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
The wiring and the assembly tasks are almost completed and the detector will be transported at PSI at the beginning of 2018 to be tested during the 2018 pre-engeneering run.
The main activities of the Rome group are:
Wiring of the chamber and measurement of the mechanical wire tension
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.
Drift Chamber high voltage system
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.
Drift Chamber gas system
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.
Cluster timing studies
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.
Drift chamber hit reconstruction and calibration
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.
Target postion measurement system
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 reached are currently underway.
A relevant issue in this application is the resistance of the camera to the BField and to the high radiation environmenent.
Active target prototypes
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.
Research of the dark photon X(16.7 MeV) in Be transitions
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.
The MEG experiment utilizes a Cockroft-Walton (CW) accelerator for producing 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. Preliminar simulations show that values of resolution and efficiency sufficient to confirm or exclude the ATOMKI result are reachable in few weeks of data taking.
The Rome group activity in the MEG experiment was relative to the TC commissioning/reconstruction/calibration
,the scintillating bars detector which measured the positron time.
We also took care of the DCH reconstruction software at all level (time and gain calibration, pattern recognition, tracking, momentum and vertex measurement). The detector studiees have beeen published in Detector Paper, published in EPJC.
The group has also developed the Bayesian analysis , which was a control analysis with the respect to the frequentistic "nominal" one (MEG Technical Note 56).