SPECTRAP - The SPECtroscopy TRAP

As arguably one of the best and most well-known theories in contemporary physics, quantum electrodynamics (QED) has been the subject of many rigorous tests. The SPECTRAP experiment aims to be one of them by checking the QED predicted effects known as "vacuum polarization" and "self energy". These contributions to the energy of an electron become very significant in high electric fields, such as exist around the cores of high-Z atoms. Therefore, one aims to produce heavy ions fully stripped of electrons down to a single one in the ground (hydrogen-like) state and do laser spectroscopy on it.

Similar experiments with e.g. Pb⁸¹⁺ or Bi⁸²⁺ have already been performed at various places, however the accuracy was limited because of high ion velocities. For this reason, in order to minimize the Doppler effect (i.e. increase the accuracy), the ions in this experiment are going to be trapped in a Penning trap and cooled down to cryogenic (liquid helium) temperature. At the same time, the trap's axial and radial optical transparency will allow for laser irradiation and detection of fluorescent photons coming from the excited ions.

Figure 1: Cryogenic Penning trap and PMT detector for fluorescent photons

The magnetic field needed for the operation of the Penning trap is provided by a superconducting magnet of the Helmholtz type. The whole system was previously used at Berkeley for a similar experiment called RETRAP (Rare Element TRAP). In collaboration with Prof. Dieter Schneider from LBNL and LLNL, Prof. David Church from Texas A&M University the setup was consigned to GSI Darmstadt where it is currently going through necessary modifications and preparations for the upcoming experiments. Strong collaboration exists also with Imperial College London and Universities of Münster and Darmstadt. Nevertheless, as a new setup under development, SPECTRAP is still looking for ambitious students (from various fields of study) to help in the development of this interesting new experiment.

Figure 2: Overview of the superconducting magnet of the Helmholtz type