Un nouveau télescope à neutrinos pour le lac Baïkal
Les membres de la collaboration du Baïkal ont procédé au déploiement et commencé l’exploitation du premier groupe de modules optiques pour le Gigaton Volume Detector (GVD-Baïkal), détecteur d’un volume d’une gigatonne. Les modules détectent la lumière Tchérenkov qu’émettent les particules chargées engendrées par les interactions des neutrinos dans l’eau du lac. Il est prévu que, d’ici à 2020, le GVD compte 10 à 12 groupes de modules, couvrant un volume total d’environ 0,4 km3 (GVD phase 1). Une autre extension est prévue ; elle permettrait de passer à une deuxième étape, avec 27 groupes et un télescope à neutrinos, dispositif qui couvrirait un volume total d’environ 1,5 km3.
In early April, members of the Baikal collaboration deployed and started operation of the first cluster of the Gigaton Volume Detector (Baikal-GVD). Named “Dubna”, the cluster comprises 192 optical modules arranged at depths down to 1300 m. The modules are glass spheres that house photomultiplier tubes to detect Cherenkov light from the charged particles emerging from neutrino interactions in the water of the lake. By 2020, GVD is set to consist of 10–12 clusters covering a total volume of about 0.4 km3 (GVD phase-1). This is about half the size of the present world leader – the IceCube Neutrino Observatory at the South Pole (CERN Courier December 2014 p30). A planned further extension should then lead towards a second stage containing 27 clusters in a telescope with a total volume of about 1.5 km3.
Neutrino detection in Lake Baikal will be an important part of the effort to understand better the high-energy processes that occur in far-distant astrophysical sources, to determine the origin of cosmic particles of the highest energies ever registered, to search for dark matter, to study properties of elementary particles, and to learn a great deal of new information about the structure and evolution of the universe as a whole. Together with KM3NeT in the Mediterranean Sea, the other future Northern-hemisphere neutrino telescope (CERN Courier July/August 2012 p31), GVD will allow an optimal view to the central parts of the Galaxy.
The start of the Baikal neutrino experiment dates back to 1 October 1980, when a laboratory of high-energy neutrino astrophysics was established at the Institute for Nuclear Research of the former Academy of Sciences of the USSR in Moscow – now the Institute for Nuclear Research of the Russian Academy of Sciences (INR RAS). This laboratory later became the core of the Baikal collaboration, including at various times the Joint Institute for Nuclear Research (JINR) in Dubna, Irkutsk State University, Moscow State University, DESY-Zeuthen, the Nizhni Novgorod State Technical University, the Saint Petersburg State Marine Technical University, and other scientific research organizations in Russia, Hungary and Germany. At present, the participation of institutes from the Czech Republic, Slovakia and Poland is under discussion.
The idea to register neutrinos in large-scale Cherenkov detectors in natural water was expressed for the first time by Moisey Markov, then at Dubna, at the 10th International Conference on High-Energy Physics, in 1960. Two decades later, Alexander Chudakov, of INR, proposed using Lake Baikal as a site both for tests and for future large-scale neutrino telescopes. The choice of this lake – the largest and deepest freshwater reservoir in the world – was determined by the high transparency of its water, its depth, and the ice cover that allows the installation of deep-water equipment during two months in winter.
The predecessor of GVD was constructed during 1993–1998. Named NT200, it comprised 192 photodetectors placed on eight vertical strings at a depth of 1100–1200 m. NT200 covered some 100,000 m3 of fresh water (an order of magnitude less than the present Dubna cluster). Already in 1994, data taken with only 36 of the final 192 photodetectors showed two neutrino events. These two events were the first of several-hundred-thousand atmospheric neutrinos since recorded by deep-underwater and under-ice experiments. Scientific research with NT200 covered a wide programme, most notably the search for a cosmic diffuse neutrino flux leading to tight limits on that flux (CERN Courier July/August 2005 p24). Moreover, limits were derived on the flux of magnetic monopoles and on muons from dark-matter annihilation in the centre of the Earth and the Sun. Last but not least, the NT200 infrastructure was used for innovative environmental studies.
A notable breakthrough in the field came in 2012, when IceCube detected the first high-energy “astrophysical” neutrinos, i.e. high-energy neutrinos generated beyond the solar system (CERN Courier July/August 2013 p35). That marked the birth of high-energy neutrino astronomy, and underlined the need to develop neutrino telescopes of similar capacity in the Northern hemisphere, to be able to study high-energy neutrino sources across the whole celestial sphere. JINR, with many years of experience as a participant in the Baikal neutrino project, recognized this opportunity and decided to treat activities related to Baikal-GVD as a scientific priority.
Baikal-GVD will have a modular structure formed from functionally independent clusters of vertical strings of optical modules. This modular structure will allow data acquisition at early stages in the construction of the facility. The choice of the telescope structure will also allow adjustment of its configuration in response to changes in scientific priorities at different times.
Prototypes of all of the basic elements of the GVD telescope system were designed, manufactured and tested during 2006–2010. The final stage of complex in-situ testing started in 2011 and finished in 2015 with the development of the Dubna cluster. Its 192 optical modules are arranged down to depths of 1300 m on eight vertical strings, each 345-m long. Different from NT200, the optical modules are not grouped in pairs, resulting in 192 space points per cluster (instead of only 96 for NT200). Moreover, the former custom-made, hybrid QUASAR phototube has been replaced by a conventional 10-inch photomultiplier with a high-sensitivity photocathode. The mechanical structure has been simplified compared with NT200, and a totally new system for front-end and trigger electronics and for data acquisition has been designed and implemented.
Deployment of the Dubna cluster is an exciting step towards a next-generation neutrino telescope in Lake Baikal. Such a telescope will be a cornerstone of a future worldwide neutrino observatory, with detectors at the South Pole, in the Mediterranean Sea and in Lake Baikal. The Baikal collaboration pioneered this technology in the 1980s and 1990s, and measured neutrinos generated in the Earth’s atmosphere. Two decades later, the long-awaited discovery by IceCube of the first high-energy neutrinos from far beyond the Earth and the solar system has given increased motivation to projects for similar large detectors in the Northern hemisphere. IceCube has lifted the curtain that hides the high-energy neutrino universe, but just by a little. In the future, Baikal-GVD will help to chart this new cosmic territory fully.