The storage of polarized atoms inside the T-shape cells of the polarized internal targets like at ANKE or PAX is sophisticated, because the hydrogen atoms are radicals and most chemical reactions will destroy the nuclear polarization. In the last 40 years a few materials are found, i.e. aluminum, titanium, Teflon and water ice, that avoids the recombination into molecules very efficiently and preserved the nuclear polarization. In parallel it was observed that the nuclear polarization even during the recombination process on some surface materials was preserved at least partially.
In a collaboration of the St. Petersburg Nuclear Physics Institute, the University of Cologne and the IKP an apparatus for the investigation and optimization of storage-cell targets was developed. This device is able to measure the polarization of hydrogen and deuterium atoms and their molecules after the recombination on a surface. During the last measurements a material was found that allows the full polarization conservation during the recombination process. Therefore, the production of highly polarized hydrogen molecules is now possible.
For experiments like ANKE or PAX the target density of the polarized nucleons in the storage cell is very important. Due to the fact that the beam intensity of polarized atomic beam sources was not increased much in the last 30 years’ other options were used to increase the target density:
- Cooling down the storage cell increases the time the atoms stay inside. But temperatures below 100 K were not possible due to huge polarization losses. Nevertheless, the target density was increased by a factor of √3.
- The geometry of such a storage cell must be optimized: Longer cells with smaller diameter increase the average useful lifetime of the atoms inside. Of course, the COSY beam is not allowed to hit the wall of the storage cell and, therefore, there was a defined border for this method at an inner diameter of 15 mm.
- The diameter of the COSY beam shrinks during acceleration to higher energies. Therefore, a moveable cell with a smaller diameter was developed that will be closed after the acceleration of the beam. In this case, the necessary inner diameter shrinks to 12 mm and the target thickness is increased by a factor 2.
Another opportunity is the recombination of polarized hydrogen or deuterium atoms into polarized molecules. Thus, a dedicated apparatus was developed and built to investigate the recombination of hydrogen and deuterium atoms on different surfaces and different temperatures in magnetic fields up to 1 T. During these experiments it could be shown that a Fomblin surface at temperatures between 50 and 100 K allows full polarization conservation for hydrogen. If the magnetic field is above 0.2 T even the polarization losses during wall collisions can be neglected. With this method the target density is increased, because the velocity of the molecules is smaller by a factor √2 compared to the atoms at the same temperature. A further increase is possible due to the fact that the polarization conservation does not depend on the temperature at least down to 50 K. But one problem still exists: Every cold surface in the vacuum of the accelerator will absorb water and after some time this surface is covered by ice. For this case could be shown that water avoids the recombination very effectively at 100 K and at lower temperatures the recombination rate increases and the polarization of the atoms and molecules is lost rather fast.
Meanwhile, this experiments gain insights into different recombination processes on many surfaces that are even helpful in other fields. E.g., in astrophysics the recombination of hydrogen atoms in stellar clouds into molecules is not well understood. A “direct” recombination of two atoms is not allowed due to energy and momentum conservation and, thus, a third partner is needed. Due to the very low density in such clouds three atoms will not hit each other at the same time. One solution can be small carbon particles where the atoms can recombine on the surface, but chemistry shows that this is not very efficient. Nevertheless, first tests of carbon-coated cells, like they might be used for a storage cell at CERN in front of the LHCb detector, have shown that the recombination in such cells is large. The reason for this effect seems to be a huge amount of Lyman-α photons coming from the dissociator of the atomic beam source that are able to deliver the necessary energy to crack the C-H binding before the recombination with another hydrogen atom is possible. Another interesting feature is the observation of the coupling between the nuclear spin of the proton with the rotational magnetic moment of the molecule. Due to this coupling the polarization is lost during wall collisions, but an external magnetic field is able to overcome this coupling and preserve the nuclear polarization. When now the polarization is measured as a function of the magnetic field and the cell temperature several details are observable, i.e. the average amount of wall collisions and, therefore, the interaction between the molecules and the surface materials or the shift of the rotational moment in HD molecules compared to D2 and H2.
In the future these experiments can be extended and the range of the temperatures will be expanded. Until it is now possible to produce highly polarized deuterium and HD molecules with this method, it should be possible to freeze them out on a cold surface at liquid helium temperatures. If it would be possible to collect enough polarized deuterium than it might be useful for first tests of polarized fusion.