Assembly of PMT arrays

Interactions of particles with liquid xenon are detected by the observation of scintillation light. To observe even the smallest particle recoil-energies, the “eyes” of the experiment have to be able to detect single photons. In XENON1T, this is realized by photomultiplier tubes (PMTs) which convert the incoming photons to a measurable charge signal. In total 248 PMTs are used in the experiment, split in two arrays of 127 PMTs at the top and 121 at the bottom of the time projection chamber. It is of uttermost importance for the performance  of XENON1T that each PMTs works within the specifications and has a stable performance during the dark matter search campaign.

Each PMT was cooled down and tested typically three times at the Max-Planck-Institut für Kernphysik in Heidelberg (MPIK). In this way the materials are exposed to thermal stress before the final assembly. This ensures that only PMTs which reach the high requirements for our dark matter search are built in the detector. In addition, a selection of PMTs were operated in liquid xenon at the Universtiy of Zurich (UZH) to test not only their long term stability but also to check for leaks by an analysis of PMT afterpulsing. The assembly of the PMT arrays started in the clean room at MPIK. The arrays were designed at UCLA and are composed of copper plates for stability and Teflon for an optimized reflectivity of ultraviolet photons. Before the assembly of all components, all materials have been individually treated with dedicated cleaning procedures to reduce radioactive contaminations at the surfaces.

The distribution of PMTs in the arrays were optimized to achieve a maximal light collection and, hence, a low energy threshold. It is worth mentioning that the bottom array has an exceptional large average quantum efficiency of 36.7 %. Before installation, the PMTs were equipped with a custom-made low-radioactive base provided by UZH. The picture below shows the PMTs including its bases and cables during the installation in the clean room.

The assembly of all PMTs was accomplished within two weeks and the arrays were shipped from Heidelberg to Gran Sasso in custom-built transport boxes to ensure safe passage. Furthermore, these boxes enable a light tight environment to be able to test each signal of the PMT in its final position and configuration. All tubes showed satisfactory signals in the oscilloscope upon arrival to LNGS.  The picture below shows the complete assembled bottom (left) and top (right) arrays. On top, the picture shows the top array from the side which will be facing the liquid xenon target.