Identification of the Lowest T=2, J^{π}=0^{+} Isobaric Analog State in ^{52}Co and Its Impact on the Understanding of β-Decay Properties of ^{52}Ni

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.

Mass measurements of the neutron-deficient 41Ti, 45Cr, 49Fe, and 53Ni nuclides: first test of the isobaric multiplet mass equation in f p-shell nuclei

Isochronous mass spectrometry has been applied to neutron-deficient 58Ni projectile fragments at the HIRFL-CSR facility in Lanzhou, China. Masses of a series of short-lived T(z)=-3/2 nuclides including 41Ti, 45Cr, 49Fe, and 53Ni have been measured with a precision of 20-40 keV. The new data enable us to test for the first time the isobaric multiplet mass equation (IMME) in fp-shell nuclei. We observe that the IMME is inconsistent with the generally accepted quadratic form for the A=53, T=3/2 quartet. We perform full space shell model calculations and compare them with the new experimental results.

First use of high charge states for mass measurements of short-lived nuclides in a Penning trap

Penning trap mass measurements of short-lived nuclides have been performed for the first time with highly charged ions, using the TITAN facility at TRIUMF. Compared to singly charged ions, this provides an improvement in experimental precision that scales with the charge state q. Neutron-deficient Rb isotopes have been charge bred in an electron beam ion trap to q=8-12+ prior to injection into the Penning trap. In combination with the Ramsey excitation scheme, this unique setup creating low energy, highly charged ions at a radioactive beam facility opens the door to unrivaled precision with gains of 1-2 orders of magnitude. The method is particularly suited for short-lived nuclides such as the superallowed β emitter 74Rb (T(1/2)=65  ms). The determination of its atomic mass and an improved Q(EC) value are presented.

Mass measurements of very neutron-deficient Mo and Tc isotopes and their impact on rp process nucleosynthesis

The masses of ten proton-rich nuclides, including the N=Z+1 nuclides ⁸⁵Mo and ⁸⁷Tc, were measured with the Penning trap mass spectrometer SHIPTRAP. Compared to the Atomic Mass Evaluation 2003 a systematic shift of the mass surface by up to 1.6 MeV is observed causing significant abundance changes of the ashes of astrophysical x-ray bursts. Surprisingly low α separation energies for neutron-deficient Mo and Tc are found, making the formation of a ZrNb cycle in the rp process possible. Such a cycle would impose an upper temperature limit for the synthesis of elements beyond Nb in the rp process.

Direct mass measurements of short-lived A=2Z-1 nuclides (63)Ge, (65)As, (67)Se, and (71)Kr and their impact on nucleosynthesis in the rp process

Mass excesses of short-lived A=2Z-1 nuclei (63)Ge, (65)As, (67)Se, and (71)Kr have been directly measured to be -46,921(37), -46,937(85), -46,580(67), and -46,320(141)  keV, respectively. The deduced proton separation energy of -90(85)  keV for (65)As shows that this nucleus is only slightly proton unbound. X-ray burst model calculations with the new mass excess of (65)As suggest that the majority of the reaction flow passes through (64)Ge via proton capture, indicating that (64)Ge is not a significant rp-process waiting point.

Critical-point boundary for the nuclear quantum phase transition near A=100 from mass measurements of (96,97)Kr

Mass measurements of (96,97)Kr using the ISOLTRAP Penning-trap spectrometer at CERN-ISOLDE are reported, extending the mass surface beyond N=60 for Z=36. These new results show behavior in sharp contrast to the heavier neighbors where a sudden and intense deformation is present. We interpret this as the establishment of a nuclear quantum phase transition critical-point boundary. The new masses confirm findings from nuclear mean-square charge-radius measurements up to N=60 but are at variance with conclusions from recent gamma-ray spectroscopy.

Discovery of 229Rn and the structure of the heaviest Rn and Ra isotopes from Penning-trap mass measurements

The masses of the neutron-rich radon isotopes {223-229}Rn have been determined for the first time, using the ISOLTRAP setup at CERN ISOLDE. In addition, this experiment marks the first discovery of a new nuclide, 229Rn, by Penning-trap mass measurement. The new, high-accuracy data allow a fine examination of the mass surface, via the valence-nucleon interaction deltaV{pn}. The results reveal intriguing behavior, possibly reflecting either a N=134 subshell closure or an octupolar deformation in this region.

Mass measurements beyond the major r-process waiting point 80Zn

High-precision mass measurements on neutron-rich zinc isotopes (71m,72-81)Zn have been performed with the Penning trap mass spectrometer ISOLTRAP. For the first time, the mass of 81Zn has been experimentally determined. This makes 80Zn the first of the few major waiting points along the path of the astrophysical rapid neutron-capture process where neutron-separation energy and neutron-capture Q-value are determined experimentally. The astrophysical conditions required for this waiting point and its associated abundance signatures to occur in r-process models can now be mapped precisely. The measurements also confirm the robustness of the N=50 shell closure for Z=30.

Masses and charge radii of 17-22Ne and the two-proton-halo candidate 17Ne

High-precision mass and charge radius measurements on ;{17-22}Ne, including the proton-halo candidate 17Ne, have been performed with Penning trap mass spectrometry and collinear laser spectroscopy. The 17Ne mass uncertainty is improved by factor 50, and the charge radii of ;{17-19}Ne are determined for the first time. The fermionic molecular dynamics model explains the pronounced changes in the ground-state structure. It attributes the large charge radius of 17Ne to an extended proton configuration with an s;{2} component of about 40%. In 18Ne the smaller radius is due to a significantly smaller s;{2} component. The radii increase again for ;{19-22}Ne due to cluster admixtures.

New binding energy for the two-neutron halo of 11Li

The extended radius of a halo nuclide is very sensitive to the minute binding energy of its valence nucleons. The binding energy of 11Li has been measured with high precision by using the radio-frequency spectrometer MISTRAL at CERN's ISOLDE facility. The new two-neutron separation energy of 378+/-5 keV is 25% higher than the previously accepted value with an uncertainty 5 times smaller.

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.

Isospin dependence in the odd-even staggering of nuclear binding energies

The FRS-ESR facility at GSI provides unique conditions for precision measurements of large areas on the nuclear mass surface in a single experiment. Values for masses of 604 neutron-deficient nuclides (30 < or = Z < or = 92) were obtained with a typical uncertainty of 30 microu. The masses of 114 nuclides were determined for the first time. The odd-even staggering (OES) of nuclear masses was systematically investigated for isotopic chains between the proton shell closures at Z = 50 and Z = 82. The results were compared with predictions of modern nuclear models. The comparison revealed that the measured trend of OES is not reproduced by the theories fitted to masses only. The spectral pairing gaps extracted from models adjusted to both masses, and density related observables of nuclei agree better with the experimental data.

Mass measurement on the rp-process waiting point 72Kr

The mass of one of the three major waiting points in the astrophysical rp process 72Kr was measured for the first time with the Penning trap mass spectrometer ISOLTRAP. The measurement yielded a relative mass uncertainty of deltam/m=1.2x10(-7) (deltam=8 keV). (73,74)Kr, also needed for astrophysical calculations, were measured with more than 1 order of magnitude improved accuracy. We use the ISOLTRAP masses of 72-74Kr to reanalyze the role of 72Kr (T(1/2)=17.2 s) in the rp process during x-ray bursts and conclude that 72Kr is a strong waiting point delaying the burst duration with at least 80% of its beta-decay half-life.

Direct mass measurements on the superallowed emitter 74Rb and its daughter 74Kr: isospin-symmetry-breaking correction for standard-model tests

The decay energy of the superallowed beta decay 74Rb(beta+)74Kr was determined by direct Penning trap mass measurements on both the mother and the daughter nuclide using the time-of-flight resonance technique and was found to be Q=10 416.8(4.5) keV. The exotic nuclide 74Rb, with a half-life of only 65 ms, is the shortest-lived nuclide on which a high-precision mass measurement in a Penning trap has been carried out. Together with existing data for the partial half-life as well as theoretical corrections, the decay energy yields a comparative half-life of Ft=3084(15) s for this decay, in agreement with the mean value for the series of the lighter nuclides from 10C to 54Co. Assuming conserved vector current, this result allows for an experimental determination of the isospin-symmetry-breaking correction deltaC.

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.

Masses of 32Ar and 33Ar for fundamental tests

Masses of the short-lived radionuclides 32Ar (T(1/2)=98 ms) and 33Ar (T(1/2)=173 ms) have been determined with the Penning trap mass spectrometer ISOLTRAP. Relative uncertainties of 6.0x10(-8) (deltam=1.8 keV) and 1.4x10(-8) (deltam=0.44 keV), respectively, have been achieved. At present, these new mass data serve as the most stringent test of the quadratic form of the isobaric-multiplet mass equation. Furthermore, the improved accuracy for the mass of 32Ar will allow for a better constraint on scalar contributions to the weak interaction. New mass values have also been measured for 44Ar and 45Ar, and a 20sigma deviation for 44Ar from the literature value was found and interpreted.

Breakdown of the isobaric multiplet mass equation at A = 33, T = 3/2

Mass measurements on (33,34,42,43)Ar were performed using the Penning trap mass spectrometer ISOLTRAP and a newly constructed linear Paul trap. This arrangement allowed us, for the first time, to extend Penning trap mass measurements to nuclides with half-lives below one second ( 33Ar: T(1/2) = 174 ms). A mass accuracy of about 10(-7) (deltam approximately 4 keV) was achieved for all investigated nuclides. The isobaric multiplet mass equation was checked for the A = 33, T = 3/2 quartet and found to be inconsistent with the generally accepted quadratic form.