This is an accepted version of a paper published in Nature. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the published paper: Hinke, C., Boehmer, M., Boutachkov, P., Faestermann, T., Geissel, H. et al. (2012) "Superallowed Gamow-Teller decay of the doubly magic nucleus 100 Sn" Nature, 486 (7403): [341][342][343][344][345] Access to the published version may require subscription.

A precision mass investigation of the neutron-rich titanium isotopes 51−55 Ti was performed at TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN). The range of the measurements covers the N = 32 shell closure and the overall uncertainties of the 52−55 Ti mass values were significantly reduced. Our results conclusively establish the existence of weak shell effect at N = 32, narrowing down the abrupt onset of this shell closure. Our data were compared with state-of-the-art ab initio shell model calculations which, despite very successfully describing where the N = 32 shell gap is strong, overpredict its strength and extent in titanium and heavier isotones. These measurements also represent the first scientific results of TITAN using the newly commissioned Multiple-Reflection Time-of-Flight Mass Spectrometer (MR-TOF-MS), substantiated by independent measurements from TITAN's Penning trap mass spectrometer.Atomic nuclei are highly complex quantum objects made of protons and neutrons. Despite the arduous efforts needed to disentangle specific effects from their many-body nature, the fine understanding of their structures provides key information to our knowledge of fundamental nuclear forces. One notable quantum behavior of bound nuclear matter is the formation of shell-like structures for each fermion group [1], as electrons do in atoms. Unlike for atomic shells, however, nuclear shells are known to vanish or move altogether as the number of protons or neutrons in the system changes [2]. Particular attention has been given to the emergence of strong shell effects among nuclides with 32 neutrons, pictured in a shell model framework as a full valence ν2p 3/2 orbital. Across most of the known nuclear chart, this orbital is energetically close to ν1f 5/2 , which prevents the appearance of shell signatures in energy observables. However, the excitation energies of the lowest 2 + states show a relative, but systematic, local increase below proton number Z = 24 [3]. This effect, characteristic of shell closures, has been attributed in shell model calculations to the weakening of attractive proton-neutron interactions between the ν1f 5/2 and π1f 7/2 orbitals as the latter empties, making the neutrons in the former orbital less bound [4,5]. Ab initio calculations are also extending their reach over this sector of the nuclear chart, yet no systematic investigation of the N = 32 isotones has been produced so far.

Mass measurements of fission and projectile fragments, produced via 238 U and 124 Xe primary beams, have been performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the Fragment Separator (FRS) Ion Catcher with a mass resolving power (FWHM) of up to 410 000 and an uncertainty of down to 6 × 10 −8. The nuclides were produced and separated in flight with the fragment separator FRS at 300 to 1000 MeV/u and thermalized in a cryogenic stopping cell. The data-analysis procedure was developed to determine with highest accuracy the mass values and the corresponding uncertainties for the most challenging conditions: down to a few events in a spectrum and overlapping distributions, which can be distinguished from a single peak only by a broader peak shape. With this procedure, the resolution of low-lying isomers is increased by a factor of up to 3 compared to standard data analysis. The ground-state masses of 31 short-lived nuclides of 15 different elements with half-lives of down to 17.9 ms and count rates as low as 11 events per nuclide were determined. This is the first direct mass measurement for seven nuclides. The excitation energies and the isomer-to-groundstate ratios of six isomeric states with excitation energies of as little as 280 keV were measured. For nuclides with known mass values, the average relative deviation from the literature values is (4.5 ± 5.3) × 10 −8. The measured two-neutron separation energies and their slopes near and at the N = 126 and Z = 82 shell closures indicate a strong element-dependent binding energy of the first neutron above the closed proton shell Z = 82. The experimental results deviate strongly from the theoretical predictions, especially for N = 126 and N = 127.

For a detailed study of the accuracy of the Penning trap mass spectrometer ISOL-TRAP all expected sources of uncertainty were investigated with respect to their contributions to the uncertainty of the final result. In the course of these investigations, cross-reference measurements with singly charged carbon clusters 12 C + n were carried out. The carbon cluster ions were produced by use of laser-induced desorption, fragmentation, and ionization of C 60 fullerenes and injected into and stored in the Penning trap system. The comparison of the cyclotron frequencies of different carbon clusters has provided detailed insight into the residual systematic uncertainty of ISOLTRAP and yielded a value of 8 · 10 −9 . This also represents the current limit of mass accuracy of the apparatus. Since the unified atomic mass unit is defined as 1/12 of the mass of the 12 C atom, it will be possible to carry out absolute mass measurements with ISOLTRAP in the future. The ISOLTRAP ExperimentThe ISOLTRAP experiment is a Penning trap mass spectrometer for accurate highprecision measurements of atomic masses. It is installed at the on-line isotope separator ISOLDE at CERN (Geneva, Switzerland) [1], and it is mainly intended for mass measurements of short-lived radioactive nuclides. The masses of close to two hundred radionuclides have already been measured with the ISOLTRAP apparatus [2].ISOLTRAP consists of a radiofrequency quadrupole (RFQ) ion beam cooler and buncher [3] and two Penning traps, each located in the field of a superconducting magnet [4]. The RFQ ion beam cooler and buncher (RICB) serves the purpose of decelerating, accumulating, cooling, and bunching the 60-keV ISOLDE ion beam. The first Penning trap is a cylindrical trap which is used for the cooling and isobaric cleaning of the ion bunches. The actual mass measurement is carried out in the second Penning trap, a hyperboloidal trap. It is based on a determination of the cyclotron frequency ν c of an ion with charge q unknown mass m ion in a magnetic field of magnitude B [4]:

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We have measured the total cross sections for electron capture by bare Pb 821 ions and ionization of hydrogenlike Pb 811 ions at 33 TeV (160 GeV͞A, g 168) in solid targets of Be, C, Al, Cu, Sn, and Au. The total capture cross sections are dominated by electron capture from pair production and are compared with theoretical calculations. The 1s ionization cross sections obtained are significantly smaller than those predicted by Anholt and Becker [Phys. Rev. A 36, 4628 (1987)]. The Pb radiative lifetimes extended by g 168 have a strong effect on the survival probability of excited states against ionization in high-Z solid targets. [S0031-9007(97) PACS numbers: 34.50. Fa, 34.80.Lx Interactions involving high-Z ions in the ultrarelativistic regime ͑.10 GeV͞amu͒, where the relevant physics is best described in terms of the Lorentz factor g, are currently a frontier in high-energy atomic collision physics [1]. A theoretical description of electron capture and ionization processes has been challenging in this regime because the interaction of high-Z projectile and target species (where Za ϳ 0.5) is strong enough at small impact parameters and large g to potentially invalidate perturbation treatments. Numerous methods for treating these processes using quantum electrodynamics (QED) in the ultrarelativistic regime now exist [1][2][3][4][5][6][7][8][9][10].An ultrarelativistic ion can capture an electron via three mechanisms: (i) radiative electron capture (REC), (ii) nonradiative capture (NRC), and (iii) electron capture via e 1 e 2 pair production (ECPP), in which the e 1 e 2 pair is produced by the intense electromagnetic pulse that arises when the projectile ion passes near a target nucleus. Capture cross sections s REC , s NRC , and s ECPP scale roughly as ϳZ t ͞g, ϳZ 5 t ͞g, and ϳZ 2 t ln g, respectively, where Z t is the target atomic number [2]. Each process has approximately the same dependence on the projectile atomic number, i.e., Z 5 p . The REC and NRC mechanisms, which dominate below the ultrarelativistic regime [11][12][13], become insignificant compared to ECPP when g . 100 even for high Z t . We report the first highenergy measurements ͑g 168͒ where s ECPP dominates the capture cross sections of competing mechanisms. Ionization cross sections are several orders of magnitude larger than capture, and our measurements test theory at the highest energy reported to date [2,10].The development of new relativistic ion colliders such as the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory or the Large Hadron Collider at CERN [2,8,14] requires knowledge of the capture cross sections at high enough g so that beam lifetimes can be accurately predicted. The cross section for the ECPP process is of practical interest to collider designers because the lower charge-state projectiles produced are lost from the beam circulating in a ring. A significant loss rate of these ions by ECPP and also by nuclear loss processes decreases the ion storage time. These machines will operate at an effective g of 2.3 ...

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