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The facility includes a Penning trap mass spectrometer consisting of a hyperbolic precision Penning trap located inside a superconductive magnet (Oxford Instruments, NMR Division, Type 200/130, Cryostat Family Type 3) having 4.7 T central field, and a cylindrical Penning trap in an 0.25 T electromagnet used for ion beam retardation. The trap setup is connected to an electron beam ion source EBIS named CRYSIS which can deliver ions of stable (not radioactive) species with high charge states, low energy and low energy spread. The objective is to weight ions of a certain chemical element with very high precision at the level of 1 part in 109 or better.
Rather than weighing the atom in a gravitational field, the "weighing" is done in an electromagnetic field where the mass comparison is turned into a frequency comparison.
The ions are created in the ion source transported and later captured in the magnetic field inside the Penning trap. Here the ion moves in circular-like orbits, a motion which is called cyclotron motion. The number of revolutions per second (the frequency) is proportional to the magnetic field strength and inversely proportional to the ion's mass. We can place the ion of interest in the magnetic field of the Penning trap and count how many revolutions per second it executes. Then we can place a different ion in the same magnetic field and measure how many revolutions per second that executes. The ratio of the two numbers gives the ratio of the masses. If one of the ions has a well known mass (e.g. 12C) the other ion mass can be obtained from this ratio. To get the mass of the neutral atom, the mass of the missing electrons and their binding energies have to be added to the measured ionic mass. This is a rather simplified description, however, it gives the basic idea of the mass measurement procedure at SMILETRAP.
The frequency measurement is carried out using the destructive time-of-flight technique (TOF). Another alternative way for frequency measurement is the use of a non-destructive Fourier-Transform-Ion-Cylotron-Resonance (FTICR) technique. The traps are at room temperature, and there is no buffer gas cooling applied in the cylindrical trap. The coldest ion is rather sorted out by using an ion evaporation technique called boiloff.
SMILETRAP has been the first experimental facility in the world where ions with charges q>8+ have been used for mass measurement purpose. In 2010, TRIUMF’s Ion Trap for Atomic and Nuclear Science (TRIUMF#TITAN) has performed mass measurements of radioactive ions with charges up to q=15+.
SMILETRAP started as a collaboration between the Manne Siegbahn Laboratory at Stockholm University and the Physics Department of the Johannes Gutenberg University in Mainz, Germany. The project was initiated by Prof. em. Ingmar Bergström. The main experimental objective is to perform mass measurements relevant for fundamental physics exploiting the precision gain by the usage of highly charged ions.
The 1989 Nobel Prize in Physics was related to ion trap technique, therefore it had a great importance for the financing of the SMILETRAP project.
The construction started in Mainz in the summer of 1990, and in December 1991 the first cyclotron resonance spectra was recorded.
The apparatus was moved to Stockholm and it was connected to the electron beam ion source CRYSIS at the Mannes Siegbahn Laboratory in 1993. Since this time it is operational and deleivers high-precision ionic and atomic mass values necessary to solve fundamental physics problems. Today the scientific leader is Prof. Reinhold Schuch and Dr. Tomas Fritioff. Former members are Dr. Szilard Nagy (2001-2005), Dr. Birgit Brandner, Dr. Henrik Bluhme, Dr. Guilhem Douysset, Dr. Gary Roleau, Dr. Conny Carlberg, Dr. Håkan Borgenstrand (1993-1997), Tobias Schwartz (1993-1996), Dr. Roland Jertz (1991-1994).
Recent highlights are:
1. A new mass value for 7Li with an unprecedented relative uncertainty of 6.3x10−10, important for nuclear charge and mass measurements of the prominent short-lived halo nuclei 9,11Li.
2. The masses of the hydrogen- and lithium-like 40Ca ions have been determined These mass values are indispensable when determining the g-factor of the bound electron in 40Ca17+,19+ for a test of quantum electrodynamics (QED) in strong fields.
3. The mass difference between 3He and 3H, is of utmost importance in the search for a finite rest mass of the electron antineutrino with the KArlsruhe TRItium Neutrino (KATRIN) experiment.