CAST Micromegas detectors: progress in 2013

During 2013, there were many upgrades in the Micromegas detectors of CAST, concerning muon veto system for the Sunset detectors, new readout electronics & DAQ, and a new detector design carefully conceived for low background. The performance of the Sunset Micromegas detectors was very good during all the 2013 CAST data taking, achieving the lowest background levels ever of the experiment. Although many problems arose in the Sunrise Micromegas installation, we finally managed to make it work. During the installation we learned many new strategies in which we are currently working for the 2014 upgrade.

For a more detailed description of CAST Micromegas activities in 2013 continue reading!

1.   2013 UPGRADES

• New muon veto on Sunset Micromegas
  • Increased the coverage from 44% (2012) to 95% (2013).
  • The background reduction increases from 25% (2012) to 40% (2013).
  • Vacuum system modified to facilitate the veto assembly & to fit the new electronics.

• AFTER-chip based electronics + new DAQ (Sunset & Sunrise Micromegas)
  • The Gassiplex-based readout was substituted by theAFTER based electronics (T2K).
  • Now a pulse/strip is recorded.
  • More handles for better background discrimination.
  • Automatic data-taking for Sunset and Sunrise. Possibility for an online check.

• Uprade of the Sunrise Micromegas
  • Chamber:
    • radiopurity of materials (chamber out of Cu), also serves as inner shielding.
    • Field shaper: electric drift field configuration improvement.
    • Front End Electronics: new connectors and cards.
    • Inner teflon layer to minimize 8 keV Cu-peak.
  • New Detector:
    • Design idea: Incorporate the HV lines in the detector layout in order to
      simplify the connections.
    • Three detectors delivered and tested. Very good performance in CAST Lab.
  • Shielding: 
    • Platform for the shielding installed.
    • Improvement in copper and lead shielding despite restrictions in dimensions.

2.   SUNRISE MICROEMGAS INSTALLATION

Here,  we summarize the problems we faced during the installation of the Sunrise Micromegas, and the solutions/changes we plan for the 2014 installation. Although we only managed to take one week of data  with the Sunrise Micromegas in 2013 (see below: 2013 DATA TAKING ), the experience was very useful for gaining experience and comissioning  all the new systems installed (electronics, DAQ, shielding, windows, etc.).

• SRMM Problems
  • Detector fabrication and design:
    • All of the detectors had shortcircuited strips.
    • HV lines for the field shaper and drift to close to mass.
    • Imperfections in construction lead to HV breaking and consequent short-circutis.
  • Chamber aspects: All major components are made of Cu. Gasket out of Teflon. Multiple mounting and dismounting resulted in:
  1. degradation of the screws and holes;
  2. deformation of the gasket, gas leaks.
  3.  further difficulties in mounting.
  • Other aspects: 
  1. interface card: incorrect footprint for the connectors.
  2. Noise: bad connection with the interface card, unsatisfying cablings, Faraday cage not suitable for the new detector.
  3. X-ray windows: not electro-polished. They don’t stand pressure/vacuum cycles.

• Changes for 2014
  • Detector fabrication and design:
    • Mass plane moved away from the HV lines for the field shaper and
      drift.
    • HV cable connections: mass plane retracted. Box with possibly
      integrated filter will be placed.
    • New design already validated. Detectors under construction.
  • Chamber aspects: Two options are being pursued:
  1. Make stainless steel screws out of low radioactivity batch.
  2. Silver-plate the copper screws.
  • Other aspects: re-design of the interface card with correct footprint, new Faraday cage design, cable trays, improve cabling, studies on grounding schematics to avoid loops.

3.   2013 DATA TAKING

The 2013 CAST data taking started on 22^nd September 2013 and finished on 7^th December 2013.  In this period, there were 63 sunset solar trackings.

• SUNSET: Sunset Micromegas showd greater than 95 % tracking efficiency, i.e. there was almost no data lost due to bad detector behaviour. Both detectors showed large active area and gain stability. The energy threshold of the detectors was around 1.5 and 2 keV for Sunset 1 and Sunset 2, while the energy resolution at 5.9 keV was 22% and 25% FWHM respectively. The background level of the detectors are the lowest values obtained in CAST thanks to the new electronics and the new veto system. 

• SUNRISE: Due to the problems described above with the new detectors, CAST-C2 Micromegas was installed in the Sunrise line the 3^rd December 2013, performing only 4/63 sun trackings. Finally, the detector worked, showing large area, good energy resolution (16% FWHM at 5.9 keV). There are not enough statistics yet to drawn conclusions on the background levels achievable, but preliminary results look promising! Moreover, many lessons were extracted from all the efforts invested in the Sunrise 2013 installation. These lessons are already being used:

1. in the design and fabrication of 3 new Micromegas detectors (solving the HV lines and field shaper issues)
2. in the desing of a new interface card, with new flat cables for strips and improved electronics connections.
3. in the search for new techniques and materials for the chamber installation,  and the x-ray windows.
4. in the improvement of the electronic noise (new Faraday cage, cable trays, grounding studies, etc.)