Simulations of the novel double-Penning trap for MLLTRAP: Trapping, cooling and mass measurements

Abstract

MLLTRAP is a double Penning trap mass spectrometer that was initially designed to be located at the Maier-Leibnitz-Laboratory (MLL) in Garching (Germany) for high precision mass measurements of exotic nuclei. A second double-trap assembly, dedicated this time to in-trap $\alpha$ decay spectroscopy, has been developed and is the object of this paper. This assembly can optionally replace the current mass-measurement trap electrode assembly during future spectroscopy campaigns at DESIR/SPIRAL2 in France. Though the previous assembly will be the instrument of choice for high-precision mass measurements, the new double-trap has been designed to be compatible with the ToF-ICR and PI-ICR mass measurement techniques in addition to its initial decay spectroscopy purpose, as it would be a significant operational advantage not to have to switch between assemblies on a regular basis. We have undergone a number of simulations to design this double trap system and estimate its future capabilities. These simulations concern the cooling of ions of interest in the first trap, mass measurements in the second trap and the application of the new Decay and Recoil Imaging (DARING) technique to measure lifetimes of first excited nuclear states populated by $\alpha$ decay. The latter will be described in details in a forthcoming publication and thus the present contribution should be considered as ”part one” of a two-part article on the design and simulation of the upcoming double-trap system for MLLTRAP. While the cooling and measurement techniques presented in this contribution have been extensively described and simulated in the past, this specific double-trap has a unique geometry, as the central electrode of the second trap has been replaced by a cubic arrangement of four Si-strip detectors. We study here the expected impact of this geometry on the mass measurement capabilities of the future trap.

Introduction

Investigating exotic nuclei far-off stability is necessary to validate or constrain many nuclear models operating in different regions of the chart of nuclei. It is a step towards understanding the structure of exotic nuclei and explaining complex astrophysical phenomena. The technological challenges of creating and studying these nuclei are being met in all major radioactive beam facilities in the world through the development of new beams and instruments. Of all the properties of a nucleus, its mass is one of the most fundamental ones, since the mass defect accounts for the sum of all nuclear interactions within. This makes mass measurement an essential (although incomplete) test for any predictive nuclear model.

Electromagnetic traps have been the most successful devices used in mass separation and measurement, owing to long observation times and predictable particle trajectories. A fairly recent and thorough overview of traps and their use in experimental nuclear physics can be found in [1]. In particular, Penning traps have been known to measure the mass of radioactive nuclei with a precision down to $10^{-8}-10^{-9}$ [2], [3], [4], [5], [6]. Many such setups are already in operation [7], [8], [9], [10], [11], [12], [13] or under construction [14], [15], [16], [17], showing both the need for measuring the ground-state properties of exotic nuclei and the success of the Penning trap in doing so. The review presented in [18] overviews recent ion traps and the masses they have allowed to measure.

As the most recently operational and upcoming facilities explore regions of the chart of nuclei further away from the valley of stability, the increasingly exotic nuclei produced are now always accompanied by large amounts of contaminants, complicating the study of the species of interest. Isobaric contaminants in particular are deleterious to trap-based mass measurements as they are hard to resolve and their own space charge can alter the trajectory of the ion of interest. For this reason most measurement Penning traps used in nuclear physics are preceded by a purification trap to clean the incoming ion bunch. While some systems use an independent magnet for the purification trap [2], [9], most second-generation setups have mounted both traps within the same magnet [8], [11], [13], [14], [17], [19].

MLLTRAP was initially designed for high precision mass measurements of exotic nuclei. The setup uses a 7-T superconducting magnet and was built at the Maier-Leibnitz-Laboratory (MLL) of the Ludwig-Maximilians-University of Munich. It was commissioned off-line using a surface ionisation Rb ion source and proved capable of separating ions with a resolving power of $1.39\ast 10^5$ and measuring masses with a statistical precision of $2.9\ast 10^{-8}$ [19]. MLLTRAP is currently installed at ALTO in Orsay, France, pending on-line commissioning [20].

Complementary to mass measurements in traps, decay spectroscopy can provide direct insight into the nuclear structure of the daughter nuclei (nuclear level energy, spin, parity) or even the deep electronic structure of an atom. If the setup allows so, measuring the lifetime of some nuclear levels can bring additional information on the nuclear wavefunction. The combination of a Penning trap and particle/$\gamma$ detectors perfectly befits decay spectroscopy: the Penning trap can be used either to clean the bunch from isobaric or isomeric contaminants and thus provide a purified sample to a post-trap decay station (trap-assisted decay spectroscopy), or to simply confine an ion bunch that then behaves like a point-like source of decay radiation in vacuum (in-trap decay spectroscopy). Of course a double Penning trap can be used for both by performing purification in the first trap and in-trap decay in the second trap.

An alternative double-trap assembly for MLLTRAP is being developed at CSNSM, Orsay, aiming to perform both in-trap decay spectroscopy of heavy $\alpha$-particle emitters and mass measurements. In order to avoid confusion from here on, we will refer to this assembly as the second/novel tower and to the mass measurement double trap as the first tower. In the novel tower, the usual central ring of the second trap is replaced by a cubic arrangement of four silicon-strip detectors (SSD), and the operational bias of the detectors provides the trapping potential. Because of the strong magnetic field, conversion electrons are very efficiently guided outside the trap, while the comparatively heavy $\alpha$ particles can be detected by the in-trap SSD, allowing unperturbed detection of the $\alpha$ particles. When combined with a position-sensitive detector placed in the fringe field of the 7-T magnet to detect the electrons, this setup allows to implement the new DARING method [21] to measure the lifetime of excited states populated in the $\alpha$ decay of exotic nuclei. This first-of-its-kind double-trap system has already been described by Weber et al. in [22], [23], [24], and recent simulations strongly support the feasibility of this lifetime measurement technique [21].

Both assemblies will be commissioned at ALTO using existing or future beams. The neutron rich beams produced by photofission will allow to probe the mass surface near the magic numbers 50 and 82. ALTO currently lacks the necessary beams to test the second tower, although a fusion-evaporation target ion-source system is being studied to synthesise ¹⁹⁶Po ($t_{1∕2}=5.8$ s) in the framework of the TuLIP project (Target Ion Source For Short Lived Ion Production, ANR grant 18-CE31-0023). The magnet and its two towers will ultimately be installed at DESIR [25], the low-energy experimental hall of the SPIRAL2 facility [26] at GANIL, Caen, France. There, MLLTRAP will receive beams from the existing SPIRAL facility as well as from the S3 spectrometer [27], which will include very heavy and super-heavy elements as well as neutron-deficient nuclei at or near the N=Z line.

This contribution describes the design of the novel tower and the simulations related to mass measurements with this assembly. Although this tower was not initially intended for mass measurements, we decided to design it with this objective in mind, as having an all-in-one system capable of decay spectroscopy, lifetime measurements and mass measurements would be an operational advantage. Simulations of the DARING technique completing those already covered in [21] will be detailed in a forthcoming publication. The next section describes the geometry of the double-trap and the features allowing for mass measurements. A brief introduction to Penning trap theory is then given in order to cover the basic equations used in this article. The following section details how the traps were optimised in order to enable mass measurements with the detector-trap. The last section covers the simulation of a typical mass measurement cycle in the second tower with a bunch of $100$ ions, from its injection into the first trap to its ejection from the second trap.