Speaker
Description
A cosmic muon veto detector (CMVD) is being built around the mini-ICAL detector at the IICHEP transit campus in Madurai. The CMVD aims to study the feasibility of building a cosmic muon veto for a shallow depth neutrino detector. For this purpose, the CMVD needs to have a muon detection efficiency of more than $99.99\%$ and false positive rate of less than $10^{-5}$. The CMVD consists of veto walls on 3 sides and the top of mini-ICAL. The veto walls are made of extruded plastic scintillator (EPS) strips of 4.5 - 4.7 m long, 10 and 20 mm thick and 50 mm wide in dimension. Each EPS strip has two holes separated by 25 mm and running along length of the EPS. One wavelength shifting (WLS) fibre of 1.4 mm diameter is inserted though each hole for light collection. One Hamamatsu made SiPM with an active area of 2 mm $\times$ 2 mm is placed at the two ends of each fibre for signal readout. In order to minimise the number of electrical connectors, two EPS strips are glued together to form the smallest unit of the CMVD, called a di-counter (DC). Each DC is fitted with a counter mother board (CMB) to connect the SiPMs with the processing electronics using an HDMI cable.
For the ease of veto wall construction, an aggregate unit, called a Tile, is made by combining four DCs together. The DCs are glued together - sideways - onto a base plate of aluminium honeycomb panel to provide mechanical support and rigidity. The EPS strips have a coating of $TiO_{2}$ for maximising internal light reflection, but the $TiO_{2}$ does not help in stopping external light from entering the scintillator. The tiles must be extremely light-leak proof to achieve the goal of having a false positive rate of less than $10^{-5}$. To achieve light-leak proofing, the tiles are wrapped with either a black Tedlar or a black Low-density polyethylene (LDPE) sheet. Another layer of heat shrinkable PVC sleeve is added to the wrapped tiles to prevent any abrasive damage to the Tedler or LDPE layer. 95 such tiles, made from 380 di-counters will be used in the CMVD.
A tile thus fabricated, must be qualified for use in the CMVD. Since the individual di-counters used in making a tile are pre-qualified in all other aspects, the tiles need to be tested only for light leak. A tile is declared as light-leak proof, if the noise rate of each of the di-counters is less than a pre-determined value. There are two components in the noise rate measured from a DC, one is the noise because of an actual light leak through the wrapping, and the other is the dark noise produced by the SiPM. In order to negate the effect of the dark noise of the SiPM, the dark noise rate of the SiPM is first established by isolating it form the scintillator strips, and wrapping it in a black cloth before taking he noise rate measurement. This value is subtracted from the noise rate of the tile, to arrive at the noise rate caused due to light leak if any.
This paper will discuss the fabrication procedure of scintillator tiles and the testing procedure used to qualify them as well as summarise the test results.
| Session | Future Experiments and Detector Development |
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