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The strongly spin-dependent absorption of neutrons in nuclear spin-polarized (3)He opens up the possibility of polarizing neutrons from reactors and spallation sources over the full kinematical range of cold, thermal and hot neutrons. This paper gives a report on the neutron spin filter (NSF) development program at Mainz. The polarization technique is based on direct optical pumping of metastable (3)He atoms combined with a polarization preserving mechanical compression of the gas up to a pressure of several bar, necessary to run a NSF. The concept of a remote type of operation using detachable NSF cells is presented which requires long nuclear spin relaxation times of order 100 hours. A short survey of their use under experimental conditions, e.g. large solid-angle polarization analysis, is given. In neutron particle physics NSFs are used in precision measurements to test fundamental symmetry concepts.

Magnetic Resonance Imaging (MRI) usually relies on magnetization of hydrogen nuclei (protons) in water or molecules in tissue as source of the signal. Biological environments with low proton content, notably the lungs, are difficult to image. Inhaling of hyperpolarized 3He gas opens the possibility to investigate ventilated spaces by MRI. To overcome the loss in signal due to the low density of the gas the nuclear polarization of the 3He spins is greatly enhanced by laser Optical Pumping. For more than three decades Optical Pumping of noble gases has been investigated, using spin exchange scattering (SE) or metastability exchange scattering (ME). Since powerful resonant laser light is available for Optical Pumping, large quantities of 3He gas can be operated. The original interest was the development of dense spin polarized targets for fundamental research in physics. As a spin off, the possibility of MRI of lung tissue filled with hyperpolarized 129Xenon was demonstrated in 1994. Later 3He was used for M...

The strongly spin-dependent absorption of neutrons in nuclear spin polarized 3He opens the possibility to polarize beams of thermal and epithermal neutrons. An effective 3He neutron spin filter (NSF) requires high 3He nuclear polarization as well as a filter thickness corresponding to amount of gas in the order of 1 bar 1. We realized such a filter using direct optical pumping of metastable 3He∗ atoms in a 3He plasma at 1 mb. Metastable exchange scattering transfers the angular momentum to the whole ensemble of 3He atoms. At present 3.6 × 1018 3He-atoms s−1 are polarized up to 68%. Subsequent polarization preserving compression by a two-stage compressor system enables to prepare NSF cells of about 300 cm3 volume with 3 bar of polarized 3He within 2 h. 3He polarizations up to 53% were measured in a cell with a filter length of about 15 cm. By this cell a thermal neutron beam from the Mainz TRIGA reactor was polarized. A wavelength selective polarization analysis by means of Bragg scattering revealed a neutron polarization of 84% at a total transmission of 12% for a neutron wavelength of 1 A.

A Rb-magnetometer was constructed for a wide range of application. It uses a very small optically pumped resonance cell with an active volume of less than 0.1 cm3 as the sensor element. The cell is housed in a small probe containing polarizer, rf-coil, heater and photoelement. The pumping light is brought to the resonance cell by use of a 3 m long fibre optics. The magnetometer allows continuous monitoring or stabilizing magnetic fields from about 3·10−8T to beyond 0.3T. The ultimate sensitivity of the instrument is of the order of 10−10T. Observing the Hanle signal in the transverse mode of optical pumping the instrument can be used for zero field detection and for the measurement for weak fields by means of field compensation.

The nuclear Zeeman levels of the ground state of Barium (6s21S0, F=I=3/2) were polarized by means of optical pumping with the resonance line (6s21S0−6s 6p1P1, λ=5535 A). The magnetic field was calibrated by means of optical pumping of sodium. The magnetic moments, calculated from the nuclear Hf-Zeeman-transitions at 228 Gauss are μ135=0,83651 (6) μN μ137=0,93573 (6) μN (with diamagnetic correction).

The3 P 1 state of Ca (τ≈0,5 msec) was populated by resonance-absorption of the intercombination line (λ=6572,8 A). The radio-frequency-transitions between the Zeeman-sublevels of the excited state were observed by means of optical double-resonance. Because of the oscillator-strength of the intercombination line being extremely small, a second excitation method, namely with electron impact, was tried. Both methods yielded equivalent results. In order to determine theg J -value of the3 P 1-state the static magnetic field was calibrated by Zeeman-transitions in the ground state of Na, the transitions being detected by optical pumping,g J was found to be 1,50105(7) in agreement with the theoretical value 1,50108.

The nuclear Zeeman levels of the odd isotopes of Ytterbium (171Yb,171 I=1/2;173Yb,173 I=5/2) were polarized in the atomic ground state (6s 2 1 S 0) by means of optical pumping with the resonance line (6s 2 1 S 0 ↔ 6s6p 1 P 1,λ=3988 a). The magnetic moments, calculated from the nuclear Hf-Zeeman transitions in a magnetic field of 228 Gauss are171 Μ I=0.49188 (2)Μ K 173 Μ I=0,67755 (3)Μ K (with diamagnetic correction applied).

In this talk I will review optical pumping (OP) experiments performed by our group to ISOLDE, CERN, in a series of Hg isotopes spanning from mass number 181 to 205 (first results in refs. [1] and [2], final in refs. [3] and [4]. I will then discuss what impact these measurements had on nuclear structure research far off stability and how the results of the latter are retroacting again on the optical work. In particular they ask for isotope shifts (IS) of light even Hg isotopes which we are prepared to measure by laser spectroscopy. Further I would like to communicate briefly first results of optical pumping experiments on “exotic” Rb and Cs isotopes. Finally I will discuss plans for laser spectroscopy on fast mass-separated beams which might favourably be applied to isotopes far off stability.