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Protons and neutrons – commonly summarized as nucleons – are the building blocks of the atomic nuclei. The nucleons themselves do not yet represent the most fundamental layer in the structure of matter. Nucleons are composed of elementary particles called quarks, which are bound together by messenger particles called gluons. All these particles possess spin and this physical quantity gives the experimentalists a unique handle to study structure details on the microscopic scale at accelerator facilities.

At high energies (or microscopic scale), perturbative Quantum Chromodynamics (pQCD) is the theoretical basis for the description of the strong interaction, whereas at a distance scale of the order 1 fm, the color force is too strong for perturbative solutions. Progress therefore will only come through the coherent interplay of experiments at all energy scales with a variety of theoretical approaches, from modeling to lattice QCD.

At high energies three distribution functions are necessary to describe the spin structure of the nucleon: the momentum distribution f₁(x), the helicity distribution g₁(x) and the transversity distribution h₁(x), which all are of equal importance. In the energy range up to several GeV, differential cross sections have been the most commonly measured observables. The strongly decaying excited states of the nucleon have a large intrinsic width and therefore polarization observables are of absolute necessity to disentangle the excitation spectrum of the nucleon. Huge progress was made in the field of polarized electron and photon beams as well as on polarized solid target materials, especially on polarized solid deuterated materials. The long missing part in all these activities is a polarized solid target with a spin orientation in x- or y-direction - perpendicular to the beam - together with particle detection in 4π-geometry. The final goal is a solid target, which is polarized by continuous Dynamic Nuclear Polarization (DNP) and operates in a 4π-detection system. This requires technical solutions presently at the limit of feasibility.

The objectives of this Joint Research Activity are twofold:

1.      Development of equipments, in which frozen target materials can be oriented in arbitrary directions by means of modern superconductivity techniques.

2.      Development of a “4π continuous-mode” polarized target system by means of low-mass solenoids

In both objectives, substantial technological innovation will be realized.

1.      The development of the Bonn polarized target group during the last decade enables double polarization measurements with a 4π particle detector. This frozen spin operation scheme with a so-called internal holding coil is adopted from the TJNAF and Mainz polarized target groups. Due to the geometry of the internal holding coil a spin orientation only in the z-direction (beam direction) can be maintained. This state-of-the-art frozen spin target operation for tagged photon beams will now be completed by an internal holding coil set allowing a spin orientation in x- or y-direction.

2.      Up to now huge superconducting magnets are used to employ the DNP process in various target materials. They are operated either at 2.5 T or at 5 T. All of them allow only a limited angular acceptance for the outgoing particles. With the construction of low-mass polarizing solenoids (> 2 T) placed inside the refrigerators, a permanent operation (continuous DNP) of the target in a 4π-detector is possible, resulting in a much better data taking efficiency.

The design of low mass solenoids with the required high homogeneity field relies on a new approach in terms of geometrical shape. The field map will be numerically simulated by means of a new code, where new optimization algorithms to define the parameters of the geometry will be implemented. This code will also allow studying the effect on the field homogeneity of the limited mechanical accuracy in the winding and of irregularities like random variations in the coil number in a layer. Properties like the thermal propagation in the target material could be studied and optimized through new mathematical models as well as efficient solution methods and discretization schemes for the simulation. These will be based on finite element methods and on modern solution techniques as multigrid methods. Multigrid (MG) methods are a most efficient way for solving differential equations. The efficiency relies on the optimal complexity as linear solver (the complexity scales as O (n), n being the number of unknowns). Commercial finite element software generally uses a direct sparse solver, with a complexity in 3D that scales with the square of the number of unknowns (~ n²). The reason is that MG methods require some parameter tuning which makes them more difficult to handle for the common user.

T1 – Spin orientation in arbitrary directions

Due to its horizontal geometry, the present frozen-spin target system allows a polarization only in the beam direction (z). This will be changed by replacing the internal solenoidal holding coil by a saddle coil geometry which gives access to x- and y-polarization directions. These developments allow the measurements of additional double-polarization observables urgently needed e.g. in photoproduction experiments.

The UBonn and FZJ groups will design the prototype saddle coils, while the RUB, UBonn, UMainz and RBI groups will test them in ³He/⁴He dilution refrigerators.

To optimize this operation mode, the measurement of polarization (based on Nuclear Magnetic Resonance – NMR) during the data taking in the frozen-spin mode has to be ensured as well. Problematic is the relaxed homogeneity of the magnetic field of the saddle coil. A small NMR coil configuration has to be connected to an upgraded NMR circuit.

The RUB, RBI and CUNI groups will be in charge of the NMR measurements in saddle-coil fields.

T2 – Low-mass polarizing solenoids

Present polarization experiments, where 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 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 10³³ cm^-2s^-1,

-   no moving system required for the polarization process,

-   good beam time efficiency (fast re-polarization 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.

The initial tests of existing low-mass solenoid (with high-temperature superconductors) will be carried on by RUB, UBonn and FZJ. The UBonn and FZJ groups will design the prototype low-mass solenoids, while the RUB, UBonn and UMainz groups will test them.

1.      Novel computing methods for magnetic field maps (delivery month from start date: 8)

2.      Simulation of target properties based on finite elements and multigrid methods (delivery month from start date: 8)

3.      Prototypes of low-mass saddle coils with dedicated geometries (delivery month from start date: 15)

4.      Dedicated NMR systems for polarization determination in saddle coil configuration (delivery month from start date: 30)

5.      Prototypes of low-mass superconducting solenoids for high field (delivery month from start date: 22)

Spin oriented nuclei in form of solid target materials are in use at the research infrastructures ELSA (Bonn) and MAMI (Mainz), as well as at CERN (Geneva). During their last campaigns at CERN between 2002 and 2007, the COMPASS collaboration for instance was using about 4000 hours per year of polarized muon beam for data taking on polarized lithium deuteride and ammonia nuclei. Further data taking is proposed for the next years. A large-scale spin program is underway at the accelerators ELSA and MAMI, where double-polarization experiments with polarized beams on polarized protons and deuterons (neutrons) will be performed. The European hadron physics community in the COMPASS (CERN), A2 (Mainz) and CB-ELSA (Bonn) collaborations will profit without doubt from the future developments.

The outcomes of the project should lead to substantial improvements and will facilitate polarization measurements in hadron physics that are beyond present technologies. The essential parameters are spin orientation of the nuclei in arbitrary directions, low mass magnetic coils of small dimensions to fit inside the refrigerators (polarizer) and high magnetic field homogeneity. This will put the users of the European research infrastructures in the field of hadron physics in the position to access new polarization observables. In this way, the description of the nucleon structure, and of the whole spectrum of nucleon excitations, will be greatly enhanced.