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Description
Summary
Novel on-detector cooling systems with very low material budget, of the order of 0.1% X0, are being developed by the PH/DT group at CERN. They consist of silicon wafers in which microchannels are etched and closed by bonding another wafer, and placed in direct contact with the sensors and FE electronics.
When designing cooling systems for HEP detectors three issues have to be addressed: (i) the material budget should be reduced, (ii) for a given material budget the cooling power should be enhanced and (iii) the temperature difference between heat source and heat sink should be minimized. Microfluidics is a good candidate for addressing the first two issues given the high heat transfer coefficient in laminar flows which is inversely proportional to the size of the channel. The natural minimization of material and thermal barriers between the heat source and the cooling fluid effectively addresses the third issue.
Three application cases are presented with quite different operational modes, specifications and constraints.
NA62-GTK - The first one, which has been selected by the NA62 collaboration for the cooling of their GTK pixel detectors, is optimized for single-phase liquid flow in 150 parallel silicon microchannels of a radiation hard coolant (C6F14) to dissipate the total power produced by the read-out chips (about 48 W per station) and to keep the sensor at a temperature of about -20ºC with a maximum temperature difference of the order of 6ºC. Six modules from a pre-production run with the final configuration have been delivered at CERN by an external supplier.
LHCb VeLo – The second case is studied in the context of the upgrade programme of LHCb. The upgraded detector modules of the VeLo detector will be placed only few mm away from the LHC beam and will be subject to very intense radiation levels. To be robust against radiation damage the sensors should be maintained at a temperature below -20ºC. The active electronics of the detector modules is expected to dissipate about 2 W/cm2 over a surface of 2.8×4.2 cm2 per each half of the 42 detecting planes, with a total estimated power dissipation in the detector volume close to 2 kW. The present cooling configuration relies on evaporative CO2 in metal pipes connected to the modules periphery by thermal ledges. The approach for the upgrade is to integrate the microchannel cooling together with two-phase CO2 as the coolant. 400 µm thick silicon micro-channel prototypes have been fabricated and successfully tested at temperatures down to -30ºC and pressure greater than 160 bars.
ALICE ITS – The third case presented is being considered for the upgrade of the ALICE ITS. In order to minimize the material budget contribution of the cooling system, a silicon frame with embedded microchannels has been developed. This design eliminates any material contribution in the detection region while keeping all the efficiency advantages linked to microchannel cooling for the thermal management of the on-detector electronics.
Tests have shown that such silicon frames with evaporative C4F10 are able to remove the power dissipated by the FE chips (of the order of 0.3 W/cm2) while maintaining the sensor below 30ºC with a maximum temperature gradient over the sensor area of 5ºC.
