Yu. A. Litvinov

Schottky mass measurements of stored and cooled neutron-deficient projectile fragments in the element range of 57≤Z≤84

Abstract A novel method for direct, high precision mass measurements of relativistic exotic nuclei has been successfully applied in the storage ring ESR at GSI. The nuclei of interest were produced by projectile fragmentation of 930 MeV / u bismuth ions, separated in-flight by the fragment separator FRS, stored and cooled in the ESR. The mass values have been deduced from the revolution frequencies of the coasting cooled ions. We have measured 104 new mass values with a precision of about 100 keV and a resolving power of 3.5×10 5 for the neutron-deficient isotopes of the elements 57≤Z≤84 . This paper presents the experimental method, the mass evaluation and a table of the experimental mass values.

Observation of non-exponential orbital electron capture decays of hydrogen-like 140Pr and 142Pm ions

Abstract We report on time-modulated two-body weak decays observed in the orbital electron capture of hydrogen-like 140 Pr 59+ and 142 Pm 60+ ions coasting in an ion storage ring. Using non-destructive single ion, time-resolved Schottky mass spectrometry we found that the expected exponential decay is modulated in time with a modulation period of about 7 seconds for both systems. Tentatively this observation is attributed to the coherent superposition of finite mass eigenstates of the electron neutrinos from the weak decay into a two-body final state.

Measurement of the {beta}{sup +} and Orbital Electron-Capture Decay Rates in Fully Ionized, Hydrogenlike, and Heliumlike {sup 140}Pr Ions

We report on the first measurement of the beta+ and orbital electron-capture decay rates of 140Pr nuclei with the simplest electron configurations: bare nuclei, hydrogenlike, and heliumlike ions. The measured electron-capture decay constant of hydrogenlike 140Pr58+ ions is about 50% larger than that of heliumlike 140Pr57+ ions. Moreover, 140Pr ions with one bound electron decay faster than neutral 140Pr0+ atoms with 59 electrons. To explain this peculiar observation one has to take into account the conservation of the total angular momentum, since only particular spin orientations of the nucleus and of the captured electron can contribute to the allowed decay.

Superallowed Gamow-Teller decay of the doubly magic nucleus 100Sn

The shell structure of atomic nuclei is associated with ‘magic numbers’ and originates in the nearly independent motion of neutrons and protons in a mean potential generated by all nucleons. During β+-decay, a proton transforms into a neutron in a previously not fully occupied orbital, emitting a positron–neutrino pair with either parallel or antiparallel spins, in a Gamow–Teller or Fermi transition, respectively. The transition probability, or strength, of a Gamow–Teller transition depends sensitively on the underlying shell structure and is usually distributed among many states in the neighbouring nucleus. Here we report measurements of the half-life and decay energy for the decay of 100Sn, the heaviest doubly magic nucleus with equal numbers of protons and neutrons. In the β-decay of 100Sn, a large fraction of the strength is observable because of the large decay energy. We determine the largest Gamow–Teller strength so far measured in allowed nuclear β-decay, establishing the ‘superallowed’ nature of this Gamow–Teller transition. The large strength and the low-energy states in the daughter nucleus, 100In, are well reproduced by modern, large-scale shell model calculations.

Mass Measurement of 45Cr and Its Impact on the Ca-Sc Cycle in X-Ray Bursts

Masses of neutron-deficient 58Ni projectile fragments have been measured at the HIRFL-CSR facility in Lanzhou, China employing the isochronous mass spectrometry technique. Masses of a series of short-lived Tz = –3/2 nuclides including the 45Cr nucleus have been measured with a relative uncertainty of about 10–6-10–7. The new 45Cr mass turned out to be essential for modeling the astrophysical rp-process. In particular, we find that the formation of the predicted Ca-Sc cycle in X-ray bursts can be excluded.

High-resolution measurement of the time-modulated orbital electron capture and of the β+ decay of hydrogen-like 142Pm60+ ions

Abstract The periodic time modulations, found recently in the two-body orbital electron capture (EC) decay of both, hydrogen-like 140Pr58+ and 142Pm60+ ions, with periods near to 7 s and amplitudes of about 20%, were re-investigated for the case of 142Pm60+ by using a 245 MHz resonator cavity with a much improved sensitivity and time resolution. We observed that the exponential EC decay is modulated with a period T = 7.11 ( 11 ) s , in accordance with a modulation period T = 7.12 ( 11 ) s as obtained from simultaneous observations with a capacitive pick-up, employed also in the previous experiments. The modulation amplitudes amount to a R = 0.107 ( 24 ) and a P = 0.134 ( 27 ) for the 245 MHz resonator and the capacitive pick-up, respectively. These new results corroborate for both detectors exactly our previous findings of modulation periods near to 7 s , though with distinctly smaller amplitudes. Also the three-body β + decays have been analyzed. For a supposed modulation period near to 7 s we found an amplitude a = 0.027 ( 27 ) , compatible with a = 0 and in agreement with the preliminary result a = 0.030 ( 30 ) of our previous experiment. These observations could point at weak interaction as origin of the observed 7 s -modulation of the EC decay. Furthermore, the data suggest that interference terms occur in the two-body EC decay, although the neutrinos are not directly observed.

Precision experiments with relativistic exotic nuclei at GSI

Exotic nuclei generated via projectile fragmentation and fission are characterized by a large phase-space population. Thus precision experiments based on kinematical observables are difficult and require special methods. The experiments described in this contribution are based on special ion-optical spectrometer modes or on phase-space reduction via cooling of stored ions. The studies are performed with the projectile fragment separator FRS and the storage-cooler ring ESR at GSI.

Energy and range focusing of in-flight separated exotic nuclei - A study for the energy-buncher stage of the low-energy branch of the Super-FRS

Abstract The relative momentum spread of in-flight separated exotic nuclear beams produced in fragmentation and/or fission reactions is of the order of a few percent. A new technique is presented, which reduces the momentum spread significantly, and first experimental results obtained with relativistic projectile fragments are shown. This technique is the key to experiments with slowed-down and stopped beams, in particular for the efficient stopping of relativistic exotic nuclei in gas-filled stopping cells. It will be employed at the energy-buncher stage of the low-energy branch of the Super-FRS facility. The ion-optical design of the energy buncher is presented and a brief outlook to the experimental program is given.

Application of the relativistic mean-field mass model to the r-process and the influence of mass uncertainties

A new mass table calculated by the relativistic mean-field approach with the state-dependent BCS method for the pairing correlation is applied for the first time to study r-process nucleosynthesis. The solar r-process abundance is well reproduced within a waiting-point approximation approach. Using an exponential fitting procedure to find the required astrophysical conditions, the influence of mass uncertainty is investigated. The r-process calculations using the FRDM, ETFSI-Q, and HFB-13 mass tables have been used for that purpose. It is found that the nuclear physical uncertainty can significantly influence the deduced astrophysical conditions for the r-process site. In addition, the influence of the shell closure and shape transition have been examined in detail in the r-process simulations.

Identification of the Lowest T =2, Jπ =0+ Isobaric Analog State in 52Co and Its Impact on the Understanding of β-Decay Properties of52Ni

Masses of ^{52g,52m}Co were measured for the first time with an accuracy of ∼10  keV, an unprecedented precision reached for short-lived nuclei in the isochronous mass spectrometry. Combining our results with the previous β-γ measurements of ^{52}Ni, the T=2, J^{π}=0^{+} isobaric analog state (IAS) in ^{52}Co was newly assigned, questioning the conventional identification of IASs from the β-delayed proton emissions. Using our energy of the IAS in ^{52}Co, the masses of the T=2 multiplet fit well into the isobaric multiplet mass equation. We find that the IAS in ^{52}Co decays predominantly via γ transitions while the proton emission is negligibly small. According to our large-scale shell model calculations, this phenomenon has been interpreted to be due to very low isospin mixing in the IAS.