Wed·Mar 14, 2018Wednesday·March 14, 2018

1. 01*

   Effect of direct reaction channels on deep sub-barrier fusion in asymmetric systems

   Md. Moin Shaikh🇮🇳 · S. Nath · J. Gehlot · Tathagata Banerjee · Ish Mukul🇮🇱 · R.Dubey · A. Shamlath · P. V. Laveen · M. Shareef · A. Jhingan🇮🇳 · N. Madhavan · Tapan Rajbongshi and 5 other authors

   A steeper fall of fusion excitation function, compared to the predictions of coupled-channels models, at energies below the lowest barrier between the reaction partners, is termed as deep sub-barrier fusion hindrance. This phenomenon has been observed in many symmetric and nearly-symmetric systems. Different physical origins of the hindrance have been proposed. This work aims to study the probable effects of direct reactions on deep sub-barrier fusion cross sections. Fusion (evaporation residue) cross sections have been measured for the system ${}^{19}$F+${}^{181}$Ta, from above the barrier down to the energies where fusion hindrance is expected to come into play. Coupled-channels calculation with standard Woods-Saxon potential gives a fair description of the fusion excitation function down to energies $\simeq 14\%$ below the barrier for the present system. This is in contrast with the observation of increasing fusion hindrance in asymmetric reactions induced by increasingly heavier projectiles, \textit{viz.} ${}^{6,7}$Li, ${}^{11}$B, ${}^{12}$C and ${}^{16}$O. The asymmetric reactions, which have not shown any signature of fusion hindrance within the measured energy range, are found to be induced by projectiles with lower $\alpha$ break-up threshold, compared to the reactions which have shown signatures of fusion hindrance. In addition, most of the $Q$-values for light particles pick-up channels are negative for the reactions which have exhibited strong signatures of fusion hindrance, \textit{viz.} ${}^{12}$C+${}^{198}$Pt and ${}^{16}$O+${}^{204,208}$Pb. Thus, break-up of projectile and particle transfer channels with positive $Q$-values seem to compensate for the hindrance in fusion deep below the barrier. Inclusion of break-up and transfer channels within the framework of coupled-channels calculation would be of interest.

   nucl-exnucl-th0 citations
2. 02*

   Strong neutron pairing in core+4n nuclei

   A. Revel🇫🇷 · F.M. Marques🇫🇷 · O. Sorlin🇫🇷 · T. Aumann🇩🇪 · C. Caesar🇩🇪 · M. Holl🇩🇪 · V. Panin🇩🇪 · M. Vandebrouck🇫🇷 · F. Wamers🇩🇪 · H. Alvarez-Pol🇪🇸 · L. Atar🇩🇪 · V. Avdeichikov🇸🇪 and 91 other authors

   The emission of neutron pairs from the neutron-rich $N=12$ isotones ${}^{18}$C and ${}^{20}$O has been studied by high-energy nucleon knockout from ${}^{19}$N and ${}^{21}$O secondary beams, populating unbound states of the two isotones up to 15~MeV above their two-neutron emission thresholds. The analysis of triple fragment-$n$-$n$ correlations shows that the decay ${}^{19}$N$(-1p{)}^{18}$C${}^{\ast }{\to }^{16}$C+$n$+$n$ is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a ${}^{14}$C core surrounded by four valence neutrons arranged in strongly correlated pairs. On the other hand, a significant competition of a sequential branch is found in the decay ${}^{21}$O$(-1n{)}^{20}$O${}^{\ast }{\to }^{18}$O+$n$+$n$, attributed to its formation through the knockout of a deeply-bound neutron that breaks the ${}^{16}$O core and reduces the number of pairs.

   nucl-exPRL(2018)·16 citations
3. 03*

   Current status and desired accuracy of the isotopic production cross sections relevant to astrophysics of cosmic rays I. Li, Be, B, C, N

   Yoann Genolini🇧🇪 · David Maurin🇫🇷 · Igor V. Moskalenko🇺🇸 · Michael Unger🇩🇪

   The accuracy of the current generation of cosmic-ray (CR) experiments, such as AMS-02, PAMELA, CALET, and ISS-CREAM, is now reaching $\sim$1--3\% in a wide range in energy per nucleon from GeV/n to multi-TeV/n. Their correct interpretation could potentially lead to discoveries of new physics and subtle effects that were unthinkable just a decade ago. However, a major obstacle in doing so is the current uncertainty in the isotopic production cross sections that can be as high as 20--50\% or even larger in some cases. While there is a recently reached consensus in the astrophysics community that new measurements of cross sections are desirable, no attempt to evaluate the importance of particular reaction channels and their required accuracy has been made yet. It is, however, clear that it is a huge work that requires an incremental approach. The goal of this study is to provide the ranking of the isotopic cross sections contributing to the production of the most astrophysically important CR Li, Be, B, C, and N species. In this paper, we (i) rank the reaction channels by their importance for a production of a particular isotope, (ii) provide comparisons plots between the models and data used, and (iii) evaluate a generic beam time necessary to reach a 3\% precision in the production cross-sections pertinent to the AMS-02 experiment. This first roadmap may become a starting point in the planning of new measurement campaigns that could be carried out in several nuclear and/or particle physics facilities around the world. A comprehensive evaluation of other isotopes $Z\le 30$ will be a subject of follow-up studies.

   ↳ astro-ph.HEhep-exhep-thnucl-ex+1PRC(2018)·121 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.