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Ben Jones, MIT

Scintillation Detector Development for LArTPC Experiments ICATPP, Villa Olmo , Como, Thursday, 26 th of September 2013. Ben Jones, MIT. Liquid Argon Scintillation Light. J Chem Phys vol 91 (1989) 1469 E Morikawa et al.

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Ben Jones, MIT

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  1. Scintillation Detector Development for LArTPC ExperimentsICATPP, Villa Olmo, Como, Thursday, 26th of September 2013 Ben Jones, MIT

  2. Liquid Argon Scintillation Light J ChemPhys vol 91 (1989) 1469 E Morikawa et al Liquid argon produces scintillation light at a wavelength of 128 nm. Llight yield ~ few 10,000’s of photons per MeV (dependences on E field, particle type and purity) Argon is transparent at 128nm, which makes LAr scintillation detectors very scalable. Coupling scintillation detection with charge detection (eg in a TPC) offers many benefits

  3. On paper… Our Motivation: MicroBooNE OpticalSystem 32 cryogenic Hamamatsu R5912-02mod PMTs (withplatinumundercoating) Mounted on a rackbehindthe TPC wireplanes Each PMT has a magneticshieldandwavelengthshiftingplate A 14m optical fiber runstoeach PMT, coupedto an LED outsidethecryostat 2.5m Drift For more info on MicroBooNE, see M. Webers plenary talk

  4. Installation of final PMT rack And in real life! Left to right: Matt Toups BJPJ Eric James Janet Conrad not shown: TeppeiKatori Photograph as installed, illuminated with amber lights

  5. Our Motivation: R&D towards large detectors (in collaboration with Indiana University) • PMT-and-plate strategy not scalable to an N-kiloton scale, multi-TPC detector like LBNE • We arealsoworking on lightguidebaseddetectors to slidebetweenTPC units • MicroBooNE will contains 4 prototypes as a long term R&D exercise • We have a dedicated lightguide test stand at MIT, and work is performed in collaboration with Indiana University Matt Toups, MIT

  6. One MicroBooNE PMT assembly Wavelength shifting plate (TPB) Mu-metal shield + mount

  7. Tetraphenyl Butadiene • 128 nm light will not penetrate, glass, air, acrylic, etc. • This is a problem for the design of optical liquid argon detectors. • Common solution is to use a fluorescent chemical like tetraphenyl butadiene (TPB) • TPB absorbs 128nm light and emits it in the visible. • In MicroBooNE we use a coating of 50% TPB in polystyrene, dissolved in toluene brush coated on acrylic

  8. Environmental sensitivity • As part of the MicroBooNE system development we optimized coatings for performance, robustness and stability • During these investigations we found that TPB is very sensitive to UV light and degrades in performance • We also sometimes observe a yellowing of the coating, after ~days of lab light exposure

  9. Photodegradation Mechanism Working with GCMS we also identified the degradation mechanism – radical mediated photo-oxidation to benzophenone

  10. Radical Mediator Studies Some stabilization of the coating is possible using a radial mediator Here we find a 20% admixture of 4-tert butylcatechol improves performance + somewhat slows degradation rate But certainly, there is much room for improvement + further work here 4-tert butylcatechol

  11. Cryogenic PMT

  12. Testing MicroBooNE PMTs Every PMT for MicroBooNE has been characterized both warm and in liquid nitrogen Measurements include gains, dark rates, and stability over few days of operation Largely the work of TeppeiKatori, MIT. Full report published in JINST.

  13. Full assembly characterization

  14. Bo Vertical Slice Test • A long term, high purity liquid argon test stand • Used to make detailed characterization of a few PMTs and supporting hardware: • Cryogenic PMTs • Base electronics • Wavelength shifting plate • High voltage system + interlocks • Cables and splitters • Readout electronics • Cryostat feedthrough • Trace impurity monitors • Etc… uB style PMT assembly

  15. Full assembly characterization: Lots of results, but no time to tell you about them…

  16. But also a LAr scintillation R&D detector Understanding light yields in scintillation detectors UV photon

  17. The Effects of Nitrogen in LAr • Unlike oxygen and water, nitrogen does not disturb charge drift in LArTPCs, and is difficult to remove from argon. • Nitrogen is an expected contaminant in any present or future large LArTPC detector (especially with vacuum-free purge) • Nitrogen at the ppm level leads to : • 1) Scintillation Quenching measured in a detailed study by the WArP collaboration in small test cells (R Acciarri et al 2010 JINST 5 P06003) • 2) Absorption of Scintillation Light Absorption effects of N2 in LAr have not previously been measured. Very important to know for big detectors! • From(R Acciarri et al 2010 JINST 5 P06003)

  18. Add nitrogen, monitor light yield at 2 source positions

  19. Light loss due to N2 in 8” source configuration 27ppb N2 3.7ppm N2 7.4ppm N2 15.5 ppm N2 Measure intensity of polonium alpha peak

  20. Divergence of 2 curves shows absorption effect 14.5” 8” PMT

  21. Nitrogen Results: Attenuation strength : Some characteristic LAr samples :

  22. Underground Argon for DM Experiments • Dark matter LAr experiments suffer from pervasive 39Ar background • 39Ar is a beta emitter with endpoinr 565 keV and a half-life of 269 years • Produced by cosmic ray interactions in air • Industrial argon distilled from air contains significant 39Ar • Underground argon extracted from carbon dioxide wells has a much lower 39Ar concentration. Underground argon distillation column at Fermilab For more information: arXiv1204.6024, 1204.6061, 1204.6011

  23. Methane as a contaminant • Unlike industrial argon, UAr contains methane as a contaminant • Concentration of argon through distillation also concentrates methane • Can be removed using hot getters – but very expensive • Methane has been shown not to harm charge collection • No spec exists on the allowed methane concentration in a LAr scintillation detector Gas composition from CO2 well Distillation concentrates both methane and argon

  24. + submitted to JINST We made a study of absorption, quenching and visible re-emissions of methane / argon mixtures. Key conclusions: Purity spec seems to be about 10ppb We see no signs of visible re-emission features At higher concentrations (50-100ppb) some quenching is observed Most losses are due to UV absorption Light yield from alpha source (PE)

  25. Just for fun: Methane was discovered by local Como hero Alessandro Volta! On vacation in 1776, Volta collected gas he noticed bubbling from mud in Lake Maggiore Interest piqued by a recent paper from Benjamin Franklin on “flammable air”, he discovered the gas was flammable By 1778, he had isolated methane from the marsh gas. Statue of Volta in Piazza Volta, Como Volta’s summer holiday activities

  26. Summary • At MIT we have been developing scintillation detectors for current and future liquid argon TPC experiments • This includes development and installation the MicroBooNE optical systems, and work on prototype systems for LBNE • We have made studies of the performance and photochemistry of wavelength shifting coatings • Using high purity test stands we have both characterized detector elements and made R&D measurements • Nitrogen absorption is important for large LArTPCs- purity spec of 2 ppm is sufficient for MicroBooNE • Methane absorption is important for DM detectors - purity spec of 10 ppb is likely appropriate.

  27. Thank you for your attention!

  28. Backups and / or no time

  29. LED pulses read through uB electronics Some technical achievements of Bo VST… • Measurement of global collection efficiency of PMT assembly • Linearity of PMT / base / splitter system up to 300 PE • Development of PMT gain and timing calibration methods • Successful operation of MicroBooNE PMT readout and trigger electronics • And more Scintillation spectrum from source to extract collection efficiency (more on this in next slides)

  30. LArTPC Detectors Ability to build massive detectors with long drift distances make LArTPCs appealing for neutrino detection A LArTPC also offers bubble-chamber level position resolution and excellent calorimetric resolution with a large active volume and electronic readout ICARUS at LNGS Simulated MicroBooNE event, reconstructed in 3D MicroBooNE at FNAL

  31. Why do TPCs need Optical Systems? Simulated MicroBooNE event with on-beam reco flash position from optical system overlaid • Typical LArTPC has a finite drift time (~ms). A priori we don’t know the interaction position. • So a LArTPC in a beam integrates milliseconds of cosmics around the beam gate • A correlated flash from the optical system allows timing of subevents to be specified to the few nanosecond level • This timing information can be used to reject cosmic rays (+other uncorrelated BGs)

  32. MicroBooNE Optical Calibration System An LED driven optical fiber calibration system will be used to: Calibrate gains, Time in the PMTs Test PMT functionality during commissioning. Pulser LED (One fiber per PMT) Feed through Fiber

  33. Fiber installation was completed 1 week ago We have already used parts of the system to verify the connection and functionality of all 32 PMTs PMT waveform Trig pulse In situ MicroBooNE PMT illuminated using calibration system LEDs Light spot fromfiber on PMT

  34. Experimental Configuration for This Study

  35. Prompt peak window

  36. - + + e Ar p + - Ar Ar Ar e Light in Liquid Argon • The scintillation light in liquid argon is produced copiously alongside all ionization charge deposits. • There are two scintillation pathways, with different time constants – a fast component with t=6ns and a slow time constant with t=1500ns. + γ 1Σu excimer Ar * p * Ar Ar Ar 6ns Ar Ar 1590ns 3Σu excimer * Ar Ar

  37. Special bonus – possible PID information * Ar Ar Ar Ar * Scintillation process Ar Ar Ar Ar Competing Excimer Dissociation Process Ar γ * Ar Ar Ar • Utlized in darkmattersearches (MiniCLEAN, DEAP), and weareinvestigating the applications of thistechnique to augment TPC basedparticle ID in MicroBooNE.

  38. Pulse shape discrimination – a vital tool in dark matter detection, also useful to us!

  39. Fit function for alpha + background Individual components (separated using PSD)

  40. General Idea: 14.5” • Source set in one of two possible positions. • Controlled amounts of N2 injected into the liquid • Quenching affects both source positions equally • Absorption hinders the further more than the nearer source. • If fractional losses from each source deviate we see an N2 absorption length effect. • A future analysis will address the effects of quenching (more extensively studied by other groups) separately.

  41. PPM amounts of nitrogen are injected into the liquid from a gas canister, charged to a known pressure. • From known volume of canister and known pressure we can calculate how many ppm we injected. • Nitrogen concentration monitored in both liquid and gas phases using LDetek8000 N2 monitor • We also monitor H20 and O2 to ~10ppb precision from the same sample lines. Trace nitrogen monitor Injection Canister Kindly loaned by Jong HeeYoo – Thanks!

  42. Attenuation Data Preliminary Divergence of these two lines is clear evidence for the nitrogen absorption effect!

  43. Stability of 1PE • SPE scale stable to within 1% for each run • This is similar to the precision of our SPE measurements • Therefore we assume constant and fold in variations as a systematic error on each point

  44. Just to be sure its really the nitrogen… Preliminary No light loss during periods with no nitrogen injection – gives confidence in system stability, constrains outgassing effects, etc.

  45. Getting to the Attenuation Strength Measured Attenuation Strength: Measured Absorption Cross Section: Preliminary

  46. Comparison to N2 gas absorption cross section world data Preliminary

  47. Nice result, but whats it gonna do for me? Preliminary $$$$ $

  48. Summary + Prospects • Bo VST has been constructed to test elements of MicroBooNE optical system – also an R&D detector for LAr scintillation light. • Detailed studies of alpha source response have been made and area used in various Bo VST studies • We have measured the effects of nitrogen absorption of 128nm argon scintillation light in liquid argon. We find that the effect is on the order 0.015% / (ppm cm) • This means absorption is no problem for MicroBooNE, and could be useful information for the design of cryo systems for large LArTPCs

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