PHY 599: How it is usually done ● This is Particle Physics and Astro-Cosmology Seminar ● We meet every Wednesday from 3:30 pm to 4:30 pm in Room 506 Nielsen. ● This seminar series is mainly intended for graduate students to participate and give talks so they gain some experience giving talks ● All registered students are required to give a 25+5 minute presentation and are also expected to actively participate and ask questions (required to ask at least 1 question per student in a given session to get the credit)
PHY 599: How it is usually done ● Course is on Canvas, I sent an announcement earlier today asking you to sign up (so start thinking about what dates/days will work best for you) ● Typically, students will be asked to fill out later slots (mid-March to April) of the seminar, but you can always sign up earlier if you think you have enough material to cover . ● Since there are only 5 students, you are also welcome to give a 1 hr talk if you have enough material to cover. 2
PHY 599: How it is usually done ● For non-student slots, you will hear from various faculty/postdoc/students in PP, A & C about their research – already some great speakers lined up! ● We will try to collaborate with OakRidge as much as possible so we can invite their Colloquium speakers as well if there is mutual interest for both groups (applies to our colloquium as well). ● Working with Catherine Longmire to bring out a HEP seminar webpage (like the Nuclear Physics folks have). Soon to come.... 3
Calibrating Liquid Argon Time Projection Chamber (LArTPC) Detectors: focusing on electron lifetime measurement Sowjanya Gollapinni UTK HEP seminar January 18, 2017 4
Reminder: The LArTPC principle ● Incoming neutrino interacts with argon producing charged particles which then ionize argon ● Scintillation light produced as well – collected by light collecting devices such as PMTs. ● Ionization electrons drift to the anode (typically several planes) under the influence of the electric field. ● 3D reconstruction by combining signals from the wire planes and drift time. 5
LArTPC Calibration ● LArTPC calibration is required to extract the ionization signal (charge and position) as precisely as possible with minimal bias – There are many detector effects such as impurities in argon, diffusion of signal, recombination of argon ions, and space charge that can introduce bias in the signal and can also effect the resolution. ● Many current and future experiments use or will use the LArTPC technology such as MicroBooNE, SBND, ICARUS, DUNE and DUNE prototype experiments. ● Establishing a calibration scheme to precisely reconstruct the the signal and extract the charge is critical for achieving the physics goals set out by these experiments. 6
The MicroBooNE experiment ● I will use some of the recent results from the MicroBooNE experiment as an example wherever possible to introduce these concepts since it is the largest currently operating LArTPC in the world. ● Neutrino experiment located at Fermilab ● 89-ton active volume ● Cathode voltage at -70 kV resulting in 273 V/cm E field ● Three anode wire planes, two induction (U,V) and one collection (W) plane ● 8256 150 μm thin wires with 3 mm wire pitch C A ● Has a PMT system and a Laser System. 7
Physics goals of MicroBooNE Other non-beam physics It is important to do reconstruct the energy precisely to reach these physics goals! 8 Slide credit: M. Mooney
Calorimetric reconstruction ● Goal is to convert ADC to GeV ● Several steps involved – Electronic calibration factor – Lifetime correction – Recombination correction 9
Calorimetric reconstruction ● Goal is to convert ADC to GeV ● Several steps involved – Electronic calibration factor – Lifetime correction – Recombination correction We will focus on this for this talk. 10
Electron drift in a LArTPC: impurities in argon ● Once you have an ionization electron (signal) formed in the TPC, there is a lot of drift physics that happens around it. ● The presence of electro-negative impurities such as (O 2 and H 2 O) can capture drifting electrons and result in signal loss N(t0) is the charge at the start; N(t drift ) is the charge at a given drift time; τ is the electron lifetime; ● In 100% pure liquid argon, electrons will drift for ever. ● Electron lifetime is the half life of electrons in liquid argon and is what is measured experimentally to understand the impurity levels in argon 11
Electron drift in a LArTPC: impurities in argon & charge loss ● For example, for a maximum distance of 2.56 m (MicroBooNE drift) and for a maximum drift time of 2.3 ms (the time taken for an electron to cross from anode to cathode at 273V/cm), what is the maximum charge loss crossing the entire TPC for a given lifetime? MicroBooNE requirements for Purity : how did we arrive at this? ● < 100 ppt O2 (ppt = parts per trillion) ● Nitrogen can also absorb scintillation light N2 < 2 ppm (this has to come from the manufacturer) 12
Relation between lifetime and purity ● Purity (electron lifetime) and impurity contamination are inversely proportional Lifetime (in ms) x O2 contamination in ppb = 0.3 Or, Lifetime (in ms) = 0.3/(O2 contamination in ppb) (based on several purity test stand measurements at Fermilab and ICARUS) ● So, a 3ms lifetime means 100 ppt of O2, and 6ms lifetime means, 50 ppt of O2 etc. If you measure electron lifetime, you can make statements about: 1. ionization Signal loss at a given electric field 2. O2 contamination in liquid argon. 13
Measuring electron lifetime ● There are many ways to measure lifetime: – Purity Monitors (ICARUS,MicroBooNE) – Long Cosmic muon tracks (ICARUS) – Laser (ArgonTube) Purity Monitors ● Mini drift chambers immersed in LAr and consist of a field cage, photo cathode and anode. Purity Monitors used in MicroBooNE ● A quartz fiber optic cable carries UV light from a xenon flash lamp to the photocathode which then produces electrons via photo-electric effect. ● The electron drift over the few cm of distance before reaching the anode. 14
MicroBooNE purity measurement ● MicroBooNE has 3 purity monitors: two inside the cryostat and one in the recirculation system. Measurement below is from the one inside the cryostat. charge ratio Purity is really good! Anode to Purit at < 60 Cathode parts per tilion (ppt) O2 Design goal Purit at < 100 ppt of O2 15
Purity Monitor: Advantages & short comings Advantages: ● can provide quick (online) measurement ● Doesn't require 3D reconstruction of tracks ● Great for the initial commissioning and data running Shortcomings ● More localized measurement. ● Cannot be extrapolated to the entire liquid argon volume in the TPC if there is a variation of the impurity distribution over the TPC volume. ● Cannot be used to study purity variations in the TPC. ● Typically is at a lower E-field which means we are not measuring the effects of the contaminants at the relevant electric field (different electron capture cross sections for fields above 0.2 kV/cm) These devices require a lot of R&D to go: ● In almost all experiments they were used they failed for various reasons: longevity a problem. Cannot rely on using them for DUNE . Will require alternate more reliable methods for the lifespan of the experiment. 16
Using Laser for purity measurement Laser: ● Would be great if one can do it. ● MicroBooNE, again is the first large scale experiment that used Laser, our initial runs had several problems: controlling the power of laser, tuning the directionality etc. ● Biggest issue one can face: if the ionized charge is not uniform along the track, it cannot be used to determine the electron drift-lifetime without improved understanding of the charge distribution. ● MicroBooNE managed to take good laser data over last summer, will produce results soon. ● But, Laser method still shares some of the short comings of the Purity monitor method. 17
Measuring purity using cosmic muons MicroBooNE sees a ton of cosmic ● By far the best way to measure purity rays since it is surface-based is using long cosmic muons tracks ● Can represent purity through out the TPC and can be used to understand purity variations. ● One can extract the charge at the beginning and end of the track and compare them track-by-track to extract lifetime. ● A more sophisticated method is to combine all tracks and bin dQ/dx them in drift time to remove track by track fluctuations. dQ/dx: charge per unit length of the track. 18
UTK is spearheading this effort for MicroBooNE and future LArTPC experiments ● First results to be shown at APS with a publication in Summer! (APS is just a milestone, the results are publication ready) Unfortunately cannot show them today, we are going through the final approval within MicroBooNE 19
Space charge & Lifetime ● There are various other factors that can effect the lifetime measurement: biggest effect come from Space charge and recombination ● Space charge is the build up of slow moving positive Ar+ ions in the detector due to, for example, cosmic rays ● The surplus of ions result in local variations of Electric field and spatial distortions in ionization position. 20 Spatial distortions in Y
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