Pair-instability (PI) is expected to open a gap in the mass spectrum of black holes (BHs) between $\approx{}40-65$ M$_\odot$ and $\approx{}120$ M$_\odot$. The existence of the mass gap is currently being challenged by the detection of GW190521, with a primary component mass of $85^{+21}_{-14}$ M$_{\odot}$. Here, we investigate the main uncertainties on the PI mass gap: the $^{12}$C($\alpha$, $\gamma$)$^{16}$O reaction rate and the H-rich envelope collapse. With the standard $^{12}$C($\alpha$, $\gamma$)$^{16}$O rate, the lower edge of the mass gap can be 70 M$_\odot$ if we allow for the collapse of the residual H-rich envelope at metallicity $Z\leq{}0.0003$. Adopting the uncertainties given by the STARLIB database, for models computed with the $^{12}$C($\alpha$, $\gamma$)$^{16}$O rate $-1\, \sigma$, we find that the PI mass gap ranges between $\approx{}80$ M$_\odot$ and $\approx{}150$ M$_\odot$. Stars with $M_{\rm ZAMS}>110$ M$_\odot$ may experience a deep dredge-up episode during the core helium-burning phase, that extracts matter from the core enriching the envelope. As a consequence of the He-core mass reduction, a star with $M_{\rm ZAMS} =160$ M$_\odot$ may avoid the PI and produce a BH of 150 M$_\odot$. In the $-2\,{}\sigma{}$ case, the PI mass gap ranges from 92 M$_\odot$ to 110 M$_\odot$. Finally, in models computed with $^{12}$C($\alpha$, $\gamma$)$^{16}$O $-3\,{}\sigma{}$, the mass gap is completely removed by the dredge-up effect. The onset of this dredge-up is particularly sensitive to the assumed model for convection and mixing. The combined effect of H-rich envelope collapse and low $^{12}$C($\alpha$, $\gamma$)$^{16}$O rate can lead to the formation of BHs with masses consistent with the primary component of GW190521.

Formation of GW190521 from stellar evolution: the impact of the hydrogen-rich envelope, dredge-up and $^{12}$C($α$, $γ$)$^{16}$O rate on the pair-instability black hole mass gap

The primary aim of experimental nuclear astrophysics is to determine the rates of nuclear reactions taking place in stars in various astrophysical conditions. These reaction rates are an important ingredient for understanding the elemental abundance distribution in our solar system and the galaxy. The reaction rates are determined from the cross sections which need to be measured at energies as close to the astrophysically relevant ones as possible. In many cases the final nucleus of an astrophysically important reaction is radioactive which allows the cross section to be determined based on the off-line measurement of the number of produced isotopes. In general, this technique is referred to as the activation method, which often has substantial advantages over in-beam particle- or $ \gamma$ -detection measurements. In this paper the activation method is reviewed from the viewpoint of nuclear astrophysics. Important aspects of the activation method are given through several reaction studies for charged particle, neutron and $ \gamma$ -induced reactions. Various techniques for the measurement of the produced activity are detailed. As a special case of activation, the technique of Accelerator Mass Spectrometry in cross section measurements is also reviewed.

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The activation method for cross section measurements in nuclear astrophysics

The JENSA gas-jet target was designed for experiments with radioactive beams provided by the rare isotope re-accelerator ReA3 at the National Superconducting Cyclotron Laboratory. The gas jet will be the main target for the Separator for Capture Reactions SECAR at the Facility for Rare Isotope Beams on the campus of Michigan State University, USA. In this work, we describe the advantages of a gas-jet target, detail the current recirculating gas system, and report recent measurements of helium jet thicknesses of up to about 1 0 19 atoms/cm 2 . Finally a comparison with other supersonic gas-jet targets is presented.

Status of the JENSA gas-jet target for experiments with rare isotope beams

The work aims to investigate the distribution versus depth of natural (238U, 232Th, 226Ra, 40K) and anthropogenic radionuclides (137Cs, 239Pu, 240Pu) along the Domitia coastal zone (Campania Region, Southern Italy) downstream the Garigliano and Volturno rivers, to value the influence of the decommissioned Garigliano Nuclear Power Plant (GNPP) releases. Despite the coastal systems are in continuous evolution, they hold memory of the radioactive contamination to be monitored in order to disseminate new knowledge both about the geochemistry of the isotopes and on their possible interactions with the geologic-sedimentologic context. Given the limited scientific investigations during the years in this field, the present work attempted to fill the gap of knowledge, trying to investigate the presence of radionuclide pollution in the Campanian coastal littoral. A constant distribution of natural radioisotopes in the medium-fine sand fraction is found, with an increase in the coarse fraction, showing an interesting correlation between the radioactive content with the lithology and mineralogy. About the anthropogenic cesium, the low concentrations (due to its strong solubility in the water) allowed to exclude any contribution of the GNPP. Furthermore the 239,240Pu actinides results, detected for both the ivestigated areas and for the used techniques, falls completely in the reference range values: the isotopic ratio is in the global fallout range allowing to exclude any contribution from the GNPP. The extraction and purification method has allowed to obtain excellent results to detect 239,240Pu actinides both for the ICP-MS, at Riso National Laboratory, and for the AMS technique, at CIRCE laboratory. For these reason the method is so reliable and reproducible that it could be considered as a reference for future environmental radioactivity studies on the sandy matrix.

New analytical methods for the assessment of natural (238U, 232Th, 226Ra, 40K) and anthropogenic (137Cs) radionuclides as actinides (239Pu, 240Pu): The case study of the Garigliano NPP releases along the Domitia sandy beaches (Southern Italy)

The study of stellar burning began just over 100 years ago. Nonetheless, we do not yet have a detailed picture of the nucleosynthesis within stars and how nucleosynthesis impacts stellar structure and the remnants of stellar evolution. Achieving this understanding will require precise direct measurements of the nuclear reactions involved. This report summarizes the status of direct measurements for stellar burning, focusing on developments of the last couple of decades, and offering a prospectus of near-future developments.

The Status and Future of Direct Nuclear Reaction Measurements for Stellar Burning

12C(α, γ)16O cross section plays a key-role in the stellar evolution and nucleosynthesis of massive stars. Hence, it must be determined with the precision of about 10% at the relevant Gamow energy of 300 keV. The ERNA (European Recoil mass separator for Nuclear Astrophysics) collaboration measured, for the first time, the total cross section of 12C(α, γ)16O by means of the direct detection of the 16O ions produced in the reaction down to an energy of Ecm = 1.9 MeV. To extend the measurement at lower energy, it is necessary to limit the extension of the He gas target. This can be achieved using a supersonic jet, where the oblique shock waves and expansion fans formed at its boundaries confine the gas, which can be efficiently collected using a catcher. A test version of such a system has been designed, constructed and experimentally characterized as a bench mark for a full numerical simulation using FV (Finite Volume) methods. The results of the commissioning of the jet test version and the design of the new system that will be used in combination with ERNA are presented and discussed.

L. Morales Gallegos

Papers

Reprint of: A supersonic jet target for the cross section measurement of the 12 C(α, γ) 16 O reaction with the recoil mass separator ERNA