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Aktivitäten in Transregio 16

Aktuelle Forschungsaktivitäten in Transregio 16


Overview: »CB-frozen spin target« at Crystal Barrel


The Crystal Barrel detector is the central part of the experimental program at ELSA until 2012. One of the major contribution of the physics output will base on single or double polarization experiments using the existing GDH-frozen spin target, which from now on will be labeled as CB-frozen spin target. Here we will briefly report on the setup of the CB-frozen spin target at the Crystal Barrel detector for the first round of polarization experiments at ELSA.

Up to now there was no need to do any modifications on the dilution refrigerator for the planned new experiments with the Crystal Barrel detector. The target has been dismounted after the last measurement in Mainz and was moved back to Bonn and has been prepared for the experimental program at ELSA. The main pumping unit was dismounted for revision and has been reinstalled to have a better access to the roots blowers to reduce the breakdown time in case of a failure. In parallel the control unit was upgraded to guaranty a reliable operation of the system at ELSA, especially in front of the background of a reduced number of operators for the running period of the target.

The installation included a reconstruction of the mechanical support structure of the target, the railway system to move the polarizing magnet and the Crystal Barrel detector and the main vacuum system. Actually, the target is completely installed at the experimental area and fully tested. To fulfill the requested transverse polarization a new transverse holding coil is under construction and will be operational available with the beginning of 2008.


Overview: »4π-continous mode« polarized solid state target


The frozen spin target is a powerful tool for polarization experiments at low intensity beams but the price for the good acceptance is firstly a loss in beam time, which is needed to polarize-up or repolarize the target material, and secondly a dedicated railway system to move the detector and external polarizing magnet for the polarization procedure.

In the best cases a beam time efficiency of 80% is attainable. But the reproduceable moving of all components is limited by the weight and size of the detector system. The second restriction from the »figure of merit« point of view is the decay of the polarization during the data taking in nearly all target materials. The overall polarization of an experiment using a frozen spin target is given by the relaxation times and limited to approximately 0.8 · Pmax. A reduction of the target temperature to increase the relaxation times is suppressed by the Kapitza-resistance in the target material at a given beam intensity. Commonly used intensities up to 107/sec are setting the effective target material temperature independently of the lowest refrigerator temperatures (e.g. 20 mK) to about 50-60 mK. Beam intensities above 108/sec are unattainable at all.

Our 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, which can be described as a continuous mode operating target with a large angular acceptance, leads to an improvement in the figure of merit by a factor of 2 compared to the existing CB-frozen spin target.

To condense the new scheme the focal points of the target are:

  • large angular acceptance ≈4π (in the horizontal case homogeneous mass distribution) 
  • high average polarization during the data taking (continuous DNP at high polarizing field)
  • high luminosities up to 1033cm−2sec−1
  • good beam time efficiency (no maintenance work for the moving of components nor repolarization)
  • no moving system for the polarization process required (no detector size restriction, fixed target-detector setup)

Once the system will have been installed in the Crystal Barrel detector it will provide highly polarized nucleons with good access to the target region for scattering experiments with real and virtual photons.

Starting from the existing » internal superconducting holding coil« a new coil capable of providing an increased field has to be implemented into the refrigerator as an internal polarizing magnet. It has to fulfill the requirements of the high homogeneity of the external polarization magnets and the low mass distribution of the internal holding coils to ensure a good detection probability for the outgoing particles. A first version of a » thin high field internal magnet« (total thickness 1.3 mm) has been wound for the existing CB-frozen spin target and used in the actual Bonn measurements. Summarizing the first experience and tests with the internal superconducting high field magnet, magnetic fields of 2 Tesla are achievable in a thin coil with only 4 to 6 layers of superconducting wire, and currents up to 120 A can be handled in a dilution refrigerator with a proper current lead system.

The most crucial point which has to be considered in the development of the internal polarizing magnet is the homogeneity of the small coil. Because of the minimized dimensions of the solenoid the magnetic volume is only a factor of 17 larger than the target volume, which needs the good homogeneity of ΔB/B ≤ 10−4. This problem is actually under investigation. The challenge here is to find a way between additional correction layers and an upper limit for the homogeneity. In collaboration with the Institute of numerical simulations of the Bonn University a new code is under development to optimize the homogeneity of the coil. the code allows to study the effect of the field homogeneity as a function of the limited accuracy in the winding, irregularities like random variations in the coil number in a layer, the SC wire diameter and it allows the implementation of optimization algorithms.

To experimentally verify the calculations and simulations of the design of the internal superconducting magnet a computer controlled field mapping system is nearly completed. A three dimensional fluxgate magnetometer is used to measure the magnetic field of the coil in a normal conducting mode under warm conditions. For cold tests of the small superconducting magnet a test facility is under construction and will be completed at the end of 2007. It is an easy handling 4He-cryostat which allows to cool down the new internal solenoids to about 1.2 K, the normal operational temperature of the internal magnets in the later 3He/4He-dilution refrigerator. By this the critical parameters of the wire as well as the field parameters could be measured. The system can also be used to dynamic polarize a target sample in the field of the test magnet. Both test facilities are designed and constructed by the approved engineer position (Dipl. Ing. Thomas Ludwig) of the last request.

To follow the idea of the 4π-continuous mode target for (double) polarization experiments in combination with the Crystal Barrel detector a completely new design of a horizontal 3He/4He-dilution refrigerator was indispensable.

The design, construction and setup will be the main work of the Bonn polarized target group until 2012.

The construction and setup of vertical as well as horizontal dilution refrigerators has a long and experienced tradition in the Bonn precision workshops. Most of the components of the new one will be machined and built by staff members of our Institute. We expect roughly 9 man-years from the beginning of the design stage to first tests with the completed system. To strengthen the manpower of the project team, we ask for an additional position (BAT IVb) for a precision engineer with special skills in precision machining, soft soldering and welding. The plan is to setup a project oriented group within the existing machine shop.

The design studies and calculations for the new refrigerator are nearly completed. In general we followed the design of the existing horizontal dilution refrigerator. Differently from the existing type we focused on high cooling power (200 mW) in the temperature range at 250 - 350 mK for the DNP-mode of the target. Nevertheless, the expected base temperature is set to about 60 mK to run the system as a frozen spin target if transverse polarization is needed.

The beam line is the central part of the cryostat and has to fulfill the function of the internal isolation vacuum. The target material will be inserted into the refrigerator along the beam line by an insert that will have the same length as the existing one. It will be vacuum tight against the mixing chamber by means of a cold indium seal in the still region. The precooling of the incoming 3He/4He mixture, liquifying at the thermal sink (1.2 K sink) are turbine like counter current heat exchanger. The final heat exchanger and the still are in first order conditioned by the outer scheme of the cryostat. From the outside, the system will look like a typical »Roubeau- cryostat«, the cylindric mixing chamber which is surrounded by the internal polarizing magnet is followed by the final heat exchanger. To the backward region the diameter opens conical to the pumping tee. The smooth transition from the larger diameter of the 70 K region to the low temperature part of the cryostat allows one to use HTSC-current leads for the internal polarizing magnet. An additional heat sink cooled by a separated 4He-circuit provide the required cooling power to cool the normal conducting current leads at the transition point to the HTSC down to 50 K. Taking the parameter of a standard HTSC wire into account, a maximum current of 150 A could be feasible for an internal magnet.

First parts of the cryostat are already machined, like the outer vacuum envelope, the turbine like heat exchangers, the internal beam line and parts of the still region. Due to a leak of man power and technical problems to realize the designed shape of the heat exchangers we are two years behind our expected time schedule of the construction of the cryostat.


Schedule: »CB-frozen spin target« at Crystal Barrel


2008 - 2010

Since the CB-frozen spin target is an essential part of the experimental program at ELSA, one of the major tasks of the »polarized target group« at Bonn is to operate the target for the approved double polarization experiments at the Crystal Barrel detector. Depending on the requested polarization observable the target will be equiped with the longitudinal or the vertical holding coil, respectively. Once the target is running in a production experiment at the beam, approximately 2 operators per week are necessary to guaranty a reliable operation, one for the standby-service during the night and the other for maintenance and repolarizing during the day. Most of the control of the target will be done automatically.


Schedule: »4π-continous mode« polarized solid state target



The machining of parts of the cryostat will continue over the the year. The refrigerator setup starts from the backward flange to the front parts. The outer vacuum jacket, the cooling shields and most of the inner precooling heat exchangers should be available at the end of the year. For first cold test of parts of the new refrigerator a test facility is needed and will be installed at the Bonn polarized target laboratory. The design and field calculations for the internal polarizing magnet should be completed in the thrid quad of the year to place the order for the winding of the coil at least end of the year.


The set-up of the test facility should be competed mid of the year. Assembling and testing of various components of the refrigerator. First cold tests of the precooling stages are aspired for the mid of the year but they are ultimately conditioned by the progress of the overall project. Nevertheless, the test of the separated 4He circuit and the connected internal coil should be feasible mid of the year. In case of successfully cold tests the assembling of the cryostat will continue. The construction and manufacturing of the final heat exchangers is scheduled for the end of the year.


The complete assembling and commissioning of the cryostat is planned for the first half of the year. First test measurements of the assembled refrigerator should be performed with the installed test facility in the laboratory of the Bonn polarized target group.


After final test measurements and proving the reliability of the total system a unique experimental tool for (double) polarization experiments in combination with the Crystal Barrel detector will be available. Through which it makes no difference if the Crystal Barrel detector is surrounded by an outer magnetic field or not. The new system will cover nearly all future eventualities one actually could have in mind. As soon as it fits in the general time schedule of the detector setup at ELSA we will start with the installation of the infrastructure of the horizontal cryostat at the experimental area.


We will continue the installation in the experimental area and perform first test with the system to prove the reliability and to measure the polarization of suitable target materials for the planned scattering experiments. First double polarization experiments with the new »4π-continuous mode« polarized solid state target.


Schedule: Irradiation facility and irradiated target materials



Chemically doped target materials like the actually used butanol suffer from a number of shortcomings. Firstly, the limited radiation resistance restricts the use of these materials to tagged photon beam fluxes not higher than 107/cm2s. Secondly, the rather modest frozen spin relaxation times imply a frequent repolarization of the target. Together with the comparatively low dilution factors of these materials the possible target figure of merit is considerably reduced. All of these drawbacks can be avoided by the use of radiation doped materials like ammonia or the lithium hydrides.

Thus it is planned to reactivate the irradiation facility at the Bonn injection linac 1, which includes a refurbishment of the two respective irradiation cryostats as well as the completion of the overhaul process of the injection linac itself. Both measures are scheduled for 2008, so that the radiation doped materials mentioned above can be produced from 2009 on.