1. INTRODUCTION

Task 8.6 fosters knowledge sharing and common tools in the neutrino community regarding state-of-the-art magnetization schemes. These schemes concern both finely segmented magnetized iron detectors and the possibility of magnetizing large liquid argon detectors. Magnetization of neutrino detectors is required to determine the charge of leptons from charged current (CC) interactions in order to identify neutrinos and antineutrinos. There are two possible ways of doing this. The first way is to magnetize large volumes with iron, in order to create a magnetic spectrometer downstream of a non-magnetized detector, such as a liquid argon or fully active scintillator detector. This is the concept behind Baby MIND, which was developed to be installed as a magnetic spectrometer downstream of a liquid argon detector in the LBNO scheme [1,2]. This concept evolved and a new magnetization scheme was designed and built at CERN to be tested at dedicated test beams [3]. Subsequently, the Baby MIND detector was constructed at CERN in 2016 and 2017 and its performance was determined on a low energy charged particles test-beam in the context of the neutrino platform activity at CERN in 2017. This was the first prototype built with a novel magnetization scheme. Baby MIND was transported to the J-PARC neutrino facility in Japan at the end of 2017 and was assembled behind the WAGASCI experiment at the beginning of 2018. Baby MIND and WAGASCI were commissioned on the neutrino beam of the T2K ND280 near detector location in April and May 2018 [4-15]. Overall, this networking activity was very effective in testing innovative aspects in the field and forming the community.

The construction of liquid argon detectors at the 10 kton scale is an essential ingredient of the future international long-baseline neutrino programme being built in the USA. The DUNE long-baseline experiment [16-18] relies on building large-scale underground liquid argon detectors to search for CP violation in the neutrino sector. Prototype liquid argon detectors are being constructed at CERN as a first step towards this goal [19-20]. Charge-identification capabilities could be carried out by magnetizing the large liquid argon modules, using appropriate cryogenic superconducting coils. WP8 activities focus on the most challenging aspects related to this detector development.

2. NOVEL MAGNETIZATION SCHEME: BABY MIND

2.1. DESIGN OF BABY MIND

The Baby MIND detector that is the subject of this task, is built on the basis of a novel magnetization scheme for finely segmented magnetized iron detectors. Baby MIND is designed with 33 ARMCO steel magnet modules (~65 t) of dimensions 2000 x 3500 x 30 mm³ interleaved with 18 scintillator detector modules. The magnetization design was carried out by A. Dudarev and the ATLAS Magnet Group at CERN. The ARMCO plates were produced by a company in Spain and machined at CERN. Following coil assembly, the first plate was tested showing a B-field of 1.5 T for a coil current of 140 A, with low power consumption 11.5 kW and excellent uniformity across the tracking region, as expected from simulations (see Fig. 1). Two slots in the plates allow the threading of the conductor on either side of the plate, creating three uniform horizontal dipole regions. The flux return on the edge of the plates is designed to fully contain the magnetic field lines. Stray fields outside the iron were found to be less than 15 mT. The assembly of the Baby MIND detector was completed in June 2017.

Fig. 1 Magnetization scheme for ARMCO plates in Baby MIND.

Particle hits are detected by scintillating bars providing horizontal and vertical position information. There are 18 scintillator modules. Each scintillator module is constructed from 95 horizontal bars (providing two horizontal planes, 3000 × 31 × 7.5 mm³) and 16 vertical bars ( 1950 × 210 × 7.5 mm³ for two vertical planes). They provide a total size of the scintillator module of 3000 × 1950 × 30 mm³. The finer resolution along the vertical direction is important to determine the curvature of tracks in the spectrometer. The bars are arranged in 4 planes, of horizontal, vertical, vertical, horizontal, with an overlap between planes to achieve close to 100% hit efficiency for minimum ionizing muons. INR Moscow built and designed the scintillator bars, providing a good light yield. The bars are polystyrene based, 1.5% PTP, 0.01% POPOP, with a reflective coating of 50 to 100 μm thickness etched on the surface, and are mechanically held together by an aluminium support frame. The light is collected by embedded Kuraray wavelength-shifting (WLS) fibers (200 ppm, S-type, diameter 1.0 mm) inside a groove. They contain a reflective coating of 50 to 100 μm thickness. A schematic view of the horizontal bars can be seen in Fig. 2 and the vertical bars in Fig. 3.

Fig.2 Schematic of the horizontal bar and light yield distributions.

Fig.3 Schematic of the vertical bar and light yield distributions.

The Kuraray scintillating fibres are read out using Hamamatsu Multi Pixel Photon Counters, MPPC S12571-025C and S10943-5796, of size 1 × 1 mm²(65% fill factor) and 25 μm cell size. The operating voltage is ~ 67.5 V with photon detection efficiency (PDE) of 35%, gain 5 × 10⁵ and dark count rate of 100 kcps. The MPPC signals, sampled at 400 MHz, are powered (HV/LV) and read out by custom-made 96 channel Front-End Boards (FEB), see Fig. 4, using CITIROC ASICs. These rack-mounted FEBs have been designed by Geneva University containing 3 × 32 channel connectors, three CITIROC ASICs with 32 channels each. The FEBs are installed in mini-crates which can connect up to seven FEBs via readout/slow control on USB3 and/or Gigabit using a backplane, seen in Fig. 5. Data is sent from the mini-crates to DAQ computers via USB3 and passed onto the data acquisition computer.

Fig.4 Schematic view of FEB layout (left) and photograph of FEB inside rack (right).

Fig.5 Front-end electronics minicrate (left) and block diagram of backplane (right).

2.2. BABY MIND TESTBEAM AND PERFORMANCE

Baby MIND was extensively tested and qualified with cosmic rays prior to beam tests. The evaluation of track reconstruction and of the charge identification efficiencies was performed at the experimental zone of the CERN Proton Synchrotron East Hall in June-July 2017 with a (mainly) muon beam from 0.5 to 5 GeV /c. The fully constructed detector is visible in Fig. 6.

Fig. 6 Exposure of the Baby MIND detector to a low energy muons beam at the CERN PS.

The magnet was operated for 5 weeks at ~1.5 T, 11.5 kW during the beam test phase. Event displays of muon tracks reconstructed in the detector can be seen in Fig. 7. Preliminary results from the analysis of the test beam data show charge identification efficiencies in line with expected values (see Fig. 8).

Fig. 7 Event display of muon tracks in the Baby MIND detector exposed to the test-beam line at the CERN PS.

Fig. 8 Charge reconstruction efficiency comparing beam test data and simulation. The bottom plot is a zoom of the top plot for momenta below 2 GeV /c [15].

2.3. INSTALLATION BABY MIND DETECTOR AT J-PARC NEUTRINO BEAM

The Baby MIND detector was transported to Japan in December 2017 in order to be installed at the ND280 near detector location on the T2K neutrino beamline in combination with the WAGASCI neutrino detector (see Fig. 9). The transport of the Baby MIND detector to Japan was widely publicised in the nTrack AIDA2020 Newsletter and the CERN Courier [21-23]. The logistics for transporting the Baby MIND muon spectrometer from CERN to J-PARC required extensive planning. The detector left CERN on 17th and 18th October in 4 containers, weighing roughly 20 tons each. The installation phase took 2 weeks in February 2018, with each of the 33 magnet modules being lowered down a narrow shaft one-by-one. A new 400 V 3-phase power line was installed on 21st February 2018 to supply the magnet. The magnet power supplies were switched on for the first time at J-PARC on 14th March, with readings for operational parameters consistent with operation at CERN during summer 2017.

Fig. 9 The Baby MIND detector installed in Japan at the T2K ND280 location.

Scintillator detector module and readout electronics commissioning began around the 9th March, a day of trials for the T2K beam line. Several days were spent tuning the electronics to synchronize to the T2K beam line trigger, and separately to the WAGASCI DAQ Start/Stop signals. Events from the first few runs, synchronized in time with the arrival of the neutrino beam, show the typical T2K neutrino beam bunch structure (Fig. 10).

Fig. 10 Events triggered in the Baby MIND detecto, showing the bunch structure of the J-PARC neutrino beam.

Data taking with neutrinos with the Baby MIND detector at the T2K ND280 location took place in May 2019 as part of a commissioning run for Baby MIND. The data taking mode was with a reverse horn current (RHC) in which there is a majority of muon antineutrinos, rather than muon neutrinos, in the beam. A preliminary analysis to reconstruct separately antineutrino and neutrino events is shown in Fig. 11, showing that the Baby MIND spectrometer can successfully reconstruct the charged muons from CC interactions.

Fig. 11 Energy spectrum of muon antineutrinos (left) and neutrinos (right) reconstructed with BABy MIND at the J-PARC neutrino beam in reverse horn current (RHC) during Baby MIND commissioning [15].

3. MAGNETIZATION OF LIQUID ARGON VOLUMES

Initial ideas on how to perform the magnetization of liquid argon volumes were based around the concept of the Superconducting Transmission Line (STL) developed for the Design Study for a Staged Very Large Hadron Collider [24], which was being used as an example of a possible magnetization cable for future Magnetized Iron Neutrino detectors (MIND) at a Neutrino Factory [25], which could potentially carry 100 kA current. The STL concept consists of a cylindrical superconducting braid inside a pipe cooled by supercritical helium. The superconductor and cryo-pipe are coaxial to a cyclindrical cryostat/vacuum vessel (see Fig. 12).

Fig. 12 Schematic of Superconducting Transmission Line (STL) showing construction details [25].

A more promising technology includes the use of ReBCO Coated Conductors, which are ceramic flat tape High Temperature Superconductors (HTS) that can operate at a temperature between 20 K and 60 K. The CERN ATLAS Magnet group (EP-ADO), have been studying HTS cables under the coordination of A. Dudarev and H.H.J. ten Kate. ReBCO cables have been studied in the disserattions of J. van Nugteren and T. Mulder [26,27]. Of particular interest is the study of Conductor on Round Core ( CORC) cables. The structure of a CORC cable features a round cable, with ReBCO tapes twisted in many layers around a central metallic core, which reduces AC loss. The CORC cable includes a CORC conductor with a typical diameter between 5 and 8 mm, with 3 or 4 mm wide ReBCO tapes. The mean current density of such a CORC cable could reach 150 A/mm² at 4.2 K and could achieve magnetic fields of 20 T in some applications. A CORC Cable-In-Conduit Conductor ( CICC) consisting of six-around one bus bar was designed as part of T. Mulder’s thesis [27]. This design (Fig. 13) was shown to carry a current of 48 kA at 4.2 K and 10 T. Future developments in this area show the potential to reach currents up to 80 kA at 5 K and 12 T (with a current density of 89 A/mm²).

Fig. 13 Schematic of CORC six-around-one bus bar, described in [27].

Alexey Dudarev from CERN has supplied a design for the magnetization of a large liquid argon detector volume, by integrating high temperature superconducting lines as part of the design, commissioned in the context of the CERN Neutrino Platform. Fig. 14 shows the schematic of this concept for a large volume liquid argon cryostat vessel and magnet. The racetrack pair of HTS coils in the upper and lower part of the cryostat generate a magnetic field, depending on the current carried by the coils. The cryostat needs to be reinforced in order to allow it to sustain the 30 MN that it has to stand due to the magnetic field and the 10 MN force due to the vacuum pressure. In this design, the volume of liquid argon is 500 m³, for a mass of 900 t. The magnetic field at the centre of the volume with 50 kA current running through the two coils is ~0.1 T.

Fig. 14 Schematic of magnetization of large volume liquid argon cryostat (image courtesy A. Dudarev).

4. CONCLUSIONS

Task 8.6 has been studying novel magnetization schemes for neutrino detectors. A simple and novel scheme to magnetize iron plates, by threading a conductor through two slots cut into the plate, produces three uniform dipole fields that are very well defined and require small currents. The iron modules exhibited a B-field of 1.5 T for a coil current of 140 A, for a total power consumption of 11.5 kW. The Baby MIND spectrometer, consisting of 33 iron modules alternating with 18 scintillator modules to track the trajectory of particles, was constructed at CERN and was deployed in a test beam at the PS to characterise the charge identification efficiency fo the spectrometer. The Baby MIND spectrometer was transported to Japan and installed in the J-PARC neutrino beam at a distance of 280 m from the target. First muon antineutrinos and neutrinos were reconstructed with Baby MIND when it was exposed to the T2K neutrino beam, showing that the spectrometer can differentiate between both types of events.

The second part of the task consisted on performing a study of the potential for magnetization of large liquid argon modules with high temperature superconducting magnets. Recent developments with ReBCO Coated Conductors, which are ceramic flat tape High Temperature Superconductors ( HTS) that can operate at a temperature between 20 K and 60 K, can potentially carry very large currents up to 80 kA or beyond. Conductor on Round Core ( CORC) cables offer a promising design for a Cable-In-Conduit Conductor ( CICC). A possible configuration using HTS cables can be used to magnetise a large magnetic volume of ~500 m³, which would allow charge identification with liquid argon detectors.

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