India-based Neutrino Observatory

  The mini-ICAL Prototype Detector    
The Pottipuram Research Centre (PtRC) (the erstwhile India-based Neutrino Observatory (INO)) is a multi-institutional project aimed at building a world-class underground laboratory at the Bodi West Hills near Theni in Tamil Nadu. The collaboration of universities and research institutes is deeply engaged in the design and construction of a mega science experiment called Iron Calorimeter (ICAL) for studying many key open questions involving the elusive particles called neutrinos. The magnetised ICAL will consist of more than 50000 tons of 56mm thick iron plates arranged in stacks with gaps in between, where around 28800 Resistive Plate Chambers (RPCs) of 1.95m x 1.92m size will be inserted as active detectors.

A prototype called mini-ICAL, with 1/600 of the weight of ICAL, has been constructed and installed in the PtRC transit campus at Madurai, to gain experience in the building of a large-scale electromagnet, to study the detector performance, and to test the ICAL electronics in the presence of fringe magnetic fields. This 4m × 4m × 1.1m detector has 11 iron layers, and 20 RPCs of 1.95m x 1.92m have been inserted in the 10 layers of gaps, in the central region. Mini-ICAL has been in operation since 2018, and is collecting cosmic ray muon data. The general construction, magnet, RPC design, gas system, electronics, trigger, DAQ and backend software systems of the mini-ICAL are exactly the same as those planned for the ICAL and its engineering prototype (e-ICAL) detector. Apart from its own physics motivations, mini-ICAL is largely meant to validate the engineering designs of the ICAL and e-ICAL.

Photograph alongside shows construction activities of mini-ICAL, with active contributions by INO graduate students and also large number of internship students from local educational institutions. A large number of students were invited to get the hands-on experience of building of particle detectors, assembly, testing, installation and commissioning activities, as well as to learn from experts the basic science concepts associated with various sub-systems of mini-ICAL.

The major components of the mini-ICAL detector are as follows:

  1. The 85-ton electromagnet (producing magnetic field of 1.3Tesla ), including the mechanical tiling of the 11 layers using 56mm thick iron plates, and stainless steel spacers for maintaining the required gaps for insertion of the RPCs.
  2. The copper coils and high current DC power supply for energizing the magnet.
  3. The RPC-based muon detectors, each of which is made of two glass plates of dimensions 1.95m x 1.92m x 8mm, sealed gaps for the gas, analog electronics consisting of 128 channels of fast amplifiers and discriminators, a digital data acquisition board, low voltage supply lines, a DC-DC ±6 kV high voltage supply, data and gas lines – all packed in a steel reinforced epoxy tray.
  4. Data acquisition systems, including trigger system, calibration system, back-end data concentrator, event builder, run consoles, etc.
  5. A closed loop gas system to circulate the specified gas mixture in the RPCs and a gas chromatograph to monitor the gas proportion.
  6. A closed loop, Low Conductivity Chilled Water Supply (LCWS) system to cool the magnet power supply and the copper coils.
  7. Many other ancillary technical services, including the measurement systems for magnetic field and the gap between iron plates, etc.

Millions of cosmic ray particles are recorded every day by the mini-ICAL detector, the data of which are analysed and understood by the graduate students of PtRC project. Due to the presence of the magnetic field, the mini-ICAL detector can distinguish between negatively and positively charged cosmic ray muons and also measure their momentum. These scientific results from mini-ICAL detector are presented in many national/international conferences, published in prestigious journals and also form the material for several Ph.D. theses.