St. Becker

A new cooling technique for heavy ions in a Penning trap

A new cooling technique for heavy ions stored in a Penning trap has been developed. The axial and cyclotron motions are cooled by buffer gas collisions. The outward radial diffusion caused by the buffer gas is counteracted by an azimuthal quadrupole rf field at the sum frequency of the magnetron and cyclotron motions. A mass selectivity of 500 in the cooling is achieved while the axial energy distribution is observed to be in equilibrium with the buffer gas temperature (T = 300 K).

ISOLTRAP: a tandem Penning trap system for accurate on-line mass determination of short-lived isotopes

The tandem Penning trap mass spectrometer ISOLTRAP has been set up at the on-line mass separator ISOLDE at CERN/Geneva for accurate mass measurements of short-lived nuclei with T12 ≥ 1 s. The mass measurement is performed via the determination of the cyclotron frequency of an ion in a magnetic field. The design of the spectrometer matches the particular requirements for on-line mass measurements on short-lived isotopes. With the ISOLTRAP spectrometer masses of more than 70 radioactive nuclei have so far been determined with resolving powers exceeding one million and an accuracy of typically 10−7.

External-ion accumulation in a Penning trap with quadrupole excitation assisted buffer gas cooling

Abstract A pulsed ion beam from an external source is injected into a Penning trap and accumulated by repeatedly lowering during ion capture to prevent the ions already captured from escaping. For the same reason the newly captured ions have to be cooled, which achieved by buffer gas collisions. To prevent radial on loss, the ions are exposed to azimuthal quadrupole excitation. By choosing the appropriate frequency (range) this method (selective quadrupole excitation assisted capture and centering (SQUEACE) allows a mass selection during the capture process and leads to a centering of those ions in the Penning trap. The multiple ion bunch capture results in a significant improvement in signal-to-noise ratio and a decrease in experiment duration.

A Penning trap mass spectrometer for the study of cluster ions

A Penning trap system has been set up for storing and investigating cluster ions over time ranges from microseconds up to minutes. This enables studies of cluster reactions with extremely low cross sections and the observation of their time dependence in a new regime. The ions are created externally by laser vaporization, cooled by adiabatic expansion of a supersonic beam, and injected into the Penning trap. Detection of reaction products is achieved by combining the advantages of two complementary approaches, viz. the high resolution of Fourier transform mass spectrometry and the high sensitivity of single‐ion counting with a time‐of‐flight mass spectrometer. The performance of the apparatus is illustrated by results of recent cluster experiments.

Ground-state properties of neutron-deficient platinum isotopes

The hyperfine structure splitting and the isotope shift in the λ=266 nm transition of Pt isotopes within the mass range 183 ≦A≦ 198 have been determined by Resonance Ionization Mass Spectroscopy (RIMS) in combination with Pulsed-Laser Induced Desorption (PLID). The Pt isotopes were obtained at the on-line isotope separator ISOLDE-3/CERN as daugthers of the primarily produced Hg isotopes. Magnetic moments, quadrupole moments, and changes in the mean-square charge radii are deduced and compared with results of a particle-triaxial rotor model and mean field calculations. Good agreement with experimental data (including nuclear level schemes and transition probabilities) can only be obtained if triaxial shape is admitted. The calculations yield a smooth transition in the shape of odd-A Pt nuclei from a slightly deformed, nearly oblate195Pt via triaxial197-187Pt to a strongly deformed nearly prolate177Pt.

Trapped metal cluster ions

An overview is given of experiments with stored metal cluster ions in a Penning trap system. The setup allows axial injection of clusters produced in an external source and a time-of-flight mass analysis of the reaction products after axial ejection. The systems options include the selection of stored ions, the manipulation of their orbits, addition of reactant and buffer gases and axial optical access for laser spectroscopic studies. As described by various examples, investigations have been made with respect to the development of trapping techniques and the characterization of metal clusters in terms of their physical and chemical properties.

Resonance lonization mass spectroscopy with a pulsed thermal atomic beam

Resonance ionization mass spectroscopy (RIMS) and pulsed-laser induced desorption (PLID) have been combined for ultrasensitive detection and spectroscopy of very small samples of refractive elements. The method has been tested and applied to laser spectroscopy of 5×109 atoms (1.5 pg) of195Au (T1/2= 183d) implanted at the ISOLDE online mass separator with 60 keV into graphite. A pulsed thermal atomic beam was formed by laser desorption with a 10 ns Nd∶Yag laser pulse. Subsequently the atoms were photoionized in a three-colour, three-step resonant excitation to an autoionizing state. The selectivity was enhanced by a time-of-flight measurement of the photo ions. In resonance, one ion was detected per 105 atoms implanted resulting in a gain in detection efficiency by three orders of magnitude in comparison to the use of a continuous atomic beam. In the course of the experiments several unknown autoionizing states were found, and the lifetime of the 6d2D3/2 state of gold was determined to beτ=10.7(6) ns.

Mass measurements of very high accuracy by time-of-flight ion cyclotron resonance of ions injected into a penning trap

Abstract The possibility of absolute mass measurements using time-of-flight detection of ion cyclotron resonance on ions injected into a Penning trap has been demonstrated. Resolving powers of 2 million have been achieved, with accuracies of about 0.5 ppm. Absolute accuracy is obtained by direct observation of the sum frequency of the cyclotron and the magnetron motions through the use of an azimuthal quadrupole r.f. field to transform initial magnetron motion into cyclotron motion. Imperfections of the Penning trap leading to systematic errors are discussed. The system has been designed specifically to measure the masses of radionuclides produced at the on-line isotope separator ISOLDE. With further developments this system will provide mass measurements of 0.1 ppm accuracy on nuclei which are available in quantities such that on average one will survive nuclear decay for the duration of the measurement cycle.

Collision induced dissociation of stored gold cluster ions

The stability of gold cluster ions Aun+ (2≦n≦23) has been investigated via collision induced dissociation in a Penning trap. Threshold energies and dissociation channels have been determined. The cluster stability exhibits a pronounced odd — even alternation: Clusters with an odd number of atoms,n, are more stable than the even-numbered ones. Enhanced stabilities are found for Au3+, Au9+, and Au19+ in accordance with the Clemenger-Nilsson and the deformed jellium model of delocalized valence electrons. Excited odd cluster ions withn≦15 predominantly decay by evaporation of dimers; all others decay by monomer evaporation. From the dissociation channels estimates of the binding energies are deduced.

Thermionic electron emission of small tungsten cluster anions on the milliseconds time scale

Small tungsten cluster anions Wn− (n=4–9 and 18–20) are stored in a Penning trap and electronically excited by photoabsorption (Ehν=1.815, 2.33, 3.5, and 4.66 eV). Delayed electron emission is observed on the milliseconds time scale by systematic variation of the storage duration between laser excitation and ion detection. Even if the photon energy exceeds the electron detachment energy, electrons are emitted several milliseconds after laser excitation. The electron emission time constant is determined as a function of the laser pulse energy. An Arrhenius analysis suggests that the observed delayed electron emission is a thermal process in analogy to thermionic emission of bulk materials. As shown by these experiments there is a simple rule for the dominating cooling channel of laser excited clusters: thermionic emission generally occurs as long as the electron binding energy is lower than the dissociation energy.