This papers describes the alignment of the West Area Neutrino Beam Line at CERN to the two neutrino experiments CHORUS and NOMAD. The T9 neutrino (n) target position and the position of the magnetic horn were optimised using the secondary muon intensity profiles from the muon pits in the shielding. In the experiments the improved geometry provides a better centred beam (< 5 cm) and a measured increase in the n flux of 8%

Catanesi, M G

de Rijk, G

Falaleev, V P

Guglielmi, A M

Palladino, Vittorio

Plothow-Besch, Hartmute

Péraire, S

Tovey, Stuart N

Tsesmelis, E

Winton, L J

CERN Document Server

EUROPEAN LABORATORY FOR NUCLEAR RESEARCHCERN - SL DIVISIONCERN-SL-96-24 OPCERN WEST AREA NEUTRINO FACILITYBEAM LINE ALIGNMENTG. Catanesi, Bari Univ., V. Falaleev, S. Péraire, G. de Rijk, E. Tsesmelis, CERN,H. Plothow-Besch, Dortmund Univ., S. Tovey, L. Winton, Melbourne Univ.,V. Palladino, Naples Univ., A. Gugliemi, Padova Univ./INFNAbstractThis papers describes the alignment of the West Area Neutrino BeamLine at CERN to the two neutrino experiments CHORUS and NOMAD.The T9 neutrino  ( n ) target position and the position of the magnetic hornwere optimised using the secondary muon intensity profiles from themuon pits in the shielding. In the experiments the improved geometryprovides a better centred beam (< 5 cm) and a measured increase in the nflux of 8%.Paper presented at the 5th European Particle Accelerator Conference (EPAC’96),10-14 June 1996, Sitges (Barcellona), Spain.Geneva, Switzerland26 June, 1996CERN West Area Neutrino Facility Beam Line AlignmentG. Catanesi, Bari Univ., V. Falaleev, S. Péraire, G. de Rijk, E. Tsesmelis, CERN,H. Plothow-Besch, Dortmund Univ., S. Tovey, L. Winton, Melbourne Univ.,V. Palladino, Naples Univ., A. Gugliemi, Padova Univ./INFNAbstractThis papers describes the alignment of the West AreaNeutrino Beam Line at CERN to the two neutrinoexperiments CHORUS and NOMAD. The T9 neutrino( n ) target position and the position of the magnetic hornwere optimised using the secondary muon intensityprofiles from the muon pits in the shielding. In theexperiments the improved geometry provides a bettercentred beam (< 5 cm) and a measured increase in the nflux of 8%.1 INTRODUCTIONThe  CERN West Area Neutrino Facility (WANF) [1],has been in operation for almost 20 years. A majorreconstruction [2] of the line took place in 1992 and 1993for a new round of nm-> nt oscillation experimentsCHORUS [3] and NOMAD [4]. Part of the proton beamline and the neutrino 'cave' were completely dismantledand cleared. A new target station with improved coolingand a new small angle collimator with highly improvedradiation containment properties were installed. Thefocusing elements (the magnetic horn and reflector) weredisplaced downstream by about 8 m in order to provide aharder neutrino beam spectrum [5]. Helium tubes, of totallength 80 m, replace wherever possible air in the focusingregion before the decay tunnel and a new large areacollimator was installed to reduce the ‘n  contamination.A n  beam monitoring system ('the muon pits') [1],based on the detection of muon yields at several depths inthe iron shield, was built in the line from the beginningof its operation. A reduced version of that system is stillin use today with new or rejuvenated solid state detectors.The hardware and software of the data acquisition  andcontrol system was replaced by a more modern state-of-the-art configuration [6]. The beam line has been nowoperating for 2 years at record proton intensities, up to~ 3•1013 protons on target (pot) per 14.4 s acceleratorcycle.In order to make best use of the very high protonintensities necessary for the neutrino oscillation searches,a complete re-tuning of the upgraded neutrino beam linewas necessary. BCTSEMsupstreamSEMsdownstreamTargetAl CollimatorHorn ReflectorDecay tunnel Iron shieldMuon pits Earth shieldCHORUSNOMADCu CollimatorFigure 1. Schematic lay-out of the WANF beam line (not drawn to scale)2BEAM LINE, DATAACQUISITION AND ANALYSIS2.1  Beam LineProtons are extracted from the SPS ring on 2 spillsduring a SPS proton cycle. The extraction is a fast/slowextraction on a combined 2nd-4 th order resonance, with anominal length of 6 ms per spill [7]. The proton beam istaken up to the T9 neutrino production target by the TT66beam line. The beam is focused on the target such that thebeam spot is circular with a 2 mm base-to-base width.The target consists of 11 Be rods of 3 mm diameter and10 cm length, spaced by 9 cm voids [8]. Total chargedparticle fluxes and the transfers beam profiles areconstantly measured with several types of detectors.• Beam Current Transformers are used to measure theproton intensity at two places in the proton beam line.• Secondary Emission Monitors are used to measurethe proton intensity in front of the target, the multiplicityof charged particles behind the target and the profile of theproton beam in front of the target.• Solid state detectors are used to measure the totalmuon flux and the profile inside voids in the muonshielding. A schematic lay-out of the beam line can befound in Figure 1.2.2  Data AcquisitionThe data coming from the beam line are acquired by 4users: proton beam operations, the WANF control systemand the experiments CHORUS and NOMAD. The protonbeam properties are written on an online ORACLEdatabase. The WANF control system writes all protondata and the data from the solid state muon detectors ontodisk. The experiments write summary information of aspill onto their raw data tapes. Displays are availableonline in the SPS control room and the experimentcontrol rooms.2.3 Optimisation and Analysis MethodsThe parameters to be optimised are:• The beam direction in horizontal and vertical suchthat the n  beam is centred on the CHORUS andNOMAD detectors.• The target position and angle with respect to theincoming proton beam such as to maximise thequantity of protons interacting with the target.• The horn position such that only a focusing effect isachieved on the positive secondaries.The sequence of actions is to optimise the beam lineis as follows. First, the target position and angle arescanned and the charged particle multiplicity as measuredwith the SEM detectors is maximised. Second, it ischecked that the above optimisation also maximised thepeak of the muon flux distribution in the muon pitshorizontaly and verticaly. For this, in both planes, aGaussian fit on readings of the individual muon detectorsin a pit is done and the peak value extracted. Third, theposition of the horn is scanned and the peak of the muonflux distribution in the muon pits is sought to bemaximum. Additionally, these distributions should becentred onto the muon pits. Assuming that the alignmentbetween muon pits and experiments is correct this willalso optimise the properties of the n  beam in theexperiments. Fourth, it is checked that the n  distributionsin the experiments are centred by looking at the spatialdistribution of the n  interactions in the CHORUS andNOMAD detectors.3ALIGNMENT OF TARGET,BEAM AND FOCUSING ELEMENTS3.1  Alignment  of  Target  with  BeamThe first step was the optimisation of the targetposition and angle with respect to the beam using thesecondary particle multiplicity as measured with theupstream and downstream SEMs. Maximum secondaryparticle multiplicities of about 18 were found for:xtarget = 2.8 mm ytarget = 0.3 mmq x = 0.0 mrad q y = 0.0 mrad.In Figure 2 the dependence of the SEM multiplicityon the target position is shown.Figure 2. The dependence of the SEM multiplicity on thetarget position relative to the beam for parallel horizontal(top) and vertical (bottom) displacement.The second step was to check the muon flux in themuon pits for these scans. The optimum for the muonpits coincide nicely with that found with the SEMmultiplicities. In Figure 3 this is illustrated for thehorizontal plane.Figure 3. Peak muon flux per pit ( m /cm2/1010 pot) as afunction of the target horizontal position relative to thebeam.3.2  Alignment  of  Horn  and  Reflector  withthe Target  and  BeamThe third step consisted of aligning the horn to thebeam, with the target at the optimum position as found inthe previous step. A misalignment of the horn will reducethe muon flux in the muon pits and displace thedistribution there. This is due to the net bending effect ofa mis-aligned focusing element. The reflector is notmotorised so no scan could be performed on it. During thehorn scan it was switched off. The muon profiles shiftwhen the horn is switched on, indicating possible mis-alignment of the horn. The horn can be moved by – 2.5mm in both directions. The first scan indicated anoptimum at:xhorn = -2.5 mm yhorn = -2.0 mm.This could indicate that the optimum might be atlarger values. The beam and target were thus movedtogether in parallel by 3 mm in both planes. After whichthe scan was repeated. In this new situation the optimumposition of the horn was found to be:xhorn = -1.5 mm yhorn = -2.0 mm.At the optimum beam/target/horn positions the peakmuon flux in pit 2 was about 3% higher than before witha much better centred profile. Switching on the reflectorprovides a better centred beam suggesting that thiselement has the correct alignment. The muon profiles forthree situations: before scan, after the scan and with thereflector switched on again, are summarised in the Tables1,2 and 3.<x> cm <y> cm peak flux/cm2/1010potpit 1 -4.8 9.6 24012pit 2 -5.0 9.5 7577pit 3 -1.3 8.7 1596Table 1. Muon profile parameters with nominalbeam/target/horn positions and reflector off.<x> cm <y> cm peak flux/cm2/1010potpit 1 1.7 2.4 25560pit 2 1.2 2.1 7780pit 3 3.3 1.5 1600Table 2. Muon profile parameters with optimumbeam/target/horn positions and reflector off.<x> cm <y> cm peak flux/cm2/1010potpit 1 0.9 0.3 46847pit 2 1.2 0.2 12136pit 3 3.1 0.6 2289Table 3. Muon profile parameters with optimumbeam/target/horn positions and reflector on.4NEUTRINO FLUX IN THEEXPERIMENTSThe result of this alignment operation is an 8%increase in n  flux in the experiments and a better centredn  beam in the detectors. The <xn> improved from-5,5 – 1.1 cm to 2.6 – 0.6 cm, the <yn> improved from14.7 – 1.2 cm to 3.8 – 0.5 cm. In Figure 4 this isillustrated for the NOMAD detector.Figure 4. Distribution of the horizontal and vertical ninteraction position in the ECAL of NOMAD. Thebroken line are the data before, the full line after the beamline optimisation. The Gaussain fit is superimposed onthe optimised distribution.5 CONCLUSIONSA successful alignment of the CERN n  beam line wasperformed. A 8% gain in neutrino yield and a bettercentred n  beam on the detectors were achieved.REFERENCES[1] E.Heijne, CERN Yellow Report, CERN 83-06[2] G.Acquistapace et al., CERN Internal Note, CERN-ECP 95-14[3] CHORUS Proposal, CERN-SPSLC/90-42;CHORUS Collaboration, M. de Jong et al., CERN-PPE/93-131;CHORUS Collaboration, G. Rossa et al., Nucl.Phys. Peoc. Suppl. B40 (1995) 85[4] NOMAD Collaboration, J.Altegoer et al., in Proc. ofIntern. Europhysics Conf. on H.E.P., Brussels 1995,(in Press);NOMAD Collaboration, A. Rubbia, Nucl. Phys.Proc. Suppl. B40 (1995) 93[5] V. Palladino, CHORUS Internal Report, March 1994[6] H. Butler et al., CERN Internal Note, CERN-ECP93-22, November 1993[7] M. Gyr et al. Proc. EPAC Berlin (1992) 1501[8] S. Péraire et al, these proceedings.

CERN West Area neutrino facility beam line alignment

CERN West Area neutrino facility beam line alignment

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