High-Precision Experiments with Trapped Radioactive Ions Produced at Relativistic Energies

Abstract

Research on radioactive ion beams produced with in-flight separation of relativistic beams has advanced significantly over the past decades, with contributions to nuclear physics, nuclear astrophysics, atomic physics, and other fields. Central to these advancements are improved production, separation, and identification methods.The FRS Ion Catcher at GSI/FAIRexemplifies these technological advancements. The system facilitates high-precision experiments by efficiently stopping and extracting exotic nuclei as ions and making these available at thermal energies. High-energy synchrotron beams enhance the system’s capabilities, enabling unique experimental techniques such as multi-step reactions, mean range bunching, and optimized stopping, as well as novel measurement methods for observables such as beta-delayed neutron emission probabilities. The FRS Ion Catcher has already contributed to various scientific fields, and the future with the Super-FRS at FAIR promises to extend research to even more exotic nuclei and new applications.

1. Introduction

In-flight fragmentation and fission have generated the majority of new nuclides this century [1]. Research with radioactive ion beams is finding widespread use in nuclear physics, nuclear astrophysics, atomic physics, chemistry, fundamental research, and various applications. This is largely due to significant advancements in production, separation, and identification methods, particularly isotope separation online and in-flight separation. In-flight separation of relativistic beams is particularly effective, providing high-purity beams for all elements up to uranium; even Np and Pu isotopes can be produced through charge–exchange reactions.

To identify and produce high-Z isotopes with high intensity, primary beam energies of approximately 1 GeV/nucleon are required. The GSI facility, with its combination of the heavy-ion synchrotron SIS18 [2] and the projectile fragment separator FRS [3], specializes in this area. It has been used to discover new isotopes [4], measure stopping powers [5,6] and charge state distributions [7] at various energies, advance hadron therapy [8,9], conduct fission research [10], discover new decay modes [11], and was used to make the first measurement of ¹⁰⁰Sn [12]. High beam energy benefits the identification of nuclei and allows for flexible operations of the separator because, at the focal planes, more matter—including detectors and degraders—can be inserted. However, the high primary beam energy also leads to challenges for certain types of experiments. For example, in experiments with slowed-down or stopped beams, the high primary beam energy results in large range distributions due to range straggling. Consequently, devices designed to thermalize the produced nuclei need to be large and have high areal densities to efficiently stop the beams.

Once the ions are slowed down, a variety of high-precision experiments can be conducted, including half-life [13] and mass measurements [14,15,16], investigations of decay modes [17], nuclear reaction studies [18], and laser spectroscopy [19]. The FRS Ion Catcher at GSI/FAIR is the first system designed for these relativistic ions and is utilized for numerous applications [20,21]. The research and technical developments as well as matching to the special challenges and possibilities of ion trapping experiments with exotic ions produced at relativistic energies with this system are detailed below.

2. Setup of the FRS Ion Catcher

The FRS Ion Catcher [20] is placed at the final focus of the symmetric branch of the FRS. It is designed to cope with the special needs of ion beams produced at relativistic energies. It consists of a degrader system, a cryogenic gas-filled stopping cell (CSC), and an RFQ beamline, including a laser ablation ion source and a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS), see Figure 1.

The uniquely high beam energies at GSI/FAIR allow for a specialized solution and very flexible operations. To fully exploit this, special devices are needed and are described in the following.

2.1. Multipurpose Degrader System

A degrader is used to slow down the ion beam, separate different species by their energy loss, reduce the energy spread of the ions, and modify the ion-optics of the separator. The high beam energy at the FRS allows the use of thick degrader systems and, thus, high flexibility. The multipurpose degrader system of the FRS Ion Catcher allows for various operational modes and consists of the following components: (i) a double wedge degrader that creates a variable homogeneous degrader; (ii) a double disk degrader that forms a variable wedge; and (iii) two ladders equipped with fixed wedges, thick aluminum plates, and tantalum plates. The homogeneity of the wedge is shown in Figure 2. It is evident that very high homogeneity is achieved for aluminum. This is due to two factors: first, the surface roughness is small (about 5 times less than that of tantalum); second, aluminum has a density more than six times lower than that of tantalum. As a result, the homogeneity of the areal density is 30 times better for aluminum compared to tantalum. A double wedge degrader has a minimal areal density of a few hundred mg/cm², as the two wedges must overlap with the beam. This may be too much for high-Z, low-energy beams. In this case, a gas degrader with variable density [22] can be used. It has a minimal areal density of only a few tens of mg/cm².

2.2. Cryogenic Gas-Filled Stopping Cell

The cryogenic gas-filled stopping cell of the FRS-IC is the first cryogenic gas cell for exotic nuclei in operation and is used for experiments [23,24,25]. It features a beam window measuring 200 × 100 mm² and has an active length of 1 m. The cell operates at temperatures between 80 and 90 K and pressures between 30 and 150 mbar of helium, resulting in an areal density of up to 10 mg/cm² [26], which is significantly higher than any other system. These unique performance parameters are essential to cope with the challenges induced by the high beam energies. The range straggling in a typical experiment is 10 mg/cm², allowing stopping efficiencies of up to 50%. Once the ions are stopped in the gas, they are transported by DC fields, usually around 30 V/cm, to the system’s extraction side, resulting in a mean transport time of 20 ms to the RF carpet. Here, the RF carpet with a 250 µm pitch generates a repelling field by applying a sinusoidal RF signal at 100 Vpp and 6 MHz. More than 50% of the stopped ions can be extracted from the CSC, with most of them being singly or doubly charged atomic ions [21,27]. Additionally, the RF carpet can suppress molecular background through ion mobility selection by finely adjusting the RF amplitude [28].

2.3. RFQ Beamline

The RFQ beamline offers differential pumping; ion transport and cooling, separation; background suppression; diagnostics; as well as ions for calibration, testing, and optimization of the downstream system, all within a compact 1.5-m-long setup [29,30,31] (see Figure 1). The first mass selection occurs right after the nozzle in the extraction RFQ, which can also function as an RF mass filter. This setup can achieve mass-resolving powers of up to 150, albeit with significantly reduced transmission efficiency (around 1%). Typically, a mass-resolving power of 5 to 10 is achieved at efficiencies above 90%, sufficient for selecting the desired mass range [32].

In the subsequent section of the beamline, various detectors for both stable and radioactive ions, as well as ion sources, are introduced. Further downstream, an RFQ switchyard is used to connect ion sources and detectors to the beamline [33]. This is followed by a dedicated RF mass filter with a mass-resolving power greater than 50 at 90% transmission. The molecular background can be reduced by using the isolation–dissociation–isolation method. In this technique, a voltage step (typically 60 V) is applied to dissociate weakly bound adducts or molecules. Mass selection is performed both before and after this dissociation step [34] with the two mass filters described earlier.

Recently, the system was upgraded with a laser ablation carbon cluster ion source (LACCI), which provides the downstream MR-TOF-MS with various calibration ions and additional capabilities [31]. After the mass filter, another RFQ captures and cools the selected ions at the end of the RFQ beamline before delivering them to the trap system of the MR-TOF-MS.

2.4. MR-TOF-MS

The final component of the FRS Ion Catcher is the MR-TOF-MS, where the quasi DC beam extracted from the CSC (typical time structure from the SIS18 is a few-second beam followed by a 2-s pause) is converted into a bunched beam using a trap system [35,36,37]. Bunches of ions with a time spread of 5–10 ns at the time focus and with an energy spread of ±10 eV are generated and travel with an average kinetic energy of 1300 eV in the time-of-flight analyzer for the selected number of turns. Subsequently, they are ejected via the time-focus shift (TFS) reflector spatially and temporally, focused on the detector or the fast-switching ion gate [38]. The system has achieved a mass resolving power of one million and mass accuracy of low ${10}^{-8}$ [39]. A few ions are sufficient to determine a mass with this level of mass accuracy. Due to the unique design of the MR-TOF-MS of the FRS Ion Catcher, which includes the TFS reflector, the system has an unprecedented mass range [40], which has been utilized in offline and online measurements [41,42]. The MR-TOF-MS can also function as an isobar and isomer separator [43] by replacing the time-of-flight detector with a fast-switching ion gate, i.e., a Bradbury–Nielsen Gate [44].

3. Examples of Advantages for Experiments with Thermalized and Trapped Exotic Ions Produced at Synchrotron Facilities

A synchrotron offers inherent advantages: (i) rapidly variable beam energy, (ii) high beam energy for heavy ions, and (iii) pulsed beams. The following examples illustrate how these advantages are utilized for ion trapping experiments.

3.1. Multi-Step Reactions in a Very Thick Target

Due to the high beam energies from the SIS18, very thick targets can be used without stopping the fragments of interest within the target. This increases the rates for all isotopes (neutron-rich and -deficient) produced in fragmentation and fission. Depending on the primary beam, target, and fragment combination, beryllium targets as thick as ≤16 g/cm², approximately 86 mm, can be used, and the interaction probability of the beam can reach up to 90%. As a result, the produced fragments also have a high probability of undergoing additional nuclear reactions within the thick target. Generally, such thick targets lead to an increased longitudinal energy spread that exceeds the acceptance of the separator due to the varying energy loss of the projectile and the fragments. However, for many neutron-deficient nuclei, this is not the case, resulting in an additional substantial increase in production yields. The key criterion is that the beam and fragments should have similar energy losses, leading to a narrow momentum spread after passing through the target. This condition is met when $A_{\mathrm{p}}/Z_{\mathrm{p}}^2\approx A_{\mathrm{f}}/Z_{\mathrm{f}}^2$, where $A_{\mathrm{p}}$ and $Z_{\mathrm{p}}$ are the mass and proton number of the primary beam, and $A_{\mathrm{f}}$ and $Z_{\mathrm{f}}$ are the mass and proton number of the fragment.

Figure 3 shows the nuclear chart, the isotopes for which the optimal choice of the primary beam meets this condition. Evidence of this effect and the energy dependence of the apparent production cross-sections has been observed in the production of ¹⁰⁰Sn at RIKEN (using a ¹²⁴Xe primary beam at 345 MeV/u) and GSI (using a ¹²⁴Xe primary beam at 1000 MeV/u), with the higher energy yielding an increase in the apparent cross-section by a factor of about 8 [12,45]. From simulations, it is expected that the rate of nuclides of interest could increase by up to an order of magnitude for the proposed mass measurements, as illustrated in Figure 4. This approach may extend the reach towards more exotic nuclides at the FRS and the Super-FRS Ion Catcher.

When multi-step reactions are used for medium and heavy primary beams, the secondary beams have low energy and increased background. Therefore, only detection systems that can handle high rates and do not rely on the FRS particle identification, such as the FRS Ion Catcher, can fully exploit multi-step reactions.

3.2. Beta-Delayed Neutron Emission

The high and variable energy enables the production of a very clean stopped beam. By using the CSC as an ion trap and extracting from the SIS18 in short pulses of about 10 ms, it becomes possible to measure beta-delayed neutron emission without needing to detect the neutron directly. This method is particularly effective for multi-neutron detection [46]. For this approach, the FRS is used to provide a clean beam of the ions of interest, which are stopped and thermalized in CSC. At variance to the normal operation, the electric fields in the CSC are switch off; thereby, the CSC acts as a ion trap. The ions are stored for up to 1 s in the dense and cold He gas. Now, the ions of interest decay and their recoils are thermalized in the CSC. After a selected time, the electric field is switched on, and ions are extracted, detected, identified, and counted with the MR-TOF-MS.

3.3. Mean Range Bunching

When the FRS is used in achromatic mode, the disk degrader positioned in front of the CSC is used to achieve mean range bunching. This technique minimizes the range differences between various nuclides, allowing them to be stopped simultaneously. This is possible as there is an almost linear dependence of the mean range of nuclides with their position in the dispersive plane. Now, a wedge-shaped degrader is inserted to compensate the difference in ranges. When combined with the MR-TOF-MS capabilities, this approach becomes a highly effective measurement method to investigate the mass surface over a broader region on the nuclear chart [42] or for decay experiments with stopped beams [47].

3.4. Optimize Stopping

A synchrotron provides the benefit of easily adjusting primary beam energy, which helps in optimizing the stopped beam. Fine-tuning these adjustments allows us to produce a particularly clean stopped beam [17,48,49] or to stop specific isotopes of interest simultaneously, see Figure 5. Another method to improve the rate of stopped beams is to use a degrader made from a high-A material. This choice minimizes nuclear reactions and reduces losses during the deceleration process. From a practical perspective, tantalum is ideal for this purpose, although producing highly uniform plates from it is technically challenging, see discussion in Section 2.1.

3.5. Using the Same Exotic Ions for Multiple, Simultaneous Experiments

Due to the high beam energy, the ions can interact with significant amounts of matter at the mid-focal plane and remain available for experiments at the final focal plane. Consequently, combined experiments were conducted where the EXPERT setup [50] was positioned at the mid-focal plane and the FRS Ion Catcher was placed at the final focal plane. The experiment requires the mid focal plane to be achromatic and the mid focal plane to the final focal plane to be dispersive. Thus, the target to final focal becomes dispersive. To achieve good stopping of the fragments transmitted to the final focal plane, a disk degrader is used to achieve a monochromatic setting. With such a setup, multiple experiments can be conducted simultaneously with the same beam.

3.6. The FRS Ion Catcher as a Tool for the Fragment Separator

By measuring the mass of the stopped ions, the MR-TOF-MS can provide an independent particle identification based solely on mass, complementing the FRS method. This technique is known as mass tagging. This fast and robust method can be implemented in different ways; for details, see [51]. The FRS and the MR-TOF-MS data streams can also be merged to obtain additional information in the FRS or provide background suppression for the MR-TOF-MS [41]. To set up the experiment, the capability to calculate the range for each ion based on the information measured with the particle identification of the FRS has been implemented.

4. Scientific Program and Outlook

The FRS Ion Catcher has already been used to address various scientific questions, from nuclear structure effects for heavy N = Z isotopes [15,16,39] to mid-shell nuclides above Sn and around ²⁰⁸Pb [13,14]. It also searches for exotic decay modes, such as fission isomers [52] and double alpha decay [53]. Current research includes multi-nucleon transfer reaction studies with slowed-down relativistic primary and secondary beams, beta-delayed neutron emission, and the evolution of the mass surface toward the third peak of the r-process [21]. These topics will remain relevant for years to come. With the Super-FRS becoming operational, there will be many new opportunities to study even more exotic nuclei; the Super-FRS Ion Catcher will play a crucial role in this research. The new stopping cell [54] is expected to achieve near-unity stopping efficiency, operate faster, and handle a higher rate, thereby further extending the reach. The system is currently under construction at Justus-Liebig University Giessen.

5. Conclusions

The development and implementation of the FRS Ion Catcher at GSI/FAIR have enabled precise and versatile experiments with thermalized relativistic ions for the first time. Important upgrades include the multipurpose degrader system at the final focal plane, the cryogenic stopping cell’s exceptional areal density and ion extraction efficiency, and the RFQ-based beamline’s capabilities for mass selection and background suppression.

The unique concepts enabled by high-energy beams from the SIS18 synchrotron, such as multi-step reactions in thick targets, mean range bunching, and optimized stopping, have significantly broadened the scope of the research with the FRS Ion Catcher.

The coupling of the FRS Ion Catcher with the fragment separator data acquisition provides independent particle identification and enhanced background suppression, it has proven to be a powerful tool for both current and future experiments.