First spatial separation of a heavy ion isomeric beam with a multiple-reflection time-of-flight mass spectrometer

The 211 Po ions were spatially separated in ﬂight from the primary beam and other reaction products by the fragment separator FRS. The ions were energy-bunched, slowed-down and thermalized in a gas-ﬁlled cryogenic stopping cell (CSC). They were then extracted from the CSC and injected into a high-resolution multiple-reﬂection time-of-ﬂight mass spectrometer (MR-TOF-MS). The excitation energy of the isomer and, for the ﬁrst time, the isomeric-to-ground state ratio were determined from the measured mass spectrum. In the subsequent experimental step, the isomers were spatially separated from the ions in the ground state by an ion deﬂector and ﬁnally collected with a silicon detector for decay spectroscopy. This pioneering experimental result opens up unique perspectives for isomer-resolved studies. With this versatile experimental method new isomers with half-lives longer than a few milliseconds can be discovered and their decay properties can be measured with highest sensitivity and selectivity. These experiments can be extended to studies with isomeric beams in nuclear reactions.


Introduction
Isomers are excited metastable states of nuclei with relatively long half-lives, due to hindrance in their decay modes [1]. The interest in isomers ranges from nuclear structure [2] and nuclear astrophysics [3] to their possible use for energy storage devices [1].
So far, experiments with isomers mostly have been performed using gamma-ray spectroscopy techniques [4][5][6][7]. Isomers with half-lives in the μs range and below are especially well suited for coincidence measurements in gamma-ray spectroscopy. In complement to this, long-lived isomers can be well investigated by highresolution mass spectrometry. Furthermore, high-accuracy mass measurements rely on the fact that ions in the ground and isomeric states are clearly identified and resolved in the mass spectrum, otherwise the results can be erratic. This requirement is a permanent strong motivation to increase the experimental mass resolving power.
The first resolution of ground and isomeric states in a mass measurement was achieved with the ISOLTRAP Penning trap [8]. At LEBIT, the first discovery of an isomer with such a device was achieved [9] and a measurement of an isomeric-to-ground state ratio was performed at JYFLTRAP [10]. By use of ions in higher charge-states in a Penning trap, the resolution of ground and isomeric states could be increased at the TITAN facility [11]. Isomers have also been observed and resolved from their ground state in mass measurements of projectile fragments stored in the Experimental Storage Ring ESR, using Schottky Mass Spectrometry (SMS) [12] and Isochronous Mass Spectrometry (IMS) [13] their decay has been studied. In storage-ring experiments, the ions are mainly fully ionized or carry only a few electrons, whereas in traps the measured ions are typically singly or doubly charged. Therefore, storage ring and ion trap experiments are highly complementary, because atomic charge states can have a strong effect on the nuclear decay properties of isomers, see [14] and references therein.
Experimental methods employing pure isomeric beams can reveal new nuclear properties. Such experiments can be isomerresolved decay spectroscopy or investigations via nuclear reactions with isomers. There are different ways to produce pure isomeric beams. The first nuclear reactions with a pure isomeric beam were performed at ISOLDE by selection of the isomer with laser resonance ionization and subsequent post-acceleration [15]. Using a novel excitation method for ion motion in a Penning trap, isomerically clean ion samples could be separated [16] and gamma-ray spectroscopy of pure nuclear states was performed at JYFLTRAP [17]. It was also shown that pure isomeric beams may be provided using storage rings [18,19]. However, these methods are limited to isomers with half-lives of one second or even longer. For the special case, where the half-life of the isomer is significantly longer than the corresponding ground state, such as in 212 Po, where the isomeric state has a half-life of 45 s, but the half-life of the ground state is 300 ns only, one does not need an experimental method for separation; one can simply wait until the ions in the ground state have decayed.
Multiple-reflection time-of-flight mass spectrometers (MR-TOF-MS) [20,21] have recently been developed into powerful tools for research with exotic nuclei [22][23][24]. They are mass spectrometers, and in combination with a Bradbury-Nielsen Gate (BNG) [25,26], a fast electrostatic ion deflector, they can also be employed as high-resolution spatial mass separators [27]. Because of their high mass resolving power, measurement speed, sensitivity and broadband characteristics, they have recently been proposed for efficient experiments with isomers [21]. In this work, such pioneering measurements have been exemplarily performed with 211 Po ions for the first time.

Experiment and data analysis
The experiment was performed with the FRS Ion Catcher setup [23] at GSI. 211 Po nuclei in ground and isomeric states were produced with the FRS [28] via projectile fragmentation of a 238 U primary beam at an energy of 1000 MeV/u in a beryllium target with an areal density of 1.629 g/cm 2 , that was followed by a 0.223 g/cm 2 Nb backing. The fully ionized polonium ions were separated in flight from the primary beam and other reaction products and were energy-bunched using two-fold magnetic rigidity analysis and a 4.063 g/cm 2 Al monoenergetic degrader, located at the central focal plane of the FRS (Fig. 1, upper panel). The isotope identification of the ions was performed using the particle detectors of the FRS. After further slowing-down in a homogenous degrader at the final focal plane, the ions were injected into a gasfilled cryogenic stopping cell (CSC) [29][30][31]. In the CSC, they were thermalized as singly-charged ions in helium gas at a pressure of 95 mbar and a temperature of 86 K, corresponding to an areal density of 5.6 mg/cm 2 . They were guided to the exit side of the CSC by electric fields, radially focussed onto an extraction nozzle by an RF carpet and extracted with the gas flow into an RFQ quadrupole beam line [23].
From there, the ions were transmitted to the MR-TOF-MS [27,32,33] (Fig. 1(a)-(c)). The MR-TOF-MS is a powerful and universal mass spectrometer with single-ion sensitivity as well as a spatial mass separator. Mass resolving powers (FWHM) as high as 600,000 and a mass accuracy of 10 −7 have been achieved off-line [32]. In the present experiment, a maximum mass resolving power of 370,000 has been obtained for ions with a mass of 133 u after 430 turns in the analyzer, corresponding to a time-of-flight of only 15.3 ms. In the MR-TOF-MS, the ions were accumulated, cooled by collisions with a helium buffer gas in an RF trap system and injected as bunches into the time-of-flight analyzer with a kinetic energy of 1.3 keV. The analyzer is formed by two electrostatic reflectors, between which the ions traveled for 192 turns, corresponding to a time-of-flight of 8.7 ms and a total path length of about 300 m. During their storage in the analyzer, the ions were dispersed in time according to their mass-to-charge ratios. After ejection from the analyzer, the ions alternatively (i) impinged on a TOF detector (isochronous secondary electron multiplier) for measurement of their time-of-flight and hence their mass-to-charge ratio (mass measurement mode) or (ii) passed through the BNG, in which unwanted ions can be deflected, and only the ions of interest were transmitted (mass separator mode). These latter ions were implanted into a Si detector mounted behind the BNG. In the Si detector their alpha decay pattern was measured. The TOF detector and the BNG with the Si detector are mounted on a remotecontrolled platform, which can be moved such that either the TOF detector or the BNG and the Si detector are in the beam line.
A mass spectrum of 211g Po and 211m Po was recorded by the TOF detector for a duration of 500 s. The detector signal was digitized by a time-to-digital converter. Next, the BNG and the Si detector were moved into the ion beam. Energy spectra from the alpha decay of 211g Po and 211m Po ions were acquired by the Si detector with the BNG voltages turned off for transmission of all ions, as well with the BNG voltages turned on for a duration chosen such that the 211g Po ions were deflected and only the 211m Po ions were allowed to reach the Si detector. The acquisition time amounted to 300 s each. The Si detector signals were amplified and digitized by an analog-to-digital converter.
The time-of-flight spectra acquired in the mass measurement mode were calibrated with 211 Pb ions obtained by the consecutive decay from a 223 Ra source mounted in the CSC. The calibration spectrum was acquired before the measurement of 211 Po. Since the mass distributions of 211m Po and 211 Pb ions would overlap, the transport of ions from the 223 Ra source was disabled for the duration of the measurement of 211 Po by applying a negative voltage relative to the surrounding electrodes of the source. In this mode, the amount of 211 Pb ions extracted from the cell is only about 10% of the 211 Po ions. The energy spectra from the alpha decays were calibrated with measurements of the emitted alpha particles from 211 Pb and 219 Rn from the same source.  [34] and the excitation energy of the isomeric state [35] as 211g Po + (9/2 + , half-life T 1/2 = 516 ms) and 211m Po + (25/2 + , T 1/2 = 25.2 s). The exact results for the measured mass excess values in the present experiment and the respective uncertainties require a more extensive data analysis procedure and will be the subject of a forthcoming publication. However, the excitation energy, i.e., the mass difference, of the isomeric state could be determined for the first time using mass spectrometry; it is (1472 ± 120) keV, which is in good agreement with the literature value of (1462 ± 5) keV [35]. The literature value is derived from the measured alpha decay energies of both states to the daughter 207 Pb. Two more isomeric states are known in 211 Po nuclei (31/2 − , T 1/2 = 243 ns and 43/2 + , T 1/2 = 2.8 μs) [35]. However, these states do not survive the extraction time from the CSC and the measurement time in the MR-TOF-MS and are therefore not observed in our mass spectrum. From the mass spectrum, the ra- tio of ions in the isomeric state with respect to the ground state has been determined. It amounts to (2.5 ± 0.8). This value includes a correction for decay losses during separation, thermalization and measurement by a factor of 0.84. The isomeric ratio is specific to the production mechanism. The isomeric ratio for the 43/2 + state in 211 Po produced by projectile fragmentation at 1000 MeV/u was measured recently using gamma ray spectroscopy [36]. However, because of its long lifetime, the 25/2 + state could not be measured.

Results and discussion
In the mass separator mode, the energy spectra from the alpha decay of 211g Po and 211m Po were measured with the Si detector behind the BNG. Fig. 3 shows the section of the spectra with the two dominant alpha-decay peaks with (a) the BNG turned off, i.e. with transmission of all ions, and with (b) the BNG voltages turned on for a period chosen such that the 211g Po ions were deflected. The measured data were fitted with a Gaussian convoluted with an exponential tail on the slope of the low-energy side [37]. The alpha particle energies in the spectrum (a) amount to (7269 ± 15) keV and (7442 ± 15) keV for 211m Po and 211g Po, respectively. These values are in good agreement with the literature values of (7275 ± 15) keV and (7450.3 ± 0.5) keV [38], respectively. The measurement of the alpha-particle energies serves as an independent identification of the ions, in addition to the identification with the FRS detectors and by use of mass spectrometry with the MR-TOF-MS. The spectrum (b) shows that with the BNG operated as an ion deflector, ions in the isomeric state could be spatially separated and transmitted as a beam of pure isomers to the Si detector. In this experiment, a pure isomeric beam was achieved for the first time using an MR-TOF-MS. Only two counts of ground state ions were transmitted and detected in this mode. In the future, this can be completely avoided by an optimization of the MR-TOF-MS settings in order to improve the peak shape or by a further increase of the mass resolving power. Even at the present performance level, a suppression ratio of at least four orders of magnitude can be achieved, albeit at larger binding energy differences [32]. Therefore, even weakly populated states can be investigated and separated. Note, that for a given excitation energy the required mass resolving power increases linearly with mass. Furthermore, the time needed to obtain a required mass resolving power also increases linearly with mass for most high-resolution mass spectrometers. For MR-TOF-MS, the required time-of-flight only increases with the square-root of the mass. Thus, the successful separation of an isomer in the mass region beyond 200 u within a time-of-flight of 8.7 ms only is an important achievement.

Perspectives
These pioneering measurements demonstrate the potential of MR-TOF-MS for isomer research. MR-TOF-MS are fast mass spectrometers with mass resolving powers in excess of several 10 5 , i.e. the capability to resolve essentially all nuclear isobars as well as isomers with excitation energies of several hundred keV or more. As non-scanning mass spectrometers, they can perform measurements, which cover several mass units, and during which all ion species are measured simultaneously. They can therefore very efficiently be used to search for hitherto unknown isomeric states with life times down to a few milliseconds. The discovery of isomers, identification, determination of the excitation energy and the determination of the isomeric ratio can result from the same measurement. Furthermore, measurements with MR-TOF-MS are complementary to gamma ray spectroscopy experiments, which are limited to half-lives below one millisecond without loss of the correlated particle identification.
Further research potential arises from the use of MR-TOF-MS in combination with a BNG as isomer separators, which supply pure isomeric beams. At variance to laser-ionization methods, the MR-TOF-MS mass separator can be employed universally, i.e. is applicable to all elements. The reach is limited only due to the cycle time of a few milliseconds. Experiments that are envisaged include decay measurements of isomerically separated ions, e.g. to investigate also weakly populated decay channels of isomers. The search for new fission isomers [39,40] and a direct determination of the excitation energy of known fission isomeric states could also be performed. So far, only one fission isomer with a half-life of several milliseconds has been discovered, i.e. in 242 Am with a half-life of 14 ms; this is accessible by the MR-TOF-MS technique. Furthermore, the post-acceleration of isomeric beams separated with an MR-TOF-MS and reactions with isomeric beams are now on the horizon.
In the present experiment, projectile fragmentation at relativistic energies was used, which gives access to all nuclides up to the heaviest projectiles. A major advantage of in-flight separation at high velocities is the unambiguous particle identification and spatial isotopic separation. By employing fusion and transfer reactions, isomers almost up to the heaviest known elements can be produced. At these low energies, direct particle identification can be performed by the MR-TOF-MS only. Furthermore, MR-TOF-MS can be applied at ISOL or IGISOL facilities as well. Thus, all nuclides on the nuclear chart can be accessed. Efficient large-scale measurements of isomers are within reach, as performed recently by SMS with electron-cooled beams at the FRS-ESR facility at GSI [41]. However with the MR-TOF-MS technique the half-life limits will be lower by three orders of magnitude than presently with SMS.

Summary
Ions of the nuclide 211 Po in the ground and isomeric states were produced by projectile fragmentation at 1000 MeV/u and separated in flight with the FRS. 211g Po and 211m Po ions were resolved in the mass spectrum measured with the MR-TOF-MS. The excitation energy of the isomer and the isomeric-to-ground state ratio were determined by mass spectrometry. The measured value of the excitation energy is in good agreement with the literature value. The isomeric-to-ground state ratio has been determined in this experiment for the first time. The separation of a pure isomeric beam using an MR-TOF-MS represents a novel experimental achievement. These pioneering measurements demonstrate the great potential of the MR-TOF-MS combined with an ion deflector (BNG) for research with isomers, including large-scale searches for new isomers and decay and reaction experiments with pure isomer beams.