Abstract
A 2.5m long liquid hydrogen target has been developed for the COMPASS experiment at CERN to investigate the nucleon spin structure via the Deeply Virtual Compton Scattering (DVCS) process. To recognize exclusive DVCS events, produced photons and slow protons need to be detected. In order to do so, the material budget around the target has to be minimal. A 0.125mm thick Kapton® target cell and a 1mm thick carbon fiber vacuum chamber with a Mylar® window have been constructed and tested. Finally, the target system was successfully employed during the DVCS pilot run in COMPASS at the end of 2012. The objective of this paper is to give a detailed description of this newly developed liquid hydrogen target apparatus.
Highlights
A 2.5 m long liquid hydrogen target has been developed for the COMPASS experiment at CERN to investigate the nucleon spin structure via the Deeply Virtual Compton Scattering (DVCS) process
The target apparatus comprises of a long cylindrical target cell with a diameter of 40 mm, a vacuum chamber with an end cap, a refrigerator system consisting of a cryocooler with a cooling power of 30 W at 20 K and five 1000 l hydrogen tanks
It is made of 0.125 mm thick Kaptons sheet and its end cap is a hemisphere made of 0.125 mm thick Mylars
Summary
The RPD, which surrounds the target, has a large polar and full azimuthal angular acceptance of proton detection. The ECAL0 is positioned directly downstream of the target and enlarges the polar angular acceptance for photon detection. The new target apparatus was installed and commissioned together with the RPD and an ECAL0 prototype in the COMPASS spectrometer for the pilot run of 2012, which lasted 45 days.
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