Hyperfine Interact DOI 10.1007/s10751-014-1044-5

Status of the MLLTRAP setup and future plans ¨ C. Weber · R. Meißner · P. Muller · P. G. Thirolf

© Springer International Publishing Switzerland 2014

Abstract The MLLTRAP Penning trap system serves as a development environment, both for mass spectrometry as well as for novel in-trap decay-spectroscopy experiments at the MATS facility at FAIR. This contribution gives an outline on the development work done at MLLTRAP and presents the current status. Keywords Penning trap · Mass spectrometry · In-trap decay spectroscopy · Internal conversion · E0 spectroscopy · m/q separator · Multi-passage system (MPS)

1 Introduction High-precision mass measurements and nuclear decay-spectroscopy experiments are two complementary approaches to gain relevant information on the basic properties of nuclear matter. Whereas Penning trap mass spectrometry provides nuclear masses which define the ground-state properties of an atomic nucleus, detailed nuclear structure and lifetimes of excited states can be assessed by nuclear decay spectroscopy experiments. The combination of both techniques leads to ‘trap-assisted decay spectroscopy’, which can exploit the high-resolution mass purification of a Penning trap system prior to the nuclear decay spectroscopy experiment. The latter technique can be defined as a post-trap decay spectroscopy experiment behind a Penning trap. In real in-trap decay spectroscopy experiments, the nuclear decay is observed from a stored ion sample which decays in situ. This is advantageous in comparison to conventional decay spectroscopy experiments, where short-lived samples are collected using solid carriers and thus suffer from interactions and scattering losses within the source material. For Proceedings of the 9th International Workshop on Application of Lasers and Storage Devices in Atomic Nuclei Research “Recent Achievements and Future Prospects” (LASER 2013) held in Poznan, Poland, 13–16 May, 2013 C. Weber () · R. Meißner · P. M¨uller · P. G. Thirolf Faculty of Physics, Ludwig-Maximilians University Munich, Am Coulombwall 1, 85748 Garching, Germany e-mail: [email protected]

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Fig. 1 Photo of the Penning-trap system MLLTRAP at the Maier-Leibnitz-Laboratory (MLL) in Garching. From Ref. [9]

in-trap spectroscopy, the detection systems have to be positioned next to the trap electrodes. As one example, electrons that are emitted in internal conversion decay are very efficiently guided along the magnetic field lines of the Penning trap apparatus. They can be detected with high efficiency with a detector situated within the magnetic field. This allows to perform in-trap conversion electron spectroscopy on a clean ion sample. An extension to this technique is realized, if the trap electrodes are substituted by the detector itself and its operation bias provides the storage potential of the trap system. In this way, α particles can be detected with a silicon strip detector array, which builds up the trap, while conversion electrons are registered on a position-sensitive detector in the magnet’s fringe field. Via the combination of detecting both particles, a novel type of recoil-distance experiment can be realized. For this purpose, a dedicated ‘detector trap system’ is developed at MLLTRAP in order to confine the nuclides of interest, while simultaneously detecting their decay products [1]. This setup is planned to be implemented at the MATS/LaSpec facility [2] at the emerging accelerator complex FAIR [3]. Moreover, in order to extend the capabilities of the Penning trap mass spectrometer either to ion species of shorter half-lives or to allow for higher accuracy, highly-charged ions instead of singly-charged ones can be used. Coupling a charge-breeding device such as an EBIS [4] [or EBIT [5]] to the Penning trap requires not only a ‘switchyard’ of variable deflection capability, but in particular an ‘m/q’ separator is needed in view of the chargestate distribution provided in the charge-breeding process. The development of such a device forms the second topic of this contribution, which gives an outline on the scientific goals and reports on the present status of the developments at MLLTRAP.

2 The MLLTRAP Penning trap system for mass spectrometry The MLLTRAP Penning trap system [6] is presently situated and has been commissioned at the Maier-Leibnitz Laboratory (MLL) in Garching. In order to allow for on-line mass measurements with exotic ion species, it is envisaged to transfer MLLTRAP to the DESIR facility [7] of SPIRAL2 [8] at GANIL, once this new low-energy radioactive ion beam (RIB) facility will be available. Figure 1 shows a photograph of the MLLTRAP setup. It is based on a double Penning-trap system contained within a two-center, 7-T superconducting solenoid.

Status of the MLLTRAP setup and future plans

Fig. 2 Location of the DESIR experimental hall among the existing GANIL facilities, the SPIRAL2 production building, and the S3 separator-spectrometer. From Ref. [9]

During commissioning measurements with stable alkali ions from an off-line ionization source, a mass resolving power of about 105 in the first (purification) trap and a relative precision of δm/m = 2.9 × 10−8 for 87 Rb ions (statistical uncertainty) was determined in the second (precision) trap. The commissioning of the system and a detailed description of the setup have been published in Ref. [6]. As further steps in improving the setup for mass spectrometry, the effects of systematic uncertainties have been minimized. First of all, the inherent magnetic field fluctuations arising from large temperature fluctuations in the experimental hall have been characterized [10]. The contribution of these magnetic field fluctuations to the systematic uncertainty was determined to be σB (νref )/νref = 7.36(38) × 10−9 / h. These fluctuations have been minimized by stabilization systems of the ambient temperature and pressure conditions [11]. In consequence, the temperature fluctuations can now be stabilized to peak-to-peak variations of Tpp = ±6 mK and pressure variations in the liquid helium reservoir of the magnet to about ppp = ±0.2 hPa, such that MLLTRAP is readily commissioned for its installation at DESIR. As soon as the experimental areas of DESIR will become available, MLLTRAP’s superconducting solenoid with the Penning traps for mass measurement will move to the DESIR low-energy experimental facility. 2.1 MLLTRAP at the DESIR facility of SPIRAL2 The future DESIR facility [7] at GANIL will facilitate a variety of nuclear-physics experiments with short-lived exotic isotopes that will become available from as many as all three different production areas of GANIL: Experiments can exploit beams from the existing SPIRAL1 facility, as well as from the newly constructed SPIRAL2 [8], where beams will be created either via fission or fusion-evaporation reactions. A particularly promising perspective for experiments is opened up by the access to extremely heavy nuclides after fusion-evaporation reactions and mass separation in the separator-spectrometer S3 [12]. Figure 2 shows the location of the DESIR low-energy building with respect to the access to beams from the different GANIL accelerator and ion beam production facilities.

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Low-energy experiments at DESIR will, for example, comprise the study of nuclear moments via laser spectroscopy at LUMIERE [13] and nuclear decay-spectroscopy experiments of purified, mass-selected nuclides behind the PIPERADE Penning trap setup [9]. The latter is specifically designed to have a large ion-storage capability in order to fulfill the needs of sufficiently high counting statistics in nuclear decay-spectroscopy experiments. High-precision mass measurements will be performed employing the MLLTRAP setup [6]. 2.2 Perspectives for mass measurements at DESIR Atomic masses, as being one of the fundamental properties of a nucleus, exhibit the strength of the underlying binding forces among the contributing nucleons and thus manifest themselves in the existence and the stability of particular nuclides. These can be assessed by a systematic high-precision mass spectrometry [14]. A broad physics program for the DESIR facility and especially for mass spectrometry has already been formulated and documented in a number of detailed letters of intent (http://www.cenbg.in2p3.fr/desir/ -Letters-of-intent-for-DESIR-) for experiments to be conducted at DESIR. Due to the variety of complementary production sites for exotic isotopes (see also Fig. 2), experiments with exotic species range from nuclei close to both nucleon driplines, the very neutron-deficient, as well as the neutron-rich region and to experiments towards the region of the heaviest elements. The detailed perspectives for mass spectrometry at DESIR were given in Ref. [9] and are only shortly outlined here: As every region of the nuclear chart is characteristic for a particular physics motivation, the neutron-deficient area close to the N = Z line is the region where superallowed β emitters reside. These undergo pure vector-type beta decays, which turns them into test cases of the conserved vector current (CVC) hypothesis [15]. From data of each decay, the vectorcoupling constant can be derived and, with the coupling constant from muon decay, the first matrix element Vud of the quark-mixing (CKM) matrix is extracted for its unitarity test. The most recent evaluation of Vud from nuclear beta decay results in an agreement to unitarity of the CKM matrix to 0.06 % [16]. For future studies, heavier superallowed β emitters, such as 66 As and 70 Br, are of prime interest, since the nucleus-dependent corrections are particularly large in these nuclei [17]. Exploiting particular synergies between different experimental installations available at the upcoming DESIR facility, all observables required for these studies can be contributed: Q values will be measured with MLLTRAP, while the branching ratios and half-lives will be determined by the PIPERADE setup via trap-assisted decay spectroscopy [9]. In addition, a further set of input data, the so-called nuclear mirror transitions between T = 1/2 isospin doublets, help to place the test of CVC on a larger database [20]. For these cases also beta-neutrino correlation factors are additionally required, which will be determined with the LPC-trap setup [18]. One of the central questions in modern research on our Universe refers to the creation of the chemical elements. Most of the heavy elements above mass A = 70 are created far out towards the neutron dripline along the pathways of the rapid neutron-capture process (r-process). Hence, the understanding of the exact location of its most probable pathways and its timescale are required to verify the relative abundance of isotopes as extracted from different models compared to (astronomical) observations. For this purpose, accurate mass values, decay half-lives and photodisintegration rates are required as part of the input data to model these processes. Here, the so-called waiting-point nuclei within the regions around the shell-closures lie within a particular focus, since their enhanced binding leads to an accumulated production of the nuclei that are produced with high abundance. Thus, one of

Status of the MLLTRAP setup and future plans

the main goals of mass measurements at DESIR are those nuclei relevant for astrophysical nucleosynthesis processes [19]. The ultimate limits of nuclear existence near the island of stability of superheavy elements is characterized by an enhanced stability against their disintegration via fission or α decay. In accelerator-based fusion reactions, these nuclides are produced in minute quantities and identified via their decay in long-term, low-statistics experiments. On the route to these superheavies, still convenient production rates can be obtained for transuranium nuclides in proximity to the heaviest elements. Thus, the determination of their nuclear binding can support the understanding of the development of the strength of shell closures in this region and can assist predictions for further experiments with rarely produced isotopes. An experimental campaign in this mass region has been pioneered by the SHIPTRAP facility [21] behind the separator SHIP [22] at GSI. Here, the sensitivity of a Penning trap mass spectrometer for high-accuracy mass spectrometry of transuranium isotopes of nobelium [23, 24] and lawrencium [25] down to production rates of 60 nb for 256 Lr has been proven feasible. With respect to the improved production scenario becoming available at S3, the measurement of 256 Rf (Z = 104) with a production cross section of ≈40 nb is envisaged at DESIR [26].

3 A multi-passage spectrometer (MPS) system as m/q separator In Penning trap mass spectrometry, an ion’s cyclotron frequency νc = qB/(2π m) in a strong magnetic field of strength B is determined via the time-of-flight ion cyclotron resonance (ToF-ICR) method [27]. Since the ion’s frequency, with charge q and mass m, is proportional to the ionic charge states of q = z ∗ e, the obtainable relative mass uncertainty δm/m can be reduced by using highly-charged ions [28], according to the relation, 

δm m

 ≈

m , √ q · T · B · Nion

(1)

where T and Nion denote the total observation time and the number of detected ions. Thus, the concept of charge breeding is envisaged at future radioactive ion beam (RIB) and Penning trap facilities, such as MATS [2]. Highly-charged ions can be created via charge breeding of short-lived, on-line produced ions, which have to be transported further to the final experimental setup within short timescales. A so-called multi-passage spectrometer (MPS) is a device that can be operated a switchyard between an ion source, a charge breeder and a Penning trap. It will direct beams of singly charged ions from the production area to a charge breeder and allows to re-extract these as multiply charged ions. Figure 3 shows the layout of an MPS system. Ions delivered from an ion source or the on-line production process are bent by 90◦ and directed into the charge breeder. After a typical breeding time of few ten to one hundred ms, their immediate extraction in a selected ionic charge state and transfer towards the trap setup requires a versatile magnetic m/q separator, which is presently being developed in Garching. This system is based on the design of a multi-passage spectrometer (MPS) originally developed at the University of Frankfurt [30, 31], which is now in operation at the HITRAP setup at GSI [32, 33]. It consists of an open, round-pole dipole magnet, providing a maximum field strength of 1.2 T. It will be equipped with three arms housing the electrostatic mirror lenses, each arranged under an angle of 90◦ . This allows either for a direct passage of

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Fig. 3 Schematic layout of a multi-passage spectrometer. The system consists of a central, round-pole dipole magnet for 90◦ deflection of an ion species with selected m/q and of electrostatic lenses for ion passage from the ion source (top), via an EBIT for charge breeding (left), to the Penning trap system (bottom). From Ref. [29]

singly-charged ions through the magnet (field off, B = 0 T) towards the traps, or their intermediate deflection by an angle of 90◦ into a charge-breeding device and their subsequent extraction in a desired charge state q = z ∗ e from the charge breeder. In contrast to the system at GSI, the magnet employed at MLLTRAP is equipped with a laminated yoke, consisting of 1-mm iron slices. Thus, it allows for a fast pulsing of the magnetic field strength of 1.2 T within a time period of 50 ms, to quickly adapt the required field to a selected m/q ion species. In preparation for the ion-optical design of the electrostatic mirror systems, a detailed magnetic field map of the dipole magnet has been obtained for ion-optical simulations. Ionoptical studies as well as the corresponding design of the ion-optical system and its vacuum chamber between the pole tips are presently ongoing.

4 Development of a novel in-trap setup for MATS Completely new types of decay spectroscopy experiments can be realized, provided that the disintegration of a nucleus is directly observed for a trapped ion, compared to radioactive species embedded in surrounding solid media, as targets or sources. In this case, clean

Status of the MLLTRAP setup and future plans

storage conditions allow for real in-situ decay-spectroscopy experiments on stored ions, while their decay products are observed with adjacent detector systems. In consequence of such an experimental setup, different advantages follow for novel types of experiments to be performed at future RIB facilities such as MATS [2] at FAIR [3]. One of the important advantages is the storage condition within a Penning trap, which keeps ions in a pure UHV environment, such that unperturbed decay spectroscopy can be performed with the prototype of an ideal, carrier-free source. The second advantage is the capability of a (socalled purification) Penning trap to select one particular nuclide among all isobars delivered by the on-line production process. Here, a high mass-resolving power of up to R ≈ 105 is routinely achieved in Penning trap experiments for mass spectrometry [34]. Therefore, mass-selected isotopes are often prepared for trap-assisted decay spectroscopy experiments behind the trap, where radioactive nuclei of interest are implanted on a tape-drive system, where their decay is registered. A remarkable number of such experiments has been performed behind the JYFLTRAP setup in Jyv¨askyl¨a [35–39], while at the upcoming DESIR facility at GANIL the PIPERADE trap setup is particularly planned as a storage device for trap-assisted decay spectroscopy experiments [9]. Moreover, the strong and homogeneous magnetic field acts as a magnetic guide for charged particles emitted in the decay process. This transportation is particularly efficient for the ultra-light electrons. Thus within about half of the relevant solid angle, electrons emitted during a conversion-electron decay can be collected to a detector spot placed perpendicular to the magnetic field lines. This advantage has been employed and first demonstrated at the REXTRAP facility at ISOLDE/CERN as well as at JYFLTRAP, where conversion electron spectra of on-line produced trapped ions were observed at a detector placed within the intermediate field region [40, 41]. The type of experiments that are envisaged in our setup presently developed at MLLTRAP will combine the possibility of a high-resolution mass purification in a first purification Penning trap with a direct in situ decay observation of a sample of masspurified, stored ions. Therefore, a second so-called detector trap will be placed in the second homogeneous field region of the superconducting magnet. This detector trap setup is particularly developed as part of the future MATS facility [2] at FAIR. 4.1 Experimental setup and scientific goals The key idea of our novel ‘detector-trap’ setup is to combine the storage of short-lived ions and the detection of their decay products within a single apparatus. Thus, a type of (Penning) trap is developed, where the central electrodes of the trap itself are composed of a cubic array of silicon (Si)-strip detector modules that are integrated in between the (conventional) cylindrical trap electrodes. The detector bias serves as the minimum of the storage potential. Figure 4 illustrates the concept of the detector-trap setup. The Si-strip detectors are fabricated with 1-mm wide strip segmentation that enables a position-sensitive detection along the horizontal (ˆz)-axis. This setup allows for a detection of emitted α particles for a determination of their characteristic energy and, furthermore, the decay axis of the recoiling nucleus. Furthermore, electrons emitted in a subsequent conversion decay are not kept within the storage potential for ions, they are guided along the magnetic field lines from the strong-field region to a position-sensitive electron detector in the magnetic fringe field. This setup allows to combine the coincident detection of α particles and electrons for a new type of recoil-distance experiment, in order to determine nuclear lifetimes. Particular candidates are very heavy (A ≥ 220), even-even α emitters. Their decay results with a large probability of 10 to 20 % in the population of first rotational 2+ states [42]. The

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Fig. 4 Schematic of the detector-trap setup for a novel type of a recoil-distance experiment: an array of silicon-strip detectors is employed for trapping the ions of interest and to detect their decay products. First, the direction of α particles is registered at the strip detectors, while conversion or shake-off electrons are efficiently guided by the strong magnetic field of the superconducting solenoid to a position-sensitive pixel detector in the fringe-field region. Combining the spatial information of both decays allows for a reconstruction of the recoil ions’ flight path. From Ref. [1]

remaining recoil nucleus travels a short distance within the trap (≈ 50 μm), until it deexcites predominantly via L-conversion. Since both decay processes are accompanied by the emission of electron clouds, their signals can be inferred to derive the creation of the 2+ states via α decay and their subsequent de-excitation in the internal conversion process. Moreover, the position of both electron clouds at the electron detector are magnified with respect to each other, when electrons are guided adiabatically by the magnetic field [43]. In this case, the electron gyration radii r follow the relation,  Bi ro = , (2) ri Bo with respect to the magnetic field strengths B at the position inside (Bi ) the strong 7 T field and in the outer fringe field Bo . From the field ratio at the respective positions, a magnification factor of about 30 is derived. Thus, the coincident detection of the α particle with both electron clouds allows for a reconstruction of the recoil ion’s flight distance. In consequence, the nuclear 2+ -level lifetime, and subsequently the corresponding nuclear quadrupole moment, can be derived. This method is also suited to perform lifetime measurements of 0+ states, provided that these are the first excited states. In cases of low excitation energies, i.e., below the threshold for pair creation, these states exclusively decay via 0+ to 0+ (E0) transitions. Therefore, they provide a simple system for the study of the electric monopole transition strength ρ 2 (E0). This particular application of in-trap decay spectroscopy requires a high-resolution conversion electron spectroscopy, which is realized by employing a cooled Si(Li) detector with an energy resolution of E ≤ 2 keV. Hence, nuclear structure information on shape coexistence and shape changes, occurring between

Status of the MLLTRAP setup and future plans

Fig. 5 Photograph of a Si-strip sensor within its customized printed circuit board. Number (1) denotes goldplated sections that will act as correction electrodes for the storage potential of the trap. The Si-array will be integrated via tenon joints (2) within the trap electrodes. Contact pads (3) and (4) are used for wire bonding and a customized connector. From Ref. [1]

spherical and deformed 0+ nuclear configurations, can be derived, since these shapes reflect the spatial overlap of the contributing wave functions [44]. Thus, the corresponding ρ 2 (E0) matrix elements allow to quantify shape changes between 0+ band heads and the shape mixing between low-lying 0+ states. At MLLTRAP, we are investigating the use of a Si(Li) detector system (ESLB 300-3000 from Canberra EURISYS) with an active area of 300 mm2 and a thickness of 4.3 mm. The detector size was chosen to allow for its integration within the trap electrode structure near the trapping region. Thus, this spectroscopy setup is complementary to the one previously described. The detector components for α detection have been particularly developed and, together with position-sensitive electron detectors, are currently tested and characterized. 4.2 In-trap detection of α particles As one part of the detection system, Si-strip sensors have been individually developed and characterized. Since they will be placed in the UHV environment of a cryogenic Penning trap system at MATS, the ambient conditions of a strong magnetic field and low temperatures substantially determine the design. Such Si-strip sensors are commercially not available and were therefore customized to our needs by the manufacturer Maxwell Semiconductor [45]. The bare sensors are glued onto a specially designed ceramic printed circuit board (PCB) (aluminum nitride, AlN). With a thermal expansion coefficient similar to the one of silicon, it is suited to operate the detector system in a cryogenic environment. Figure 5 shows a photograph of a mounted Si-sensor, which is described in more detail in Refs. [1, 29]. All detector modules have been commissioned with a mixed-nuclide αcalibration source and their energy resolution has been quantified for the 5.8 MeV line of

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Fig. 6 Test source for a position-sensitive electron detection. An electron source with variable intensity and extraction energy is employed to characterize the specific response of the RadEye pixel detectors

244 Cm, where an average energy resolution E FWHM of better than 25 keV was obtained throughout all strip detectors.

4.3 Detection of low-energy electrons For the detection of electrons in recoil-distance experiments, a position-sensitive electron detector is required in order to distinguish the arrival spot of shake-off electrons emitted in the initiating α decays from the cloud of electrons from the final conversion-electron decay, typically occurring a few 100 ps later. Therefore, a pixel detector system was chosen, where the total detector area is read out within single frames. A commercially available Si-pixel, X-ray detector system from Rad-icon Imaging Corp. [46] is currently under investigation. Since these detectors can provide a minimum readout time of 370 ms, their application seems feasible within the typical time cycle of a trap experiment, which is of about one second. The RadEye sensors can be tiled next to each other, such that we can create a total active detector surface of 50 × 50 mm2 with 1024 × 1024 pixel elements with a size of 48 × 48 μm2 . This offers a position resolution that is about a factor of thirty better than the expected distance between the arriving electron clouds of shake-off and conversion electrons of about 1.5 mm (see Fig. 4). In first tests, we have investigated the applicability of these X-ray detectors for detection of low-energy electrons emitted from a positionable 133 Ba conversion-electron source. As the kinetic energies of electrons emitted from this nucleus are rather high (up to Ekin = 320 keV), a customized test setup with an energy-variable electron source will be used for further investigation. It is based on an electron cathode with an extraction and a focusing electrode. Here, the kinetic energy of electrons is determined by the extraction potential of up to 5 keV. Figure 6 shows the setup of this electron test source. In order to avoid a direct line of sight of the light-sensitive detector to the filament, the electron source is skewed with respect to the optical axis. Thus, extracted electrons will be directed back towards the optical axis employing split pair coils in both transversal directions.

Status of the MLLTRAP setup and future plans

5 Conclusion and outlook The MLLTRAP Penning trap setup is presently employed as a development environment for future low-energy experiments at the DESIR facility of GANIL or the MATS facility at FAIR. In order to conduct on-line mass measurements, the Penning trap mass spectrometer will be installed at DESIR. In addition, a multi-passage spectrometer is under development to serve as an m/q separator and a switchyard for highly-charged ions. In this way, the potential use of MLLTRAP can be extended in future to measurements of highly-charged ions. A novel type of Penning trap setup is presently constructed, which will allow for a first direct in-situ observation of α and internal conversion decays of stored ions. It is realized as a ‘detector trap’ made from Si-strip detector modules, which can simultaneously provide the storage potential and register the decay products. In this way, recoil-distance experiments on populated excited nuclear states become feasible, in order to study nuclear lifetimes and quadrupole moments of excited states in very heavy α emitters. Acknowledgements This project is partly funded by the German Ministry for Education and Research BMBF (05P09WMFNE, 05P12WMFNE), the Deutsche Forschungsgemeinschaft DFG (HA 1101/14-1, TH 956/2-2), and the Maier-Leibnitz Laboratory MLL/Garching.

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