2012: The update of the shielding of Sunset detectors
We have just finished the planned upgrade of the Sunset Micromegas detectors. Its main goal has been to reduce the background contribution of the 6 keV peak, originated in the vacuum pipes and the cathode. For this purpose, the lead thickness and the shielding closure was increased and steel was replaced by copper in the setup.
The copper innermost layer of the shielding was increased from 0.5 to 1 cm, to attenuate the natural radiation originated in the lead. Polyethylene was removed to leave space to the extension of the lead shielding up to 10 cm. Moreover many lead bricks were machined to assure a better enclosure than in previous setup, mainly at the region where the electronics is plugged and around the connection to cold bores. Apart from that, the cathode, the gas connections and the pipe were replaced by new copper components. The inner of the last piece was also coated with Teflon to suppress the copper fluorescence.
- Left: View of the installation of Sunset detectors in 2012. The copper pipe with an inner teflon coating is shown at the left bore. At the right bore, a micromegas detector is installed using teflon and delrin screw, nuts and gaskets and the new copper gas connections. Right: A later step of the installation, showing the careful design to adjust the copper and lead shielding to detectors and the Faraday cage.
Commissioning data confirms the disappearance of the 6 keV peak, leading to a background level of ~2 x 10-6 s-1 keV-1 cm-2., i.e., a factor 3 better than in previous setup.
Later on a cosmic veto, composed of a plastic scintillator and a photomultiplier, has also been installed. The (delayed) coincidence of veto signal and the micromegas trigger was used to reject events generated by muons in the offline analysis. This feature further reduce background down to ~1.5 x 10-6 s-1 keV-1 cm-2, the lowest level so far at CAST!