R&D with very forward calorimeters for linear colliders

The luminosity of the International Linear Collider will be measured by counting Bhabha scattering events, recognized by coincident detection of two low angle electrons in two calorimeters located symmetrically on both sides of the interaction point. The calorimeters are designed as sampling calorimeters with tungsten absorber disks interspersed with silicon sensors. For a precise measurement of the polar angle, it is essential to achieve a small Molière radius. The design thus foresees ultra-thin sensors that allow 1 mm spacing between the absorbers. In 2014, a first multi-layer prototype of the calorimeter was tested in a beam of 5 GeV electrons and muons at CERN. Excellent performance of the sensors was demonstrated. The development of electron showers has been measured and compared to simulations. Good agreement is found. In the next step, different connectivity schemes have been explored allowing to reach sensor module thickness below 1 mm. Eight thin sensor layers, fully equipped with front-end electronics, have been succesfully tested in a 5 GeV electron beam at DESY. In parallel, the development of a novel ultra-low power SoC type front-end ASIC called FLAME has been started. Prototypes of its two main blocks have been designed, fabricated, and partially tested.