CALICE ECal is a novel type of electromagnetic calorimeter employing tungsten as a radiator and square silicon diode pixels called pads as sensors. It is made of 30 sandwich layers of alternating tungsten layers and PCBs. The mechanical structure is made of tungsten wrapped in a carbon fiber. Almost each layer consists of 3 by 3 matrix of silicon wafers, each wafer has 6 by 6 square Si pads. The very front end electronics is located outside of the active area, mounted on a PCB. CALICE ECal prototype has more than 9 000 active sensor elements in total. Signals from the PCBs are read out via cables to the VME64 crate which provides digitization and readout.
[obrazok TB setupu]
CALICE ECal employs silicon diodes for detection of passage and measurement of the deposited energy of passing charged particles. Sensor is manufactured on the 4 inch wafer with high resistivity, about 5 kΩcm. Silicon wafer is 525 μm thick.
One wafer is a matrix of a 6 by 6 pads. One pad is 10x10 mm² large. Wafer is slightly larger than 60 mm because of the included 1 mm guard rings which safeguard surface currents and make electric field in the active volume homogeneous on the border. About half of the wafers for the CALICE ECal prototype was made by the On Semiconductor company in Roznov in Czech republic.
Pads are p-n junction diodes operated at reverse bias. This forms a sensitive region depleted of mobile charge carriers and sets up an electric field that sweeps charge liberated by a passage of radiation to the electrodes. High-resistivity silicon is used for detector fabrication and therefore it is possible to fully deplete the volume at voltages of about 150 to 200 V. When an ionizing particle passes through, energy loss of just 3.6eV creates an electron-hole pair. This is an order of magnitude less than energy needed to produce electron-ion pair in gasses or about two orders of magnitude less than energy necessary for production of a single photon in scintillation counter. Passage of a minimum ionizing particle through the fully depleted volume of the CALICE ECal prototype pad should create about 42000 electron hole pairs. The read-out signal is processed in the front end chips and converted to the digital form and finally stored after a processing chain on the grid.
ECal prototype consists of 3 stacks, each has 10 layers of an alternating structure of tungsten and silicon. Each stack has tungsten of different thickness: 1.4mm or 0.4 X0 per layer per for the stack, 2.8mm or 0.8 X0 for the layers in the second stack and 4.2mm or 1.2X0 for the layers in the rear stack. This selection should guarantee a good resolution at low energy due to the thinner tungsten at the front and it should contain the electromagnetic showers well because of the thick tungsten layers at the rear.
Silicon pads are mounted to the PCBs by a mean of conductive glue. The signal is read out from a single PCB using the FLC_PHY3 front end chips on the PCB which contain input charge-sensitive preamplifier, signal shaper and a 18:1 multiplexer. Output is analog. The silicon wafers on each side of the slab are staggered in the horizontal direction for the reasons of avoiding alignment of guard rings.
|Institute of Physics, Prague||04.12.2007, ver. v02_01|
|ECAL||©Jaroslav Zalesak (zalesak|fzu|cz)|