ISOLTRAP: An on-line Penning trap for mass spectrometry on short-lived nuclides

ISOLTRAP is a Penning trap mass spectrometer for high-precision mass measurements on short-lived nuclides installed at the on-line isotope separator ISOLDE at CERN. The masses of close to 300 radionuclides have been determined up to now. The applicability of Penning trap mass spectrometry to mass measurements of exotic nuclei has been extended considerably at ISOLTRAP by improving and developing this double Penning trap mass spectrometer over the past two decades. The accurate determination of nuclear binding energies far from stability includes nuclei that are produced at rates less than 100 ions/s and with half-lives well below 100ms. The mass-resolving power reaches 10⁷ corresponding to 10keV for medium heavy nuclei and the uncertainty of the resulting mass values has been pushed down to below 10^-8. The article describes technical developments achieved since 1996 and the present performance of ISOLTRAP.     

Observing a single hydrogen-like ion in a Penning trap at T = 4 K

We describe how a single hydrogen-like ion (C⁵⁺) is prepared, cooled with the method of resistive cooling and non-destructively detected with the image-current technique in a cryogenic Penning trap at T = 4 K. The storage time for C⁵⁺-ions in the cryogenically pumped vacuum chamber is longer than six months. The experimental techniques of preparing, cooling and detecting highly-charged ions in a Penning trap are relevant for precision experiments such as g-factor measurements, mass spectroscopy and laser spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

The LEBIT facility at MSU

The low-energy beam and ion trap facility LEBIT at the NSCL at MSU has demonstrated that rare isotopes produced by fast-beam fragmentation can be slowed down and prepared such that precision experiments with low-energy beams are possible. For this purpose high-pressure gas-stopping is employed combined with advanced ion manipulation techniques. Penning trap mass measurements on short-lived rare isotopes have been performed with a 9.4 T Penning trap mass spectrometer. Examples include ⁶⁶As, which has a half-live of only 96 ms, and the super-allowed Fermi-emitter ³⁸Ca, for which a mass accuracy of 8 ppb (280 eV) has been achieved. The high accuracy of this new mass value makes ³⁸Ca a new candidate for the test of the conserved vector current hypothesis.      

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.     

The TITAN mass measurement facility at TRIUMF-ISAC

The TITAN facility at TRIUMF-ISAC will use four ion traps with the primary goal of determining nuclear masses with high precision, particularly for short lived isotopes with lifetimes down to approximately 10 ms. The design value for the accuracy of the mass measurement is 1 ×10^???8. The four main components in the facility are an RF cooler/buncher (RFCT) receiving the incoming ion beam, an electron beam ion trap (EBIT) to breed the ions to higher charge states, a cooler Penning trap (CPET) to cool the highly charged ions, and finally the measurement Penning trap (MPET) for the precision mass determination. Additional goals for this system are laser spectroscopy on ions extracted from the RFCT and beta spectroscopy in the EBIT (in Penning trap mode) on ions that are purified using selective buffer gas cooling in the CPET. The physics motivation for the mass measurements are manifold, from unitarity tests of the CKM matrix to nuclear structure very far from the valley of stability, nuclear astrophysics and the study of halo-nuclei. As a first measurement the mass of ¹¹Li will be determined. With a lifetime of 8.7 ms and a demonstrated production rate of 4×10⁴ ions/sec at ISAC the goal for this measurement at TITAN is a relative uncertainty of 5×10^???8. This would check previous conflicting measurements and provide information for nuclear theory and models.     

Trapping positrons for low energy antihydrogen production

A method of trapping large numbers of positrons at liquid helium temperatures in a 6 Tesla magnetic field is described. Positrons from a sodium-22 source are moderated to low energies with a tungsten reflection moderator. A Penning trap with hyperbolic electrodes holds the positrons in a magnetron (EXB) orbit. The positrons are then cooled via coupling to a tuned circuit that is in resonance with the axial oscillation of the positrons. At this point, many slow positrons are permanently trapped in the Penning trap. The positrons are centered in the trap by applying a radio-frequency field at a frequency near the sum of the axial and magnetron frequencies. This method promises to produce 10⁶ trapped positrons at a density of 10⁷ to 10⁸ per cm³. Such densities of positrons would be useful in producing antihydrogen in combination with existing antiproton plasmas.     

Possible measurement of the gravitational acceleration of antiprotons in the presence of “strong” patch effect electrical fields

The role of electrical fields due to the patch effect in a Penning trap used to measure the Earth's gravitational accelerationg on antiprotons is analyzed. Theg measurement method is based on the study of the gravity-induced shift of the center of the radial orbits of particles stored in a Penning trap having the magnetic field perpendicular to the direction of the force of gravity. The analysis of the radial motion shows that forces originating from patch effect electrical fields about ten times stronger than the force of gravity, still allow a differential measurement ofg for antiprotons and matter particles (H^–). As the precision of the measurement is affected by the particle axial energy distribution, particular care must be devoted to injecting antiprotons and H^– ions into the trap with very similar initial conditions.     

Status of the Penning trap project in Munich

The MLLTRAP at the Maier-Leibnitz-Laboratory (Garching) is a new Penning trap facility designed to combine several novel technologies to decelerate, charge breed, cool, bunch and purify the reaction products and perform high-accuracy nuclear and atomic mass measurements. It is now in the commissioning phase, achieving a mass-resolving power of about 10⁵ in the purification trap for stable ions.     

Effects of space charge on the mass purification in Penning traps

The influence of space charge on the mass selection of ions stored in a Penning trap was investigated with the ISOLTRAP experiment at CERN/ISOLDE. A mixture of ^85,87Rb^?+? ions has been used to probe the change of the experimental parameters, e.g. frequencies and amplitudes of the radiofrequency excitations, as a function of the number of ions present in the trap.     

Determining isotopic distributions of fission products with a Penning trap

A novel method to determine independent yields in particle-induced fission employing the ion guide technique and ion counting after a Penning trap has been developed. The method takes advantage of the fact that a Penning trap can be used as a precision mass filter, which allows an unambiguous identification of the fission fragments. The method was tested with 25MeV and 50MeV proton-induced fission of ²³⁸U . The data is internally reproducible with an accuracy of a few per cent. A satisfactory agreement was obtained with older ion guide yield measurements in 25MeV proton-induced fission. The results for Rb and Cs yields in 50MeV proton-induced fission agree with previous measurements performed at an isotope separator equipped with a chemically selective ion source.     

Precision Penning trap mass measurements of rare isotopes produced by projectile fragmentation

The low-energy beam and ion trap facility LEBIT at NSCL/MSU is at present the only facility where precision experiments are performed with stopped rare isotope beams produced by fast-beam fragmentation. LEBIT combines high-pressure-gas stopping with advanced ion manipulation techniques to provide brilliant low-energy beams. So far these beams have mainly been used for mass measurements on short-lived rare isotopes with a 9.4T Penning trap mass spectrometer. Recent examples include ^70m Br , located at the proton dripline, ³²Si and the iron isotopes ^63-65Fe . While the measurement of ³²Si helps to solve a long-standing dispute over the validity of the isobaric multiplet mass equation (IMME) for the A = 32 , T = 2 multiplet, the mass measurements of ^65m,g Fe marked the first time a nuclear isomeric state has been discovered by Penning trap mass spectrometry.     

Half-open Penning trap with efficient light collection for precision laser spectroscopy of highly charged ions

We have conceived, built and operated a ’half-open’ cylindrical Penning trap for the confinement and laser spectroscopy of highly charged ions. This trap allows fluorescence detection employing a solid angle which is about one order of magnitude larger than in conventional cylindrical Penning traps. At the same time, the desired electrostatic and magnetostatic properties of a closed-endcap cylindrical Penning trap are preserved in this configuration. We give a detailed account on the design and confinement properties, a characterization of the trap and show first results of light collection with in-trap produced highly charged ions.     

Parametric excitation of oscillations of charged particle bunches in a Penning trap

A self-similar solution is obtained for the self-consistent hydrodynamic equations describing the motion of an ellipsoid of charged particles in a Penning trap and in an rf trap. The conditions are determined for which a small periodic variation of the confining magnetic field in the Penning trap drives oscillations of the bunch. Zh. Tekh. Fiz. 67, 27–29 (January 1997)     

Ion bunch stacking in a Penning trap after purification in an electrostatic mirror trap

The success of many measurements in analytical mass spectrometry as well as in precision mass determinations for atomic and nuclear physics is handicapped when the ion sources deliver “contaminations”, i.e., unwanted ions of masses similar to those of the ions of interest. In particular, in ion-trapping devices, large amounts of contaminant ions result in significant systematic errors—if the measurements are possible at all. We present a solution for such cases: The ions from a quasi-continuous source are bunched in a linear radio-frequency-quadrupole ion trap, separated by a multi-reflection time-of-flight section followed by a Bradbury–Nielsen gate, and then captured in a Penning trap. Buffer-gas cooling is used to damp the ion motion in the latter, which allows a repeated opening of the Penning trap for a stacking of mass-selected ion bunches. Proof-of-principle demonstrations have been performed with the ISOLTRAP setup at ISOLDE/CERN, both with ¹³³Cs⁺ ions from an off-line ion source and by application to an on-line beam of ¹⁷⁹Lu⁺ ions contaminated with ¹⁶³Dy¹⁶O⁺ ions. In addition, an optimization of the experimental procedure is given, in particular for the number of ion bunches captured as a function of the ions’ lifetimes and the parameters of the experiment .     

Progress with the MPIK/UW-PTMS in Heidelberg

The precise determination of the ³He/³H mass ratio, and hence the tritium ??-decay endpoint energy E ₀, is of relevance for the measurement of the electron anti-neutrino mass performed by the Karlsruhe Tritium Neutrino experiment (KATRIN). By determining this ratio to an uncertainty of 1 part in 10¹¹, systematic errors of E ₀ can be checked in the data analysis of KATRIN. To reach this precision, a Penning Trap Mass Spectrometer was constructed at the University of Washington and has been transferred to the Max Planck Institute for Nuclear Physics in Heidelberg at the end of 2008. Since then it is called MPIK/UW-PTMS. Special design features are the utilization of an external ion source and a double trap configuration. The external Penning ion source efficiently ionizes the helium and tritium gas and can give superior elimination of unwanted ion species compared to the previously utilized in-trap-ionization by electrons from a field-emission point. The design as a double Penning trap allows a faster measurement procedure. This should help to avoid problems resulting from long-term drifts in the experimental conditions. Additionally, the laboratory in Heidelberg was carefully prepared to have very stable environmental conditions. Experimental challenges and the first Heidelberg results with the new spectrometer are presented.     

Extremely cold positrons for antihydrogen production

The storage of extremely cold (4 K) antiprotons in a Penning trap is an important step toward the creation and study of cold antihydrogen. The other required ingredient, the largest possible number of comparably cold positrons, is still lacking. These would be recombined in a high vacuum with the trapped antiprotons, already stored at a pressure below 5×10⁻¹⁷ Torr, thereby avoiding annihilation of the antihydrogen atoms before they can be used in high accuracy measurements or in controlled collision experiments. In an exploratory experiment, positrons from a 18 mCi²²Na source follow fringing field lines of a 6 T superconducting solenoid through tiny apertures in the electrodes of a Penning trap to strike a tungsten (reflection) moderator. The positron beam is chopped mechanically and a lock-in directly detects a positron current of 2.5×10⁶e⁺/s on the moderator. The use of a moderator, unlike an earlier experiment in which < 100 positrons were confined in vacuum, should greatly increase the number of positrons trapped in high vacuum.     

Precise spectroscopy for fundamental physics

We have applied experimental techniques that were developed for use in atomic frequency standards andclocks to investigations of local Lorentz invariance, the linearity of quantum mechanics, andanomalous long-range spin-dependent forces. These experiments used a hyperfine transition in⁹Be⁺ ions in a Penning trap. Recently, we have studied hyperfine transitions in¹⁹⁹Hg⁺ ions in a linear rf trap. Hg⁺ ions might be used for similar investigations in the future.Work of the National Institute of Standards andTechnology. Not subject to US copyright.     

Collision induced dissociation of doubly charged silver clusters Agn 2+, n=21,22,23

Doubly charged silver clusters Ag_n ²⁺, n=21, 22, 23, are produced by electron bombardment of an Ag_n ⁺ ensemble stored in a Penning trap. After mass selection the clusters are subjected to collision induced dissociation. The fragmentation channels are determined by time-of-flight mass spectrometry after ejection of the resulting ion ensemble from the trap. Monomer evaporation is the only decay path observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

The SMILETRAP (Stockholm-Mainz-Ion-LEvitation-TRAP) facility

Described in this paper is an experimental facility which measures atomic masses by using multiply charged ions from an electron beam ion source. The ions are injected into a Penning trap and the cyclotron frequencies measured. A precision of 2×10^–9 has been reached using highly charged carbon, nitrogen, oxygen and neon.