Multi-reflection Time-of-flight Research Articles

Simulations of a cryogenic, buffer-gas filled Paul trap for low-emittance ion bunches

Many experiments with pulsed ion beams benefit from or even require ion bunches with both small temporal width as well as small energy spread. To achieve optimal ion-beam preparation, a buffer-gas filled cryogenic Paul trap is being developed in the context of the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS). There, ion bunches of short-lived radionuclides are trapped in a Multi-Reflection Time-of-Flight (MR-ToF) device. Thus, the ions can be repeatedly probed by a laser beam compared to only once in conventional, single-passage collinear laser spectroscopy. To fulfill MIRACLS’ opposing requirements of a small temporal ion-bunch width and small energy spread, a buffer-gas filled cryogenic Paul trap is envisioned. Ion-optical simulations confirm the advantages of cryogenic temperatures and the linear scaling of the beam emittance as a function of the buffer-gas temperature. Beyond MIRACLS, high-quality ion beams from a cryogenic Paul trap will be beneficial for other precision experiments at radioactive ion beam facilities.

Tandem mass spectrometry using continuous-wave infrared multiphoton dissociation in an electrostatic linear ion trap.

The electrostatic linear ion trap (ELIT) can be operated as a multi-reflection time-of-flight (MR-TOF) or Fourier transform (FT) mass analyzer. It has been shown to be capable of performing high-resolution mass analysis and high-resolution ion isolations. Although it has been used in charge-detection mass spectrometry (CDMS), it has not been widely used as a conventional mass spectrometer for ensemble measurements of ions, or for tandem mass spectrometer. The advantages of tandem mass spectrometer with high-resolution ion isolations in the ELIT have thus not been fully exploited. A homebuilt ELIT was modified with BaF2 viewports to facilitate transmission of a laser beam at the turnaround point of the second ion mirror in the ELIT. Fragmentation that occurs at the turnaround point of these ion mirrors should result in minimal energy partitioning due to the low kinetic energy of ions at these points. The laser was allowed to irradiate ions for a period of many oscillations in the ELIT. Due to the low energy absorption of gas-phase ions during each oscillation in the ELIT, fragmentation was found to occur over a range of oscillations in the ELIT generating a homogeneous ion beam. A mirror-switching pulse is shown to create time-varying perturbations in this beam that oscillate at the fragment ion characteristic frequencies and generate a time-domain signal. This was found to recover FT signal for protonated pYGGFL and pSGGFL precursor ions. Fragmentation at the turnaround point of an ELIT by continuous-wave infrared multiphoton dissociation (cw-IRMPD) is demonstrated. In cases where laser power absorption is low and fragmentation occurs over many laps, a mirror-switching pulse may be used to recover varying time-domain signal. The combination of laser activation at the turnaround points and mirror-switching isolation allows for tandem MS in the ELIT.

Increased beam energy as a pathway towards a highly selective and high-flux MR-ToF mass separator

Many experiments at radioactive ion beam (RIB) facilities suffer from isobaric contamination, i.e. unwanted ions of similar mass. During the last decade, Multi-Reflection Time-of-Flight (MR-ToF) devices have gained remarkable attention for mass separation of short-lived, low-intensity beams of radionuclides at RIB facilities throughout the world. They exceed mass resolving powers m/Δm of 105 within a processing time of some (tens of) milliseconds. Due to space-charge effects, however, the mass separation remains an experimental challenge when many ions are simultaneously confined in the MR-ToF device. This limits the wider application of MR-ToF mass separators at RIB facilities. By performing ion-optical simulations including space-charge effects, we investigate different schemes of ion preparation in a Paul trap upstream of the MR-ToF device as well as MR-ToF operation and study their influence on mass separation and maximal ion flux. The validity of these simulations are benchmarked by time-of-flight and collision-induced fluorescence measurements with a 1.5 keV MR-ToF device. More advanced ion-beam preparation techniques such as the use of laser cooling, buffer-gas cooling at cryogenic temperatures or specific electric-field parameters for ion trapping and ejection from the Paul trap can significantly reduce the processing time needed to reach a given mass resolving power. However, the simulations of these methods also indicate that space-charge effects in the MR-ToF device become relevant at lower ion numbers compared to ’standard’ ion preparation. Thus, the overall amount of mass separated ions per unit of time remains essentially the same. In contrast, the simulations suggest that increasing the kinetic energy of typically just a few kiloelectronvolts in present MR-ToF instruments to 30 keV results in a significant increase of the attainable maximal ion flux.

Simulation studies of a 30-keV MR-ToF device for highly sensitive collinear laser spectroscopy

The Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS) seeks to extend the reach of high-resolution collinear laser spectroscopy (CLS) to more exotic radionuclides. In this novel technique, ion bunches of short-lived radioisotopes are trapped between two electrostatic mirrors of a Multi-Reflection Time-of-Flight (MR-ToF) device at 30-keV kinetic energy. The same ion bunch can be probed by a spectroscopy laser for thousands of times compared to a single probing in the traditional CLS measurement scheme. Thus, the experimental sensitivity is increased by more than one to two orders of magnitude. Extensive simulations are presented, demonstrating the feasibility of high-resolution collinear laser spectroscopy (CLS) in the newly envisioned MR-ToF apparatus operating at ion energies of 30 keV. Once the mechanical design and operational parameters are optimized for the requirements of CLS, the spectral line is neither significantly broadened nor distorted by the combination of CLS and MR-ToF operation. According to the simulations, the storage efficiency and the ion–laser overlap are suitable for laser excitation of the majority of the trapped ions. In summary, >90% injection and storage efficiency, >75% ion–laser overlap and a line width approaching the natural line width of the transition of interest are reached in the simulation.

An accuracy benchmark of the MIRACLS apparatus: Conventional, single-passage collinear laser spectroscopy inside a MR-ToF device

Collinear laser spectroscopy (CLS) has been performed in a multi-reflection time-of-flight (MR-ToF) device operated in single-pass mode, i.e., without confining the ions in the ion trap. While our Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS) aims to increase the CLS sensitivity by storing ions in the MR-ToF device, the present work characterises conventional single-passage CLS as a preparatory step for the upcoming comparison with MIRACLS’ multi-pass mode. To this end, the isotope shift in the 3s2S1/2→3p2P3/2 transition (D2 line) between ions of the magnesium isotopes 24Mg and 26Mg has been measured under varying experimental conditions. Our result agrees with the precise literature value. Associated studies of systematic uncertainties demonstrate a measurement accuracy of better than 20 MHz in this new apparatus. This value will serve as the reference for analogous studies to be performed in the MIRACLS approach in which ions are trapped in the MR-ToF device for thousands of revolutions and probed by the spectroscopy laser during each passage.

Multiple active voltage stabilizations for multi-reflection time-of-flight mass spectrometry

The performance of a multi-reflection time-of-flight (MR-ToF) mass spectrometer is evaluated under the use of four voltage feedback loops to actively regulate its mirror potentials. Different electronic hardware is characterized to find the most useful configuration for parallel regulation of all of the MR-ToF analyzer's reflecting potentials. The gain in mass resolving power for low-abundance ion species is demonstrated by measuring pairs of molecular isobars of zinc clusters and analyzed in the context of expected flight-time fluctuations. For higher-abundance species, the resolving powers reached in short- and long-term measurements are probed with bismuth-cluster ions, resulting in values up to 500 000 and 200 000, respectively, in the absence of offline corrections. Additionally, feedback-loop regulation is found to be advantageous for changes of experiment cycles in which voltages are switched for, e.g., ion ejection.

Concatenated multi-reflection time-of-flight spectra for wide-band mass spectrometry

A series of multi-reflection time-of-flight (MR-ToF) spectra is recorded with increasing ion flight times and added up to a single transient. It shows periodic ToF-signal trains for each ion species revolving between the MR-ToF analyzer’s mirrors. This is used to perform a “comb-sampling” evaluation previously devised for analysis of capacitive-pickup signals (Greenwood et al., Rev. Sci. Instrum. 82 (2011), 043103). The recording and evaluation focuses on simultaneously measuring the full range of trapped ions for wide-band mass spectrometry. The comb-sampling technique is revisited for the present transients, as it is the first time it is used for comparatively background-free single-ion-counting data.

Radioactive ion beam manipulation at the IGISOL-4 facility

The IGISOL-4 facility in the JYFL Accelerator Laboratory of the University of Jyvaskyla (JYFL-ACCLAB) produces low-energy radioactive ion beams, primarily for nuclear spectroscopy, utilizing an ion guide-based, ISOL-type mass separator. Recently, new ion manipulation techniques have been introduced at the IGISOL-4 including the application of the PI-ICR (Phase-Imaging Ion Cyclotron Resonance) technique at the JYFLTRAP Penning trap, as well as commissioning of a Multi-Reflection Time-Of-Flight (MR-TOF) separator/spectrometer. The successful operation of the MR-TOF also required significant improvement of the Radio-Frequency Quadrupole (RFQ) cooler and buncher device beam pulse time structure. In addition, laser ionization techniques have been developed for particular cases, for example, a hot cavity laser ion source for silver production. A new stable isotope ion source and a beam line has been introduced for tuning and calibration purposes. In addition to the installations at the IGISOL-4 facility, the extension of the vacuum-mode recoil separator MARA (Mass Analysing Recoil Apparatus), MARA-LEB (MARA Low Energy Branch) has been under development. MARA-LEB will utilize the gas-cell technique and laser ionization to convert MeV-scale radioactive beams to low-energy ones.

A multi-reflection time-of-flight setup for the improvement and development of new methods and the study of atomic clusters

A multi-reflection time-of-flight (MR-ToF) device has been set up for the development and test of new MR-ToF techniques and for future applications in atomic cluster research. The instrument, consisting of a laser-ablation ion source, a quadrupole bender, the MR-ToF analyzer (with ion mirrors and in-trap lift), and a channeltron detector, is described in detail and characterized with respect to preliminary results of its performance parameters. In addition, cluster ions were mass selected in the MR-ToF device and photodissociated. The charged fragments were stored and mass analyzed in a proof-of-principle MS/MS experiment where both MS steps were performed in the MR-ToF operation mode.

Improved stability of multi-reflection time-of-flight mass spectrometers through passive and active voltage stabilization

Multi-reflection time-of-flight (MR-ToF) spectrometers are devices in which ions are reflected between two electrostatic mirrors to prolong their flight path. The performance of the apparatus relies on the stability of the voltages that are used to define the mirror potentials. Especially the voltage of the mirror electrodes at the point where the ions reverse the direction of their axial motion needs to be stable in order to minimize fluctuations of their total flight time. Here we present a method to increase the short- and long-term stability of suitable voltage supplies for enhanced performance of the spectrometer.

Simulations of a proof-of-principle experiment for collinear laser spectroscopy within a multi-reflection time-of-flight device

For nearly four decades Collinear Laser Spectroscopy (CLS) has been employed to determine ground-state properties of short-lived radionuclides. To extend its reach to the most exotic radionuclides with very low production yields, the novel Multi Ion Reflection Apparatus for CLS (MIRACLS) is currently under development at ISOLDE/CERN. In this setup, 30-keV ion bunches will be trapped between two electrostatic mirrors of a multi-reflection time-of-flight (MR-ToF) device such that the laser beam will probe the ions during each revolution. Thus, the observation time will be extended and the experimental sensitivity will be increased significantly while maintaining the high resolution of conventional CLS. A proof-of-principle experiment is currently being performed to demonstrate the potential of CLS within a low-energy MR-ToF device. Its first experimental results benchmark the validity of ion-optical simulations from the CLS perspective, which will also be applied to MIRACLS’ 30-keV apparatus.

First steps in the development of the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy

Collinear laser spectroscopy (CLS) has been combined with the multi-reflection time-of-flight (MR-ToF) technique. To this end, a photodetection system has been implemented at the drift region of a MR-ToF apparatus and a laser beam has been sent along the path of the ions that are stored between the two ion-optical mirrors. The main goal of the present proof-of-principle (PoP) experiments, is the confirmation of the expected increase in sensitivity compared to conventional fluorescence-based CLS due to the repeated probing of the trapped ion bunches. The novel method will be used for the precise measurement of nuclear ground- and isomeric-state properties of exotic nuclei with low production yields at radioactive ion-beam facilities. A significant sensitivity improvement of CLS is expected, depending on the half-life and mass of the nuclide of interest. The status of the PoP setup and future improvements are discussed.

Current status of experimental facilities at RAON

The Rare Isotope Science Project (RISP) was established in December 2011 for the construction of the accelerator complex RAON (Rare isotope Accelerator complex for ON-line experiments) which will be used for the rare isotope science in Korea. The rare isotope accelerator at RAON will provide both stable and rare isotope (RI) beams with an energy range from a few keV to a few hundreds of MeV per nucleon for research in the fields of basic and applied sciences.At the moment, there are 7 experimental facilities considered at RAON: KOrea Broad acceptance Recoil spectrometer and Apparatus (KOBRA) and Large Acceptance Multi-Purpose Spectrometer (LAMPS) for nuclear physics, High Precision Mass Measurement System (HPMMS) with Multi-Reflection Time-of-Flight (MR-ToF) and Collinear Laser Spectroscopy (CLS) for nuclear physics using atomic physics techniques, Nuclear Data Production System (NDPS) for nuclear reaction data, Muon Spin Rotation/Relaxation/Resonance (μSR) for material science, and Beam Irradiation System (BIS) for bio-medical science.In this paper, the current status including detail design and research goal of 7 experimental facilities at RAON will be discussed.

Multiple ion capture and separation in an electrostatic storage device

The present study introduces new approaches for the capture/injection and selection of ions with large mass-to-charge (m/q) differences in an electrostatic ion beam trap (EIBT) or multi-reflection time-of-flight (MR-ToF) device. While no m/q-dependent storage criterion exists for devices of this nature, the process of injecting ions into the trap relies on a timed potential switching and is therefore mass-selective to some extent. To partially circumvent this restriction, the approach of sequential capture is presented, where additional ion species are captured while already-stored ones are unaffected by the switching pulses. Specifically, it is possible to perform a full in-trap lift capture pulse while a given, already-stored ion species is reflected in a mirror potential or passes through the lift electrode itself. Additionally, the technique of ion selection by means of transversal ejection from the trap is extended to allow ion species with large m/q differences to be retained. To this end, deflector functions with different frequencies are added with an AND logic and supplied to an in-trap deflector electrode. The combination of both techniques allows the simultaneous storage of size-selected ion species with an m/q ratio surpassing 4 : 1 at the present setup, where the single-pulse lift capture is restricted to a ratio of only 1.9.

Non-isobaric time-of-flight correction for isobar resolving in MR-ToF mass spectrometry

A development in multi-reflection time-of-flight (MR-ToF) mass spectrometry is presented with which isobars are resolved by means of an offline correction with the signal of a non-isobaric ion species. The method is demonstrated for a pair of Cr2+ isobars with a relative mass difference of 1.8∙10-5, with either an additional, non-isobaric Cr2+ or a CrO+ ion appearing in the same time-of-flight spectrum, although at a different number of revolutions during the same storage time. This is in contrast to earlier applications where ToF shift corrections are performed with ions at identical revolution numbers, typically isobars that are also acting as mass-reference ions. It is shown that ions of virtually any mass simultaneously stored in the MR-ToF analyzer can be employed. This is particularly useful in situations where both the ions of interest and the reference ions for mass calibration have only small count rates not sufficient for the ToF-shift corrections. For the present system, the resolving power increases by more than a factor of two when the offline drift correction is applied.

Multi-reflection time-of-flight mass spectrometry with combined in-trap lift capture and mirror-switch ejection

Multi-reflection time-of-flight (MR-ToF) devices are established as mass separators and analyzers with high resolving powers and fast processing times. For ion injection and ejection, either the ion mirrors or an in-trap lift electrode has to be switched. In the present work, these two methods are combined with in-trap lift switching for ion capture and exit-side mirror switching for ejection with higher information content. Measurements are performed with small lead clusters to illustrate individual advantages of both techniques and the gain of combining them with focus on the ions’ ToF ejection window.

First operation and mass separation with the CARIBU MR-TOF

The recent installation of a Multi-Reflection Time-of-Flight (MR-TOF) isobar separator at the CARIBU facility has the promising potential to significantly improve the mass separation and selection of short-lived neutron-rich beams. Ions cycled in the km-long isochronous trajectories between two electrostatic mirrors can be separated to high levels of mass-resolving power within a short time (tens of ms). The installation process is described and results from the first operation are discussed. Following an optimization of the mirror voltages a mass-resolving power of 6.8·104 was achieved and a separation of isobars was demonstrated. The higher purity beams provided by the MR-TOF and delivered to the Canadian Penning Trap (CPT) will provide access to further measurements of neutron-rich nuclei along the astrophysical r-process path.

A multi-reflection time-of-flight mass spectrograph for short-lived and super-heavy nuclei

A multi-reflection time-of-flight (MRTOF) mass spectrograph has been implemented at RIKEN to provide high-precision mass measurements of very short-lived nuclei. Of particular interest are mass measurements of r-process nuclei and trans-uranium nuclei. In such nuclei, the MRTOF can perform on par with or better than traditional Penning trap systems. We demonstrate that the MRTOF-MS is capable of accurately attaining relative mass precision of δm/m<10-7 and describe it’s utility with heavy, short-lived nuclei.

On-line separation of short-lived nuclei by a multi-reflection time-of-flight device

A multi-reflection time-of-flight (MR-ToF) mass analyzer has been integrated into ISOLTRAP, the precision mass spectrometer for on-line mass determinations of short-lived nuclides at ISOLDE/CERN. The new instrument improves ISOLTRAP by providing a fast separation of isobaric contaminant species as well as subsequent ion selection using the fast Bradbury–Nielsen gate. Suppression ratios of up to 104 and mass-resolving powers of over 105 have been reached in off-line experiments. Preliminary data from on-line applications illustrate the benefit and performance of the device and its potential in the context of the ISOLTRAP setup.

Static-mirror ion capture and time focusing for electrostatic ion-beam traps and multi-reflection time-of-flight mass analyzers by use of an in-trap potential lift

Capture and ejection of ions in electrostatic ion-beam traps and multi-reflection time-of-flight (MR-ToF) devices can be accomplished by pulsing the potential of only a single drift tube inside of the device in contrast to the conventional switching of the ion-mirror voltages. In addition to its simplicity, the new method allows to set the position of the time-focus plane at a given ion detector or ion-beam separator without the need of any further hardware. The position can be adjusted easily by the choice of the pulse height of the potential switch.