Nuclear Astrophysical Reaction Studies Using Heavy Ion Storage Rings

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Abstract / Description of output

Nuclear astrophysics is a fascinating field requiring ­exploration of nuclear reactions and properties of key importance for fundamental questions on the origin of the elements and the life cycle of stars. Performing such measurements represents a very challenging experimental endeavor, particularly when these reactions and properties involve unstable nuclei. These nuclei play a key role in hot stellar environments such as recurrent nova explosions taking place in binary systems, or cataclysmic supernovae and neutron star mergers. While significant progress was made over the decades since the rallying cry of Willy Fowler for such studies in his Nobel lecture of 1984, many stellar puzzles remain frustratingly unsolved. This is often due to our inability to determine accurately key reaction rates involving radioactive beam species in the astrophysical energy range for explosive burning (typically ∼1-10 MeV/u). Daunting challenges arise in terms of luminosity and beam intensity, but also in terms of beam quality and beam and target purity. Development of appropriate new techniques and detection systems is required. Heavy ion storage rings offer a new arena to address these challenges.

Heavy ion rings have already been used extensively to study properties of exotic nuclei. These studies were pioneered at the Experimental Storage Ring (ESR) at the ­Society for Heavy Ion Research (GSI; Germany). A key discovery was the first observation of examples of bound-state beta-decay [Citation1]. Studies of electron capture decay in hydrogen-like ions, mass measurements, and identification of long-lived isomeric states of exotic nuclei were also performed on the ESR [Citation2]. Mass measurement programs are also presently being conducted at the heavy ion storage ring facilities, CSRe, Lanzhou, China [Citation3] and the Rare RI-Ring, RIKEN, Japan [Citation4].

Rings also offer several attractions for nuclear reaction studies. First, the unreacted beam is recycled and reaccelerated on the target approximately 105 times per second, ­resulting in a five orders-of-magnitude boost to the luminosity. Repeated passage through the ring magnets greatly purifies the isotopic composition of the stored beam, as only isotopes with charge over mass ratios within the acceptance of the magnets can remain stored. Finally, beam emittance is improved with the use of an electron cooler. These high-intensity, high-quality beams impinge on an ultra-thin (1010–14 atoms/cm2) ultra-pure cryogenic jet target [Citation5]. The ultra-thin target limits the effect of losses of recirculating ions in the ring and results in low energy loss for both the beam and charged particle reaction products induced in the target.

Pioneering experiments performed by the EXL collaboration on the ESR at GSI exploited thin gas jet targets to study low-energy target recoils detected using a ultra-high vacuum (UHV)–compatible Si array centered around 90 degrees in the laboratory frame. In Ref. [Citation6], for example, a recirculating radioactive beam of 390 MeV/u 56Ni ions injected into the ESR from the Fragment Recoil Separator (FRS) was used to study forward angle (in the Centre of Mass [CoM] frame, near 90 degrees in the lab) proton elastic (and inelastic) scattering and determine the matter radius of doubly magic 56Ni. The EXL collaboration similarly studied near zero degree (CoM) inelastic scattering of 4He target recoils for excitation of the Giant Monopole Resonance in 58Ni (a similar reaction study was recently reported at CSRe [Citation7]). Finally, still at the ESR, the 20Ne(p,d)19Ne transfer reaction was studied in inverse kinematics [Citation8]. Silicon detectors were positioned at forward angles to detect coincidences between protons and 19Ne/15O ions as a prototype technique for branching ratio determinations of astrophysical resonances in X-ray burster reactions.

In this review we will focus in particular on recent developments and exciting new opportunities at the ESR and the new CRYRING [Citation5] low-energy storage ring at GSI/FAIR (
). Over the last decade, significant advances in postdeceleration of in-flight ions down to astrophysical energies have opened the path to investigation of nuclear reactions directly at the energies at which they occur in stars at these two rings. Paired with a radioactive beam facility, this offers world-unique and unprecedented possibilities for direct and indirect measurements of stellar cross-sections of pivotal importance to solve long-standing stellar puzzles.
Original languageEnglish
Pages (from-to)23-26
Number of pages4
JournalNuclear Physics News
Volume33
Issue number3
DOIs
Publication statusPublished - 8 Sept 2023

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