The final goal is a polarized solid target, which is polarized by continuous Dynamic Nuclear Polarization (DNP) and operates in a 4π-detection system.

Description of work and role of partners

1) Development of a “4π-DNP continuous mode” polarized target system by means of low mass polarizing solenoids for small size targets.
Present polarization experiments, where a 4π particle detection geometry is used, suffer from the fact that the data taking is interrupted due to the refreshment of the decaying polarization. The refreshment procedure, the polarization loss as well as the required low temperatures during the data taking lead to a reduced efficiency (‘figure of merit’) of the polarized target operation in the experiments. The goal for the future is to combine the advantages of the frozen spin technique with those of the ‘continuous mode’ operating target to a so called ‘4π continuous mode’ polarized target. This new polarized target scheme leads to an improvement in the ‘figure of merit’ by a factor of 2 compared to the existing frozen spin target operation. The important points of such a target system are:
• large angular acceptance close to 4π
• high average polarization during the data taking (continuous DNP at high polarizing field and low temperature)
• high luminosities up to 10**33 cm-2s-1
• no moving system for the polarization process required
• good beam time efficiency (fast repolarization in case of radiation damage).
In practice, starting from the existing ‘internal superconducting holding coil’ a new coil capable of providing an
increased field has to be implemented into the 3He/4He refrigerator as an internal polarizing magnet. It has to fulfill the requirements of the high homogeneity of the external polarization magnet and the low mass distribution of the internal holding coil to ensure a good detection probability for the outgoing particles. The most crucial point which has to be considered in the development of the internal polarizing magnet is the homogeneity of the small coil.

(2) Development of a “4π-DNP continuous mode” polarized target system by means of low mass polarizing coils for large size targets.
The world largest polarized target system is working at CERN since more than 20 years in deep inelastic scattering experiments with polarized muons. The present polarized target setup bases on a combination of two superconducting magnets (1) a 2.5 T large-diameter superconducting polarizing solenoid with high field homogeneity and (2) a 0.6 T superconducting dipole for transverse spin experiments. These two magnets are hosted in one huge vessel which leaves no space for a recoil proton detector (RPD) of similar space as the one foreseen for the unpolarized GPD measurement. Meanwhile solutions to integrate a transversely polarized target into a complete RPD are under discussion. They involve the construction of thin internal superconducting coils needed to perform the DNP process and to hold the polarization. Their structure would have to be compatible with that of the cryogenic and microwave equipments. R&D activities have to be initiated,
where all the cooperation partners in this DNPMag project with their manifold expertise have to be integrated.

The qualifications of the participating institutions and their major roles in the project are:
• University Bochum:
refrigerators, pulsed and continuous wave NMR, X- and V-band ESR, DNP, microwave systems, material preparation techniques
Experiments at CERN/COMPASS, ELSA and MAMI
• University Bonn:
superconducting magnet technology, superconductivity, DNP, cryogenics
Experiments at ELSA
• University Mainz:
magnet design optimization and technology, microwaves, cryogenics
Experiments at MAMI
• University Prague:
pulsed NMR technologies, cryogenics
Experiments at CERN/COMPASS
• University of Zagreb:
continuous wave NMR
Experiments at MAMI
• Saclay:
magnet field calculations and design
Experiments at CERN/COMPASS