E. Radtke

A new frozen-spin target for 4π particle detection

Abstract A new frozen-spin target has been developed, that allows the detection of emitted particles in an angular acceptance of almost 4π in the laboratory frame. The central part of this new target represents a 3He/4He dilution refrigerator that is installed horizontally along the beam axis. The refrigerator includes an internal superconducting holding coil to maintain the nucleon polarization in the frozen-spin mode longitudinally to the beam. The design of the dilution refrigerator and the use of an internal holding coil enabled for the first time the measurement of a spin-dependent total cross section in combination with a polarized solid state target. This new frozen-spin target was used successfully to measure the helicity asymmetry of the total photoabsorption cross-section at the Mainz accelerator facility MAMI. This experiment has been performed in order to verify for the first time the GDH sum rule.

First results of the large COMPASS 6LiD polarized target

The COMPASS (NA58) experiment at CERN operates with a large solid polarized target (PT) to study the spin structure of the nucleon. The COMPASS PT system started its operation with the target material 6 LiD in 2001. Deuteron polarizations of þ54:2% and � 47:1% were achieved in a 3 He= 4 He dilution refrigerator at a magnetic field of 2: 5T : The equal spin temperature (EST) concept was found to hold among the deuteron, the 6 Li and the 7 Li nuclei during the dynamic nuclear polarization (DNP) process. The agreement with the EST concept allows the permanent monitoring of only one nuclear species by the nuclear magnetic resonance (NMR) method. r 2003 Elsevier Science B.V. All rights reserved.

Experimental determination of the complete spin structure for (p)over-barp -g(Lambda)over-bar Lambda at p((p)over-bar)=1.637 GeV/c

The reaction anti-proton + proton -> anti-\Lambda + \Lambda -> anti-proton + \pi^+ + proton + \pi^- has been measured with high statistics at anti-proton beam momentum of 1.637 GeV/c. The use of a transversely-polarized frozen-spin target combined with the self-analyzing property of \Lambda/anti-\Lambda decay allows access to unprecedented information on the spin structure of the interaction. The most general spin-scattering matrix can be written in terms of eleven real parameters for each bin of scattering angle, each of these parameters is determined with reasonable precision. From these results all conceivable spin-correlations are determined with inherent self-consistency. Good agreement is found with the few previously existing measurements of spin observables in anti-proton + proton -> anti-\Lambda + \Lambda near this energy. Existing theoretical models do not give good predictions for those spin-observables that had not been previously measured.

The COMPASS Polarized Target in 2006 and 2007

The COMPASS experiment has been taking data since 2002. Its polarized target was upgraded during the 2005 CERN SPS shutdown. With the high acceptance magnet we obtained +56.0% and −53.0% deuteron polarization in 6LiD. In 2007 ammonia is used as a proton target which has a relaxation time of ∼4000 h at 0.6 T.

Measurement of spin-transfer observables in pp → ΛΛ at 1. 637 GeV/c

Spin-transfer observables for p p-->Lambda Lambda have been measured using a transversely polarized frozen-spin target and a beam momentum of 1.637 GeV/c. Current models of the reaction near threshold are in good agreement with existing measurements performed with unpolarized particles in the initial state but produce conflicting predictions for the spin-transfer observables Dnn and Knn (the normal-to-normal depolarization and polarization transfer), which are measurable only with polarized target or beam. Measurements of Dnn and Knn presented here are found to be in disagreement with predictions from these models.

Experimental determination of the complete spin structure for anti-proton + proton -> anti-\Lambda + \Lambda at anti-proton beam momentum of 1.637 GeV/c

The reaction anti-proton + proton -> anti-\Lambda + \Lambda -> anti-proton + \pi^+ + proton + \pi^- has been measured with high statistics at anti-proton beam momentum of 1.637 GeV/c. The use of a transversely-polarized frozen-spin target combined with the self-analyzing property of \Lambda/anti-\Lambda decay allows access to unprecedented information on the spin structure of the interaction. The most general spin-scattering matrix can be written in terms of eleven real parameters for each bin of scattering angle, each of these parameters is determined with reasonable precision. From these results all conceivable spin-correlations are determined with inherent self-consistency. Good agreement is found with the few previously existing measurements of spin observables in anti-proton + proton -> anti-\Lambda + \Lambda near this energy. Existing theoretical models do not give good predictions for those spin-observables that had not been previously measured.

Measurement of spin-transfer observables in p p-->Lambda Lambda at 1.637 GeV/c.

Spin-transfer observables for p p-->Lambda Lambda have been measured using a transversely polarized frozen-spin target and a beam momentum of 1.637 GeV/c. Current models of the reaction near threshold are in good agreement with existing measurements performed with unpolarized particles in the initial state but produce conflicting predictions for the spin-transfer observables Dnn and Knn (the normal-to-normal depolarization and polarization transfer), which are measurable only with polarized target or beam. Measurements of Dnn and Knn presented here are found to be in disagreement with predictions from these models.

Recent developments in the field of polarized solid state target materials

With the help of systematic EPR and NMR studies under various conditions the polarization properties of some established target materials could be considerably improved. In 6LiD preliminary polarization results of the still ongoing investigations are 52%1 at 150 mK and 18% at 1 K both at a magnetic field of 2.5 T. Radiation doped d-butanol was polarized up to 48%1 and 13% under the same conditions. Additionally first investigations were started into a group of materials, the alkanes, which have not been tested for polarization purposes so far. Difficulties specific to these compounds could be overcome with the result of promising +16.2%/−13.5% at 1 K/2.5 T in a hybrid sample of chemically doped n-pentane/butanol. In this context new insights could be also gained into the polarization physics of radiation doped polyethylene.

The Bochum Polarized Target

The Bochum ‘Polarized Target’ group develops the target material 6LiD for the COMPASS experiment at CERN. Several different materials like alcohols, alcanes and ammonia are under investigation. Solid State Targets are polarized in magnetic fields higher than B=2.5T and at temperatures below T=1K. For the Dynamic Nuclear Polarization process, paramagnetic centers are induced chemically or by irradiation with ionizing beams. The radical density is a critical factor for optimization of polarization and relaxation times at adequate magnetic fields and temperatures. In a high sensitive EPR—apparatus, an evaporator and a dilution cryostat with a continuous wave NMR—system, the materials are investigated and optimized. To improve the polarization measurement, the Liverpool NMR—box is modified by exchanging the fixed capacitor for a varicap diode which not only makes the tuning very easy but also provides a continuously tuned circuit. The dependence of the signal area upon the circuit current is measured and it i...