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Detection techniques for future neutrino observatories

23 February 2015

The discovery of high-energy astrophysical neutrinos initially announced by IceCube in 2013 provided an added boost to the planning for new, larger facilities that could study the signal in detail and identify its origins. Three large projects – KM3NeT in the Mediterranean Sea, IceCube-Gen2 at the South Pole and the Gigaton Volume Detector (GVD) in Lake Baikal – are already working together in the framework of the Global Neutrino Network (CERN Courier December 2014 p11).

In December, the RWTH Aachen University hosted a workshop on these projects and their low-energy sub-detectors, ORCA and PINGU, which aim at determination of the neutrino-mass hierarchy through precision measurements of atmospheric-neutrino oscillations. Some 80 participants from 11 different countries came to discuss visionary strategies for detector optimization and technological aspects common to the high-energy neutrino telescopes.

Photodetection techniques, as well as trigger and readout strategies, formed one particular focus. All of the detectors are based on optical modules consisting of photomultiplier tubes (PMTs) housed in a pressure-resistant glass vessel together with their digitization and read-out electronics. Representatives of the experiments shared their experiences on the development, in situ performance and mass-production of the different designs. While the baseline design for IceCube-Gen2 follows the proven IceCube modules closely, KM3NeT has successfully deployed and operated prototypes of a new design consisting of 31 3″ PMTs housed in a single glass sphere, which offer superior timing and intrinsic directional information. Adaption of this technology for IceCube is under investigation.

New and innovative designs for optical modules were also reviewed, for example a large-area sensor employing wavelength-shifting and light-guiding techniques to collect photons in the blue and UV range and guide them to a small-diameter low-noise PMT. Presentations from Hamamatsu Photonics and Nautilus Marine Service on the latest developments in photosensors and glass housings, respectively, complemented the other talks nicely.

In addition, discussions centred on auxiliary science projects that can be carried out at the planned infrastructures. These can serve as a test bed for completely new detection technologies, such as acoustic neutrino detection, which is possible in water and ice, or radio neutrino detection, which is limited to ice as the target medium. Furthermore, IceCube-Gen2 at the South Pole offers the unique possibility to install detectors on the surface above the telescope deep in the ice, the latter acting as a detector for high-energy muons from cosmic-ray-induced extensive air showers. Indeed, the interest in cosmic-ray detectors on top of an extended IceCube telescope reaches beyond the communities of the three big projects.

The second focus of the workshop addressed the physics potential of cosmic-ray detection on the multi-kilometre scale, and especially the use of a surface array as an air-shower veto for the detection of astrophysical neutrinos from the southern sky at the South Pole. The rationale for surface veto techniques is the fact that the main background to extraterrestrial neutrinos from the upper hemisphere consists of muons and neutrinos produced in the Earth’s atmosphere. These particles are correlated to extended air showers, which can be tagged by a surface array. While upward-moving neutrinos have to traverse the entire Earth and are absorbed above some 100 TeV energy, downward-moving neutrinos do not suffer from absorption. Therefore a surface veto is especially powerful for catching larger numbers of cosmic neutrinos at the very highest energies.

The capabilities of these surface extensions together with deep-ice components will be evaluated in the near future. Presentations at the workshop on various detection techniques – such as charged-particle detectors, imaging air-Cherenkov telescopes and Cherenkov timing arrays – allowed detailed comparisons of their capabilities. Parameters of interest are duty cycle, energy threshold and the cost for construction and installation. The development of different detectors for applications in harsh environments is already on its way and the first prototypes are scheduled to be tested in 2015.

• The Detector Design and Technology for Next Generation Neutrino Observatories workshop was supported by the Helmholtz Alliance for Astroparticle Physics (HAP), RWTH Aachen University, and Hamamatsu Photonics. For more information, visit hap2014.physik.rwth-aachen.de.

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