Magnetic Shielding Studies of the LHCb RICH Photon Detectors Mitesh Patel, Marcello Losasso, Thierry Gys (CERN )

Large Samples of b decays at LHC –N bb = / year with Luminosity 2×10 32 cm -2 s -1 for  ×10 7 s –b production predominately at small polar angle LHCb Experimental Goals Precision Measurements of CP Violation in b decays Many pure hadronic final states Particle identification (  /K) essential Employ two RICH detectors Use high efficiency photon detectors, sensitive to magnet fields This talk calculation of the residual field seen by photon detectors Example Decays LHCb optimized as single forward arm spectrometer

Two RICH detectors either side of dipole magnet Detectors experience stray field from the magnet Stray field largest in upstream RICH detector (RICH1) – talk will focus on this upstream detector LHCb Experiment RICH1 Z ~ m RICH2 Z ~ m Calorimeters Z ~ m Muon System Z ~ m Aerogel and C 4 F 10 CF 4 Dipole Magnet 4 Tm

Magnetic Requirements Must satisfy two conflicting requirements : |B|/ G z/cm B.dl > 100 kGcm Deliver field between Vertex Locator and Trigger Tracker (charged particle momentum determination for efficient triggering) Shield the RICH photon detectors from the field such that they operate efficiently LHCb Magnet Hybrid Photon Detector Magnetic shield developed that meets these requirements The LHCb Magnet

Magnetic Shielding in RICH 1 Primary Magnetic shield used to channel flux around the photon detector array –Shield made from high permeability iron : armco –Lab tests bare hybrid photon detectors (HPDs) operate efficiently up to ~15G –Simulations indicate residual field inside primary shield ~25G Use set of secondary shields (one for each photon detector) to reduce this to acceptable level –Shields made from high permeability material, baseline solution: Mu-metal –Alternative possibility : supranhyster-36 Primary Magnetic Shield Secondary shields/Photon detectors Readout electronics Beam pipe Spherical mirrors Flat mirrors

Now have a complicated magnetic environment : Armco primary magnetic shield Would like to check that, in this environment : Magnetic flux does not short circuit through the detector array Residual field inside shields acceptable (<10G) Field in secondary shields not close to saturation (7kG) Array of 14×7 individual secondary shields within this primary shield Finite element calculation Magnetic Model

Opera-3d software made by Vector Fields First measurements (RICH2) show good agreement simulation/data Model of Magnet and primary RICH1 shield uses up all elements software can cope with –Large volume – elements must extend into air far enough outside to encompass majority of field –Complicated conductor and shims Wish to introduce into this model elements to form ~1mm thick secondary shields – ~1mm thick shields ~1m magnet 4m 11m 2.5m

Sub-modelling Take complete model with (relatively) coarse meshing in region of interest Extract potential at nodes in region of interest Apply this potential to nodes of a part(ial) model (boundary such that don’t expect potential to change with introduction of secondary shields) The new part model is then solved separately (potential allowed to evolve except at boundary where fixed) –Part model then a much smaller volume –More elements available to model secondary shields complete model part model magnet primary shield nodes

Potential at boundary fixed perturbation from adding secondary shields inside the primary shield must not spread to edges Even with sub-modelling still limited by no. of elements rectangular rather than circular cross-section secondary shields Magnetic Potential : complete model Magnetic Potential : (complete model – part model) Without Mu-metal With Mu-metal

Build array of 14×7 shields, 0.9 mm thick with staggered geometry cf circular shields have 10% more material If shields close to saturation, fact have : Put this array inside part model of primary shield : Solve part model twice : With secondary shield elements made from mu-metal With the same elements but made from air may become relevant rather than P 0.866P P

Effect on Field at Detector Plane With mu-metal shieldsWithout mu-metal shields Field inside HPD volume falls from ~20G to <10G Field even lower in set of shields closest to internal shelf… X=0cmX=35cmX=0cmX=35cm 20G 0G

With mu-metal shieldsWithout mu-metal shields Side view shows same difference in |B| in tubes closest to internal shelf (White regions: field greater than 20G maximum) |B| / G Position / cm 20G 0G

With mu-metal shieldsWithout mu-metal shields (White regions: field greater than 20G maximum) Field has transverse component in the tube closest to shelf – very well shielded In all other tubes – field mostly longitudinal 0G 20G

Field highest inside shields at the centre of the array |B| / G Position / cm Peak field ~8G

Field in the Mu-metal Fields ≤2kG – well away from saturation (7kG) Field highest at bottom of array, closest to internal shelf and the side walls of the shield Peak field ~3.9kG |B| / G Position / cm

With mu-metal shields 0G 20G 0G With extended mu-metal shields Extending the Shield Protrusion 1cm - Field at HPD plane |B| / G Position / cm Peak field at HPD plane : 8GPeak field at HPD plane : 6G Essentially no change in flux channelled through Mu-metal

Changing Shield Material from Mu-metal to Supranhyster-36 Supranhyster-36 lower permeability material but expect similar shielding performance Supranhyster-36 has higher saturation value : 11kG (cf mu-metal, saturation at 7kG) gives more margin Model indicates that mu-metal is far from saturation so not critical No discernable change in field seen at HPD photocathode plane See similar peak field to that seen in the mu-metal With supranhyster-36 shields Position / cm

Conclusions Using an array of individual secondary shields around the photon detectors inside the primary RICH1 magnetic shield : Field at photon detector photocathode plane falls from ~20G to <8G B mostly longitudinal- apart from in set of shields closest to internal shelf Field channelled through Mu-metal shields ≤2kG, peak field 3.9kG – half way from saturating Extending mu-metal shields by 1cm 6G peak field instead of 8G Changing the material of the secondary shields to supranhyster-36 similar shielding efficiency/peak field in shields but saturation then at 11kG The magnetic environment in the (more sensitive) upstream RICH detector of LHCb is such that the photon detectors will operate efficiently A method of mapping the field in-situ and compensating for the effect of the residual field is being developed (see the talk of A. Van Lysebetten)

Require : Cover total area ~ 2.8 m 2 Single photon sensitivity Granularity ~ 2.5mm x 2.5mm Visible and near-UV sensitivity 25ns time resolution Photon Detector : The LHCb Pixel HPD Solution : The LHCb Pixel Hybrid Photon Detector Electron optics: Cross-focussed, demagnification ~ 5 Anode: 16×16mm 2 Si pixel detector, bump-bonded to 40MHz binary readout chip Pixel cell:62.5  m x 500  m:256 x 32 matrix Effective pixel size at photocathode: 2.5mm x 2.5mm: 1024 channels