Prepared for submission to JINST Photon Detection System Performance in the DUNE 35 ton prototype LAr-TPC detector F. Irst, a , b , 1 S. Econd, c T. Hird a , 2 and Fourth c , 2 on behalf of the DUNE collaboration a One University, some-street, Country b Another University, different-address, Country c A National Laboratory, some-location, Country E-mail: first@one.univ Abstract: The 35 ton (35t) prototype for the Deep Underground Neutrino Experiment (DUNE) far detector is a single phase liquid argon time projection chamber (LAr-TPC) integrated detector. The 35t took cosmic data for a six week run from the start of February to the middle of March 2016. The 35t had two drift volumes on either side of its anode plane assembly (APA) and utilized wire planes with wrapped wires and a photon detection system (PDS) read out by silicon photomultipliers (SiPMs). The PDS of the 35t demonstrated time resolution less than 100 ns, within the requirements of the DUNE far detector. 1 Corresponding author. 2 Also at Some University.
Contents 1 Introduction 1 2 The 35-ton Prototype 2 3 Photon Detector Operations 6 4 Photon Detector Performance 7 5 Conclusion 9 1 Introduction Introduce argon scintillation and the role of the photon detectors in the intro. The Deep Underground Neutrino Experiment (DUNE) is an international, dual-site experiment that will study neutrino physics and search for proton decay. A beam of neutrinos with 2.5 GeV mean energy will be produced by the Long Baseline Neutrino Facility (LBNF) at Fermilab National Accelerator Laboratory (FNAL) and aimed 1300 km through the earth at a 40 kiloton liquid argon time projection chamber (LAr-TPC) far detector located in the Homestake Mine in SD. The beam produced at Fermilab will consist of ν µ and will be measured with a near detector 574 m from the target; the far detector (FD) will measure ν µ disappearance and ν e appearance. [1] The DUNE FD will be the largest LAr-TPC ever constructed and will present multiple engineering and data- processing challenges. The cryostat, electronics, and field cage will need to be scaled up by a factor of 285. Cold digital electronics will be required to minimize the number and lengths of readout cables. In order to prototype and test the necessary technologies and solutions, the DUNE 35 ton prototype detector was constructed and run at FNAL. The DUNE 35 ton prototype (35t) is a prototype single phase LAr-TPC integrated detector which tested DUNE far detector design and components. [2] Phase 1 of the 35t was a test of the membrane cryostat only from Dec. 20, 2013 to Feb. 15, 2014 and achieved the LAr purity required for LAr-TPC running, a 3 ms drift electron lifetime (equivalent to 100 ppt of O 2 ). Phase 2 of the 35t tested new LAr-TPC features in a fully integrated system including both TPC and PDS to characterize the technology’s performance with cosmic ray observations. The 35t’s Phase 2 primary data-taking period was from Feb. 1, 2016 to March 12, 2016. Because the DUNE far detector’s TPC will have drift time on the order of milliseconds, it will be necessary to use another method to precisely measure the time of interaction T0 and match the interactions in the far detector with the neutrino beam spill timing.Not really. We need to be clear that this is only a requirement for non-beam physics. Make the focus NDK and SN physics. Maybe the way to structure this is an intro paragraph about what the PDs will do, then say we will use the 35ton to measure time resolution and coarse attenuation length. – 1 –
Figure 1 . External muon counter positions on 35 ton 2 The 35-ton Prototype The 35t was located on the Fermilab grounds in building PC4. This section will cover the experi- mental design. The primary focus will be on the photon detection system subsection. The 35t cryostat design is described in greater detail in (35 ton design paper). We should put in a few sentences of description here along with the reference. The Time Projection Chamber (TPC) consisted of two drift volumes inside the LAr-filled cryostat between the APAs and the CPAs on either side of them. High energy particles passing through the LAr ionized electrons along their path; these electrons drifted to the APAs by electric fields maintained between the CPAs and APAs. When reaching the APAs, the drift electrons induced signals on the wires wrapped around the induction planes and were then collected by the vertical wires on the collection planes. The induction and collection plane signals were read out by cold electronics and used to reconstruct 3D paths of the particles in the drift volumes. The TPC design and operation is described in greater detail in (35 ton design paper). The 35t used plastic scintillation counters placed around the outside of the cryostat structure to detect cosmic muons passing through the 35t. The positions of the external muon counters are depicted in Figure 1. Signals of coincidences of hits on multiple external muon counters in single event windows indicating throughgoing muons were used to trigger detector readout via the Penn Trigger Board (PTB) during triggered running mode of the system. The photon detection system (PDS) of the 35t consisted of the photon detectors (PDs) installed – 2 –
inside the cryostat, the SiPM Signal Processors (SSPs) used to read out and process the signals from the PD, and the calibration system that produced and diffused UV light inside the cryostat to test the capabilities of the photon detectors. The 35t contains eight separate photon detectors installed on the APAs. The photon detectors consisted of waveguides that led scintillation light to silicon photomultipliers (SiPMs) at their ends. SiPM outputs were read out by SSPs produced by Argonne National Laboratory. Figure 3 shows an example photon detector design. The eight photon detectors installed in the APAs tested several different waveguide technologies. Photon detector positions in the 35t APAs are shown in Figure 2. Photon detectors 0, 4, and 6 were developed by Colorado State University (CSU) and used a design of an array of wavelength-shifting fibers placed behind a radiator coated with TPB. The radiator converts scintillation light incident on it to visible blue light, of which approximately half is caught by the fibers and converted again to green light for transport to the top end of the detector, where they were read out by 8 SiPMs on each PD. Photon detectors 1, 3, and 7 were developed by Indiana University (IU) and used a waveguide consisting of four acrylic bars with 12 SiPMs and readout channels each (3 per bar). Each light guide was coated via dipping with a TPB solution dissolved in DCM at 0.6% by weight. The light guides convert scintillation light entering them into photons of visible blue light; total internal reflection catches a portion of the converted photons and conveys them to the 3 SensL C-series SiPMs on the readout end. Photon detector 2 was developed by Louisiana State University and used an acrylic plank coated with TPB with an embedded bundle of three wavelength-shifting fibers. Scintillation light that enters the plank is converted to blue light, which is captured either directly or after reflections by the fiber bundle which shifts the light to green photons and guided to both ends of the plank, where the fiber bundle is read out on both ends by one SensL B-series SiPM on each end. Photon detector 5 was developed by Lawrence Berkeley National Laboratory (LBNL) and used a waveguide four bar design with 12 SiPMs and readout channels each (3 per bar). [3] Each photon detector’s SiPMs were read out by one of seven SiPM Signal Processors (SSPs) designed and constructed by Argonne National Laboratory. The SSPs received the waveform output from the SiPMs as analog voltages, passed them through a fully-differential voltage amplifier, and digitized the waveforms with a 14-bit, 150 MSPS analog-to-digital converter (ADC). A Xilinx Artix-7 Field-Programmable Gate Array (FPGA) processed the digitized data from each channel with a leading edge discriminator to detect events. SSP readout can be configured in multiple ways including using external triggers for reading out events or self-triggering on the measured waveforms when the amplitude exceeds a threshold set for each channel. Both the externally triggered mode and self-triggering mode were employed during the 35t’s data run. In externally triggered mode, waveforms with the maximum allowed 2048 samples were saved (a length of about 15.5 µ s ) when triggers from the muon counters were received. In self-triggered mode, shorter waveforms with 700 samples (about 5 µ s ) were saved in order to not overwhelm the 35t’s DAQ. Calibration of the photon detector is important for quantifying phenomena such as the energy range of interest, the scintillation light’s fast and slow components, and the propagation of photons including reflections and scattering. The 35t’s calibration system was designed to be capable of examining the above phenomena as well evaluating multiple photon detectors’ relative efficiencies – 3 –
Figure 2 . Photon detector positions in 35 ton – 4 –
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