Calorimeter Electronics Jim Pilcher 11-Dec-2008

December 11, 2008J. Pilcher2 Introduction Calorimeters essential for energy measurement in particle physics Detection over a wide range of energies Incident particles deposit their energy in a medium Tank of liquid (water or scintillator), dense medium(iron/scintillator), air Can be very large (esp. for neutrinos and cosmic rays) In many calorimeters optical EM radiation is produced Electronics converts this signal to digital information For signal processing to calculate energy, time, quality of the measurement

December 11, 2008J. Pilcher3 Introduction SNO Calorimeter Detect Cerenkov radiation from interactions of ~ 10 6 eV neutrinos Also calorimeters in Double Chooz, Daya Bay, Minos, MiniBoone

December 11, 2008J. Pilcher4 Introduction Collider detectors calorimeters (to E ~ eV) Example for ATLAS Calorimeters But also ALICE, CMS, LHCb, CDF, D0 Will not discuss noble liquid calorimeters today Tile barrel Tile extended barrel LAr forward calorimeter (FCAL) LAr EM end-cap (EMEC) LAr EM barrel LAr hadronic end-cap (HEC)

December 11, 2008J. Pilcher5 Introduction Auger observatory (to E ~ ev) Detect fluorescence radiation from air showers induced by cosmic rays  Calorimeter absorption medium is the earth’s atmosphere Detect muons from decay of hadrons produced in these air showers

December 11, 2008J. Pilcher6 Photo-detectors Role is to convert optical photons to electrical signal Photons from Cerenkov radiation or scintillation Amplify the number of electrons if possible Many types See proceedings from Beaune Conference 2005 in NIMA 567 (2006) In this talk I touch on a few important types Photomultipliers, Hybrid Photo Diode (HPD), Silicon PMT(SiPMT)  Choice depends on necessary area coverage, spatial segmentation, operation in magnetic field

December 11, 2008J. Pilcher7 Photomultiplier Tubes (PMTs) As used in ATLAS hadron calorimeter Ideal current source Operated at gain of 10 5

December 11, 2008J. Pilcher8 Photomultiplier Tubes (PMTs) Very diverse device Many sizes and shapes Much experience from years of usage and development Wide range of gains (~ 10 4 to ~10 7 )  Set by operating voltage and number of amplifying stages The ATLAS device is 10-year-old design  Now quantum efficiencies over 40%  Segmented anodes (eg. 8x8)

December 11, 2008J. Pilcher9 The Physics of PMTs Design principle Amplification by electron acceleration in electric field and secondary emission from a metallic dynode Gain =  V  V is operating voltage  is ~ number of stages Fast devices signal rise time ~1ns transit time ~10ns

December 11, 2008J. Pilcher10 Issues to consider for PMTs Gain stability Depends on geometry of dynodes Emission coefficients of photocathode and dynodes Requires careful control of temperature and HV  ATLAS PMT have  G/G ~ 0.2% /  C  1% / V Magnetic field sensitivity Low energy secondary electrons are easily deflected Must use magnetic shielding in magnetic fields  Even for earth’s field  ATLAS PMTs shielded for axial B ~ 200 G (for 1% variation) After-pulsing Electron interaction with residual gas in tube Secondary signal correlated in time with primary

December 11, 2008J. Pilcher11 Hybrid Photodiodes Used for CMS hadron calorimeter Must operate in strong magnetic fields CMS HCAL designed for ambient field of 4T Need for segmented anode Limited operational experience with large systems of HPDs

December 11, 2008J. Pilcher12 The Physics of HPDs Incident photons produce electrons in photocathode Electrons accelerated in electric field (few kV) Energetic electrons are strike the silicon Produce hole-electron pairs and hence current in reversed biased diode (gain) CMS device operates at 12 kV for gain of 2500 Can segment silicon structure for spatial information Photons  Electrical signals 

December 11, 2008J. Pilcher13 Issues to consider for HPDs Must operate at high voltage without sparking Cross talk can occur between readout pixels Sensitivity to variations in magnetic field direction modifies electron trajectories

December 11, 2008J. Pilcher14 Silicon Photomultipliers Development to useful devices in recent years Just emerging from R&D stage Pixels of ~ 30  m  ~ 30  m formed on a common silicon substrate Typical sensitive area 1 mm  1 mm ~ 1000 photosensitive pixels

December 11, 2008J. Pilcher15 Physics of Silicon Photomultipliers Photons produce hole-electron pairs in Si Quantum efficiency > 70% Applied voltage of ~ 50 V produces high local electric fields because of tiny size of structure Results in gains of 10 6 A saturated Geiger mode response (digital) for each pixel struck by photon Signal from all pixels is on a common output terminal Can count number of photons detected from size of signal

December 11, 2008J. Pilcher16 Issues for Silicon Photomultipliers Sensitive area is small Could use one per fiber Photon detection efficiency ~ 50% because of geometric filling efficiency of pixels Dark count rate is high ~ 300 kHz Still limited experience in large systems CALICE calorimeter for linear collider R&D uses them

December 11, 2008J. Pilcher17 Pulse Shaping Signal from photodetector is often conditioned prior to digitization In some applications the photodector signal is digitized at very high rate (Gsps) to record the “wave form” as in an oscilloscope  I will not discuss this option here Make pulse shape insensitive to timing variations in light collection  Fine details of waveform from photodetector may not be necessary for measurement of interest Standardized shape facilitates time measurement  Can take multiple samples of the signal to allow estimate of time –Energy and time measurement can be made with the same hardware –Sub nanosecond time measurements are possible  Digital signal processing Match bandwidth of PMT signal to bandwidth of ADC  Higher speed ADC increases power and cost –10-year-old ATLAS design was close to practical limit at the time –Higher speed now practical if needed (eg sLHC readout)

December 11, 2008J. Pilcher18 Pulse Shaping ATLAS example Very similar solution in Auger front-end electronics and E14 experiment at J-PARK Shaper integrates input pulse and produces standardized shape determined by electronics components  Output pulse area proportional to charge of input pulse  If output shape is invariant then its amplitude is proportional to input charge  Can make a very linear system

December 11, 2008J. Pilcher19 ATLAS TileCal Pulse Shaping Photomultiplier is a near-ideal current source Current is very insensitive to impedance load on anode This allows a shaper built with fully passive elements (LC filter)  Very low noise since no active components and very low resistance TileCal shaper is a 7-pole Bessel filter Can produce near-Gaussian output shape for digital sampling

December 11, 2008J. Pilcher20 ATLAS TileCal Pulse Shaping The circuit Followed by buffer amplifiers to set gain and limit maximum signal Use two gain ranges for increased dynamic range

December 11, 2008J. Pilcher21 Pulse Shaping Issues slightly different in the noble liquid calorimeter Denis will describe Tradeoff between long shaping time for low electronic noise and short shaping for low noise from multiple interactions within time window

December 11, 2008J. Pilcher22 Digitization Can follow shaper with sampling ADC For multiple samples of the shaped waveform  Extract, energy, time, quality factor for pulse shape Intense commercial development of ADCs for high volume applications  Medical instrumentation, cellular phone systems  Maxim, Analog Devices, Linear Technology, Texas Instruments Dramatic improvements in performance with time  Smaller feature size –Less power –Higher speed –Lower cost Sampling rate may be determined by application  At LHC sample at 40 MHz bunch crossing rate Required dynamic range may necessitate multiple gain scales

December 11, 2008J. Pilcher23 Digitization Now lots of choice for ADC e.g. Maxim offers bit ADCs capable of sampling at 40 Msps or greater Similar for other manufacturers Let’s examine relevant characteristics Example, MAX1126 (4 ADCs in one package)

December 11, 2008J. Pilcher24 Digitization Tiny device Difficult to probe with scope Optimized PCB design is essential to realize its performance Generally pipelined, multistage ADCs Sample every clock cycle Deliver digital output 7 cycles later  7 internal ADCs working in parallel

December 11, 2008J. Pilcher25 Digitization Pipeline

December 11, 2008J. Pilcher26 ADC Limitations These are NOT ideal devices Might expect resolution of step size / sqrt(12) if all steps uniform Step sizes NOT uniform  Differential non-linearity This is a very well behaved design

December 11, 2008J. Pilcher27 ADC Limitations Also, deviations of response from straight line fit Integral non-linearity This is a very well behaved design Should not rely on averaging ADC measurements to an accuracy better than 1 count

December 11, 2008J. Pilcher28 Data Flow ADCs produces 2 x 12 bits (if dual range system) every 40 MHz 960 Mbps Essential to filter this to extract signal of interest May require fast digital link to transport data to digital signal processor A major challenge Systems may have 1000’s of channels

December 11, 2008J. Pilcher29 Conclusions Modern readout electronics capable of high dynamic range measurements Dual range system can give 16-bit dynamic range Can measure timing with same hardware as energy Can profit from commercial pressures to improve ADCs and optical data links