ISOLTRAP: An on-line Penning trap for mass spectrometry on short-lived nuclides

ISOLTRAP is a Penning trap mass spectrometer for high-precision mass measurements on short-lived nuclides installed at the on-line isotope separator ISOLDE at CERN. The masses of close to 300 radionuclides have been determined up to now. The applicability of Penning trap mass spectrometry to mass measurements of exotic nuclei has been extended considerably at ISOLTRAP by improving and developing this double Penning trap mass spectrometer over the past two decades. The accurate determination of nuclear binding energies far from stability includes nuclei that are produced at rates less than 100 ions/s and with half-lives well below 100ms. The mass-resolving power reaches 10⁷ corresponding to 10keV for medium heavy nuclei and the uncertainty of the resulting mass values has been pushed down to below 10^-8. The article describes technical developments achieved since 1996 and the present performance of ISOLTRAP.     

Unambiguous identification of three beta-decaying isomers in 70Cu

Using resonant laser ionization, beta-decay studies, and for the first time mass measurements, three beta-decaying states have been unambiguously identified in 70Cu. A mass excess of -62 976.1(1.6) keV and a half-life of 44.5(2) s for the (6-) ground state have been determined. The level energies of the (3-) isomer at 101.1(3) keV with T(1/2)=33(2) s and the 1+ isomer at 242.4(3) keV with T(1/2)=6.6(2) s are confirmed by high-precision mass measurements. The low-lying levels of 70Cu populated in the decay of 70Ni and in transfer reactions compare well with large-scale shell-model calculations, and the wave functions appear to be dominated by one proton-one neutron configurations outside the closed Z=28 shell and N=40 subshell. This does not apply to the 1+ state at 1980 keV which exhibits a particular feeding and deexcitation pattern not reproduced by the shell-model calculations.     

The mass of 22Mg

Mass measurements with a relative precision of better than 1.5 x 10(-8) were performed on 22Mg and its reaction partners 21Na and 22Na with the ISOLTRAP Penning trap mass spectrometer at CERN, yielding the mass excesses D(22Mg)=-399.92(27) keV, D(21Na)=-2184.71(21) keV, and D(22Na)=-5181.56(16) keV. The importance of these results is twofold. First, a comparative half-life (Ft value) has been obtained for the superallowed beta decay of 22Mg to further test the conserved-vector-current hypothesis. Second, the resonance energy for the 21Na proton capture reaction has been independently determined, allowing direct comparisons of observable gamma radiation in nova explosions with the yield expected from models.     

A new Channeltron-detector setup for precision mass measurements at ISOLTRAP

Recent technical developments at ISOLTRAP include the design, installation and characterization of a new Channeltron-detector setup in order to increase the detection efficiency by about a factor of 3. Since a detection efficiency close to 100% is reached, true single-ion experiments can be performed for the first time and exotic nuclides further away from the valley of stability with lower production yields and/or shorter half-lives as compared to previous investigations are accessible. This publication comprises part of the thesis of C. Yazidjian.     

Penning trap mass spectrometry for nuclear structure studies

High-precision mass measurements as performed at the Penning trap mass spectrometer ISOLTRAP at ISOLDE/CERN are an important contribution to the investigation of nuclear structure. Precise nuclear masses with less than 0.1 ppm relative mass uncertainty allow stringent tests of mass models and formulae that are used to predict mass values of nuclides far from the valley of stability. Furthermore, an investigation of nuclear structure effects like shell or sub-shell closures, deformations, and halos is possible. In addition to a sophisticated experimental setup for precise mass measurements, a radioactive ion-beam facility that delivers a large variety of short-lived nuclides with sufficient yield is required. An overview of the results from the mass spectrometer ISOLTRAP is given and its limits and possibilities are described.      

Laser Ionization and Penning Trap Mass Spectrometry – A Fruitful Combination for Isomer Separation and High-precision Mass Measurements

We have demonstrated for the first time that element-selective laser ionization in combination with ultra-high resolution mass spectrometry can be used to prepare isomerically pure ion ensembles. Together with β–γ coincidence studies this method allowed a determination of the low-energy structure and the unambiguous identification of triple β-decaying isomerism in ⁷⁰Cu. By selective resonant ionization and measurement of the masses of these three states using ISOLTRAP at ISOLDE/CERN with a relative uncertainty of δm/m ≈ 5 ⋅ 10⁻⁸ a clear state-to-mass assignment was possible which resolved the assignment puzzle in ⁷⁰Cu.     

Restoration of the n=82 shell gap from direct mass measurements of 132,134Sn

A high-precision direct Penning trap mass measurement has revealed a 0.5-MeV deviation of the binding energy of (134)Sn from the currently accepted value. The corrected mass assignment of this neutron-rich nuclide restores the neutron-shell gap at N=82, previously considered to be a case of "shell quenching." In fact, the new shell gap value for the short-lived (132)Sn is larger than that of the doubly magic (48)Ca which is stable. The N=82 shell gap has considerable impact on fission recycling during the r process. More generally, the new finding has important consequences for microscopic mean-field theories which systematically deviate from the measured binding energies of closed-shell nuclides.     

Mass measurements and evaluation around A = 22

Frequency ratio measurements with different combinations of the singly charged ions from ^{21, 22, 23}Na , ^{22, 24}Mg , and ^{37, 39}K were performed at the on-line Penning trap mass spectrometer ISOLTRAP, CERN, Geneva. The masses and mass differences were deduced with a relative uncertainty of about or even below one part in 10⁸ for the ions of interest using a least-squares analysis of all measured relations. The results have direct consequences for weak-interaction study as they give additional input to the test of CVC, and for nuclear astrophysics, because they help to establish the minimum observable signal for a NeNa cycle in a nova burst. We report here about the measurements and the detailed evaluation.