Carlos Escobar, Paul Rubinov PD Nov. 5, 2015
1. Motivations for this study 2. History and Background 3. Recent investigations 4. Test using the SCENE cryostat at Fermilab 5. Preliminary Results 6. Using the Solid Xenon facility at PAB 7. Acknowledgements 1
Motivations: The simultaneous read-out of the charge and light signals in LNG’s detectors is an important tool for many experiments ranging from DM to large LAr TPC’s. So far the light signal used (or planned to be used) comes from the VUV scintillation with wavelengths ranging from 78 nm (LNe) to 175 nm(LXe). VUV detection presents many challenges: 1. Use of WLS (long term stability? ) 2. Complicated schemes for collecting the light 2
Motivations 3. Singlet/triplet ratio by PSD needs excellent light collection ( see DEAP Coll.) and delayed light emission from TPB does not help it . 4. Recent estimates of Rayleigh scattering length puts it at 55 cm (Grace and Nikkel)- start having pernicious effects especially for extremely large LAr TPC’s (DUNE) 5. Attempting to improve charge collection (recovering charge lost by recombination) by doping with a photosensitive chemical such as TMG (Icarus -1995) kills the VUV light . 6. VUV poorly reflected from almost any surface/material; more opaque wire meshes. GO TO NIR! 3
History and Background NIR light emission from noble gases has been known since the late 40’s with lines around 1,300 nm. In condensed noble gases NIR emission was seen in transient optical absorption experiments via electron excitation in late 60’s and early to mid 70’s by various groups. Next slide gives one example for Ar. Bressi and collaborators pursued for many years the study of NIR emission in both gaseous and liquid states for xenon and argon: clear evidence for NIR emission from gas but much lower LY in liquid. 4
History and background cont. example of spectrum obtained in argon: from Suemoto et al (1977) Top: solid Middle: liquid Bottom: gas Same results were obtained by Arai and co-workers evidence for excited neutral rare Gases molecules (1.3 eV = 954 nm) 5
Recent investigations Two groups have pursued the investigation of the NIR scintillation in LAr very vigorously in recent years Novosibirski (Bondar, Buzulutskov et al.) and TU Munich (Ulrich, Schoenert and various students and postdocs). Both use table-top setups(cubic centimeter volumes) and very intense, pulsed low energy beams (12 keV electrons Munich and pulsed X-rays between 30 and 40 keV-Bondar et al.) Next slides show pictures of their apparatuses. 6
recent investigations Munich apparatus 7
Recent investigations cont. Novosibirski apparatus 8
Summary of the results from the two groups 1 st results from the Munich group indicated NIR emission from LAr: No absolute light yield but spectral information 9
recent investigations cont. Results from the Novosibirski group: 510+/-90 photons/MeV from 400nm to 1000nm 10
Recent investigations: coda But, more recently (end of 2014 and beginning of 2015) the Munich group revisited their NIR results in LAr, claiming that impurities had caused the 1 st result. Their new line of investigation is mixing small parts of xenon into argon (10 ppm produces the best results: 10,000 NIR photons/MeV at 1,180 nm) Bondar et al Have not reconsidered their results. 11
Conclusions so far A vigorous, systematic, experimental program is still needed, one that would use larger volumes of LAr with 1)rigorous control of purity 2)spectral information 3)determination of the time structure of an eventual NIR emission 4) determination of the light yield 5) “standard” ways of exciting the LAr (radioactive sources; cosmic rays; particle test beams) 12
In the mean time Test using the Scene cryostat at Fermilab: Use CPTA SiPM’s for NIR detection (4.4 mm*2) Tagged 0.511 MeV gammas from Na22 source (1 Ci) Tag with liquid scintillator and PMT. Good purity of LAr. Expect very low rate of events due to: source intensity; geometrical acceptance; small energy loss of gammas in the LAr (mostly Comptons). But would serve as a Yes or No test 13
Test with SCENE cont. 14
Preliminary results First results started coming in August 18 th We trigger on our best SiPM (operate it at a bias voltage of 44 V- trigger on a fraction of a single p.e.) and look for the PMT pulse in a time window that spans negative (PMT fires before) and positive times (PMT signal after SiPM). Expect, if SiPM is triggering on scintillation light, that there are more early PMT pulses than late ones. This is what we see: 15
Preliminary results cont cut on PMT pulse height to select 0.511 MeV ’s 16
Preliminary results cont time distribution: Not clear what the long tail is. Fast comp . ~ 200ns time const Rate of events 0.02Hz confirmed by GEANT4 Simulation of geometry and energy loss in the LAr. Rate is compatible with light with λ in the very end of the SiPM pde curve λ > 900 nm Took data on gas and rate is compatibly lower. 17
Conclusions so far NIR scintillation from LNG is a promising alternative for the much needed light signal (provide t_0; help with PID; essential for SN ’s), one that avoids problems associated with the detection of the VUV light and opens the door for simultaneously improving the charge collection in LAr TPC’s. Simultaneous detection of VUV and NIR in smaller volumes (of the scale of DM experiments) could help separate nuclear recoils from the electromagnetic background. 18
Using the solid xenon facility at PAB to perform a: More comprehensive experimental program with: 1) Simultaneous observation of VUV and NIR light 2) Rigorous purity control 3) Spectral information 4) Time structure of the NIR light 5) Light yield determination 19
The cryostat The external stainless steel chamber A cross-sectional view of the chamber. with 3 glass windows and an outermost The glass chambers are made of Pirex, 5mm diameter of 30cm 20
Two running configurations Looking at the NIR light trough the glass: Pirex is transparent to NIR: trigger on the NIR through opposites windows - no background expected. Radioactive source on the inner chamber. 21
Second configuration: determining the light yield (LY) in the NIR Simultaneously detect the VUV and the NIR: • Trigger on the VUV light: reduces systematic errors in the NIR LY (VUV LY is known). • photon detectors plus radioactive source mounted on a Polytetrafluoroethylene (PTFE) tube: • 22
Light Detection System 23 VUV: Operate inside the cryostat; needs to survive low temperatures, options are: 1.SiPMs coated with TPB, such as SensL MicroSB- 30035 3x3 mm 2 2. Cryogenic PMT such as Hamamatsu R8520-506 (1” square) or R6041 - 506 (1” diam.) coated with TPB. 3. Hamamatsu produces VUV PMTs, with MgF2 windows such as R972 (3/4” diam.) and R1080 (1/2”diam.) with typically Q.E around 10% at 128nm. Can they operate in cryogenic environmt?
Light Detection System – cont. NIR: Use inside and outside the cryostat: InGaAs Single Photon Avalanche Photo Diodes (SPAD) operating in the Geiger mode and configured as an array have been shown to have quite uniform pixel by pixel photon detection efficiency (PDE) larger than 22% at λ = 1.5 m and dark count rates below 50,000 Hz at -20 Celsius (M. A. Itzler et al., 2010). Arrays of InGaAs SPAD’s as large as 256x64 pixels (100 m pitch) have been available since 2010. Several vendors across the world, Princeton LightWaves Inc. in the USA and Micro Photon Device (MPD) in Italy, among others. 24
Acknowldgements The work reported here was done with: Hugh Lippincott, Thomas Alexander and Michael Reid to whom we thank for the support , discussions and input. We thank Hugh for offering free access to the SCENE cryostat and for the many discussions. 27
O2 < 1 ppb H2O < 1 ppb CO < 1 ppb CO2 < 1 ppb H2 < 1 ppb N2 < 1 ppb CH4 < 1 ppb
Fast component : <0.1 us Compatible with our results
Transmissivity of MgF 2
more MgF 2
The argon excimer potential from M. Hofmann’s thesis at TUM, Germany
Emission spectrum of LAr as measured by the TUM group (electron excitation) from M. Hofmann’s thesis at TUM, Germany
Raw data w/o cutting on PH
GEANT 4 simulation of energy loss In LAr:
Geant4 sim. Cont. In gas:
A message from Prof. Andreas Ulrich, TUM (9/16/15) As you say in your slide about “future plans”, spectroscopic information will be important to learn about the origin of the NIR signals which you presented so clearly. I had asked Alexander Neumeier to go over his files to see what he might have besides the data in our publications. He found 4 spectra from gaseous and liquefied argon with a 1% nitrogen admixture. Those spectra actually show several lines between 500 and 1200nm. The data were recorded with a not very sensitive PbS detector. So the light may not so weak. Unfortunately, these spectra are so poor quality and we have no conclusive assignment for the lines that I am not sending them at the time being.
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