Fundamental applications of spin polarized gases
Tests of fundamental symmetries in 3He/129Xe clock-comparison experiments
3He/129Xe clock-comparison experiments based on free spin precession can be used to test fundamental symmetries. Therefore one has to search for non-magnetic interactions of spins σ of type
Vnon-magn = f · σ.
The most precise tests of new physics are often realized in differential experiments that compare the transition frequencies of two co-located clocks, typically radiating on their Zeeman or hyperfine transitions. An essential assumption in this so-called clock comparison experiments is that the anomalous field f does not couple to magnetic moments but directly to the sample spins σ. This direct coupling allows co-magnetometry that uses two different spin species to distinguish between a normal magnetic field and an anomalous field coupling. The advantage of differential measurements is that they render the experiment insensitive to common systematic effects, such as uniform magnetic field fluctuations. That’s why clock comparison experiments are often used to study fundamental symmetries of nature, such as:
- the interaction of spins with a hypothetical background field which implicates violation of the Lorentz Symmetry;
- the spin-dependent short-range interactions induced by light, pseudoscalar particles;
- the search for an electric dipole moment of 129Xe.
That means all these non-magnetic interactions of the spins generate an energy-splitting, respectively a frequency shift
Δω = Vnon-magn/ħ
similar to the magnetic interaction.
The signal of the precessing spins decays exponentially with the so called transverse relaxation time T2. To determine the frequency of this signal with a high accuracy we need a long spin coherence time, respectively a long T2-time. Therefore we need a magnetic guiding field B0 with small field gradients and that is why optimal conditions for our experiments are given by the magnetically shielded room (BMSR-2) of the Physikalisch-Technische Bundesanstalt (PTB) in Berlin. The BMSR-2 is built-on 7-Layer mu-Metal so that the residual magnetic field is smaller than 2 nT inside. Magnetic fields such as that of the Earth are kept out here as effective as nowhere else. The instrumental setup at the PTB in Berlin consists of a SQUID vector magnetometer system, which was originally designed for biomagnetic applications.
Provided that the magnetic guiding field B0 is constant during the measurement it is possible to analyse the above mentioned frequency shifts with our long spin precession signals. But since the field drifts are about 1pT per hour the magnetic guiding field B0 is not constant. Hence the variation of the Zeeman/Larmor-frequency is much bigger than the frequency shifts we are looking for. That is why we are using a 3He/129Xe comagnetometer. Both gases are polarized and filled into a glass cell. The frequency of the spin precession signals are measured by LTc-SQUID detectors. If we compose the frequency difference, whereby the frequency of Xenon is weighted with the gamma ratios of the two gases, the dependence on magnetic field fluctuations should drop out, and we get sensitive to the frequency shifts due to non-magnetic spin interaction:
Δω =ωHe - γHe/γXe·ωXe
The essential difference, in particular for instance to the 3He/129Xe spin masers used so far, is, that by monitoring the free spin precession, an ultra-high sensitivity can be achieved with a clock which is almost completely decoupled from the environment.
Experiments in high magnetic fields
Measurement of the precession frequency of gaseous, nuclear spin-polatized 3He in high magnetic fields (> 1 T) allows determining the magnetic field strength with high accuracy. For high magnetic fields a precession frequency is of 100 of Megahertz. The frequency can be measured by recording the signal amplitude of the free induction decay (FID) of the precessing spins after a resonant RF-pulse excitation (𝜋/2-pulse). If the coherent spin precession time T2 (transverse relaxation time) is in the range of some seconds simply counting the zero-crossing of the oscillation signal results in a precision in the order of 10-9 for the determination of magnetic flux density B. Taking the full statistic of the recorded FID signal, this result can be improved significantly reaching sensitivity of order 10-10-10-11. The achievement of this accuracy is very important especially for applications in the Penning trap mass spectrometry.
Polarized 3He targets
Polarized 3He nuclei can be used as polarized neutron target. Experiments at the electron accelerator MAinz MIcrotron (MAMI) in Mainz involve for example the extraction of the electric formfactor of the neutron Ge,n via electron scattering. In addition recently a polarized 3He target has been installed for the first time at the tagged photon beam of MAMI. It has been demonstrated that the system works reliably and that the polarization losses during handling of the polarized gas are under control.