu). The resulting chemical pattern is consistent with the abundances of the carbon-enhanced metal-poor r/s star CS29528-028, provided the ejecta of the jet model is not homogeneously mixed. Conclusions. The helium burning zones of rotating massive stars experience an efficient s-process during the evolution followed by an efficient n-process during a jet-like explosion. This is a new astrophysical site which can explain at least some of the metal-poor stars showing abundance patterns midway between the s- and r-process.

An investigative study on neutron emissions from titanium- deuterium system under thermal shock

The purpose of this study is to investigate the titanium-deuterium system under thermal shock, as a potential neutron source. The expected neutron emission is unique, i.e. it is monoenergetic with energy of 2.45 MeV, which is valuable for calibrating neutron detectors. In our study, titanium was loaded with deuterium gas at room temperature in an experimental system, and the system was subjected to rapid thermal cycling by repeated cooling with liquid nitrogen, followed by rapid warm up phases to create a non-equilibrium condition in titanium lattice. Neutron bursts were monitored using a [superscript 3]He detector, which responds to slow neutrons, a moderated [superscript 3]He detector, which responds to slow and fast neutrons, and a proton recoil detector, which responds to fast neutrons. The pressure and temperature of the system was monitored throughout the experiments. The result of this work shows that: 1) loading of titanium with deuterium gas should be done under high vacuum conditions (less than1 X 10[superscript 6] torr) to remove environmental contaminants, which was found to inhibit the titanium-deuterium reaction, 2) cracks observed in titanium samples from lattice stress varied in size and location in titanium lattice and dependent on the level of deuterium loading. The presence of cracks in some locations indicates that the titanium-deuterium reaction is a local effect, 3) low level neutron burst were observed in less than 23% of all experiments and involved the detection of a single neutron burst, suggesting that neutron emission is a statistical process occurring at low probability. The neutron burst was observed from partially deuterated titanium samples. The level of neutrons detected is consistent with what has been reported in literature. 4). A large temperature increased from room temperature to 450 [degree sign]C during phase transition from [delta]-titanium to [delta]-titanium occurred, but no neutrons were observed. The temperature increased is likely associated with the exothermic reaction that occurs during hydride formation, which does not lead to neutron emission. 5) No evidence of tritium or nuclear transmutation was observed in our experimental system.

New Attempts to Understand Nanodiamond Stardust

We report on a concerted effort aimed at understanding the origin and history of the pre-solar nanodiamonds in meteorites including the astrophysical sources of the observed isotopic abundance signatures. This includes measurement of light elements by secondary ion mass spectrometry (SIMS), analysis of additional heavy trace elements by accelerator mass spectrometry (AMS) and dynamic calculations of r-process nucleosynthesis with updated nuclear properties. Results obtained indicate that: (i) there is no evidence for the former presence of now-extinct 26Al and 44Ti in our diamond samples other than what can be attributed to silicon carbide and other ‘impurities’, and this does not offer support for a supernova (SN) origin but neither does it negate it; (ii) analysis by AMS of platinum in ‘bulk diamond’ yields an overabundance of r-only 198Pt that at face value seems more consistent with the neutron burst than with the separation model for the origin of heavy trace elements in the diamonds, although this conclusion is not firm given analytical uncertainties; (iii) if the Xe–H pattern was established by an unadulterated r-process, it must have been a strong variant of the main r-process, which possibly could also account for the new observations in platinum.