The members of this research project have a large experience in the construction and in the studies of the performance of high energy particle detectors. They are members of the ATLAS experiment at the LHC (the CERN Large Hadron Collider) and in the last years they joined the upgrade project of the two New Small Wheels, the new forward detectors of the experiment, where 128 large (2-4 m2) micromegas chambers have been considered to measure the position of the muons in presence of high fluxes of particles both from the proton-proton interactions and from the background. With other Italian colleagues they are now contributing to the construction of 32 large micromegas chambers.
The first tests of these large detectors presented some new and unexpected issues that are now under investigation. These arise from the cleanliness of the printed boards and of the meshes used and and from difficulties to meet the mechanical requirements on the components. All these determine severe high voltage instabilities that not allow at moment a safe use of these detectors in long term experiment as ATLAS at LHC. The members of the project aim to contribute to the research and development studies to recover these problems with test in laboratory and testing the detectors in a cosmic ray setup or at beams.
At the same time they will continue to contribute to the preparation of the chambers and of the software to analyze the signals from the micromegas when running in the ATLAS experiment.
In Italy the preparation of the micromegas chambers for the ATLAS Experiment is based on four construction sites. The cathode-drift panels are built in Roma (Sapienza/INFN), the readout panels in Pavia (University/INFN) and the meshes are stretched in Roma TRE (University/INFN). Afterwards the panels and the meshes are transported to the Laboratori Nazionali dell'INFN di Frascati where they are assembled together to produce the chambers.
All the steps of the construction are preceded by several tests of the components and followed by accurate measurements on the products. Finally the assembled chambers will be tested with cosmic rays in the Laboratori di Frascati.
A few chambers built so far, presented unexpected high voltage instabilities that do not permit to supply across the amplification gap the high voltage needed for the correct operation of the chambers. These problems had not been observed in the past in the many small prototypes (10 x 10 cm^2) of micromegas chambers extensively tested with beams at CERN before designing the large chambers for the New Small Wheel.
At the same time, when compared with the small prototypes, in the large chambers the width of the full efficiency plateau before the discharge onset is reduced. This is of course unsafe in detectors that have to be operated for a long time and in presence of high rate flux of particles as in LHC.
At present one of the origins of the high voltage instabilities seems to be the presence of two types of impurities in the chambers: 1) ionic contamination residual from the photolithographic processes used to produce the printed boards with the microstrips mounted in the readout panels, 2) dust or similar particles in the drift of amplification gaps.
To solve these problems a cleaning procedure of the components has been implemented and has now to be fully tested.
At the same time also the type of micromesh (30 micron wire diameter, 70 micron pitch) used in the present chambers is under investigation. It seems that calendered or polished meshes can improve the high voltage stability.
A standard gas mixture Argon-CO2 : 93-7% has been used in the micromegas prototypes studied so far. In order to improve the high voltage stability and to increase the width of the efficiency plateau, new gas mixtures are now considered: different mixtures of the two gases (Ar-CO2) or the addition of small percentages of a second quenching gas (for instance isobuthane).
All these studies have to be implemented in near future and can be performed in test beams or in cosmic rays facilities in preparation at the construction sites (in Italy at LNF).
The solution of these problems is critical for the micromegas chambers that have to be installed in large high energy experiments at LHC or in future colliders.
The physicist presenting this research project are at present involved in the first tests performed at LNF of Frascati on the first chambers built in Italy for the experiment.
With the support of this research project they want to continue their activity in the research program that has to finalize the working conditions of large micromegas chambers. They want also to take a part in the preparation of the cosmic ray facility at LNF for testing all the assembled chambers.
Many results on the performances of the small prototypes have been in the past pioneered by the physicists of the Sapienza group. This research project aims to continue to analyze the data from test beams and from cosmic rays and to extent the results in the reconstruction of the tracks in the ATLAS experiment.
A young physicist who can join the group with a dedicated financial support, can give a strong contribution to the research program presented in this project by contributions to the test of the detectors and to the analysis of the collected data.
Micromegas chambers and other MPGD are presently considered for several applications in environmental analyses and medical physics. The use of fast and large detectors with high accuracy on the position and the direction measurements as in the microTPC mode, can be very interesting in tracking muons in environmental research as for instance the detection of structures in rocks (micromegas have been used in studies on the Cheops pyramid). The good resolution of the micromegas chambers can also be exploited for imaging applications for hadron therapy monitoring. During the present research project these possibilities will be examined and proposed to colleagues of other research fields.