Frequently Asked Questions

Why is a silicon detector not used around the beampipe?

It was decided to instrument the downstream tracking stations with a single technology that can perform similarly to a silicon tracker, but cost-effective enough to cover the 30 square metres of acceptance of each layer. The result is a light and uniform tracking detector without the need for cooling or signal cables entering into the detector acceptance.

Why are the SiPM dectors cooled to -40 degrees C (and not more)?

The non-ionising radiation (neutron) damage to the SiPMs creates thermally-induced single pixel avalanches with a rate that would be too large at room temperature. Cooling reduces the noise rate by a factor of 2 for every 10 degrees reduction in temperature. Minus 40 Celsius is considered the reasonable limit to operate the cooling system due to the limited insulation available in the detector modules consisting of 20 mm of expanded foam. Space is not available for more. Vacuum insulation of the SiPM arrays is also not possible due to the connection to the scintillating fibres. Simulations have shown that below this, regions of the outer surfaces of the Read-out Boxes will have temperatures below the dew point of the cavern, creating a potential problem for water damage and electronic shorts.

Why are the SiPM detectors annealed at +40 degrees C (and not more) ?

The damage and resulting leakage current in the silicon from neutron damage can partially be reduced by heating the silicon for several hours at a high temperature. A greater reduction can be achieved at higher temperatures. However, the system is limited by the contact of the SiPMs to the plastic scintillating fibres, which begin to degrade and deform at temperatures above 50 degrees Celsius.

-- BlakeLeverington - 2015-12-08

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Topic revision: r3 - 2018-11-27 - BlakeLeverington
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