The new frozen spin target at MAMI
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he New Frozen Spin Target at MAMI1, 2 A. Thomasa, N. S. Borisovb, H.J. Arendsa, A. N. Fedorovb, G. M. Gurevichc, R. L. Kondratievc, M. Korolijad, A. B. Lazarevb, M. Martineza, W. Meyere, S. V. Mironovb, A. B. Neganovb, V. N. Pavlovb, H. Ortegaa, G. Reicherzd, and Yu. A. Usovb a
Institut für Kernphysik, Universität Mainz, J.J. Becherweg 45, D55128 Mainz, Germany b Joint Institute for Nuclear Research, Dubna, 141980 Russia cInstitute for Nuclear Research, Russian Academy of Sciences, Moscow, 119312 Russia d Rudjer Boskovic Institute, Zagreb, Croatia eInstitut für Experimentalphysik I, RuhrUniversität Bochum, D44780 Bochum, Germany Abstract—The new frozen spin polarized target for experiments at the polarized beam of the real photon facility A2 of the MAMI accelerator is described. The A2collaboration at the Mainz Microtron MAMI is measuring photon absorption cross section using circularly and linearly polarized photons up to the energy of 1.5 GeV. The photons are produced in the ‘Bremsstrahlungs’ process. In the years 2005/2006 the Crystal Ball detector with its unique capability to cope with multi photon final states was set up in Mainz. Since 2010 the experimental apparatus has been completed by a polarized target. The horizontal dilution refrigerator of the FrozenSpin Target has been constructed and is operated in close cooperation with the Joint Institute for Nuclear Research in Dubna, Russia. The system offers the opportunity to provide longitudinally and trans versely polarized protons and deuteron. In this paper the operation experience of this new FrozenSpin Tar get and first results from the runs in 2010 and 2011 are presented. DOI: 10.1134/S1063779613060221 21
1. THE ‘CRYSTAL BALL AT MAMI’ 1.1. The MAMI Accelerator
The MAMI accelerator (see Fig. 1) with its source of polarized electrons, based on the photoeffect on a strained GaAs crystal, routinely delivers polarized beams with a maximum energy of 1608 MeV. We typi cally have a degree of polarization of about 85%. Details about the new machine type can be found in reference [1]. 1.2. The A2 Real Photon Facility The A2 Real Photon Facility is one of the three major experiments using the electron beam from the MAMI accelerator, see Fig. 2. The experiments A1 and A4 use the direct electron beam, while in A2 the electron beam is converted in a beam of photons in the “Bremsstrahlungs’ process. The electrons are used to produce a secondary beam of real photons in the ‘Bremsstrahlungs’—pro cess. The energy of these photons is detected in the Glasgow–Mainz taggingsystem [2]. 1 This
work was supported by the ‘Deutsche Forschungsgemein schaft’ (SFB 443), DFGRFBR (Grant No. 090291330), the European CommunityResearch Infrastructure Activity under the FP6 ‘Structuring the European Research Area’ program (Hadron Physics, contract number RII3CT2004506078). 2 The article is published in the original.
The resulting photons can be circularly polarized, with the application of a polarized electron beam, or linearly polarized, in the case of a crystallin
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