Presentation on theme: "Scintillation Detector Development for LArTPC Experiments ICATPP, Villa Olmo, Como, Thursday, 26 th of September 2013 Ben Jones, MIT."— Presentation transcript:
Scintillation Detector Development for LArTPC Experiments ICATPP, Villa Olmo, Como, Thursday, 26 th of September 2013 Ben Jones, MIT
Liquid Argon Scintillation Light 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 J Chem Phys vol 91 (1989) 1469 E Morikawa et al
Our Motivation: MicroBooNE Optical System 32 cryogenic Hamamatsu R5912-02mod PMTs (with platinum undercoating) Mounted on a rack behind the TPC wireplanes Each PMT has a magnetic shield and wavelength shifting plate A 14m optical fiber runs to each PMT, couped to an LED outside the cryostat On paper… For more info on MicroBooNE, see M. Webers plenary talk 2.5m Drift
Installation of final PMT rack Left to right: Matt Toups BJPJ Eric James Janet Conrad not shown: Teppei Katori Photograph as installed, illuminated with amber lights And in real life!
Our Motivation: R&D towards large detectors PMT-and-plate strategy not scalable to an N-kiloton scale, multi-TPC detector like LBNE We are also working on lightguide based detectors to slide between TPC 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 (in collaboration with Indiana University) Matt Toups, MIT
Wavelength shifting plate (TPB) One MicroBooNE PMT assembly Mu-metal shield + mount
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
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
Photodegradation Mechanism Working with GCMS we also identified the degradation mechanism – radical mediated photo-oxidation to benzophenone
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
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 Teppei Katori, MIT. Full report published in JINST.
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
Full assembly characterization : Lots of results, but no time to tell you about them…
Understanding light yields in scintillation detectors UV photon But also a LAr scintillation R&D detector
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)
Add nitrogen, monitor light yield at 2 source positions
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
Divergence of 2 curves shows absorption effect 8”14.5” PMT
Nitrogen Results: Attenuation strength : Some characteristic LAr samples :
Underground Argon for DM Experiments Dark matter LAr experiments suffer from pervasive 39 Ar background 39 Ar 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 39 Ar Underground argon extracted from carbon dioxide wells has a much lower 39 Ar concentration. Underground argon distillation column at Fermilab For more information: arXiv 1204.6024, 1204.6061, 1204.6011
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 CO 2 well Distillation concentrates both methane and argon
+ 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)
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 Just for fun:
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.
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 LED pulses read through uB electronics Scintillation spectrum from source to extract collection efficiency (more on this in next slides)
LArTPC Detectors MicroBooNE at FNAL 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 Simulated MicroBooNE event, reconstructed in 3D ICARUS at LNGS
Why do TPCs need Optical Systems? 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) Simulated MicroBooNE event with on- beam reco flash position from optical system overlaid
MicroBooNE Optical Calibration System Feed through LED Fiber (One fiber per PMT) Pulser An LED driven optical fiber calibration system will be used to: 1.Calibrate gains, 2.Time in the PMTs 3.Test PMT functionality during commissioning.
In situ MicroBooNE PMT illuminated using calibration system LEDs Trig pulse PMT waveform Light spot from fiber on PMT 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
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. Ar p + * * 1Σu excimer Ar γ 6ns e Ar + - p + e - + * 3Σu excimer 1590ns
Special bonus – possible PID information Ar * * * γ Utlized in dark matter searches (MiniCLEAN, DEAP), and we are investigating the applications of this technique to augment TPC based particle ID in MicroBooNE. Scintillation process Competing Excimer Dissociation Process
Pulse shape discrimination – a vital tool in dark matter detection, also useful to us!
Individual components (separated using PSD) Fit function for alpha + background
General Idea: 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. 14.5”
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 Hee Yoo – Thanks!
Attenuation Data Preliminary Divergence of these two lines is clear evidence for the nitrogen absorption effect!
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
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.
Getting to the Attenuation Strength Measured Attenuation Strength: Measured Absorption Cross Section: Preliminary
Comparison to N2 gas absorption cross section world data Preliminary
Nice result, but whats it gonna do for me ? $$$$$ Preliminary
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
Understanding the Geometrical Effect Ray trace to understand expected light yields per percent of absorption at each position 8”14.5”
Taking ratio, any quenching effect cancels Ratio = Light loss at 8” Light loss at 14.5” Our region of interest We will measure the nitrogen absorption effect as % light loss per ppm^-1 cm^-1. First, measure the light loss ratio as a function of N2 concentration. In our region of interest the relationship should be ~linear. Absorption strength extracted by comparing the gradient of the measured line to the gradient of the line right, which gives proportionality factor for X axis scales. This factor tells us the % light loss per ppm cm of nitrogen.
2) Measurement from liquid and gas capillaries in agreement with saturation pressure based equilibrium calculation 1) Amount of N2 in liquid agrees with amount injected to within our uncertainty of the injection volume. How do we know we get N2 concentration right? Injection volume uncertainty region
Single exponent power law (cosmic background) + Poisson (alpha source) Detected light spectrum – clean argon, source at 8”
Check on functional form of fits: Power law background is great. Alpha fit needs improvement (not exactly poissonian).
Why? “Shadowing” of outer source edges leads to reduced poisson mean light yield from edge area elements This leads to an enhanced low tail of the source spectrum Disc source kindly loaned by Adam Para – Thanks!
So we Measure the Shadowing Function… Now we know how the source is shadowed, we know how to fit all points.
Improved fit from shadowing function Major improvement with new fit function. Note : no extra free parameters, since shadowing function was tuned on an independent dataset.
Aside: Pulse Shape Discrimination in Action Alpha enriched Cosmic only Saturation