The OPERA experiment Target Tracker

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Abstract

The main task of the Target Tracker detector of the long baseline neutrino oscillation OPERA experiment is to locate in which of the target elementary constituents, the lead/emulsion bricks, the neutrino interactions have occurred and also to give calorimetric information about each event. The technology used consists in walls of two planes of plastic scintillator strips, one per transverse direction. Wavelength shifting fibres collect the light signal emitted by the scintillator strips and guide it to both ends where it is read by multi-anode photomultiplier tubes. All the elements used in the construction of this detector and its main characteristics are described.

Introduction

OPERA [1] is a long baseline neutrino oscillation experiment designed to detect the appearance of ντ in a pure νμ beam in the parameter region indicated by the anomaly in the atmospheric neutrino flux. The detector is installed at the Laboratori Nazionali del Gran Sasso (LNGS) in a cavern excavated under the Gran Sasso in the Italian Abruzzes. The cavern (Hall C) is in the line of sight of the CNGS beam of neutrinos originating from CERN, Geneva, at a distance of 730 km. The beam energy has been optimized to maximize the number of ντ CC interactions. The commissioning of the beam and of the electronic components of the detector has started in August 2006. The data taking is due to start in spring 2007 and to last for 5 years.

At the nominal value of Δm232=2.5×10-3eV2 and full νμ-ντ mixing (sin22θ23=1), as measured by the Super-Kamiokande atmospheric neutrino experiment [2], OPERA will observe about 20 ντ CC interactions after 5 years of data taking, with an estimated background of only one event.

The OPERA detector consists of two identical super-modules. Each super-module has a 0.9-kton instrumented target followed by a 10×8m2 dipolar magnetic muon spectrometer. One target is the repetition of 31 6.7×6.7m2 modules each including a proper target wall followed by a tracker wall. A target wall is an assembly of 52 horizontal trays, each of which is loaded with 64 bricks of 8.3 kg each. A brick is made of 56 lead sheets, 1 mm thick, providing the necessary mass, interleaved with 57 nuclear emulsion films that provide the necessary sub-millimetre spatial resolution required to detect and separate unambiguously the production and decay vertices of the τ- lepton produced in charged current ντ interactions with the lead nuclei.

The bricks in which neutrino interactions have occurred, typically 30 per day at the nominal beam intensity, are identified by the event reconstruction in the trackers and the spectrometers. They will be extracted on a regular basis, disassembled and the emulsion films will be scanned and analysed by a battery of high speed, high resolution automatic microscopes in order to locate the interaction vertex and search for candidates of τ- lepton decay.

The main role of the Target Tracker is therefore to locate the lead/emulsion brick where a neutrino interaction has occurred. It will also provide a neutrino interaction trigger for the readout of the whole OPERA detector and be used as a calorimeter for the event analysis.

The required high brick finding efficiency puts strong requirements on the Target Tracker spatial resolution and track detection efficiency. The replacement of faulty elements of the Target Tracker is extremely difficult and this detector must therefore present a long term stability and reliability (at least for 5 years which is the expected OPERA data taking period). In case of problems, the brick finding efficiency not only of the bricks just in front of the concerned zone, but also of several walls upstream will severely be affected. The sensitive surface to be covered being of the order of 2×3000m2, a cost effective technology had to be used.

This paper describes at the beginning the detection principle and geometry. The different components and the corresponding R&D done before the detector construction are extensively presented. Elements about the construction, installation and long term detector stability monitoring are also given.

Section snippets

Overview of the Target Tracker

The technology selected to instrument the targets of the OPERA detector consists in scintillator strips, 6.86 m long, 10.6 mm thick, 26.3 mm wide, read on both sides using Wavelength Shifting (WLS) fibres and multi-anode photomultipliers (PMT). The particle detection principle is depicted by Fig. 1. The scintillator strips used have been produced by extrusion, with a TiO2 co-extruded reflective and diffusing coating for better light collection. A long groove running on the whole length and at the

Components

A description of the main components entering the Target Tracker construction is given in this section. More information can be found in the Target Tracker Technical Design Report [3].

Radioactivity in the Target Tracker

In this section, the radioactivity measurements of the materials used for the Target Tracker construction are presented. The goal of these measurements was mainly to ensure that the background caused by the activity of these materials was low enough not to introduce significant dead time during the acquisition due to high trigger rate. Using the results of these measurements, a simulation to obtain an estimation of the induced background in the scintillator strips and measured by the WLS fibres

Effect of the magnetic field on PMTs

The efficiency of a PMT is affected by a strong enough magnetic field because the Lorentz force modifies the p.e. trajectory. Similarly, the gain is reduced by the effect of the field on the multiplication process of the secondary electrons. Studies done by MINOS collaboration and Hamamatsu indicate that the efficiency of the PMT used in OPERA decreases significantly if the magnitude of the field perpendicular to the photocathode exceeds 5 Gauss.

Simulations done with TOSCA [12], based on the

Construction and installation

Two production lines have been set up at IReS-Strasbourg to construct, test and calibrate a total of 8 modules per week. The construction was done according to the following steps:

  • (1)

    Groups of 16 scintillator strips were placed on a frame on a table equipped with a 16-head glue distribution system. The frame was slightly curved along its length, upward at the centre. After mixing with the hardener, about 15 g of glue was injected in each groove. The fibres, stretched with springs at both ends, were

Module tests and calibration

The light tightness of the modules was tested using PMT's dynode 12 which is the OR of all the PMT channels. The same measurements repeated in the Gran Sasso underground laboratory under much reduced cosmic ray and ambient radioactivity background give a counting rate of the order of 20 Hz/channel in the absence of light leaks.

For energy calibration, each module was placed on a vertical scanning table equipped with two electron spectrometers (see 3.1) that may irradiate simultaneously any two

Ageing

The properties of the glues are well known as they have been used by other neutrino experiments (NEMO3 [15], MINOS). Several glue samples have been followed for more than 7 years. The strength of the double face adhesive is guaranteed by the production and selling companies (MACTAC10 and VARITAPE11) not to vary for at least 10 years. According to tests done by the AMCRYS–H company, an

Summary and conclusion

The Target Tracker of neutrino oscillation OPERA experiment is now operational (the first recorded CNGS neutrino interactions can be found in Ref. [17]). The main role of this tracking device is to locate the lead/emulsion bricks containing neutrino interactions, and also act as calorimeter for these events.

Two years of R&D have mainly been spent to select high quality plastic scintillator, long absorption length WLS fibres and efficient photodetectors. Due to relatively high fringe magnetic

Acknowledgements

We acknowledge the support of the funding agencies IN2P3–France, FNRS/IISN–Belgium, SNF–Swiss and JINR-Russia. We would like to thank all private companies which have collaborated with our institutes in developing and providing materials for the OPERA Target Tracker construction. We are grateful to the OPERA collaboration for the support provided. Finally, we would like to thank all technicians non-authors of this paper who have helped us all these last years.

References (16)

  • E.H. Bellamy

    Nucl. Instr. and Meth. A

    (1994)
  • Lucotte

    Nucl. Instr. and Meth. A

    (2004)
  • M.J. Varanda

    Nucl. Instr. and Meth. A

    (2000)
  • OPERA proposal, An appearance experiment to search for νμ↔ντ oscillations in the CNGS beam, CERN/SPSC 2000–028,...
  • Y. Fukuda

    (Super-Kamiokande Collaboration)

    Phys. Rev. Lett.

    (1998)
  • Target Tracker Technical Design Report...
  • H. Du et al.

    Photochem. Photochem

    Photobiol.

    (1998)
  • E. Ables, et al. (MINOS), Fermilab Proposal P-875,...
There are more references available in the full text version of this article.

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