Design and Performance of the DUNE 35-ton Prototype Time Projection Chamber The 35-ton Author List Addresses for the 35-ton Author List Abstract The DUNE 35-ton prototype time-projection chamber was designed to test the functionality of the components foreseen to be used in the far detector for the DUNE experiment. The Phase I run, completed in early 2014, demon- strated that liquid argon could be maintained at sufficient purity in a mem- brane cryostat. A time projection chamber was installed for the Phase II run, which collected data in February and March of 2016. The Phase II run was a test of the modular anode plane assemblies with wrapped wires, cold readout electronics, and integrated photon detection systems. While the details of the design for the DUNE far detector has since evolved, the 35-ton prototype is a demonstration of the functionality of the basic com- ponents. Measurements are performed using the Phase II data to extract signal and noise characteristics, detector alignment and performance in the gaps between modules, as well as measurements of the electron lifetime and diffusion characteristics. Prototype, Liquid Argon, Time Projection Chamber Keywords: 1. Introduction 1 The 35-ton prototype was designed to test the performance of the con- 2 cepts and components to be used in the DUNE far detector. The DUNE 3 far detector is proposed to consist of 40 kTons (fiducial) of liquid argon in 4 four 10 kTon modules located at the 4850’ level of the Sanford Underground 5 Research Facility (SURF) in Lead, South Dakota. The start of installation 6 of the first 10 kton module is scheduled to begin in 2021. The DUNE far 7 detector modules will be much larger than any previous liquid argon time 8 projection chamber (TPC), and the components must be shipped to the site, 9 Preprint submitted to Nuclear Instruments and Methods in Physics Research ADecember 7, 2016
lowered down the shaft, assembled in place, tested, and operated, all in a 10 cost-effective and time-efficient manner. These steps place constraints on 11 the design of the far detector, and compromises must be made in order to 12 satisfy these constraints. To meet the physics goals of DUNE, the perfor- 13 mance of the detector must satisfy basic requirements of spatial, time, and 14 energy resolution, signal-to-noise performance, detection efficiency and up- 15 time. The design choices must be tested in a prototype before the far detector 16 design is finalized for the far detector and resources are committed. Section 2 17 describes the design of the 35-ton prototype and which design choices for the 18 far detector are tested. Because of the rapid evolution of the far detector de- 19 sign, the choices considered when the 35-ton prototype design was finalized 20 are no longer exactly those considered for the DUNE far detector, although 21 the broad features are the same. Section 2 describes these issues in detail. 22 The data acquisition system is described in Section 3, and the running 23 conditions are summarized in Section 4. Several analyses of the data from 24 the Phase II run of the 35-ton prototype are listed in Sections 6 through 12. 25 These comprise studies of the signal and noise performance of the system, 26 the relative alignment of the external counters and the TPC using cosmic-ray 27 tracks, the measurement of the relative time between the external counters 28 and the TPC using tracks that cross the anode-plane assembly (APA) vol- 29 umes, alignment and charge characteristic measurements using tracks that 30 cross between one APA’s drift volume to another’s, a measurement of the 31 electron lifetime, and studies of diffusion of drifting electrons. A summary 32 and outlook is given in Section 13. 33 2. Detector Design 34 The 35-ton prototype was designed by the LBNE collaboration and project 35 effort in order to test the design of the LBNE far detector, a massive liquid- 36 argon TPC to be located at SURF [ ? ]. In order to ship the APA’s from 37 their manufacturing site to SURF in standard high-cube shipping contain- 38 ers, lower them down the shaft at SURF and assemble them in place, they 39 are limited in size to 6m × 2.5m. Amplifiers and digitizers are placed in the 40 cryostat in order to reduce thermal noise and simplify the cabling. It is more 41 cost-effective to place the readout electronics on the ends of the APA’s, and 42 thus the far detector has two layers of APA’s: one with electronics read out 43 on the top and one on the bottom. As the total volume of liquid argon is 44 very large and the drift length should be limited to 3.6 m in order to combat 45 2
the effects of electron lifetime and diffusion, the APA’s are installed inside 46 the active volume and read out on both sides. 47 It is predicted that a steep angle of 45 ◦ between the wires of separate wire 48 planes optimizes the physics reach [ ? ], by providing a high degree of spatial 49 resolution for measuring the displaced vertex positions in π 0 → γγ decay, 50 with subsequent showering of the photons some distance away from the π 0 51 vertex. This steep angle, coupled with the aspect ratio of the APA frames 52 and the need to read out the wires only on one edge of the APA in order to 53 reduce cable runs and make the electronics accessible, results in the design 54 choice that the angled wires are to be wrapped around from one side of the 55 APA frame to the other and back. The angled wires are the U - and V -plane 56 induction wires. There are two planes of collection wires, one on each side of 57 each APA, that do not wrap around from one side to the other. 58 The wrapped induction-plane wires induce an ambiguity in the inter- 59 pretation of the charge read out on them, as the charge could have been 60 deposited on any of the wire segments, and also at any position along the 61 wire segments. If the angles of the U and V planes were chosen to be ± 45 ◦ , 62 this ambiguity would not be possible to break in the data, even for single, 63 isolated hits, as the combinations of U , V , and Z wires cross in multiple 64 places. In order to break this degeneracy, the angles were chosen to be 45 . 6 ◦ 65 for the U plane and − 44 . 3 ◦ for the V plane, and the collection wires were 66 chosen to be vertical. 67 An unistrumented grid wire plane is situated between the U plane and the 68 drift volume, and a grounded mesh is installed between the collection plane 69 and the argon volume inside the APA frame where the photon detectors lie. 70 The 35-ton prototype is designed to test the performance of a detector 71 with these choices. In order to fit inside the membrane cryostat of the Phase I 72 prototype, however, the APA’s and the drift volumes were shortened relative 73 to the far detector design. A drift region as long as possible to fit in the 74 35-ton cryostat was designed, while still having a shorter drift region on the 75 other side of the APA in order to test the double-sided readout functionality 76 of the APA’s. The long drift length of the 35-ton prototype is 2.258 m from 77 the center of the APA to the cathode, while the short drift length is 0.302 m. 78 The APA’s are also smaller versions of the ones to be used in the DUNE 79 far detector. Two tall APA’s are mounted on the ends of the plane of APA’s. 80 They measure 2 m vertically by 0.5 m horizontally, and extend from the 81 bottom of the detector to the top. Their electronics are mounted on the top. 82 Two shorter APA’s are mounted between the two long ones, both 0.5 m wide. 83 3
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