F. Herfurth

A linear radiofrequency ion trap for accumulation, bunching, and emittance improvement of radioactive ion beams

An ion beam cooler and buncher has been developed for the manipulation of radioactive ion beams. The gas-filled linear radiofrequency ion trap system is installed at the Penning trap mass spectrometer ISOLTRAP at ISOLDE/CERN. Its purpose is to accumulate the 60-keV continuous ISOLDE ion beam with high efficiency and to convert it into low-energy low-emittance ion pulses. The efficiency was found to exceed 10% in agreement with simulations. A more than 10-fold reduction of the ISOLDE beam emittance can be achieved. The system has been used successfully for first on-line experiments. Its principle, setup and performance will be discussed.

Masses of exotic calcium isotopes pin down nuclear forces

The properties of exotic nuclei on the verge of existence play a fundamental part in our understanding of nuclear interactions. Exceedingly neutron-rich nuclei become sensitive to new aspects of nuclear forces. Calcium, with its doubly magic isotopes 40Ca and 48Ca, is an ideal test for nuclear shell evolution, from the valley of stability to the limits of existence. With a closed proton shell, the calcium isotopes mark the frontier for calculations with three-nucleon forces from chiral effective field theory. Whereas predictions for the masses of 51Ca and 52Ca have been validated by direct measurements, it is an open question as to how nuclear masses evolve for heavier calcium isotopes. Here we report the mass determination of the exotic calcium isotopes 53Ca and 54Ca, using the multi-reflection time-of-flight mass spectrometer of ISOLTRAP at CERN. The measured masses unambiguously establish a prominent shell closure at neutron number N = 32, in excellent agreement with our theoretical calculations. These results increase our understanding of neutron-rich matter and pin down the subtle components of nuclear forces that are at the forefront of theoretical developments constrained by quantum chromodynamics.

Ramsey method of separated oscillatory fields for high-precision penning trap mass spectrometry

Ramseys method of separated oscillatory fields is applied to the excitation of the cyclotron motion of short-lived ions in a Penning trap to improve the precision of their measured mass values. The theoretical description of the extracted ion-cyclotron-resonance line shape is derived and its correctness demonstrated experimentally by measuring the mass of the short-lived 38Ca nuclide with an uncertainty of 1.1 x 10(-8) using the Penning trap mass spectrometer ISOLTRAP at CERN. The mass of the superallowed beta emitter 38Ca contributes for testing the theoretical corrections of the conserved-vector-current hypothesis of the electroweak interaction. It is shown that the Ramsey method applied to Penning trap mass measurements yields a statistical uncertainty similar to that obtained by the conventional technique but 10 times faster. Thus the technique is a new powerful tool for high-precision mass measurements.

Mass measurements in the vicinity of the r p-process and the nu p-process paths with the Penning trap facilities JYFLTRAP and SHIPTRAP

The masses of very neutron-deficient nuclides close to the astrophysical r p- and {nu} p-process paths have been determined with the Penning trap facilities JYFLTRAP at JYFL/Jyvaeskylae and SHIPTRAP at GSI/Darmstadt. Isotopes from yttrium (Z=39) to palladium (Z=46) have been produced in heavy-ion fusion-evaporation reactions. In total, 21 nuclides were studied, and almost half of the mass values were experimentally determined for the first time: {sup 88}Tc, {sup 90-92}Ru, {sup 92-94}Rh, and {sup 94,95}Pd. For the {sup 95}Pd{sup m}, (21/2{sup +}) high-spin state, a first direct mass determination was performed. Relative mass uncertainties of typically {delta}m/m=5x10{sup -8} were obtained. The impact of the new mass values has been studied in {nu} p-process nucleosynthesis calculations. The resulting reaction flow and the final abundances are compared with those obtained with the data of the Atomic Mass Evaluation 2003.

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.

Recent developments at ISOLTRAP: Towards a relative mass accuracy of exotic nuclei below 10-8

During the last two years, the performance of the Penning trap mass spectrometer ISOLTRAP at ISOLDE/CERN (Geneva) has been considerably enhanced. Many technical improvements have been completed (i) to access nuclides that are produced in minute quantities of only 100 ions s−1, (ii) to increase the relative mass accuracy to ≤ 1 × 10−8 and (iii) to make accessible nuclei with a half-life of down to ≈ 5 ms. The major steps are presented, in particular the recent implementation of a magnetron phase locking mechanism which results in a significant reduction of the duration of ISOLTRAPs cyclotron measurements.

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.

High-precision mass measurements of nickel, copper, and gallium isotopes and the purported shell closure at N=40

High-precision mass measurements of more than 30 neutron-rich nuclides around the Z=28 closed proton shell were performed with the triple-trap mass spectrometer ISOLTRAP at ISOLDE/CERN to address the question of a possible neutron shell closure at N=40. The results for {sup 57,60,64-69}Ni (Z=28), {sup 65-74,76}Cu (Z=29), and {sup 63-65,68-78}Ga (Z=31) have a relative uncertainty of the order of 10{sup -8}. In particular, the mass of {sup 76}Cu was measured for the first time. We analyze the resulting mass surface for signs of magicity, comparing the behavior of N=40 with that of known magic numbers and with midshell behavior. While the classic indications from the mass surface show no evidence for a shell closure at N=40, there is evidence for a weak--and very localized--effect for Z=28, consistent with findings from nuclear spectroscopy studies.

First on-line test of SHIPTRAP

Abstract The ion trap facility SHIPTRAP is installed behind the separator for heavy ion reaction products (SHIP) at GSI, which is well known for the discovery of new super-heavy elements produced in cold fusion reactions. SHIPTRAP consists out of a gas cell for stopping the recoil ions delivered by SHIP and two linear radio frequency quadrupole (RFQ) structures for cooling and accumulating the ions. In a first Penning trap the radionuclides of interest get further cooled and isobaric contaminants are removed. The second Penning trap is intended for high-precision mass measurements or identification of the stored ions before providing them to further downstream experiments. During a first on-line experiment in 2001, ions from SHIP were stopped in the gas cell and transferred into the RFQ structures. Accumulation and cooling could be demonstrated.

Mass measurements and nuclear physics-recent results from ISOLTRAP

The Penning trap mass spectrometer ISOLTRAP is a facility for high-precision mass measurements of short-lived radioactive nuclei installed at ISOLDE/CERN in Geneva. More than 200 masses have been measured with relative uncertainties of 1 × 10−7 or even close to 1 × 10−8 in special cases. This publication gives an overview of the measurements performed with ISOLTRAP and discusses some results.