MEG II
The MEG-II drift chamber is a cylindrical chamber with a stereo angle configuration (for the measurement of the hit positions along the camera longitudinal axis). It is filled with a low mass, Helium based gas mixture, Helium (85%)-Isobutane (15%), in order to minimize the multiple Coulomb scattering of the positron along its trajectory, and it is immersed in a magnetic field to allow momentum measurements.
The chamber is made by 10 layers of square cells (with side of about 0.7cm); in each cell the signal wire is made of golden Tungsten (25 micron diameter) while the surrounding field wires are of silvered Aluminum (40-50 micron diameter). The signal wires are set at a potential of 1700 V using a power supply system and the signals are read by a custom electronic (designed and built by INFN Lecce) which amplifies the signal preserving a high band width (about 1Ghz). The signals are subsequently digitized by boards invented at PSI, the wavedream boards, based on the DRS4 chip.
The main activities of the Rome group are:
- Wiring of the chamber and measurement of the mechanical wire tension
- Drift Chamber high voltage system
- Drift Chamber gas system
- Cluster timing studies
- Drift chamber hit reconstruction and calibration
- Target position measurement system
- Active target prototypes
- Research of the dark photon X(16.7 MeV) in Be transitions
Besides the partecipation to the wiring procedures, the Rome group built a system for the measurement of the wire mechanical tension, crucial to guarantee the goodness of the wiring and assembling quality. The system is based on the acoustical excitation of the characteristic modes of the wires.
After various studies to identify the best solution in terms of precision, reliability and safety, it has been chosen a commercial high precision ISEG system. The Rome group is responsible of the integration procedure, the tests and the operations.
The Rome group built the gas system, composed of a rack with all the components to deal safely with the gas flux through the chamber (Helium, Isobutane). The system includes several monitoring tools (gas analyzers, monitoring chamber) to guarantee the purity and the stability of the mixture.
The cluster counting/timing technique has been proposed to improve a drift chamber performances in terms of particle identification capability and spatial resolution (Nucl. Instr. and Meth. A572. (2007) 198-200). The Rome group is involved in the development of this technique in the context of the FIRB (Futuro In Ricerca) grant won by a member of the group, Francesco Renga. Although the MEG-II drift chamber does not need these techniques to reach the target resolution, the high band-width of the front-end electronics would allow to utilize them.
The Rome group has the responsibility of the software for the reconstruction of the hits due to the passage of the positrons through the chamber, starting from the digitized waveforms. This is a challenging task given the very high signal rate in MEG-II. Moreover the group is responsibile of the development of the drift chamber calibration and monitoring tools.
The precision of the target alignment is crucial to reach the MEG-II sensitivity since it has a direct impact on the high level physical quantities reconstruction. The target position in MEG was determined with an optical survey at the beginning of the run and during the run itself using tracks coming from the target. The precision of these measurements was not sufficient so this was a relevant systematic effect. In order to overcome this limit the Rome group developed a system with a camera detector installed at one of the chamber endplate that regularly take pictures of the target, illuminated by a LED for the purpose. Tests to verify that the needed precision (100 micron) and stability are currently underway. A relevant issue in this application is the resistance of the camera to the BField and to the high radiation environmenent.
The possibility to install in MEG-II a scintillating fibers target (in place of a plastic foil) has been considered to have a direct measurement of the positron production vertex. An intense activity has been carried out during this years and several prototypes have been built.
The ATOMKI experiment observed an anomaly in the angular distribution of internal pair creation in transitions 7Li(p,e+e-)8Be which can be interpreted as a dark photon. (ATOMKI paper). The MEG experiment utilizes a Cockroft-Walton (CW) accelerator to produce photons for liquid Xenon calibration. The accelerator produces protons with energy of about 1 MeV impinging a Litium tetraborate target. This reaction produces also the 8Be studied at ATOMKI. The Rome group is currently studying the feasibility of such a measurement using the MEG-II CW and the MEG-II drift chamber. Preliminary simulations show that the values of resolution and efficiency are sufficient to confirm or exclude the ATOMKI result in few weeks of data taking.
MuonEDM
The muon beam monitor and the time-of-flight detector enable alignment monitoring together with the clock-wise (CW) and counter-clock-wise (CCW) symmetry checks during data collection. However, during beam commissioning, a detailed characterization of muon trajectories is essential to ensure proper alignment with the magnet and validate trajectory distributions against expectations. This is critical to optimize muon storage efficiency and confirm that magnet support and movement systems maintain trajectory symmetry between CW and CCW injections, minimizing systematic uncertainties.
The Rome Group is mainly involved in:
- Development of a precision muon tracking detector
We are developing a precision muon tracking detector for beam commissioning, temporarily placed at the muon entrance trigger detector’s location. The detector aims for a momentum resolution of 0.5% and angular/position resolutions within a few mrad, respectively mm, to resolve anomalies and biases of the phase space. Achieving such precision at 28 MeV/c initial muon momentum, requires an ultra-light detector, to minimize tracking deterioration from the multiple Coulomb scattering.
The proposed solution is a Time Projection Chamber (TPC) using an ultra-light helium-based gas mixture, potentially at sub-atmospheric pressure (0.4 bar), separated from the vacuum in the magnet bore by a vacuum-tight 300 nm Silicon Nitride window. A field-cage, an innovative technology, ensures an homogeneous electric field within the drift region. A GridPix detector is used as TPC readout structure, a gaseous detector made of a conductive mesh implanted 50 \( \mu \)m above a Timepix chip. A voltage difference between mesh and chip creates avalanche when drift electrons reach the mesh, functioning similarly as a sort of microscopic Micromegas.
