Two-body beta decay of stored few-electron ions

The combination of the projectile fragment separator FRS and the cooler-storage ring ESR at the accelerator facility of GSI Darmstadt offers the unique opportunity to study beta decay of stored highly-charged ions. Basic nuclear properties such as masses and half-lives are measured by applying the mass- and time-resolved Schottky Mass Spectrometry (SMS). The relative mass-to-charge ratio is directly correlated to the relative revolution frequency. The SMS is sensitive to single stored ions and the decay of each stored ion can be precisely determine by steady monitoring of the corresponding revolution frequencies. On this basis the single particle decay-spectroscopy has been developed which allows for an unambiguous time-resolved and background-free identification of a certain decay branch. In this contribution we discuss experiments on the orbital electron capture (EC) of radioactive ions in the ESR. Fully ionized, hydrogen- and helium-like ¹⁴⁰Pr and ¹⁴²Pm ions have been selected for these studies. These nuclei decay to stable daughter nuclei via either the three-body ?? ^?+?- or the two-body EC-decay by a single allowed Gamow?CTeller (1^?+? ??0^?+?) transition.     

Lifetime measurements of metastable states in CRYRING

Collinear ion beam laser spectroscopy has been utilized in the ion storage ring CRYRING in order to measure lifetimes of metastable states in singly charged ions. The laser light has been used for selective probing of the population decay of individual fine structure or hyperfine structure states. With the use of a mechanical shutter, time resolved studies could be performed with millisecond resolution. In another experiment, the cw laser light was used to optically pump stored ions from the ground state into a specific metastable state. With most of the stored ions in the metastable state, direct observation of the decay of the forbidden transition from the metastable state to the ground state could be observed passively. Measurements of metastable lifetimes in Ca⁺, Sr⁺, Xe⁺ and Eu⁺ will be discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

A double Paul trap system for the electronic coupling of ions

We report on the progress towards the implementation of a double trap system for the coupling of ion clouds stored in different traps through the charges they induce in a common electrode. The two traps have cylindrical symmetry and can be used either as Penning or Paul traps. The laser system for Doppler cooling of ⁴⁰Ca⁺ clouds is described. The cooling can be performed differentially in the two traps, aiming at the study of the coupling between two clouds of ions at different temperatures. This setup is the starting point for the realization of energy transfer between two Doppler-cooled ions stored in different traps. We report on the status of the experiment and evaluate the performance required for an efficient coupling.     

Laser cooling of a small number of Be+ ions in a penning trap

Be⁺ ions stored in a Penning trap were cooled by a laser beam perpendicular to the magnetic field. The cooled ions are strongly coupled and phase transitions of up to 100 ions were observed. In experiments with only a few ions stored in the trap, a stepwise decrease in fluorescence intensity was observed. All steps are of the same size and so every step is attributed to a single ion. The discrete changes in fluorescence occurred more frequently when the background pressure was increased, caused by collisions between stored ions and background neutral molecules.     

Relativistic doppler effect in199Hg+ stored ions atomic frequency standard

RF traps are widely used nowadays in high resolution hyperfine spectroscopy. The spectrum of the microwave hyperfine transition at 40.5 GHz of the fundamental level² S _1/2 of¹⁹⁹Hg⁺ is phase modulated at the secular frequency of the stored ions and it consists of a narrow central line and lateral bands broadened by the ion trajectory and velocity distribution. The central line itself is shifted by the second-order Doppler effect which is the most important systematic error of stored ions frequency standards. In this paper the relativistic Doppler effect in the case of¹⁹⁹Hg⁺ stored ions is deduced applying a mathematical formalism and using physical parameters that we have measured experimentally.     

A phase-imaging technique for cyclotron-frequency measurements

A novel approach to mass measurements at the 10^?9 level for short-lived nuclides with half-lives well below one second is presented. It is based on the projection of the radial ion motion in a Penning trap onto a position-sensitive detector. Compared with the presently employed time-of-flight ion-cyclotron-resonance technique, the novel approach is 25-times faster and provides a 40-fold gain in resolving power. Moreover, it offers a substantially higher sensitivity since just two ions are sufficient to determine the ion’s cyclotron frequency. Systematic effects specific to the technique that can change the measured cyclotron frequency are considered in detail. It is shown that the main factors that limit the maximal accuracy and resolving power of the technique are collisions of the stored ions with residual gas in the trap, the temporal instability of the trapping voltage, the anharmonicities of the trapping potential and the uncertainty introduced by the conversion of the cyclotron to magnetron motion.     

Ion storage in Kingdon trap

Abstract

The characteristics of stored ions in a Kingdon trap have been investigated. The charge distribution of stored ions was measured by a time of flight (TOF) mass spectrometer. Storage of Ar^q+ (q = 1, 2, 3, 4) produced by electron beam irradiation has been confirmed. The dependences of Ar ion yields on the trapping potential and storage time have been systematically studied.

Applying a voltage to end plates is very important for the storage of ions. Remarkable oscillations of the ion yields are found in the decay curves as a function of storage time for Ar⁺, Ar²⁺ and Ar³⁺ indicating periodical motion of each ion group about the central wire. The three dimensional orbits of ions in the trap are analysed by a computer calculation to understand the experimental results.     

Status of the project TRAPSENSOR

Penning traps are used to perform very precise mass measurements on exotic and stable nuclei covering a variety of topics. In order to reach the highest accuracy, only one ion must be stored and measured in the trap. The mass is determined from the oscillation frequencies, by detecting the current the stored ion induces on the trap electrodes. This is a well-known technique demonstrated for ions with low or medium mass-to-charge ratios. Another technique recently proposed, and referred to as Quantum Sensor, aims at extending the applicability of single-ion Penning-trap measurements through the full atomic mass scale. The technique substitutes the electronic detection by the detection of fluorescence photons from a laser-cooled ion stored in a second Penning trap, thereafter this ion interacts with the ion of interest. The new device is under completion at the University of Granada (Spain) within the project TRAPSENSOR. This paper will present the status of this project.     

Frequency stability of a mercury ion frequency standard

Observation with an improved signal-to-noise ratio of the hyperfine resonance pattern of¹⁹⁹Hg⁺ stored in a radiofrequency trap is reported. A frequency control loop which locks the frequency of a 5MHz quartz crystal oscillator to the hyperfine transition of stored mercury ions, close to 40.5 GHz, is described. In this system, pulses delivered by the photomultiplier are processed digitally. Optimal conditions for the interrogation of the hyperfine transition are specified. A fractional frequency stability σ_y(τ)=3.6×10⁻¹¹τ^−1/2 for 10s<τ<3500s has been obtained. This frequency stability is comparble to that of commercially available cesium beam frequency standards. Prospect for improvement by a factor of at least 10 are discussed. This work has been sponsored by DRET     

Additive colouring of CaF2:Yb crystals: determination of Yb2+ concentration in CaF2:Yb crystals and ceramics

When growing CaF₂ crystal doped with rare-earth ions, most of these ions are present in a trivalent state. However, due to contact with graphite crucible, a small proportion of a number of ions (Eu, Sm, Yb and Tm) are reduced to a bivalent state. A similar situation takes place during fabrication of CaF₂ ceramics doped with rare-earth metals. This fact is of particular importance for laser CaF₂:Yb crystals (ceramics), a promising material for short-pulse, high-power, high-energy diode-pumped solid state lasers since the presence of bivalent Yb ions can be a source of thermal losses. To date, there has been no technique to determine Yb²⁺ concentration in as-grown crystals. The proposed technique is based on a total reduction of Yb³⁺ ions via the heating of as-grown CaF₂ crystals with known concentration of Yb in the reducing atmosphere of metal vapour and determining the cross section of absorption bands of Yb²⁺ ions. The knowledge of these parameters allows estimation of the Yb²⁺ content in CaF₂:Yb crystals or ceramics by analysing their absorption spectra. Examples of using this technique are given. The technology of CdF₂ crystals reduction (an “additive colouring”) and features of colouring of crystals doped with rare-earth ions are considered.     

Radiative lifetimes of excited W II levels

Radiative lifetimes of 19 selected W II levels with energies between 36 000 cm^-1 and 55 000 cm^-1 have been measured with the time-resolved laser-induced fluorescence technique. The ions are generated in a hollow cathode discharge and stored in a linear Paul trap. Selected states are populated with tunable dye laser pulses and the subsequent fluorescence is measured by means of a 5 Gigasample transient digitizer and a fast photodetector with a risetime of 700 ps. By taking into account both the temporal profile of the laser pulses and the separately measured response function of the system, the lifetime can be determined from the full decay curve. A refined evaluation procedure, taking into account saturation effects in the signals, reduces the uncertainty in our data to around 1%. Received: 30 July 1998 / Revised: 18 August 1998     

A compact Penning trap for light ions

We describe the construction of a novel compact Penning trap from strong permanent magnets for trapping light ions. Our cylindrically symmetric, iron-free magnetic configuration allows fully analytical treatment, is easy to handle and to optimize. The magnetic field inhomogeneity is less than 1% in a volume of 1 cm³ at 0.7 T. The stored H⁺ and H ₂ ⁺ ions in this trap are detected electrically by the rf absorption method. The charge density, total number and storage time of the trapped ions are measured.Dedicated to H. Walther on the occasion of his 60th birthday     

3d D lifetime in laser cooled Ca : Influence of cooling laser power

We have measured the lifetime of the metastable 3D _5/2 level in Ca⁺ using the “quantum jump" technique on a single stored and laser cooled ion in a linear Paul trap. We found a linear dependence of the measured decay rate on the power of the laser which repumps the ions from the long lived 3D _3/2 level. This can be explained by off-resonant depletion of the 3D _5/2 level. The proper lifetime of this level is obtained by a linear extrapolation of the measured lifetime to zero laser power. We obtain 1100(18) ms in agreement with theoretical calculations. The observed systematic change of the decay rate resolves discrepancies between earlier experiments in which this effect had not been considered. Measurements on a linear chain of 10 laser cooled ions showed unexpected frequent coincidences of quantum jumps within our observation time of 20 ms. This indicates a so far unexplained correlation between the ions in the chain at large distances. Received 3 March 1999     

Space-Charge Effects with REXTRAP

The beam quality of radioactive ion beams produced by present target ion source technology is often not sufficient for direct post-acceleration. Furthermore, pulsed beams insure a more efficient use of an accelerator. In the case of REX-ISOLDE, the post accelerator at the CERN ISOLDE facility, a gas-filled Penning trap (REXTRAP) has been chosen for accumulation of the radioactive ions and conversion into cooled bunches. Radial centering of the ions is achieved by applying an rf field with a frequency equal to the cyclotron frequency of the desired ion species. The efficiency achieved in the first tests with different isotopes covering nearly the entire mass range was already >20%. Going to total numbers of >10⁵ stored ions in the trap a shift of the centering frequency could be observed, which is most likely due to space charge effects. Despite this, it was possible to accumulate up to 10⁷ ions and deliver them as cooled bunches. This revised version was published online in July 2006 with corrections to the Cover Date.     

Trapped positrons for antihydrogen

Positrons from a 12 mCi²²Na source are slowed by a W(110) reflection moderator and then captured in a Penning trap, by damping their motion with a tuned circuit. Because of the stability of the Penning trap and the cryogenic ultra-high vacuum environment, we anticipate that positrons can be accumulated and stored indefinitely. A continuous loading rate of 0.14 e⁺/s is observed for 32 h in this initial demonstration. More than 1.6×10⁴ positrons have thus been trapped and stored at 4 K, with improvements expected. The extremely high vacuum is required for compatibility with an existing antiproton trap, which has already held more than 10⁵ antiprotons at 4 K, for producing antihydrogen at low temperatures. The extremely cold positrons in high vacuum may also prove to be useful for cooling highly stripped ions.     

Paramagnetic centers in oak and onion extracts

We have used EPR to study oak bark, bulb onion peel, and their aqueous extracts. We have established that their paramagnetic properties are determined by at least four types of paramagnetic centers: semiquinone radicals, Fe³⁺ ions, and two types of Mn²⁺ ions. We have observed that metal ions are extracted by water from the plant raw material without any change in their original coordination environment. The aqueous extracts were subjected to oxidative processes: mild oxidation in the dried state, strong oxidation when stored as solutions. We have shown that when the solutions are stored, the oxidation reaction is catalyzed by the metal ions and is accompanied by formation of semiquinone radicals. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 440–445, May–June, 2009.     

Photon-trap spectroscopy of size-selected free cluster ions: “direct” measurement of optical absorption of Ag+ 9

An absorption spectrum of size-selected free cluster ions has been measured “directly” via extinction of light without relying on photodepletion/dissociation spectroscopy. The novel technique employs an ion trap and an optical cavity; cluster ions stored in an ion trap interact with photons trapped in a cavity. The storage lifetime of photons in the cavity provides “direct” observation of extinction of light (photon-trap spectroscopy, which is a generalized scheme of cavity ring-down spectroscopy). The first measurement is performed on ultraviolet absorption of Ag₉ ⁺. Temperature dependence of the spectrum is presented by cooling the ion trap down to 10 K.     

Recombination experiments at CRYRING

Recent advances in studies of electron-ion recombination processes at low relative energies with the electron cooler of the heavy-ion storage ring CRYRING are shown. Through the use of an adiabatically expanded electron beam, collisions down to 10^-4eV relative energies were measured with highly charged ions stored in the ring at around 15 MeV/amu energies. Examples of recombination measurements for bare ions of D⁺, He²⁺, N⁷⁺, Ne¹⁰⁺ and Si¹⁴⁺ are presented. Further on, results of an experiment measuring laser-induced recombination (LIR) into n=3 states of deuterium with polarized laser light are shown. This revised version was published online in July 2006 with corrections to the Cover Date.     

A photoelectron-ion-ion triple coincidence technique for the study of double photoionization and its consequences

A new technique in which two photoions are detected in coincidence with a photoelectron is described, and its advantages in the study of double ionization are explored. The power of the technique to distinguish decay routes is demonstrated by proof that the equal mass fragmentation of SO₂²⁺produces predominantly O₂⁺ + S⁺and not O⁺ + O⁺. The dissociation of CH₃I²⁺ following He(II) ionization is shown to involve at least twelve distinct pathways, instead of the six previously known.A major potential of the new technique is to elucidate the dynamics of three-fragment decays: it is shown that the formation of C⁺ + S⁺ from CS₂²⁺ can be modelled as a sequential, rather than a simultaneous explosion. The new technique gives the first demonstration, in the case of NO₂, of angular anisotropy in the electrons ejected in double photoionization. The counting statistics of the new method are shown to allow absolute calibration of the detection efficiencies for both electrons and ions. Finally, the technique offers a new method for the detection and investigation of slowly dissociating doubly-charged ions.