The ArDM experiment
The CIEMAT group actively participates in hardware and software aspects related to the ArDM project. The main contributions to the experiment are related to:
￼• The ArDM neutron shield. The neutron background interactions from cosmic rays and natural radioactivity can produce single-scatter nuclear recoils in the ArDM's energy region of interest. The overall flux of neutrons produced by (α, n) reactions from uranium and thorium contained in the surrounding rock is expected to be one of the main contributions, but it can be effectively reduced by using a hydrocarbon shielding. Materials with plenty of hydrogen act as shielding by reducing the neutrons' energies enough that they cannot cause a recoil above threshold. The polyethylene neutron shield of the ArDM detector reduces in a factor 105 the flux of neutrons of less than 1MeV.
The neutron shield of the ArDM detector consists of a polyethylene wall with half a meter thickness which completely surrounds the detector. This shield has an octogonal prism shape with a height of 4.47 m and a base inscribed in a circle of 2.6 m on diameter. It is divided into three sections: a lateral body, a bottom cover and a top cover removable for access to the detector. The shield has been completely constructed and mechanized at CIEMAT. The total amount of polyethylene used in the manufacture is 17 tons.
• Neutron flux measurements at LSC. Muon-induced neutrons coming from natural radioactivity can reach the detector from large distances and easily penetrate through the shielding, mimicking WIMP signals, thus a dedicated measurement of the neutron flux in the experimental Hall A at LSC is required. These measurements are being performed in collaboration with the Nuclear Innovation Unit from CIEMAT, in particular, with Daniel Cano and Trinitario Martínez.
In the first phase of operation, two BC501A liquid scintillator detectors and one He3 proportional counter were installed underground in Hall A close to the ArDM detector in November 2013. The liquid scintillators are foreseen for fast neutron spectroscopy, while the proportional counter provides a reference for comparison with previous measurements performed by CUNA Collaboration. This setup is currently being commissioned. The data are being taken continuously, analysed remotely and transferred to CIEMAT by network connection.