Mass and lifetime measurements of exotic nuclei in storage rings

Abstract

Mass and lifetime measurements lead to the discovery and understanding of basic properties of matter. The isotopic nature of the chemical elements, nuclear binding, and the location and strength of nuclear shells are the most outstanding examples leading to the development of the first nuclear models. More recent are the discoveries of new structures of nuclides far from the valley of stability. A new generation of direct mass measurements which allows the exploration of extended areas of the nuclear mass surface with high accuracy has been opened up with the combination of the Experimental Storage Ring ESR and the FRragment Separator FRS at GSI Darmstadt. In-flight separated nuclei are stored in the ring. Their masses are directly determined from the revolution frequency. Dependent on the half-life two complementary methods are applied. Schottky Mass Spectrometry SMS relies on the measurement of the revolution frequency of electron cooled stored ions. The cooling time determines the lower half-life limit to the order of seconds. For Isochronous Mass Spectrometry IMS the ring is operated in an isochronous ion-optical mode. The revolution frequency of the individual ions coasting in the ring is measured using a time-of-flight method. Nuclides with lifetimes down to microseconds become accessible. With SMS masses of several hundreds nuclides have been measured simultaneously with an accuracy in the 2 x 10(-7)-range. This high accuracy and the ability to study large areas of the mass surface are ideal tools to discover new nuclear structure properties and to guide improvements for theoretical mass models. In addition, nuclear half-lives of stored bare and highly charged ions have been measured. This new experimental development is a significant progress since nuclear decay characteristics are mostly known for neutral atoms. For bare and highly charged ions new nuclear decay modes become possible, such as bound-state beta decay. Dramatic changes in the nuclear lifetime have been observed in highly charged ions compared to neutral atoms due to blocking of nuclear decay channels caused by the modified atomic interaction. High ionization degrees prevail in hot stellar matter and thus these experiments have great relevance for the understanding of the synthesis of elements in the universe and astrophysical scenarios in general.

The Challenge of High-Resolution Spectrometer Experiments with Exotic Nuclei

“…The electron cooling time ( e ) strongly depends on the initial velocity spread (v) of the injected fragments ( e  v -3 ). With such electron-cooled fragment beams one can ideally perform high-resolution experiments [8]. Accurate mass measurements have been successfully carried out in the past for a large number of isotopes with previously unknown experimental masses [28,20].…”

“…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.…”

A new Time-of-Flight mass measurement project for exotic nuclei and ultra-high precision detector development

“…On the other hand, however, the state-of-art nuclear models, which can reproduce known masses in a precision of less than 500 keV, has been questioned about their reliability in computations with long range extrapolation from the known mass surface. All these arguments are the strong motivations for developing new precision mass measurement techniques and moreover for performing new accurate mass measurements [1][2][3][4], in particular for neutron-rich nuclides.…”

Recent Precision Experiments with Exotic Nuclei Produced with Uranium Projectiles and Experimental Prospects at Fair

“…The fragments spatially separated by in-flight facilities inject the fragments of interest into a storage-cooler ring [3][4][5]. Cooling in storage rings have been successfully applied to reach ideal conditions for precise revolution time measurements of the circulating ions.…”

“…In this case one can apply ion-optical methods to circumvent the influence of the inevitably large phase space. An elegant solution is to perform experiments with dispersion-matched spectrometers [10] or use longitudinal dispersive systems like a storage ring in the isochronous operating mode [4].…”