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The T2K ND280 Off-Axis Pi-Zero Detector

S. Assylbekovb, G. Barrd, B.E. Bergerb, H. Bernsh, D. Beznoskoc, A. Bodekf, R. Bradfordf,1, N. Buchananb, H. Buddf, Y. Caffarib, K. Connollyh, I. Dankoe, R. Dasb, S. Davish, M. Dayf,

S. Dytmane, M. Dziombah, R. Flightf, D. Forbushh, K. Giljec, D. Hansene, J. Hignightc, J. Imberc, R.A. Johnsona, C.K. Jungc, V. Kravtsovb, P.T. Lec, G.D. Lopezc, C.J. Malafisc, S. Manlyf, A.D. Marinoa,, K.S. McFarlandf, C. McGrewc, C. Metelkog, G. Nagashimac, D. Naplese, T.C. Nichollsg, B. Nielsenc, V. Paolonee, P. Paulc, G.F. Pearceg, W. Qiang, K. Ramosc, E. Reinherz-Aronisb, P.A. Rodriguesf, D. Ruterboriesb, J. Schmidtc, J. Schwehrb,

M. Siyadg, J. Steffensc, A.S. Tadepallic, I.J. Taylorc, M. Thorpeg, W. Tokib,c, C. Vaneka, D. Warnerb, A. Weberd,g, R.J. Wilkesh, R.J. Wilsonb, C. Yanagisawac,2, T. Yuana

aUniversity of Colorado at Boulder, Department of Physics, Boulder, Colorado, U.S.A.

bColorado State University, Department of Physics, Fort Collins, Colorado, U.S.A.

cState University of New York at Stony Brook, Department of Physics and Astronomy, Stony Brook, New York, U.S.A.

dOxford University, Department of Physics, Oxford, United Kingdom

eUniversity of Pittsburgh, Department of Physics and Astronomy, Pittsburgh, Pennsylvania, U.S.A.

fUniversity of Rochester, Department of Physics and Astronomy, Rochester, New York, U.S.A.

gSTFC, Rutherford Appleton Laboratory, Harwell Oxford, United Kingdom

hUniversity of Washington, Department of Physics, Seattle, Washington, U.S.A.

Abstract

The Pi-Zero detector (PØD) is one of the subdetectors that makes up the off-axis near detector for the Tokai-to-Kamioka (T2K) long baseline neutrino experiment. The primary goal for the PØD is to measure the relevant cross sections for neutrino interactions that generateπ0’s, especially the cross section for neutral currentπ0 interactions, which are one of the dominant sources of background to theνµ→νeappearance signal in T2K. The PØD is composed of layers of plastic scintillator alternating with water bags and brass sheets or lead sheets and is one of the first detectors to use Multi-Pixel Photon Counters (MPPCs) on a large scale.

Keywords: Neutrinos, Neutrino Oscillation, Long Baseline, T2K, J-PARC, Pi zero Detector

Corresponding author

Email address:[email protected](A.D. Marino)

1Presently at Argonne National Laboratory, Argonne, Illinois, U.S.A

2Also at BMCC/CUNY, New York, New York, U.S.A.

Preprint submitted to Nuclear Instruments and Methods in Physics March 19, 2012

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1. Introduction

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The Tokai-to-Kamioka (T2K) experiment is a long-baseline neutrino oscillation experiment

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designed to probe the mixing of the muon neutrino with other neutrino species and to shed light

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on the neutrino mass scale. The T2K neutrino beam is generated using the the new high-intensity

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proton synchrotron at J-PARC, which has a Phase-I design beam power of 0.75 MW. T2K uses

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Super-Kamiokande [1] as the far detector to measure neutrino rates at a distance of 295 km from

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the beam production point, and near detectors to sample the unoscillated beam. The neutrino

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beam is directed 2.5away from the Super-Kamiokande detector and travels through the Earth’s

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crust under Japan, as illustrated in Fig. 1.

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295 km

280 m J-PARC Near Detector Super-

Kamiokande

1000 m

Neutrino Beam

Figure 1: A schematic showing the path of the neutrinos in the T2K experiment, from the start of the neutrino beamline at J-PARC to Super-Kamiokande, 295 km away.

The T2K experiment [2] near detector complex (ND280), located 280 m from the start of the

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neutrino beam, contains the on-axis INGRID detector and an off-axis detector.

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The off-axis detector, shown in Fig. 2, is situated at the same off-axis angle as Super-Kamiokande

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and contains the Pi-Zero detector (PØD) a plastic scintillator-based detector optimized forπ0de-

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tection followed by a tracking detector comprising two fine grained scintillator detector modules

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(FGDs) sandwiched between three time projection chambers (TPCs). The PØD and tracker are

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surrounded by electromagnetic calorimeters (ECALs), including a module that sits immediately

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downstream of the tracker. The whole detector is located in a magnet with a 0.2 T magnetic

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field, which also serves as mass for a side muon range detector (SMRD). This paper describes

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the PØD subdetector in greater detail.

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1.1. Goals of the PØD

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The primary physics goal of T2K is to measure the mixing angleθ13or to improve the existing

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limit by an order of magnitude if the angle is too small to measure directly. This is done by

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Figure 2: An exploded view of the off-axis near detector.

looking for the appearance ofνein aνµbeam. Additional physics goals include the precision

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determination of the∆m223andθ23 parameters through aνµdisappearance measurement, where

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the parameters will be measured to a precision ofδ(∆m223) ∼104eV2 andδ(sin223)∼0.01

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respectively. In addition to neutrino oscillation studies, the T2K neutrino beam (a narrow-band

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beam with a peak energy of about 600 MeV) will enable a rich physics program of neutrino

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interaction studies at energies covering the transition between the quasi-elastic and resonance

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production regimes.

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To achieve the required precision for theνeappearance measurement (observed via the pro-

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cessνe+ne+p), the neutral currentπ0 rate (νµ+N → νµ+N0+X) must be mea-

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sured at the J-PARC site near the neutrino beam production point using the off-axis near detector.

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Events containingπ0’s are the dominant physics background to theνeappearance signal at Super-

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Kamiokande. The PØD sits at the upstream end of the off-axis detector and has been designed

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to precisely measure the neutral current processνµ+N → νµ+N0+X on a water (H2O)

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target. In addition the PØD will constrain the intrinsicνecontent of the beam flux which is an

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irreducible background to theνeappearance measurement.

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Early design studies demonstrated that understanding theπ0 andνe backgrounds required

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sensitivity to interactions containingπ0with momentum greater than 200 MeV/c. This requires

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a photon reconstruction threshold of well below 100 MeV. Both of the background processes to

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be constrained by the PØD are a relatively small fraction of the total PØD interaction rate, and

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must be measured on a water target, forcing a large water mass. In addition, sufficient energy

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resolution is needed to demonstrate the presence of aπ0 through reconstruction the invariant

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mass. The eventual design of the PØD realizes these goals by interleaving water target between

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scintillator layers which both measure charged particles and support the water target. The rate on

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water is determined using statistical subtraction with data collected during periods having water

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in the detector and out of the detector.

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1.2. Description of the PØD

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The main features of the PØD design are shown in Fig. 3. The electronics supports and

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detector mounting system are visible surrounding the active regions of the detector. In addition

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the regions of the detector are also labeled. Figure 4 shows a schematic of the active regions of the

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PØD. The central region, composed of the ”upstream water target” and ”central water target,”

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is made from alternating scintillator planes, water bags, and brass sheets. The front and rear

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sections, the “upstream ECal” and “central ECal” respectively, use alternating scintillator planes

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and lead sheets. This layout provides effective containment of electromagnetic showers and a

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veto region before and after the water target region to provide rejection of particle interactions

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that enter from outside the PØD.

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Figure 3: 3D drawing of the roughly 2.5 m cube PØD outside of the basket. Downstream face of detector shown. See Section 5 for a description of the TFB and RMM electronics.

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Upstream ECal

Upstream Water Target

Central Water Target Central ECal

Legend

Lead Light-tight Cover

Brass Water Scintillator Wavelength-shifting Fiber

Figure 4: A schematic of the four PØD Super-PØDules as installed in the detector. Beam direction: left to right.

There are a total of 40 scintillator modules in the PØD. Each PØD module, or PØDule, has

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two perpendicular arrays of triangular scintillator bars, forming a plane. There are 134 horizontal

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bars (2133 mm long) and 126 vertical bars (2272 mm long) in each PØDule. Each bar has a

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single coaxial hole through which is threaded a wavelength-shifting (WLS) fiber. Each fiber has

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a mirrored coating applied on one end while the other end is optically coupled to a Hamamatsu

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Multi-Pixel Photon Counter (MPPC) [3] for readout, as shown in Figure 5. Each photodetector

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is read out with Trip-t Front-end electronics (Section 5). There are a total of 10,400 channels for

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the entire PØD.

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The PØDules were assembled into four units called Super-PØDules. The two ECal Super-

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PØDules each consist of a sandwich of seven PØDules alternating with seven stainless steel-clad

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lead sheets (4.5 mm thick). The water target is formed from two units, the upstream and central

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water target Super-PØDules. The upstream (central) water target Super-PØDule is comprised

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of 13 PØDules alternating with 13 (12) water bag layers (each of which is 28 mm thick), and

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13 (12) brass sheets (1.28 mm thick), as shown in Fig. 6. The dimensions of the entire PØD

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active target are 2103 mm×2239 mm×2400 mm (width×height×length) and the mass of

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the detector with and without water is 15,800 kg and 12,900 kg respectively. The PØD is housed

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Scintillator Bar

Fiber

Ferrule

MPPC Signal Wires

External Shell PVC Frame

Figure 5: A close-up view of the edge of a PØDule showing how the WLS fibers exit the scintillator bars and couple to the MPPCs. The optical connectors will be described on more detail in Section 2.2.5.

inside a detector basket structure that supports the central off-axis detectors inside the magnet.

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Figure 6: Expanded view of water target PØDule, brass radiator and water bladder containment frame.

The remainder of this paper describes in detail the design, fabrication, and performance of the

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PØD. The production of the scintillator bars and their assembly into planks and PØDules will be

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presented followed by a description of how the individual PØDules were combined into the four

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Super-PØDules, and are read out using photosensors. The detector component performance,

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starting with scans of the PØDules using a radioactive source, dark noise measurements, and

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tests with the light injection system, is presented. The paper concludes with a description of the

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calibration and performance of the full detector.

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2. Design and Construction of the PØDule

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The PØDule is the basic structural element of the PØD active region, and is constructed

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of scintillator bars sandwiched between sheets of high-density polyethelene (HDPE, thickness

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6.4 mm). The entire structure is surrounded by PVC frames that support the PØDule as well as

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providing mounts for the required services such as the MPPC light sensors, and the light injection

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system.

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The polystyrene triangular scintillating bars that make up the PØDules were fabricated by

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co-extruding polystyrene with a reflective layer of TiO2 and a central hole for the WLS fiber.

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The light seal for the tracking plane is maintained by light manifolds that collect the WLS fibers

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into optical connectors. These manifolds also provide access to the fibers for the light injection

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system. Because of the large number of scintillating bars and the available space limitations, it

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was impractical to route the fibers outside the magnetic volume therefore the Hamamatsu MPPC

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photosensors, which are immune to the magnetic field, were attached directly to each WLS fiber

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just outside the PVC PØDule frame, as shown in Figure 5.

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2.1. Design of the PØDule

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The PØDule was designed to both provide the active tracking region and to serve as a struc-

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tural element. This was achieved using a laminated structure of crossed scintillator bars between

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polystyrene skins. The final PØDule has been shown to have a rigidity similar to a solid mass

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of polystyrene of similar thickness. The edge of the central scintillator and skin structure of the

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PØDule is surrounded by a machined PVC frame. Each PØDule is instrumented on one side

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(both y and x layers) with MPPCs and on the other a UV LED light injection system. The bottom

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PVC frame supports the weight of the PØDule within the ND280 detector basket. The frames

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also provide the fixed points needed to assemble the PØDule into the four Super-PØDules via

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two precision holes located in the four corners of each PØDule as well as a set of seven holes

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spaced along each side through which tensioning rods were passed.

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The PØDules, after installation into the finished PØD, are oriented such that the most up-

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stream layer of scintillator has the bars oriented approximately along the vertical axis while the

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downstream layer has its bars oriented along the horizontal axis. This arrangement results in a

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local coordinate system defined such that the x, y and z axes are approximately congruent with

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the global coordinate system where x is horizontal, y is vertical, and z points downstream toward

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Super-Kamiokande. The external dimensions of the PØDule are 2212 mm (x) by 2348 mm (y)

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by 38.75 mm (z).

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To facilitate assembly of the PØDule (described in Section 2.3), all of the components were

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prefabricated with holes that allowed alignment during assembly. The assembly tolerance was

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less than 0.5 mm on all internal dimensions, and less than 1 mm on the thickness. The rela-

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tive dimensions of the PØDules were maintained using precisely located holes in the PØDule

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assembly table.

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2.2. Assembly of PØDule Components

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The construction and assembly of the PØDule components was distributed across several

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institutions. This allowed a supply chain that could produce the required components in parallel

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and optimized use of facilities, local expertise and available personnel.

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2.2.1. PØDule Scintillator Preparation

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The polystyrene scintillator for the PØDule was manufactured in the extrusion facility at

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Fermi National Accelerator Laboratory [4] using an extrusion die and process originally devel-

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oped for the inner detector of the MINERνA experiment [5, 6]. The blue-light emitting scintil-

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lator base material was Dow Styron 663 W doped with 1% PPO and 0.03% POPOP to shift the

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UV scintillation light emitted by the styrene into the blue. The bars are triangular in cross section

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with nominal dimensions of 17±0.5 mm height and 33±0.5 mm width. Each bar also had a

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nominal 2.6 mm diameter hole centered at 8.5±0.25 mm above the widest part of the triangle

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for fiber insertion. To reflect the produced light and therefore maximize the possibility of capture

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by the wavelength shifting fiber in the center hole, a thin, 0.25 mm on average, layer of styrene

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mixed with 25% TiO2was coextruded on the outside of the bar, and both ends of the scintillator

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bar were painted with white EJ-510 TiO2Eljen paint.

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During production, physical characteristics of the scintillator were monitored by taking fre-

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quent samples and measuring their outer dimensions, the location and dimensions of the center

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hole, and the thickness and coverage of the coextrusion. At approximately twenty different

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equally spaced times during production, samples were also taken and used to characterize light

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output using a radioactive source counting setup with a reference piece of scintillator from the

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MINERνA production. Physical dimensions were held well within the tolerance, and no ev-

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idence was observed for variation in light output beyond the uncertainties in the monitoring

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measurement, roughly 5%.

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2.2.2. PØDule Plank Assembly

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The extruded scintillator bars were bundled into manageable sized “planks” to be used in the

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assembly of the full-sized PØDules. There were two sizes of planks for each of the bar lengths

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and a special jig was constructed for each of the four plank types.

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Each of the triangular scintillating bars was prepared for the plank assembly and subjected

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to quality assurance (QA) procedures prior to assembly into a plank. As each bar was unpacked

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it was inspected for signs of visible damage, such as nicks or cuts in the TiO2coating and any

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damaged bars were removed from plank production. Once a bar passed the visual inspection it

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was cut to length using a jig to ensure proper length. A mounted pneumatic drill was used to

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bore out the ends of the holes running down the center of each bar. A long stiffwire was passed

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through each bar to ensure that no debris was lodged in the hole that would prevent insertion of

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the WLS fiber. An additional check was made to ensure that the hole for the fiber was centered

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on the end of the triangular bar.

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Four separate jigs were set up on two optical tables for the gluing of the bars into planks.

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The short bars were made into two types of planks, one type containing 16 bars and one type

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containing 17 bars. The long bars were made into 15 bar and 17 bar planks. Prior to application

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of the epoxy to the bars, the bars were placed in the jig and a heavy straight-edge was used to

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ensure that the thickness of bars were within the 0.25 mm tolerance of the nominal 17.25 mm

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plank thickness. A log document, or traveler, was kept with each plank during the entire assembly

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process. The traveler contained details such as the plank serial number, the identification number

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of all bars contained in the plank, and any measurements made on the plank during assembly and

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quality assurance.

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Once the bars had been test fitted into the plank gluing jig, they were removed and epoxy

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was applied to each using an automated gluing machine (Fig. 7) that mixed the two epoxy parts

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and applied a steady stream of glue to two sides of the bar. The epoxied bars were placed back

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into the gluing jig and a vacuum sealed frame was used to apply pressure to the plank for about 2

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hours while the epoxy set. A final QA inspection was made to ensure that the planks were within

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the thickness tolerance.

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Figure 7: Automated glue machine used to apply epoxy to the triangular bars before they were placed into the plank gluing jig.

2.2.3. PØDule WLS Fiber Preparation

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The wavelength shifting fibers that are inserted into the holes in the scintillator bars are

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Kuraray multi-clad, S-35, J-type, doped with Y-11 (175 ppm) with a diameter specification of

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1.00+0.020.03 mm. The fibers were placed into the holes in the scintillator, but were not glued into

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place. Studies done for the Minerνa experiment indicated that the light yield for glued fibers was

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approximately 2 times greater than the light yield for unglued fibers [7], but unglued fibers are

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considerably easier to install. The same study also showed that the light yield did not strongly

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depend on the fiber-to-hole diameter ratio over the range of 0.3 to 0.9 [7], so the decision to use

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a 1.0 mm fiber in a 2.6 mm hole does not have a large impact on the light collection.

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The WLS fiber was delivered in unspooled “canes” pre-cut to a rough length 67 mm longer

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than the bar length in order to avoid memory effects of spooled fiber.

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The first step in processing the delivered fiber was to mirror one end. This work was per-

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formed in the Thin Film Coatings facility in Lab 7 at Fermi National Accelerator Laboratory. One

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end of the fiber was first “ice polished” with the ice providing mechanical support for a group of

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approximately 800 fibers polished with a diamond polisher in a single batch. The polished end

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was then coated with aluminum using a sputtering vacuum deposition process. After completion

186

of the mirroring, each fiber was coated with a thin layer of epoxy to protect the mirror.

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The reflectivity of three fibers from each batch of 800 was determined by measuring the light

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output of a fiber with the mirror end placed into a piece of scintillator with an attached radioactive

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source, and then remeasuring the light output after cutting offthe mirror with a 45 cleave and

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painting the cleft end with black paint. For individual fibers, an average reflectivity of 86% was

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measured, with a root mean square of the ensemble of measurements of 6%.

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After mirroring, the fiber was glued into one end of a ferrule (see Section 2.2.5 that was

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injection molded from Vectra A430, a Teflon-filled liquid-crystal polymer. The length between

194

the ferrule end and the fiber mirror end was required to be kept at 1 mm tolerance over the

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several meter length of the fiber. The ferrule was designed to mount into a housing that contained

196

the MPPC and kept the fiber end in contact with the pliable optical layer covering the MPPC.

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Because the Teflon in the Vectra plastic clogged the diamonds in the fiber polisher, the fiber was

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glued into the ferrule a few mm proud of the end of the ferrule, a 1 mm thick layer of optical

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epoxy was deposited for mechanical support, and then the epoxy with the embedded fiber was

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polished. The resulting finish was inspected with a microscope to establish when diamond wear

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adversely affected the finish. Typically, 1500 to 2000 fibers could be polished with a single

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diamond.

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2.2.4. MPPC Acceptance Testing

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The MPPCs used by the PØD, as well as by the ECAL, SMRD, and INGRID detectors,

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are solid-state photosensors manufactured by Hamamatsu Photonics. The active sensor for the

206

MPPC is a 1.3 mm×1.3 mm array of 50-micron pixels, totaling 667 pixels (including a small

207

inactive area for electrical contact). Each pixel operates in Geiger mode, producing a well-

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defined pulse when a photoelectron generates a cascade. The MPPC output is the sum of all

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pixel outputs, the pixels having very similar responses. As a result, the output spectrum from

210

the MPPC shows well-separated peaks corresponding to the number of pixels fired, which at low

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light levels is a good measure of the number of photoelectrons.

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A photosensor quality-control (QC) testing program was performed on all 10,400 MPPCs

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installed in the PØD, as well as 1,100 spares. The goals of the testing were to measure the

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operational characteristics of the MPPCs, to verify that their performance was acceptable, and to

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set the initial operating voltages to be used in the PØD. In particular, the gain of each device was

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measured as a function of bias voltage. A linear fit was used to extract the breakdown voltage –

217

the minimum bias voltage to produce a Geiger cascade. The MPPC characteristics were found to

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be largely consistent from device to device as a function of overvoltage (the bias voltage above

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the breakdown voltage), but there is a significant variation in breakdown voltages from device

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to device, particularly between devices originating in different batches. All tests were done at a

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controlled temperature of 20C, controlled to better than 0.2C, as the MPPC breakdown voltage

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depends on the operating temperature.

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The testing protocol required the measurement of a number of different MPPC operating

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parameters. First, a scan of gain vs bias voltage was performed to measure the breakdown voltage

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and to establish the operating voltage. Measurements were made over a 2 V wide overvoltage

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range around the predicted gain range of 5×105 and 7.5×105. Dark noise rates and relative

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detection efficiencies at these gains were also measured.

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Physics signals in the PØD range from a few to hundreds of photoelectrons. Eight light levels

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across this range were used to characterize the photosensors, with measurements made at four

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different bias voltages for each light level.

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Production testing of the photosensors began in late September 2008 and was completed by

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January 2009. Photosensors passing all QC tests were shipped for installation into the completed

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PØDules. Only 230 out of 11,500 photosensors were rejected by the quality control procedure,

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despite stringent acceptance criteria. Of those rejected, 74 were broken during assembly, 76 were

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rejected due to dirt on the photosensor surface. Only 80 were rejected for abnormal behavior in

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the photosensor testing data.

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A dedicated NIM paper describing the photosensor quality-control testing and characteriza-

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tion procedure in more detail can be found at [8]. A paper describing the PØD MPPC testing in

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greater detail is under preparation.

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2.2.5. Optical Connectors

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Custom optical connectors were fabricated to provide optical fiber alignment to the MPPC

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active area and to reduce the signal rate due to light contamination from external sources to ac-

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ceptable levels to well below the intrinsic dark noise of the MPPCs. The connector system, shown

244

in Fig. 8 consists of three injection-molded components: a fiber-alignment tube or “ferrule”, a

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housing that holds the MPPC and provides alignment to the fiber within 150µm, and an external

246

shell to provide mechanical protection and lock the connector on the ferrule. The material for all

247

three molded components is Vectra®A130, a 30% glass fiber loaded liquid-crystal polymer with

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very low shrinkage, excellent dimensional stability, and very good mechanical properties.

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Ferrule

MPPC Foam Disk

PCB External Shell

Figure 8: Optical connector: exploded (left) and assembled (right).

The MPPC is held in place against the fiber by a 3 mm thick closed-cell polyethylene foam

250

disk, acting as a spring. To ensure good optical contact between fiber and MPPC, the fiber is

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allowed to protrude from the ferrule end by 0.5 mm after polishing, so the entire compression

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force from the foam spring is applied between the fiber end and MPPC face. Electrical con-

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nection between the MPPC and the front-end electronics is provided via a small circular printed

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circuit board (PCB), with spring-loaded pin sockets making contact to the MPPC leads and a

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Hirose Electric Co. micro-coax connector.

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Connectors of this type are used in ND280 for the P0D, ECAL and INGRID sub-detectors.

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Approximately 40,000 total connectors were produced (including spares).

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2.3. Assembly of the PØDule

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The PØDule assembly was one of the main fabrication stages to produce the forty PØDules

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(plus 10% spares). This step involved gluing the main mechanical components: the pre-glued

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scintillators planks, the four outer PVC frames, and the two outer HDPE plastic skins. A key

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requirement for this process was to keep within the tolerances of the thickness of the PØDule (28

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mm) and to have the alignment of the scintillator bars match the wavelength shifting fiber holes

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in the scintillators with the holes in the PVC frames.

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PØDule construction was performed on a specially designed gluing table. This flat table

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(∼2.5×2.5 m) had precision alignment holes to keep the outer four PVC frames in the same

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location for all the PØDules. The first assembly step was the placement of the HDPE plastic

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sheet or skin on the gluing table, followed by painting HYSOL epoxy glue with paint roller

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brushes over the entire top side of the sheet. Next, the four outer PVC frames were placed on

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the edges of the glued sheet, and the position of the PVC frames were fixed by steel pins pressed

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into the holes in the table. Next, the scintillator x-layer planks were lowered onto the bottom

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HDPE sheet. They were aligned with respect to the PVC frame holes, then steel alignment pins

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were pressed through the frame holes and into the holes in the scintillator. After all the x-planks

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were pinned into place, epoxy glue was painted over the upper surface of the x-plank scintillator.

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Then the y-planks were lowered onto the epoxied x-planks, and again the y-plank positions were

276

aligned with steel alignment pins. Next the upper HDPE outer sheet or “skin” was lowered on to

277

the glue-painted y-plank surface.

278

After all the gluing was completed, a vacuum sheet was lowered onto the PØDule and vacuum

279

was applied so the PØDule was uniformly compressed on all sides with about 0.5 atmosphere

280

of pressure. The PØDule was left for about 12 hours to cure overnight. After the epoxy had

281

cured, the vacuum sheet and the pins were removed and the PØDule was loaded with 260 WLS

282

fibers that were each attached to the snap-on optical connector containing an MPPC. Finally the

283

260 MPPCs were connected via mini-coax cables to the scanner readout electronic boards. The

284

PØDule was scanned overnight with a60Co source and read out as described in Section 6.2.

285

During the peak of the construction phase, five PØDules were produced per week. After the

286

PØDules were constructed and successfully scanned, they were stacked together vertically into

287

the four Super-PØDules.

288

2.4. The PØDule Light Injection System

289

The purpose of the PØD Light Injection System (LIS) is to provide monitoring for all 10,400

290

channels in the PØD. The LIS is capable of exposing the MPPC photosensors to light intensities

291

covering a range of more than two orders of magnitude with flash-to-flash intensity stability of

292

less than 2% [9]. This allows for monitoring of the photosensor response over the full range of

293

energy deposition expected for neutrino interactions in the PØD.

294

The main design challenge of the LIS was the geometrical constraint that it be embedded

295

within the 3 cm×4 cm×220 cm PØDule layer support frame for each of the 80 scintillator

296

planes. Each PØDule support frame has a cavity that allows 5 mm segments of WLS fibers to

297

be exposed to light produced by a pair of LEDs (Fig. 9). The LIS uses 80 pairs of fast-pulsed

298

400 nm UV LEDs as light sources; each pair illuminates a cavity in a single PØDule support

299

frame. LEDs exhibit minimal pulse-to-pulse fluctuations in intensity, so the temporal response

300

of an MPPC photosensor is dominated by photoelectron statistics.

301

14

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The LIS provides the capacity to vary the LED light intensity over the required dynamic

302

range through control of both the height and the width of the pulses, by varying the current pulse

303

applied to the LED. The LEDs are driven by electronics originally designed for the MINOS

304

experiment LIS system [9]. The LIS electronics consists of four pulser boxes, a control box, a

305

distribution box and a power supply. The pulser boxes are mounted on the bottom and the north

306

sides on the PØD in close proximity to the LEDs. The control box is situated under the PØD ,

307

mounted on the detector basket. The distribution box and the power supply are located in a rack

308

about 10 meters from the PØD. Each pulser box contains two LED driver boards, a controller

309

board, and an LVDS to TTL converter board. Each driver board has ten channels that can be

310

programmed to pulse simultaneously in group of of 5, 10, or 20 channels. In contrast to the

311

MINOS light injection system, each channel is connected to a pair of LEDs by a 60 cm long

312

shielded cable. Communication between a PIC16F877 microprocessor located on the controller

313

board and a control computer is handled via ASCII commands over a serial RS232 link. Signals

314

from the control computer are carried over an Ethernet link and converted for the pulser boxes

315

by an Ethernet-RS232 converter.

316

During normal operation the PØD LIS control computer instructs the system to pulse a spe-

317

cific combination of LEDs at a specific height, width, and frequency to monitor the temporal

318

behavior of the PØD and associated readout. Dedicated LIS runs are taken periodically to allow

319

more precise measurements of properties, such as the relative timing between the PØD channels.

320

Figure 9: Cross-section view of the LIS cavity. Not shown are transparent strips with an opaque band to shadow nearby fibers.

15

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3. Water Targets

321

3.1. Water Target Design

322

The water target modules were designed to provide layers of water approximately 3 cm thick

323

in the beam direction, with transverse dimensions of approximately 2 m×2 m. In order to

324

provide such large but thin layers without excessive container mass, high-density polyethylene

325

(HDPE) bladders were fabricated using materials and techniques derived from experience with

326

the Pierre Auger Observatory Water Cherenkov detector liners [10]. Adjacent scintillator bar lay-

327

ers provided the structural support required, with water pressure transmitted through successive

328

PØDules to end plates attached to the ND280 basket frame.

329

Since the water bladders are not rigid, it is necessary to protect against excessive deflection of

330

adjacent scintillator bar layers due to unbalanced pressure, such as the case where one water layer

331

is emptied while the others remain full. A vertical HDPE center strut to limit such deflections

332

was required, so each water layer consists of two side-by-side bladders, each approximately 1 m

333

wide by 2 m high by 3 cm thick.

334

The bladders were heat-welded and sealed into HDPE frames that provide ports for fill, drain,

335

and sensor tubes. The frames also accommodate the mounting and support hardware used to as-

336

semble the Super-PØDules. The frames were edged with silicone sponge gasket material (Stock-

337

well Elastomerics R-10470), such that each bladder was effectively encased within a waterproof

338

seal when compressed between adjacent PØDules. This provides a second level of water con-

339

tainment to protect against leaks. The water bladder frames were equipped with drain ports to

340

direct any water captured within the gaskets into two drip pans mounted below the PØD and

341

basket, one on either side of the basket centerline. Each target layer was filled with water and left

342

overnight prior to acceptance and shipment for integration with scintillator layers and assembly

343

into Super-PØDules. The upstream target Super-PØDule had 13 water layers (26 bladders) and

344

the central target Super-PØDule had 12 layers (24 bladders), for a total of 25 water target layers

345

and 50 bladders. After integration, water fill/drain testing was repeated and repeated a third time

346

when the Super-PØDules arrived at J-PARC.

347

In order to meet the physics analysis requirements, a water pump and monitoring system was

348

designed to allow individual bladders to be filled, drained, and also provide water depth data.

349

A pump rack with 50 self-priming bellows pumps (Gorman-Rupp Industries (GRI) 16001-005

350

F-009 T-007), one for each bladder, was designed with valves arranged to allow each pump to

351

16

(17)

be used either to fill or drain a single bladder. The pump rack is located outside the magnet.

352

The pumps are connected to fill and drain tubes on the bladders using Polyflo 66P 3/8-inch OD

353

polyethylene tubing.

354

Water is supplied from a 3000 liter high-density polyethylene tank (DenHartog model VT0900-

355

46), next to the pump rack, coupled to the pumps through two 20-liter buffer tanks on the rack.

356

The main tank is filled with filtered tap water, with commercial chlorine bleach added as a biocide

357

at an effective concentration of 0.025% sodium hypochlorite by volume.

358

Each bladder is equipped with separate drain and fill tubes made of 3/8-inch schedule 40

359

PVC pipe, and has two additional PVC pipes containing water monitoring sensors of two kinds:

360

binary level sensors, which simply report their state as wet or dry, and depth (pressure) sensors.

361

3.2. Fabrication of Water Targets

362

The water target bags are made from a 1 meter wide continuous roll of seamless polyethylene

363

plastic tubing. The plastic sheet is 0.1 cm thick, so the combined thickness of 2 walls of the water

364

bag is 0.2 cm. The tubing is cut to the length of the PØDule height, and is heat welded to make

365

a bag with a leak proof seal at the bottom with no seams on the sides of the bag. Two HPDE

366

frames are then attached, one on the top and one on the bottom section of the bag. The top

367

frame is slotted to provide entrance holes from the top for the water tubes and the pressure and

368

level sensor pipes. The top HDPE frame length is held in position by the HDPE gasket frames

369

on the PØDule. The bottom frame is threaded to allow for through-screws, protruding from the

370

bottom HDPE frame of the gasket frame, to pull the bag taut in the enclosure. This arrangement

371

allows for a leaky bag to be pulled straight from the top of the water target Super-PØDule and

372

replaced by lowering a new bag from the top without removing the Super-PØDule out of the

373

basket frame.

374

3.3. Instrumentation of the water targets

375

In order to properly monitor the water level in each water target layer, sensors are inserted

376

into each water target bladder. Accurate water level readings are needed not only for safety and

377

engineering concerns, but also to determine the water mass inside the fiducial volume of the PØD

378

to the desired accuracy of 3%.

379

17

(18)

The monitoring system consists of sensors that are inserted into the bladder and operated in

380

the water, an external monitoring sensor for the environment, and a DAQ system. The entire

381

system is designed to be independent of the ND280 Global Slow Control system in order to be

382

independent of shutdowns and to provide additional flexibility needed during PØD fill or drain

383

operations.

384

Two sensor pipes are installed in each of the 50 water bladders. In each bladder, both pipes

385

have a Global Water WL400 water depth sensor at the bottom end of the pipe. Each pipe also

386

have one Honeywell LLE series Liquid Level Sensor binary wet-dry level sensors placed near its

387

top, for calibration and back-up purposes. All new sensor pipes were water tested overnight or

388

longer prior to deployment. The depth sensors are calibrated by filling test pipes to a series of

389

measured heights, and logging repeated depth readings. These data are then used to fit calibration

390

curves relating depth to current-loop mA. Results are consistent with factory calibration data.

391

The calibration process is checked with a 1-point measurement using an identical test stand after

392

shipment to J-PARC.

393

The water sensors required an auxiliary, custom built, connection board to distribute DC

394

power and convert current-loop signals to voltage signals to be read out by the sensor boards.

395

The sensor boards are mounted on the top of the PØD, above the water target bladders. This

396

board contained a SenSym ICT series ASDX Pressure Transducer, a Texas Instruments TMP275

397

Digital Temperature Sensor, 12-bit ADCs to read the sensor outputs, and digital logic to com-

398

municate with the rest of the DAQ. Communication between the DAQ and system hardware

399

components is performed using I2C. Signals are converted to RS-232 serial data for transmission

400

due to the distance between the PØD and the DAQ computer. Sensor signals were digitized on

401

the Sensor Boards, which were in turn connected to a custom built, 8 port Multiplexer (Mux)

402

Board using the I2C bus. A total of four Mux boards are required. The Mux Board converted

403

the I2C signal to RS232 and also supplied power to the sensors and electronics. The sensors are

404

controlled and monitored by a graphical interface using the Qt 4.0 application and user interface.

405

The monitoring program controls the sensor readout, interprets the data, feeds the data to the

406

GSC system, and then stores the data locally.

407

An identical WL400 sensor and readout board is installed in the main water storage tank in

408

order to monitor its water level during filling and draining.

409

18

(19)

4. Super-PØDules

410

The full PØD detector is constructed of four Super-PØDules assembled from PØDules. Fig-

411

ure 4 shows the arrangement of the four Super-PØDules in the assembled detector as well as

412

details of the structure within each type of Super-PØDule.

413

4.1. The Super-PØDule Design and Tooling

414

The four Super-PØDules that make up the PØD were designed to simplify the installation

415

and shipping from the assembly site to the installation site at J-PARC while allowing for effi-

416

cient assembly. This was achieved by assembling all four modules from a set of standardized

417

components (PØDules, water target modules, brass radiators, and lead radiators) that could be

418

preassembled prior to the Super-PØDule assembly. The components were assembled onto their

419

final mounting hardware, which was held on custom rolling carts. The carts were designed to sup-

420

port the Super-PØDules and allow them to be moved. After arrival in Japan, the Super-PØDules

421

were kept on their individual carts which allowed them to be moved and tested independently.

422

Table 1 shows the mass, dimensions and the depth in radiation lengths of each Super-PØDule.

423

During assembly the component masses were sampled and used to estimate the dry mass of

424

each Super-PØDule. The mass of the water added to the target Super-PØDule is accounted for

425

separately. We estimate that the dry mass uncertainty is approximately 0.8%. The length of

426

each Super-PØDule along the neutrino beam axis was measured after assembly and is estimated

427

to have an accuracy of 0.5 mm. The width and height dimensions of the Super-PØDule are

428

perpendicular to the beam axis and have a tolerance of 5 mm. These dimensions include space

429

for the TFB read out electronics.

430

4.2. ECal Super-PØDule Assembly

431

An ECal Super-PØDule is assembled from seven PØDules interleaved with seven layers of

432

lead plates. The assembly began with the construction of the lead panels. The first step was

433

the preparation of a 0.05 cm thick, or 0.03 radiation lengths (R.L.), stainless steel (S.S.) sheet

434

which was mounted on a flat assembly table. A thin aluminum frame was then epoxied and

435

screwed on all four sides of the sheet. Epoxy was applied to the surface of the S.S. sheet and

436

25 3.45 mm thick (0.67 R.L.) lead strips were gently positioned side-by-side onto the epoxied

437

surface. The lead plates were precut to size to minimize any gaps. The lead plates were then

438

19

(20)

Super-PØDule Mass Dimensions Depth (kg) (mm×mm×mm) in R.L.

Upstream ECal 2900 2298×2468×305 4.946 Upstream Water 2298×2468×888

Target:

Empty 3600 1.370

Filled 5100 2.379

Central Water 2298×2468×854 Target:

Empty 3500 1.356

Filled 4900 2.287

Central ECal 2900 2298×2468×304 4.946

Table 1: The mass, dimensions, and depth in radiation lengths for each Super-PØDule.

painted with epoxy and two stainless steel half-width sheets were placed onto the pre-epoxied

439

lead plates, creating a panel that contained lead plates sandwiched between a pair of S.S. sheets.

440

A vacuum cover was placed over the entire assembly and evacuated prior to allowing the epoxy

441

to set overnight. Figure 10 is a photo of a S.S. plate being placed onto a layer of lead panels that

442

have been painted with black epoxy.

443

Once the epoxy had set overnight, a U-channel beam was placed across the top of the com-

444

pleted lead panel and screwed into position. The U-channel beam was then lifted with a forklift

445

to position the lead panel and rotated to hang vertically. The lead panel was then moved to a

446

rollabout cart and mounted to a frame. Once the panel was positioned and held into place by the

447

cart frame, the U-channel beam was removed. The completed PØDule, was then lifted vertically

448

and mounted onto the lead panel. This process was repeated seven times to complete an ECal

449

Super-PØDule. After the seventh PØDule was added, threaded stainless steel rods were screwed

450

through clear holes around the periphery of the PØDules and secured in place with a special flat

451

nut.

452

Finally the electronics mounting rail assemblies were attached to the top and on one side of

453

the ECal Super-PØDules where the MPPCs were mounted. Once the readout electronics (see

454

20

(21)

Figure 10: A stainless steel plate being placed onto a lead panel

Section 5) were installed, the mini-coax cables from 1820 MPPCs were connected to the TFBs

455

and labeled, completing the assembly. Figure 11 is a photo of a fully assmbled PØDule, mounted

456

vertically on a rollabout stand.

457

4.3. The Water Target Super-PØDule Assembly

458

The water target bag Super-PØDule assembly on the PØDules consisted of two water blad-

459

ders that were mounted on the face of one PØDule and encased by HDPE frames on four sides

460

(top, bottom, left, and right). This procedure was performed on 25 PØDules to form two separate

461

water target Super-PØDules that held 12 or 13 water layers.

462

The assembly began with a PØDule placed flat on an assembly table. Two brass sheets of

463

thickness, 0.15 cm (0.1 R.L.) were placed on the PØDule. A thin weather strip gasket was placed

464

along the PØDule edges on two sides and the bottom. Three HDPE frames were then placed over

465

the weather strips to form a water tight gasket seal and then a vertical HDPE center divider strip

466

was placed in the middle of the PØDule. Next, the two water bladders were placed on the brass

467

sheet. Each water bag had a top and bottom HDPE frame. The next step was to place the weather

468

strip gasket in a groove in the three HDPE frames.

469

Finally a thin HDPE cover sheet was placed over the entire surface to form a water-tight seal

470

and to keep the water bladders from protuding out when the PØDule was mounted vertically and

471

moved. The upper water bladder HDPE frame was supported by slots in the upper HDPE gasket

472

21

(22)

Figure 11: The fully-assembled upstream ECal Super-PØDule including the TFB readout electronics boards.

frame. The lower water bladder HDPE frame had mounting holes that were screwed down with

473

screws extending through the bottom HDPE gasket frame. The bottom HDPE gasket frame had

474

drain holes or ports, which were connected to a drain hose to allow any water leak to drain out in

475

a controlled manner.

476

4.4. Shipping and Installation of the Super-PØDules

477

Each of the four Super-PØDules was mounted on a solid wooden base and enclosed in a

478

wooden crate for shipping. The four Super-PØDules were flown from JFK airport in New York

479

to the Narita International Airport near Tokyo by the Nippon Express shipping company in April

480

2009. The crated Super-PØDules were then delivered to J-PARC on two flatbed trucks and

481

22

(23)

offloaded into the LINAC building where they were unpacked, removed from their wooden bases

482

and checked out in preparation for installation. All electrical and plumbing utilities were installed

483

in the detector hall and attached to the off-axis detector basket while the Super-PØDules were

484

being checked out in the LINAC building, which simplified the installation of the Super-PØDules

485

into the basket.

486

Each Super-PØDule remained bolted to custom rolling carts until just prior to installation.

487

Aluminum covers were attached to the side and top electronics of the Super-PØDule to prevent

488

damage during installation into the detector basket. Super-PØDules were installed in order from

489

the most upstream side of the detector basket. For installation each Super-PØDule was discon-

490

nected from its cart and lifted by a crane using a custom lift fixture (Fig. 12) that allowed precise

491

positioning of the Super-PØDule as it was lowered into the basket about 20 m below the staging

492

area. Figure 12 shows the first Super-PØDule, the upstream ECAL, being lowered into position

493

with guidance from a local contractor.

Figure 12: The left figure shows an engineering drawing of an ECAL module being lifted by the custom lift fixture, while the right picture shows the first Super-PØDule (upstream ECAL) being lowered into the detector basket.

494

Once a Super-PØDule was set into position in the basket, it was bolted into place and util-

495

ity connections were made to it. Precise positioning of each Super-PØDule in the basket was

496

accomplished using adjustment screws. Following the utility connections and the filling of the

497

water targets, aluminum covers were attached to the PØD in preparation for data taking.

498

23

(24)

5. Readout Electronics and DAQ

499

The PØD, like the SMRD, ECal, and INGRID detectors in the ND280, uses Trip-t-based

500

front-end electronics [11] to read out its 10,400 photosensors. Each Trip-t Front-end Board

501

(TFB) contains four 32-channel Trip-t ASIC chips [12], originally developed at Fermilab for the

502

DØ experiment, and can serve up to 64 MPPC sensors, which are connected by miniature coaxial

503

cables to the TFB. The signal from the photosensor is divided capacitively in the ratio of 10:1

504

and routed into separate Trip-t channels to provide a high and a low gain response to the same

505

input, thereby increasing the dynamic range of the electronics. The high gain channel provides

506

measurement for up to a 50 photoelectron (p.e.) signal with∼10 ADC/p.e. resolution, while the

507

low gain channel can be used to measure larger signals up to about 500 p.e.

508

The Trip-t chips integrate the charge in 23 consecutive integration cycles that are synchro-

509

nized with the accelerator beam spill such that each 58 ns wide beam bunch, separated by∼

510

580 ns, falls into a separate integration window 100 ns away from the start of each integration

511

cycle. The length of the integration cycle and the reset period between them can be programmed

512

and are set to 480 ns and 100 ns respectively for beam operation. These values will virtually

513

eliminate deadtime for in beam spill interactions but will result in some deadtime for out of spill

514

events such as Michel electrons. The Trip-t stores the results of the 23 integration cycles in its

515

analog pipeline, which are digitized using a 10-bit ADC when the data are transferred offthe

516

board following the end of the integration period. The high gain channels also feed an internal

517

discriminator that measures the time when the integrated signal exceeds a programmable preset

518

threshold of about 2.5 p.e. in each integration cycle. The operation of the TFB is controlled

519

by a Field Programmable Gate Array (FPGA), which also provides time stamping for the dis-

520

criminator output with a 2.5 ns resolution, and moves the data from the Trip-t to the back-end

521

electronics.

522

The TFB operation requires four low voltage levels: 5.0 V, 3.3 V, 2.5 V, and 1.2 V. The bias

523

voltages for the MPPCs are also provided through the TFB with the central core of the mini-

524

coax cables connecting the sensors to the board being at the same uniform voltage level (∼70 V).

525

Individual voltage adjustments are achieved by setting the voltage level of the shield sheath of

526

the coax cable between 0 to 5 V in 20 mV increments using an 8-bit DAC.

527

The entire PØD readout requires 174 TFBs; 29 for each of the upstream and central ECal

528

Super-PØDules and 58 for each of the water target Super-PØDules. The back-end electronics of

529

24

(25)

the PØD consist of 6 read-out merger modules (RMM), a cosmic trigger module (CTM) shared

530

with the SMRD and the downstream ECal (DsECal), a slave clock module (SCM), and one mas-

531

ter clock module (MCM) for the whole ND280 detector. Each RMM serves as a communication

532

interface between the 29 TFBs and the data acquisition (DAQ) system, by passing control com-

533

mands, clock, and trigger signal in one direction, and data in the other. Signals between the

534

RMM and TFBs are transmitted using the LVDS protocol via shielded Ethernet cables, while the

535

RMMs are connected to the DAQ computers with optical Gigabit Ethernet links.

536

Cosmic trigger primitives are formed from the 29 TFBs on the upstream ECal Super-PØDule

537

based on coincidences between some number of MPPCs. These trigger primitives are transmitted

538

to the CTM and combined with other trigger primitives from the SMRD and the DsECal to create

539

a global cosmic trigger decision when any side pairs of the ND280 detector are traversed by a

540

cosmic ray muon. The MCM receives the accelerator timing signals when a spill happens and

541

transmits trigger as well as periodic clock synchronization signals to the RMMs and TFBs via

542

the SCM. The MCM can also generate pedestal and calibration triggers, such as for synchronous

543

operation of the PØD light injection system. The SCM duplicates most of the functionality of

544

the MCM and allows the configuration and operation of each sub-detector, such as the PØD, in

545

standalone mode independent of the other detectors.

546

The global ND280 DAQ [2] [13] uses the MIDAS framework [14], developed at TRIUMF,

547

operating on computing nodes running Scientific LINUX operating system. The TFBs are con-

548

trolled and read out by the front-end processing nodes (FPN), each of which serves two RMMs

549

(3 FPN are used for the PØD). The read-out and configuration is provided by the read-out task

550

(RXT), while the raw data from the TFBs are decoded and formatted for output by the data

551

processing task (DPT). The DPT also performs per-channel histogramming of specific trigger

552

types before zero suppression and inserts the histograms to the output data stream periodically.

553

An event builder process collects the fragments from the sub-detectors, and writes the fully-

554

assembled events to a buffer after basic consistency checks. Finally, an archiver process transfers

555

the completed files to mass storage and creates a preview copy on a local semi-offline system for

556

fast-turnaround calibration and data quality checks.

557

A global slow control (GSC) complements the global DAQ using the same MIDAS-based

558

software framework. Front-end tasks running on the sub-detector computers connect to various

559

equipment of the sub-detectors and collect monitoring data that are stored in a MySQL database

560

25

Сурет

Figure 1: A schematic showing the path of the neutrinos in the T2K experiment, from the start of the neutrino beamline at J-PARC to Super-Kamiokande, 295 km away.
Figure 2: An exploded view of the off-axis near detector.
Figure 3: 3D drawing of the roughly 2.5 m cube PØD outside of the basket. Downstream face of detector shown
Figure 4: A schematic of the four PØD Super-PØDules as installed in the detector. Beam direction: left to right.
+7

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ISSN 1991-3494 Print ҚАЗАҚСТАН РЕСПУБЛИКАСЫ ҰЛТТЫҚ ҒЫЛЫМ АКАДЕМИЯСЫНЫҢ Х А Б А Р Ш Ы С Ы ВЕСТНИК НАЦИОНАЛЬНОЙ АКАДЕМИИ НАУК РЕСПУБЛИКИ КАЗАХСТАН THE BULLETIN OF THE NATIONAL