Study of the proton-neutron interaction strength far from stability

  G. Kripkó-Koncz [1,4]  ,  I. Mardor [1,2]  ,  S. Ayet San Andrés [3]  ,  A. Shrayer [1]  ,  T. Dickel [3,4]  ,  D. Amanbayev [4]  ,  S. Beck [3,4]  ,  J. Bergmann [4]  ,  H. Geissel [3,4]  ,  L. Gröf [4]  ,  E. Haettner [3]  ,  C. Hornung [4]  ,  N. Kalantar-Nayestanaki [5]  ,  I. Miskun [4]  ,  A. Mollaebrahimi [5,4]  ,  W. R. Plaß [3,4]  ,  C. Scheidenberger [3,4]  ,  H. Weick [3]  ,  S. Bagchi [6,3,4]  ,  D. L. Balabanski [7]  ,  A. A. Bezbakh [8,9]  ,  Z. Brencic [10]  ,  O. Charviakova [11]  ,  P. Constantin [7]  ,  M. Dehghan [3]  ,  O. Hall [12]  ,  M. N. Harakeh [5]  ,  J. P. Hucka [3,13]  ,  A. Kankainen [14,15]  ,  R. Knöbel [3]  ,  N. Kurkova [8]  ,  N. Kuzminchuk [3]  ,  D. Nichita [7,16]  ,  Z. Patyk [11]  ,  S. Pietri [3]  ,  S. Purushothaman [3]  ,  M. P. Reiter [12]  ,  H. Roesch [3,13]  ,  F. Schirru [3]  ,  A. Spataru [7,16]  ,  G. Stanic [17]  ,  A. State [7,16]  ,  Y. K. Tanaka [18]  ,  M. Vencelj [10]  ,  M. I. Yavor [19]  ,  J. Zhao [3]  ,  and the FRS Ion Catcher Collaboration [3,4]  
[1] School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
[2] Soreq Nuclear Research Center, Yavne, Israel
[3] GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany
[4] II. Physikalisches Institut, Justus-Liebig-Universität Gießen, Gießen, Germany
[5] Nuclear Energy Group, ESRIG, University of Groningen, Groningen, The Netherlands
[6] Department of Astronomy and Physics, Saint Mary’s University, Halifax, Canada
[7] Extreme Light Infrastructure-Nuclear Physics (ELI-NP), Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, Bucharest-Magurele, Romania
[8] Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia
[9] Institute of Physics, Silesian University in Opava, Opava, Czech Republic
[10] Jozef Stefan Institute, Ljubljana, Slovenia
[11] National Centre for Nuclear Research, Warszawa, Poland
[12] School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom
[13] Institut für Kernphysik, Technische Universität Darmstadt, Darmstadt, Germany
[14] Department of Physics, University of Jyväskylä, Jyväskylä, Finland
[15] Helsinki Institute of Physics, Helsinki, Finland
[16] Doctoral School in Engineering and Applications of Lasers and Accelerators, University Polytechnica of Bucharest, Bucharest, Romania
[17] Johannes Gutenberg-Universität Mainz, Mainz, Germany
[18] Institute for Nuclear Physics, High Energy Nuclear Physics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama, Japan
[19] Institute for Analytical Instrumentation, RAS, St. Petersburg, Russia

The masses of exotic nuclei provide key information for the understanding of nuclear structure and astrophysics. At the FRS (FRagment Separator) at GSI Helmholtz Centre for Heavy Ion Research, exotic nuclei are produced at relativistic energies by projectile fragmentation or fission and separated in-flight. At the FRS Ion Catcher experiment the nuclei are thermalized and stopped in a Cryogenic Stopping Cell and are transported via a radio-frequency quadrupole (RFQ) beamline to a Multiple-Reflection Time-of-Flight Mass Spectrometer (MR-TOF-MS) for high-precision direct mass measurements. The MR-TOF-MS has single-ion sensitivity and features resolving powers at full width at half maximum of up to one million. Consequently, the system enables us to handle very low yields and perform mass measurements with relative mass accuracies down to 1.7 · 10−8 [1], reaching the level of Penning traps.

The average interaction strength between the last (highest energy orbitals) proton(s) and neutron(s) in a nucleus (denoted as δVpn), may be derived from differences of accurate atomic masses, and in turn point empirically to various aspects of nuclear structure and interactions. This has been investigated especially in nuclei with N=Z [2] and neutron-rich nuclei with equal proton and neutron numbers above doubly-magic cores [3].

Confirmed by mass measurements at the FRS Ion Catcher, the detailed structure of δVpn along the N=Z, Z+2 and Z+4 lines has been investigated recently [1]. In addition, a systematic search for deviations from the expected δVpn trends throughout the nuclear chart, may indicate that the mass values in the Atomic Mass Evaluation [4] are not correct or even some mass measurements are questionable. These results and planned investigations will be discussed.


[1] I. Mardor et al., Phys. Rev. C 103, 034319 (2021)
[2] P. Van Isacker et al., Phys. Rev. Lett. 74, 4607 (1995)
[3] R. B. Calkirli et al., Phys. Rev. C82, 061304(R) (2010)
[4] M. Wang et al., Chinese Phys. C 45, 030003 (2021)