TRIGA-TRAP Project

Note: for current information please also visit the corresponding experiment page of the new installed "Stored and Cooled Ion Division" at MPIK Heidelberg.

Introduction

TRIGA-TRAP is a newly developed double-Penning trap mass spectrometer, especially designed for experiments with single singly charged ions [1]. Concerning detection techniques, the system features the commonly used destructive time-of-flight resonance method as well as the narrow-band non-destructive FT-ICR technique as described here. Within the TRIGA-TRAP project we installed for the very first time a Penning trap at a nuclear reactor in order to have access to, e.g., heavy elements above uranium. Masses of these nuclides are of high importance among others for reliable nucleosynthesis calculations in nuclear astrophysics.

The Penning trap setup is finished and the commissioning is ongoing. Cesium as well as carbon cluster ions have already been stored successfully and detected by a channeltron. First time-of-flight mass measurements on heavy nuclides are planned for the end of 2008.

The Penning trap mass spectrometer is part of the TRIGA-SPEC [1] spectroscopy project, which also includes the collinear laser spectroscopy setup TRIGA-LASER.

to the top

Applications for mass measurements with TRIGA-TRAP

The chemical composition of our universe has many surprising features: Why is iron so much more abundant than heavier elements such as gold? Why are there heavy elements at all and how did they come into existence? The properties of atomic nuclei, especially their masses, play a crucial role in these fundamental questions at the interface of nuclear and astrophysics. TRIGA-TRAP aims for mass measurements on heavy nuclides above uranium, which have been partly not available for direct mass determination due to very low production rates. In addition, the TRIGA reactor will provide neutron rich nuclei that are important for the rapid neutron capture process.

Figure 1: Typical predicted path of the r-process (in red) on the chart of nuclides. Shown are nuclei, which are stable or so long lived that they naturally exist (black), unstable nuclei for which the mass is known (green) and all other unstable nuclei that are predicted by nuclear theory to exist (yellow) - click for bigger version

to the top

The Research Reactor TRIGA Mainz

The Mainz TRIGA reactor can be operated in a steady-state mode with a maximum power of 100 kWtherm or in the pulsed mode with 30 ms pulse duration (FWHM) at a peak power of 250 MWtherm. Four horizontal beam tubes give access to the reactor core. Here, a gas-jet system can be installed for continuous transport of fission products from a fission target (U-235, Pu-239 or Cf-249) mounted close to the reactor core to the ion source of the Penning trap. In addition to short-lived fission products, several actinide elements ranging from uranium up to californium are also available for precise mass measurements in off-line experiments [2].

to the top

Experimental Setup

The mass measurement setup at beam tube B of the Mainz TRIGA reactor is shown in the image below. After ionization of the nuclides of interest and mass separation, the ions are first stored in a cylindrical Penning trap to perform buffer gas cooling with helium, to reduce the motional amplitudes and to clean away contaminations. The ions of interest are then transferred to a hyperbolical precision Penning trap to perform the mass measurement based on the cyclotron frequency ν_c determination of the ion with charge-to-mass ratio q/m stored in the B = 7T strong magnetic field [3].

Figure 2: TRIGA-TRAP setup fully assembled. Two off-line ion sources are available in the high-voltage cages at the right. The 7-T magnet houses both Penning traps. At the left side from the magnet, a liquid-helium cryostat is used to cool the superconducting tank circuit for the FT-ICR detection system. - click for bigger version

Recently, the Fourier Transform-Ion Cyclotron Resonance detection technique has been optimized to reach single ion sensitivity [4,5]. Therefore, also the nuclides with very low production rates but rather long half-lives in the range of seconds can be investigated, as it is the case for heavy and superheavy elements. For the FT-ICR method a superconducting helical resonator (see figure 3) forms an LC resonance circuit with the parasitic capacitances of the trap and the wires. In resonance, the resistance has a very sharp maximum, generating a measurable voltage drop. By the means of this narrow band technique, the image current of a single singly charged heavy ion in the order of a few hundred fA is detectable.

Figure 3: Superconducting resonator for the narrow-band FT-ICR detection system - click for bigger version

The complete system is controlled by the LabView based CS, mainly maintained by GSI Darmstadt. Many other Penning trap facilities also use that system and share the knowledge.

to the top

Position sensitive detector

In addition to classical time-of-flight analysis a space-resolving delay-line detector has been set up (Fig. 4). It's a customized commercial system of the company RoentDek Handels GmbH. If the exact values of the magnetic field and the electrostatic fields are known, one is able to back-reference to the spatial distribution of the ions in the trap. Thus, the preparation of the ions in the trap can be observed and optimized directly for the first time. Furthermore, it is possible to identify some of the contaminations because of their position in the trap, since the radio-frequent quadrupole excitation for the mass-dependent conversion of the magnetron motion into cyclotron motion results in different final states of the orbital motion. This identification as well as the Ramsey method reduce the uncertainty in frequency determination and thus allow more precise mass determinations.

Figure 4: MCP delay-line detector - click for bigger version -

The principle of the space-resolving detector is that the electron cloud, which is emitted from a MCP, hits a delay-line anode consisting of copper wires, being winded perpendicular to each other. The point of impact can be determined by the measurement of the delay-time difference towards both ends of the wires. This technique allows for a space-resolution of 70 μm. The measurement principle is illustrated in Fig.5.

Figure 5: Principle of position measurement

References

[1] J. Ketelaer et al., Nucl. Instr. and Meth. A (2008), doi:10.1016/j.nima.2008.06.023
[2] K. Eberhardt, A. Kronenberg, Kerntechnik 65 (2000) 5.
[3] K. Blaum, Phys. Rep. 425, 1 (2006).
[4] C. Weber et al., Eur. Phys. J. A 25, S01, 65 (2005).
[5] R. Ferrer et al., Eur. Phys. J., accepted (2006).