A PION Beam FACILITY FOR EXPERIMENTS AT SIS

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The secondary beam is produced from primary high energy (2 A GeV) Carbon beam particles at a rate of 108 particles/s impinging on a low-Z material. Thus the secondary beam facility is composed from two major items: The production target and the collection and transport line. The production target is a set of Beryllium targets with thick nesses ranging from 2 to 18.4 g/cm2 machined to a cylindrical shape 100 mm length and 7 mm diameter. The primary beam energy loss corresponds to a heat dissipation of about 25 Watt cooled by a water heat exchanger coupled to the target holder by a copper rod. A maximum of seven target places are available. Insertion and removal of the target is remotely controlled. The target is surrounded from an effective radiation shield from blocks of iron and concrete. The construction is made to allow for removal of the highly radioactive target for exchange. The pion accumulation and transport beam line of length of 33 m is an achromatic symmetric arrangement of two dipole and four quadrupole doublet magnets (QDQQDQ) resulting in a beam spot of 1 cm in diameter. The geometrical and momentum acceptance reaches 25 mst%, the momentum acceptance amounts to 10 %. The dispersive middle (delta x/(delta p/p) = -9 mm/%) allows for momentum selection using tracking detectors. The Br value of the beam line can be chosen to select the central momentum within a momentum range from 0.4 GeV/c to 2.8 GeV/c. The pion beam yield curves show a maxima of of 5*10(6) to 10(7) at around 1 MeV/c for 2 A GeV Carbon primary beam and at around 2 GeV/c for 3.5 GeV proton beam. For high pion momenta the proton beam is more advantageous by a factor of 5 in intensity. Due to their decay the loss of pions is of about 55%. The electron yield stemming mostly from pion decay in flight reaches a maximum of about 2*10(5) at around 0.6 GeV/c. For pion proton reactions a liquid hydrogen target is being built. Its cold head is presently constructed for use in the HADES di-lepton spectrometer, where the target area can be accessed only along the beam axis. Cooling of the target is made in a feed-through mode with transport tubes coupled to the dewar. The secondary pion beam facility has a potential for hadron physics research with pion induced nuclear reactions, with the advantage of less background as compared to proton induced reactions for the fundamental processes and access to the low mass region in the time-like form factor which is kinematically forbidden for proton induced reactions. This research includes particular reaction channels like the recoil-free production of omega vector mesons. This gives access to measure hadronic in-medium mass modifications at normal nuclear matter density. Pion reactions probe fundamental channels important for the calculation of the multi-channel reaction dynamics of high-energy nucleus collisions. The secondary electron beam can excellently be used for test measurements of complex detectors and detector development. As a total the secondary pion and electron beam facility is a service device to be used from public research institutes as well as for industrial R&D work.

The secondary beam facility can be equipped with a secondary beam monitoring system on choice: -Time of flight and tracking hodoscopes. -Laser monitoring system. -Experiment target detector. -Scintillating fiber array. -Transmission threshold Cherenkov detector. An additional momentum measurement system is based on an event-wise tracking of pion trajectories. Two hodoscopes H1 and H2 are placed in the dispersive area of the transport line. There the secondary beam is defocused and distributed on a large cross section with low rate density per detector area. The scintillator hodoscopes are constructed from 16 rod of BC404 plastic sintillator of 1cm width and 0.5cm thickness coupled on both sides via Plexiglas light guides to Hamamtsu R3578 photomultipliers for light read out. For high rate ability (10(6) per sec) especially designed active transistor bases are used for the photomultipliers. The time resolution achieved between H! or H2 and a similarly constructed hodoscope H3 at the experimental target site increases from 110 ps (sigma) at low beam intensities to 150 ps at intensities equivalent to rates of 2GHz. At this resolution, pions can be discriminated from electrons by different time-of flight for pion momenta below 1.5 GeV/c. The detection efficiency of the hodoscopes ranges between 96 to 99% for the different scintillator rods independent from momentum of the pions. The momentum resolution achieved by tracking with H1 and H2 is around 0.4 %. The hodoscopes are equipped with a laser source sending light pulses to each photomultiplier tube through quartz fibres with a three-fold purpose: time reference, monitoring of the pulse electronics, and for off-line timing drift corrections. A N2 pulsed laser of 3 mW power radiating UV light at 337nm is used. A trigger for laser events is implemented by means of a reference photomultiplier tube. The direct laser light is fed into four 1 mm optical fibres which in their turn are split into bunches of 16 quartz fibres with polished faces. Intensity adjustment is made by varying the distance of fiber to the photomultiplier in the coupling device. The nuclear reaction experiments need fast start detectors for identifying the time of an incoming particle. Depending on the intensity and the type of the beam various devices are available: a 1 cm thick scintillator detector coupled to a photomultipler for rates < 10(6) and a radiation-hard CVC diamond strip detector for higher rates and high-Z beam particles. An array of 48 BCF92Bicron scintillating fibres read out in groups of 16 fibres using position sensitive Hamamatsu photomultiplier tubes can be places downstream near the experimental target position for reconstruction of the focus of secondary beams. The sensitive surface of this detector is 6.4 x 3.2 cm(2) and the spatial resolution is about 2 mm. For use of the secondary electron beams for detector tests a transmission gas threshold Cherenkov detector has been built. Mounted upstream from the test stand it is used to identify the electrons in the dominant hadronic secondary beam. The detector consists of a 193cm long radiator section separated from the vacuum of the beam line by 100 micrometers Titanium foils. A mirror positioned under 45 degrees to the beam axis intercepts the Cherenkov light cone and reflects it to Hamamtsu R2059-01 photomultipliers. The mirror consists of 20micrometres/cm(2) Aluminum on a 0.8 mm thick glass substrate, its surface being protected by 12micrograms/cm(2) MgF(2). For pure N2 radiator gas (refractive index n = 1.0003) the threshold velocity v/c = 0.0007 corresponding to a gamma-factor close to 40 or an electron momentum around 1 GeV.

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