J. R. Crespo López-Urrutia

The on-line charge breeding program at TRIUMF's Ion Trap For Atomic and Nuclear Science for precision mass measurements

TRIUMFs Ion Trap for Atomic and Nuclear science (TITAN) constitutes the only high precision mass measurement setup coupled to a rare isotope facility capable of increasing the charge state of short-lived nuclides prior to the actual mass determination in a Penning trap. Recent developments around TITANs charge breeder, the electron beam ion trap, form the basis for several successful experiments on radioactive isotopes with half-lives as low as 65 ms and in charge states as high as 22+.

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.

The magnetic trapping mode of an electron beam ion trap: New opportunities for highly charged ion research

Using x‐ray spectroscopic techniques, we have investigated the properties of an electron beam ion trap (EBIT) after the electron beam is switched off. In the absence of the electron beam, bare, and hydrogenlike Kr35+ and Kr36+ ions remain trapped due to externally applied magnetic and electric fields for at least 5 s; xenon ions with an open L shell, i.e., Xe45+–Xe52+, remain trapped at least as long as 20 s. The ion storage time in this ‘‘magnetic trapping mode’’ depends on the pressure of background atoms as well as on the value of the externally applied trapping potential, and even longer ion storage times appear possible. The magnetic trapping mode enables a variety of new opportunities for atomic physics research involving highly charged ions, which include the study of charge transfer reactions, Doppler‐shift‐free measurements of the Lamb shift, measurements of radiative lifetimes of long‐lived metastable levels, or ion‐ion collision studies, by x‐ray or laser spectroscopy, and mass spectrometry. Be...

Novel technique for high-precision Bragg-angle determination in crystal x-ray spectroscopy

A novel technique for a high-precision large acceptance determination of the Bragg angle in crystal x-ray spectroscopy is presented and demonstrated. The method exploits visible light beams as fiducials reflected on the x-ray crystal’s surface to ensure exact knowledge of the position on the crystal at which the x rays are reflected, replacing entrance slits, thus making flat crystals suitable for low x-ray fluxes. It can be shown that many error sources arising from uncertainties in the determination of geometrical properties are eliminated in this way. A flat crystal x-ray spectrometer based on this technique has been designed, built, and tested using the most precisely known wavelengths emitted by highly charged ions, namely H- and He-like argon. The result for the 1s2pP11→1s2S01 w-line of He-like argon exhibits a statistical uncertainty of 3.8ppm and an estimated systematic error of about 3ppm, thus becoming the most accurate measurement of the He-like resonance transition in highly charged ions. It i...

An unexpectedly low oscillator strength as the origin of the Fe xvii emission problem

Highly charged iron (Fe16+, here referred to as Fe xvii) produces some of the brightest X-ray emission lines from hot astrophysical objects, including galaxy clusters and stellar coronae, and it dominates the emission of the Sun at wavelengths near 15 ångströms. The Fe xvii spectrum is, however, poorly fitted by even the best astrophysical models. A particular problem has been that the intensity of the strongest Fe xvii line is generally weaker than predicted. This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions, fuelling a continuing controversy over whether this discrepancy is caused by incomplete modelling of the plasma environment in these objects or by shortcomings in the treatment of the underlying atomic physics. Here we report the results of an experiment in which a target of iron ions was induced to fluoresce by subjecting it to femtosecond X-ray pulses from a free-electron laser; our aim was to isolate a key aspect of the quantum mechanical description of the line emission. Surprisingly, we find a relative oscillator strength that is unexpectedly low, differing by 3.6σ from the best quantum mechanical calculations. Our measurements suggest that the poor agreement is rooted in the quality of the underlying atomic wavefunctions rather than in insufficient modelling of collisional processes.

Spectroscopy in the extreme ultraviolet on an electron beam ion trap

A compact grazing-incidence spectrometer was implemented on the Livermore electron beam ion trap facility for spectral measurements in the extreme ultraviolet spectral region. The spectrometer employed a 1200 l/mm grating designed for flat-field focusing and a charge coupled device camera for readout. The instrument was used to measure line emission in the range from 25 to 220 A with a resolving power as high as 600. The performance and calibration of the instrument is described and spectra from highly charged nitrogen and iron ions are presented. Measurements of the K-shell spectrum of He-like N5+ are presented that confirm earlier wavelength determinations and illustrate the accuracy achievable with the instrument. Our measurement suggests a change in the line identifications of the forbidden He-like N5+ transition 1s2s 3S1→1s2 1S0 and of the Li-like N4+ collisional satellite transition 1s2s2p 2P3/2→1s22s 2S1/2 observed on the Alcator C-Mod tokamak.

Efficient rotational cooling of Coulomb-crystallized molecular ions by a helium buffer gas

The preparation of cold molecules is of great importance in many contexts, such as fundamental physics investigations, high-resolution spectroscopy of complex molecules, cold chemistry and astrochemistry. One versatile and widely applied method to cool molecules is helium buffer-gas cooling in either a supersonic beam expansion or a cryogenic trap environment. Another more recent method applicable to trapped molecular ions relies on sympathetic translational cooling, through collisional interactions with co-trapped, laser-cooled atomic ions, into spatially ordered structures called Coulomb crystals, combined with laser-controlled internal-state preparation. Here we present experimental results on helium buffer-gas cooling of the rotational degrees of freedom of MgH+ molecular ions, which have been trapped and sympathetically cooled in a cryogenic linear radio-frequency quadrupole trap. With helium collision rates of only about ten per second—that is, four to five orders of magnitude lower than in typical buffer-gas cooling settings—we have cooled a single molecular ion to a rotational temperature of  kelvin, the lowest such temperature so far measured. In addition, by varying the shape of, or the number of atomic and molecular ions in, larger Coulomb crystals, or both, we have tuned the effective rotational temperature from about 7 kelvin to about 60 kelvin by changing the translational micromotion energy of the ions. The extremely low helium collision rate may allow for sympathetic sideband cooling of single molecular ions, and eventually make quantum-logic spectroscopy of buffer-gas-cooled molecular ions feasible. Furthermore, application of the present cooling scheme to complex molecular ions should enable single- or few-state manipulations of individual molecules of biological interest.

Physics with colder molecular ions: The Heidelberg Cryogenic Storage Ring CSR

A novel cryogenic electrostatic storage ring is planned to be built at the Max-Planck Institute for Nuclear Physics in Heidelberg. The machine is expected to operate at low temperatures (∼ 2K) and to store beams with kinetic energies between 20 to 300 keV. An electron target based on cooled photocathode technology will serve as a major tool for the study of reactions between molecular ions and electrons. Moreover, atomic beams can be merged and crossed with the stored ion beams allowing for atom molecularion collision studies at very low up to high relative energies. The proposed experimental program, centered around the physics of cold molecular ions, is shortly outlined.

A cryogenic electrostatic trap for long-time storage of keV ion beams

We report on the realization and operation of a fast ion beam trap of the linear electrostatic type employing liquid helium cooling to reach extremely low blackbody radiation temperature and residual gas density and, hence, long storage times of more than 5 min which are unprecedented for keV ion beams. Inside a beam pipe that can be cooled to temperatures <15 K, with 1.8 K reached in some locations, an ion beam pulse can be stored at kinetic energies of 2-20 keV between two electrostatic mirrors. Along with an overview of the cryogenic trap design, we present a measurement of the residual gas density inside the trap resulting in only 2 x 10(3) cm(-3), which for a room temperature environment corresponds to a pressure in the 10(-14) mbar range. The device, called the cryogenic trap for fast ion beams, is now being used to investigate molecules and clusters at low temperatures, but has also served as a design prototype for the cryogenic heavy-ion storage ring currently under construction at the Max-Planck Institute for Nuclear Physics.

Magnetic-dipole transition probabilities in B-like and Be-like ions

The magnetic-dipole transition probabilities between the fine-structure levels (1s{sup 2}2s{sup 2}2p) {sup 2}P{sub 1/2}-{sup 2}P{sub 3/2} for B-like ions and (1s{sup 2}2s2p) {sup 3}P{sub 1}-{sup 3}P{sub 2} for Be-like ions are calculated. The configuration-interaction method in the Dirac-Fock-Sturm basis is employed for the evaluation of the interelectronic-interaction correction with negative-continuum spectrum being taken into account. The 1/Z interelectronic-interaction contribution is derived within a rigorous QED approach employing the two-time Green function method. The one-electron QED correction is evaluated within framework of the anomalous magnetic-moment approximation. A comparison with the theoretical results of other authors and with available experimental data is presented.