Photodetector Calibrations: Gain & Quantum Efficiency M. Toups - PowerPoint PPT Presentation

Photodetector Calibrations: 
 Gain & Quantum Efficiency M. Toups Fermi National Accelerator Laboratory 
 Workshop on Calibration and Reconstruction for LArTPC Detectors 
 12/10/2018

Scintillation Light in a LArTPC Photodetectors 12/10/18 2 M. Toups—LArTPC Caleb Reco Workshop

LAr Light Detection Motivation • Provides interaction time (t 0 ) to reconstruct drift coordinate – Correct for e - lifetime, fiducialize volume • Trigger detector readout 
 Cosmics during drift window • Cosmic background rejection – Essential for surface LArTPCs ν • Event reconstruction/particle ID TPC 12/10/18 3 M. Toups—LArTPC Caleb Reco Workshop

LAr Scintillation light • LAr is a bright scintillator: O(10,000) γ /MeV • LAr scintillates in the VUV at 127 nm • LAr is very transparent to its own light • Rayleigh scattering length: λ≈ 66 cm • LAr light divided between two components • Fast component: τ≈ 6 ns • Slow component with τ≈ 1.5 us E Morikawa et al, J Chem Phys vol 91 (1989) 1469 • LAr scintillation light and charge anti- correlation 12/10/18 4 4 M. Toups—LArTPC Caleb Reco Workshop

LAr Scintillation light • LAr is a bright scintillator: O(10,000) γ /MeV Standard SiPMs/PMTs not sensitive at this wavelength, so use wavelength shifters • LAr scintillates in the VUV at 127 nm (WLS) • LAr is very transparent to its own light • Rayleigh scattering length: λ≈ 66 cm • LAr light divided between two components Tetraphenyl butadiene (TPB) • Fast component: τ≈ 6 ns evaporative coating • Slow component with τ≈ 1.5 us • LAr scintillation light and charge anti- correlation V. Gehman et al, NIM A 654, 116 (2011) See also C. P . Benson et al, Eur. Phys. J. C (2018) 78: 329 12/10/18 5 5 M. Toups—LArTPC Caleb Reco Workshop

Present LArTPC Photon Detection Systems 430 nm light 127 nm light PMT TPB-coated acrylic plate ARAPUCA e.g. MicroBooNE, SBND, ICARUS e.g. SBND, protoDUNE e.g. SBND, protoDUNE TPB coated acrylic light guide Commercial (EJ-280) light guide e.g. protoDUNE with TPB-coated radiator plates 12/10/18 6 6 M. Toups—LArTPC Caleb Reco Workshop

Gain & Quantum Efficiency (QE) Calibrations Photodetector WLS 
 (e.g. TPB) 430 nm light 127 nm light WLS Incident Photosensor Photosensor Measured = conversion Acceptance VUV QE/PDE gain charge vs. time e ffi ciency photon flux Photodetector quantum e ffi ciency Goal: Infer incident VUV photon flux from measured charge 12/10/18 7 7 M. Toups—LArTPC Caleb Reco Workshop

Photosensor Gain Calibration • First step in PD calibration converts measured charge to PEs • Function only of the photosensor and its operating conditions • Depends on temperature, operating voltage • Separates changes in photosensor response from other changes in photodetector response or in the LArTPC response 12/10/18 8 M. Toups—LArTPC Caleb Reco Workshop

Photosensor ex situ Gain Calibration • Warm measurements of photosensor gain often performed as part of a suite of acceptance tests • Additional cold tests of photosensor gain verify proper functioning at cryogenic temperatures • Photosensor gain measurements at cryogenic temperatures can determine nominal operating voltage for photosensors ahead of time • DUNE-specific: ganging together multiple MPPCs with similar gains may require pre-selection based on cold gain JINST 13 (2018) no.10, P10030 12/10/18 9 M. Toups—LArTPC Caleb Reco Workshop

Photosensor in situ Gain Calibration (visible light) JINST 10 (2015) no.06, T06001 • MicroBooNE system uses individual LEDs for each PMT • Similar method but with a single laser diode fanned out to all PMTs employed in ICARUS • Both can also be used to verify proper PMT functioning after installation • Single PEs from late component of LAr scintillation light can also be used to monitor gain over time while running 12/10/18 10 M. Toups—LArTPC Caleb Reco Workshop

Photosensor in situ Gain Calibration (UV light) • Used in 35-ton, protoDUNE, SBND, and DUNE JINST 13 (2018) no.06, P06022 • Pulsed external UV LEDs (245-280 nm) coupled to quartz fibers connected to di ff users on the cathode to uniformly illuminate PDs • Reference photodiode monitors LED output • Gain measurements, linearity, timing resolution, and stability over time • Does not measure (relative) QE due to di ff erent PD response at 245 nm • SiPMs/MPPCs often said to “self-calibrate”, but situation may be more complicated when cold summing boards are used and clearly separated peaks are not visible arXiv:1706.07081 [physics.ins-det] 12/10/18 11 M. Toups—LArTPC Caleb Reco Workshop

Photodetector QE JINST 13 (2018) P02034 • Defined as QE = meas /N ph N P E pred • Depends on photon wavelength, incident angle (often ignored), and hit location on photodetector • Usually defined for a given WLS emission spectrum at a given location on the photodetector or averaged over the entire surface. • Convolution of 3 main processes: JINST 8 (2013) T07005 • WLS conversion e ffi ciency (depends on surface treatment, temperature, environmental e ff ects causing aging/degradation) • Acceptance of WLS light at photosensor (depends on index of refraction, photosensor geometric coverage, surface quality) • Photosensor QE/PDE (depends on temperature, operating voltage) 12/10/18 12 M. Toups—LArTPC Caleb Reco Workshop

Photodetector QE Measurements: Three Methods • Measure the convolution of process all at once , in situ • Direct measurement of quantity of interest, but relies on poorly constrained quantities related to light propagation (e.g. material reflectivity) and production (e.g. light yield, quenching) • Measure/estimate each process individually , then combine • Isolate and understand each e ff ect individually, but brings in poorly understood quantities (e.g. WLS conversion e ffi ciency) and extrapolations (e.g. temperature) • Cross-check/validation of the first method • Measure relative QE w.r.t another absolutely-calibrated photodetector • Di ffi cult to use an identical flux of incident photons, so simulation is typically used to estimate di ff erences in the incident photon flux • While the simulation still brings in poorly constrained quantities related to light propagation (e.g. material reflectivity), this method does not depend on the predicted amount of light produced (unless it is brought in through the calibration of the reference photodetector) 12/10/18 13 M. Toups—LArTPC Caleb Reco Workshop

Proton Assembly Building Light Detection Test Stand (“Bo”) Commercial Argon 1 ppm Oxygen, H 2 O 12/10/18 14 14 M. Toups—LArTPC Caleb Reco Workshop

PMT Calibration with Radioactive Source (5.3 MeV alpha) 8” or 14.5” 2018 JINST 13 P02034 d = 14.5” d = 8” averaged over TPB plate Similar method used to measure ARAPUCA e ffi ciency of 0.37 ± 0.01 (sys) ± 0.03 (stat)% in JINST 13 (2018) no.03, C03040 12/10/18 15 M. Toups—LArTPC Caleb Reco Workshop

Light Guide Calibration with Cosmic Rays at readout end Commercial light guides with TPB-coated radiator plates “Blanche” test stand @ PAB NIMA 907 (2018) 9-21 12/10/18 16 M. Toups—LArTPC Caleb Reco Workshop

Photodetector QE Take-aways • Dedicated experiments requiring significant cryogenic infrastructure needed to measure QE of just a few PD units (expensive) • Surface and test beam LArTPCs can validate these PD QE measurements using (tagged) cosmic rays or beam particles • Scaling up in size to DUNE there are a few ways forward: • Predict PD QE in LAr based on measurements of each process individually, then validate with dedicated test stand experiments • Similar to the above, but use relative measurement of convolution of all processes to predict PD QE in LAr • Need to identify appropriate physics sources (natural, radioactive sources, electron beams?) to validate these PD QE measurements in DUNE (coupled to light yield) • Can these be performed with drift HV o ff (removing HV integration requirements?) 12/10/18 17 M. Toups—LArTPC Caleb Reco Workshop

Summary • Gain measurements and stability monitoring is relatively straightforward but important for understanding any variations in PD response seen over time • Experiments to measure PD QE done in dedicated test stand measurements (e.g. at PAB), but these are generally limited to just a few PD units • Comparison with in situ measurements from currently operating detectors (e.g. protoDUNE) important for understanding validity of these test stand measurements • DUNE will require a large QC campaign of calibration measurements to understand & measure PD QE before installation 12/10/18 18 M. Toups—LArTPC Caleb Reco Workshop