The cooling capabilities of C sub(2) F sub(6)/C sub(3) F sub(8) saturated fluorocarbon blends for the ATLAS silicon tracker

We investigate and address the performance limitations of the ATLAS silicon tracker fluorocarbon evaporative cooling system operation in the cooling circuits of the barrel silicon microstrip (SCT) sub-detector. In these circuits the minimum achievable evaporation temperatures with C sub(3)F sub(8) were higher than the original specification, and were thought to allow an insufficient safety margin against thermal runaway in detector modules subject to a radiation dose initially foreseen for 10 years operation at LHC. We have investigated the cooling capabilities of blends of C sub(3) F sub(8) with molar admixtures of up to 25% C sub(2) F sub(6), since the addition of the more volatile C sub(2) F sub(6) component was expected to allow a lower evaporation temperature for the same evaporation pressure.A custom built recirculator allowed the in-situ preparation of C sub(2) F sub(6)/C sub(3) F sub(8) blends. These were circulated through a representative mechanical and thermal setup reproducing an as-installed ATLAS SCT barrel tracker cooling circuit. Blend molar compositions were verified to a precision of 3.10 super(?3) in a custom ultrasonic instrument.Thermal measurements in a range of C sub(2) F sub(6)/C sub(3) F sub(8) blends were compared with measurements in pure C sub(3) F sub(8). These indicated that a blend with 25% C sub(2) F sub(6) would allow a reduction in evaporation temperature of around 9 super(o) C to below -15 super(o) C, even at the highest module power dissipations envisioned after 10 years operation at LHC. Such a reduction would allow more than a factor two in safety margin against temperature dependant leakage power induced thermal runaway.Furthermore, a blend containing up to 25% C sub(2) F sub(6) could be circulated without changes to the on-detector elements of the existing ATLAS inner detector evaporative cooling system.