Photo Detectors in High Energy Physics

1. Photo Detectors in High Energy Physics Dieter Renker

2. Outline Various detectors will be presented that involve photo sensors. The problems will be pointed out and the realized solutions which have been found, will be described. Calorimeters: PMT, PIN diode, APD, G-APD Ring image Čerenkov detectors: Gas detector, Multi anode PMT, Hybrid PMT Time of flight measurements: PMT, MCP, G-APD Tracker: Multi anode PMT, VLPC

3. Photomultiplier tubes in calorimeters • PMTs can be made to cover large areas with up to 20 inch diameter (SuperKamiokande). • Shown here is the crystal ball with 3 inch tubes for the measurement of + 0+e++ • Challenges are: • large area • low light yield (pure CsI) • fast response  +

4. Photomultiplier tubes in calorimeters The amplification in the dynodes of a PMT has an extremely low level of noise. Summing over a large number of coincident PMT signals is therefore possible. Shown here is the 800 liter LXe calorimeter with 800 PMTs in the+ e++ experiment. The deposited energy is derived from the sum of all PMT signals and the position of the conversion from the distribution of the individual amplitudes. 100 liter prototype

5. PMT – some remarks PMT’s are a commercial product since 70 years. This is a long time for development and optimisation. Even so the progress during the last years is remarkable: the bulky shape turned into a flat design with very good effective area coverage and PMTs became position sensitive due to segmented (pixelised) anodes. Devices with segmented anodes with 256 pixels are available. In addition the prices came down considerably. Industry reduces the number of parts of the mechanically complicated dynode structure inside the vacuum container (some 40 pieces). To some extent the manual mounting of these parts is replaced by automated processes. The price per readout channel can be below 20 US$. The quantum efficiency is relative low (~25%) but new photo cathodes can reach 40 to 50% (GaAsP and Ultrabialkali). PMT’s with small transit time spread are available and allow a time resolution smaller than 100 ps. PMT’s are very sensitive to magnetic fields. Only some proximity focused types tolerate axial fields.

6. PIN photodiodes General purpose detectors need magnetic fields for the measurement of the momentum of charged particles. The PMTs have to be replaced by solid state devices. CLEO pioneered the use of CsI(Tl) crystals and PIN photodiodes in an electromagnetic calorimeter (7800 Crystals and 4 diodes/crystal). The QE of PIN photodiodes matches the emission wavelength (550 nm) of CsI(Tl) better than PMT’s. It is ~80%. Consequently the energy resolution is very good: <2% for 1GeV ’s. The PIN photodiode is a very successful device – all B-factories use them and L3, GLAST …

7. PIN photodiodes – problems • PIN photodiodes have no gain. The operation is very stable but they need a charge sensitive amplifier which makes the signal rise time slow and introduces noise to the system (CPIN ~80 pF/cm2). Calorimeters made of materials with low light yield (pure CsI in KTeV and Čerenkov calorimeters with lead glass) cannot use PIN photodiodes. • The full thickness of the PIN photodiodes (300 m) is sensitive. Charged particles (e.g. e+ and e-) which leak out at the rear end of the crystals and pass the diode produce an unwanted addition to the signal. A MIP creates some 100 electron-hole pairs per micron in silicon. This makes 30.000 electron-hole pairs which fake ~6 MeV additional energy in a CsI(Tl) calorimeter (Nuclear Counter Effect).

8. PIN photodiodes – nuclear counter effect High energy Low energy s Each dot stands for an energy deposition of more than 10 keV 80 GeV e- beam in a 18 cm long PbWO4 crystal

9. Basic APD Structure (CMS version) • Photo-conversion electrons from the thin p-layer induce avalancheamplification at the p-n junction. • Electrons created by ionising particles traversing the bulk are not amplified. • deff ~ 6 m 50 times smaller than in a PIN diode.

10. APDs in the CMS ECAL 36 supermodules with 1700 crystals each PbWO4 crystal 2 APD’s/crystal  122.400 APD’s In the endcaps vacuum phototriodes are used because of the very high radiation levels.

11. APD Impact on Energy Resolution ECAL energy resolution: CMS design goal :a ~3%, b~0.5%, c~200 MeV APD contributions to: a: photo statistics (area, QE) and avalanche fluctuations (excess noisefactor) b: stability (gain sensitivity to voltage and temperature variation, aging and radiation damage) c: noise (low capacitance, serial resistance and dark current)

12. s / = E 5%/ E(GeV) Sampling Calorimeter of KLEO The KLEO calorimeter is made of 200 layers of scintillating fibers embedded in a lead absorber. For the readout fine mesh PMT’s are used on both ends of the 430 cm long fibers. An excellent energy resolution has been achieved: The time resolution of allows the determination of K00’s vertices from the photon arrival time. This is almost a homogeneous calorimeter because of the relative small amount of absorber material

13. Sampling electromagnetic calorimeters – LHCb • The Shashlik electromagnetic calorimeter of LHCb is a stack of scintillator- and lead tiles with wavelength shifting fibers perpendicular to the tiles and with PMT readout. • high radiation environment • good energy resolution • fast response • high dynamic range PMT: Hamamatsu R7899-20

14. Hadron Calorimeters Hadron calorimeters are almost always built as sampling devices because to some extent they allow to make the response to electrons equal to the response to hadrons (e/h=1, compensation) which is crucial for the energy resolution. The best example is the ZEUS calorimeter with uranium- and scintillator-plates. The energy sampled is typically a few percent of the total incident energy. A small number of photons have to be detected with good signal to noise ratio. Up to now all sampling hadron calorimeters which are based on the detection of light from plastic scintillators use PMT’s.

15. Sampling Calorimeters – CMS HCAL with hybrid PMT’s • CMS HCAL is made of brass, scintillator tiles, wavelength shifting fibers and proximity focused hybrid PMT’s. • needs to work in a 4 T magnetic field • with large dynamic range, • high position sensitivity and • low crosstalk CMS diode design 19 x 5.4mm 73 x 2.68mm

16. Calorimeters with SiPM readout for ILC and T2K Minical for the ILC: 11 layers of 3x3 plastic scintillator tiles (50x50x5 mm3) with 2 mm Fe in between. Readout with WLS fibers and SiPM’s. Calibration with light from a LED (shaded area) and with MIP’s from 90Sr. <N> = 25 p.e. Spectra (data and MC) of the 11 layers expressed in number of MIP’s for a 3 GeV incident e+ beam V. Andreev et al.,NIM A 540 (2005) 368

17. Ring Image Čerenkov counters RICH counter: measure photon impact point on the photon detector surface. Needed is detection of single photons with • large area coverage • good spatial resolution • high efficiency and good signal-to-noise ratio Special requirements depend on the specific features of individual RICH counter: • Operation in (high) magnetic field • High rate capability • Very high spatial resolution • Excellent timing (time-of-arrival information and background reduction)

18. Rich in CLEO • CLEO Rich – the classical example • Available space limited • Operation in a magnetic field • Low material budget – in front of the calorimeter • 13% of a radiation length Solution: The photon detector is a wire chamber filled with methan and TEA (triethylamine) which has a QE of 30% at 150 nm.

19. Gas Photo Multiplier with CsI photo-cathodes • Alice: proximity focussing CsI RICH • liquid C6F14 radiator • MWPC • cathode pads coated with CsI • total area 11 m2 • similar in HADES, COMPASS, J-LAB … • Hadron Blind Detector of PHENIX • triple Gas Electron Multiplier (GEM) • high QE • low sensitivity to charged particles • fast response (1.6 ns for single photons) No ion and photon feedback

20. e+ Support tube (Al) Quartz Barbox Compensating coil e- Assembly flange Standoff box • BABAR Rich requirements: • CsI(Tl) needs to detect photons down to 20 MeV • small radiation length < 20% small radial size required /K separation at 4 GeV/c is 6.5 mrad  3 separation requires 2.2 mrad resolution DIRC: Detection of Internally Reflected Čerenkov Light PMT’s: ETL 9125

21. HERA-B RICH with multianode PMT’s Requirements: Rates ~1MHz Long term stability High QE over ~3m2 Gas based detectors could not be usedbecause of the high rate environment Multianode PMTs: R5900-M16 and R5900-M4

22. RICH for LHCb with hybrid PMT‘s • Large area (2.8m2) with high active area fraction • Fast compared to the 25ns bunch crossing time • Have to operate in a small magnetic field • Granularity 2.5x2.5mm2 • hybrid PMT with 5x demagnification the anode is a pixel detector with 8192 channels organized in 1024 super-pixels of 500 x 500 m2 size.

23. Time of flight for particle identification The plastic scintillators (3 m long, ~4x6 cm2) with fine mesh PMT’s on both sides of the TOF detector provide a time resolution of 100 ps and allow a /K separation up to 1.2 GeV. The Aerogel Cerenkov Counter (ACC) extends the /K separation to 3.5 GeV. It also uses fine mesh PMT’s.

24. Tracking with scintillating fibers and VLPC‘s in DØ • 80000 fibers 2.5 and 1.7 m long • single photon detection capability • 80% QE • operated at 6°K • gain ~50.000 • position resolution 120 m

25. Target Tracker of Opera locate the lead/emulsion brick where a neutrino interaction occurred Total area 3000 m2 Few photo electrons: 6 to 7 p.e. per MIP 64-channel Hamamatsu H7546 PMT 6700 x 2.6 x 1 cm3

26. Summary A large variety of techniques of photo sensors has been developed and realized in present detectors for high energy physics. There is an almost infinite number of alternative designs which have been proposed and partly tested in prototypes. The working horse is still the PMT when weak light flashes need to be detected. New types of photo sensors have always quickly been adopted in high energy physics experiments. The new Geiger-mode avalanche photodiodes will for sure have a heavy impact on the design of future detectors.