The response to high magnetic fields of the Vacuum Phototriodes for the Compact Muon Solenoid endcap electromagnetic calorimeter K W Bell1, R M Brown1, D J A Cockerill1, P S Flower1, P R Hobson2, D C Imrie2, B W Kennedy1, A L Lintern1, O Sharif2, M Sproston1, J H Williams1 1CLRC - Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX UK 2Brunel University, Department of Electronic and Computer Engineering, Uxbridge, UB8 3PH, UK Poster presented at the 3rd Beaune Conference on New Developments in Photodetection Beaune, France, 17-21 June, 2002
Motivation • The endcap electromagnetic calorimeter of the Compact Muon Solenoid detector at the LHC needs fast, radiation tolerant photodetectors with moderate gain. • Neutron radiation levels, particularly as , are too high for 10 year operation of silicon photodetectors. • The photodetectors can be closely aligned with the axial 4T magnetic field inside CMS. • Photodetectors need to cover the end face of lead tungstate scintillator crystals, these will allow 26 mm diameter devices. • Lead tungstate emission spectrum is well matched to bialkali photocathodes. Solution: Vacuum Phototriodes (VPT)
Compact Muon Solenoid Electromagnetic Calorimeter 7 TeV protons Superconducting coil Total mass : 12,500t Overall Diameter: 15.0m Overall Length: 21.6m Magnetic field: 4T
ECAL design objectives Benchmark physics process: Search for ~130 GeV Higgs via H (Sensitivity depends critically on mass resoln) m /m = 0.5[E1/E1 E2/E2 / tan(/2)] WhereE/E = a/E b c/E Performance Aims: BarrelEnd cap Stochastic term, a: (p.e. statistics/shower fluctuation) 2.7% 5.7% Constant term, b: (non-uniformities, shower leakage) 0.55% 0.55% Noise term, c: (Electronic noise, event pile-up) Low L 155MeV 205MeV High L210MeV 245MeV (Angular resolution limited by uncertainty in position of interaction vertex)
=26.5 mm MESH ANODE Vacuum PhotoTriodes • Endcap B-field orientation favourable for VPTs • (Axes: 8.5o < || < 25.5o wrt to field) • More radiation hard than Si diodes • (with UV glass window) • Gain 8 -10 at B = 4 T • Active area of ~ 280 mm2/crystal • Q.E. ~ 20% at 420 nm RIE production VPT
Light Principles of VPT operation Primary photoelectron Photocathode 10 µm pitch mesh anode (1000V) Dynode (800V) Dynode gain is ~ 20 but collection efficiency is about 50% Typical tube gain is ~10
Magnet characterisation Multiple VPT holders inserted into full field of warm iron magnet 1.8T Dipole Magnet at RAL All VPTs are measured at 0 B 1.8T and -30o 30o at RAL A sample of VPTs are measured at B =4.0T and = 15o at Brunel 4.0T Superconducting solenoid at Brunel
0 to 1.8T System at RAL • Tubes normally measured at angles from -30° to +30° • Relative response measured at fields from 0T to 1.8T • Noise and dark currents measured too. • Great care taken to ensure uniform photocathode illumination with blue LED • Multiple tubes measured simultaneously (currently 24 per run) • DAQ system is RS232 based and uses LabView to provide the control and user interface • ADC system is CAMAC based
DAQ Card NI 6033E PCI bus Step Motor Maclennan RS 232 4T System at Brunel Cathode current Anode current Automated VPT tester at Brunel. Mainly uses IEEE488.2 instruments. ADC is mulitplexed and PCI based. VPT and Pre-amp
Response vs Angle at B=1.8T Very reproducible distribution Arrows indicate angular regions of end caps
Response vs B-Field Strength VPT Axis at 15o angle to the magnetic field Note: the precise details of how much reduction in relative response depends on the uniformity of photocathode illumination.
Response at 4T and 15° Measurement repeatability Of 4T/0T gain ratio on pre-production tubes was good
Anode Response Distribution Data plotted to show expected response to scintillation light as a function of incident electron energy on the PbWO4 crystal Production tubes which will be used in CMS Mean anode pulse height over the angular range 8°-25° in a 1.8T magnetic field.
Summary • A new generation of fine-mesh VPTs has been developed to satisfy the high magnetic field/radiation hardness requirements of CMS operating at the Large Hadron Collider, CERN • An automated characterisation facility based at Brunel and RAL has been commissioned to handle 15000 devices over three years • The performance of over 800 of the 1400 production VPTs from RIE has been measured to date. • Nearly all tubes pass the acceptance tests. • Only one tube passing the 1.8T tests failed at 4T We would like to acknowledge support from PPARC (UK) and INTAS (EU).