The Challenge of High-Resolution Spectrometer Experiments with Exotic Nuclei

Abstract

Exotic atomic nuclei are short-lived species and are characterized by extreme proton-to-neutron ratios. These nuclei are created in stellar reactions and are keys to understand the abundance of the elements in the universe [1]. Exotic nuclei have quite different properties compared to the well-known species close to the valley of beta stability. This observation makes them essential for the basic understanding of nuclear matter. Nowadays, these rare isotopes can also be produced with modern powerful accelerator facilities at several laboratories worldwide [2] which solve the problems of the inherent small production cross section sections via high primary beam intensities and high energies [3,4]. However, the successful production of exotic nuclei is only the first necessary step, the separation from the primary beam and from the abundant contaminants and the measurements of their properties are of equal importance [5]. Spectrometer BasicsHigh resolution experiments with energetic heavy ions can in principle be performed in-flight with lateral-dispersive and longitudinal-dispersive electromagnetic spectrometers. Lateral-dispersive systems analyze with magnetic and electric dipole fields and apply multipole fields for focusing and correction of image aberrations [6,7]. A typical lateral-dispersive spectrometer stage, such as it is used in many accelerator based heavy ion laboratories, is shown schematically in Figure 1. A lens system is used at the entrance of a dipole magnet and a second one is placed behind to determine the focal plane conditions. The physical quantity of interest is the momentum resolving power which is mainly determined by the entrance emittance of the ion beam to be analyzed and the illuminated area in the dispersive plane of the dipole magnet. Electric fields can also be applied at low energies (at and below the Coulomb barrier) instead of magnets, but the same statements on the resolving power hold in this case as well. In longitudinal dispersive devices such as ion storage rings [8] and multiplereflection time-of-flight mass spectrometers (MR-TOF-MS) [9,10] the challenge is to achieve isochronous conditions with negligible aberrations of the higher-order optics. In general, one can achieve a higher resolving power with the longitudinal-dispersive, multi-path devices compared to the lateraldispersive spectrometers. The latter spectrometers may be applied as in-flight separators for nuclear reaction products and usually consist of a combination of several dispersive stages. The advantage of these spectrometers is the superior spatial isotopic separation of rare isotopes from the intense primary beam and the abundant contaminants. Space-charge problems are of minor importance for such lateral-dispersive spectrometers. They can have an overall maximum momentum resolving power (p/p) of several 10 4 [11,12], whereas storage rings and MR-TOF-MS systems can achieve more than 50 times higher resolving powers. In lateral spectrometers, the spatial deflections and resolving power are measured by po...

Identification and separation of radioactive isotope beams by the BigRIPS separator at the RIKEN RI Beam Factory

“…Such an enhanced capability of the BigRIPS separator is significantly advancing the research on exotic nuclei at RIBF. Other new-generation in-flight RI-beam facilities, including the Super-FRS at GSI FAIR [33] and the FRIB at MSU [34], are presently being developed with the same goal.…”

“…Since isotope separation using an energy degrader depends on the energy of RI beams, we choose the thickness of the energy degraders at the F1 and F5 foci so as to optimize the purity of RI beams. This two-stage separation [10,33], which combines the isotope separation in the first and second stages, is also effective when unwanted isotopes are transmitted as contaminants due to the charge state change or the secondary reactions in the first degrader at F1. Here we introduce two typical examples.…”