DMEG

DMEG is a cooperation project between Italy and Korea,

defined as "progetto di grande rilevanza" sponsored and funded by Ministero degli Esteri e della Cooperazione Internazionale (MAECI)

The Project

The Multigap Resistive Plate Chamber (MRPC) is a gaseous detector that can cover a large area; it has a low cost of construction and a high quality of performance. Many studies done in the past years to design different geometry configurations of MRPC, led to a very performing detector often used as time-of-flight (TOF) system or tracking device. MRPC efficiency is close to 100% and time resolution is ~50 ps. MRPC detectors have been operating with the C₂F₄H₂-based gas mixture which has very high Global Warming Potential (GWP) value. In many experiments a closed loop gas system has been introduced. However, the construction cost is not negligible and the problem of gas leaks still remains. A better alternative is to use an ecological gas. Studies to find an ecological gas mixture have been performed by some groups. Their results show that the electric potential applied to the chamber should be increased over 20kV when the ecological gases were used. We propose to develop a novel design of the MRPC that could allows us to run the detector at around 10kV while maintaining good efficiency and high time resolution. MRPC is used in many high energy physics experiments as a timing device. Their construction is cheap and relatively easy. A mixture of conventional freon R134a and SF₆ guarantees high efficiency and excellent time resolution. These gases, however, have very high Global Warming Potential. For environmental reasons, these gases need to be replaced by gases with lower GWP values. A big Italian High School Project, EEE (Extreme Energy Events), has been running with 6 gaps MRPCs. EEE involves researchers and young students to build telescopes (done with three 6 gaps MRPCs each) located all over the Italian surface, to track cosmic muons. The operating potential for these MRPCs is ±10kV. Recent measurement of the MRPC performance with pure HFO-1234ze shows that a significant increase of the applied voltages by up to 4kV is required. Efforts are being exerted to reduce the operating potential by adding new types of eco-gas which has quenching property. CF3I is one of the candidates to replace the present quenching gas SF₆. The effect of adding a few percent of CF3I, however, is not big enough to reduce the operating potential significantly. Studies on adding other ecological or non-ecological gases are going on actively. We propose to design, produce and test a modification in the MRPC design: a double stack MRPC that allows us to reduce the applied potential by a factor of 2 and to operate it simply with pure HFO-1234ze. The main change is to divide the 6 gaps into 2 stacks of 3 gaps with an additional electrode in the middle. The ALICE-TOF module has already adopted the 2 stack design, but it has electrodes in square pads of a few centimeters in each side. The readout of both ends of the strip electrodes of two stacks by the NINO electronics may need careful studies of the impedance matching in order to guarantee good performance of the timing device. Another important modification should be the way in which the gas gaps are filled. A new design of the MRPC using strips as the electrodes, its production, and the tests with cosmic rays and the accelerator beams will be the key activities of this work. The first phase of the project will consist in the design and construction of a strip prototype. The MRPC will be composed with two stacks, three gaps each; the read out cards will be connected at both ends of the strips (similar to EEE chambers), in order to read the time difference and measure the impact point of the particles on the strip. In parallel studies on the economic flow of ecological gas will be performed, with the aim to minimize the gas consumption. The strategy is to use a peculiar geometry of the gas pipes inside the chamber that will allow to speed up and improve the filling of the gaps. Presently the gas gaps of the large EEE chambers are placed in a big aluminium box. The inlet and outlet of the gas are placed on the box at diagonally opposite positions. The gas mixture flows in the box and it takes some time for the gas to fill in the gas gaps by diffusion process. This works fine, but it has two disadvantages: there can be a variation of the gas purity depending on the position on the chamber, which may result in the variation of the detector performance as has been reported by the CMS muon system. At the same time, most of the gas may flow from the inlet to the outlet through the empty space inside the box without reaching the gas gaps. A good way of improving the gas supply to the gas gaps has been conceived by Professor Laktineh and used in the production of the SDHCAL detector. Gas shielding the effective volume of the MRPC on the edges and providing/extracting the gas mixture using capillaries may guarantee an even flow of the gas inside the gaps and also allow us to reduce the amount of gas flow without waisting the gas by letting it flow through the empty space in the chamber.