Fast non-explosive gases for drift chambers

Abstract

Typical gases which are stock at Fermilab are Ar:C/sub 2/H/sub 6/(50:50) and Ar:CO/sub 2/ (80:20). Argon:Ethane has the virtue of high gas gain and a saturated drift velocity. In fact, parametrizing the drift velocity as a function of electric field we find v/sub d/(E) = v/sub o/(1/minus/e/sup -E/E/o) with v/sub o/ approx. = 5.4 cm/..mu..sec and E/sub o/ = 160 V/cm. However, safety considerations make this gas somewhat inconvenient. The addition of alcohol as quencher also raises the saturation field to, for example, E/sub o/ approx. = 500 V/cm for 1.5% added alcohol. This gas also tends to break up in a high-beam flux environment and leave carbon deposits. The addition of alcohol to avoid such aging often takes a unit cell out of saturation over its entire volume. Finally, for collider applications it is useful to exclude free protons from the gas in order to reduce the sensitivity to the sea of slow neutrons which are present in the collider environment. In contrast, Ar:CO/sub 2/ (80:20) is a gas with more moderate gas gain. The drift velocity at high field is v/sub d/(E > 1.5 kV/cm) approx. = 5.8 cm/..mu..sec. For most field configurations this gas does not saturate,more » causing a long tail in the drift time distrubtion due to low field regions in the unit cell. The virtues of this gas mixture are that it is cheap, not flammable, and stable under high-beam flux. However as the Collider Upgrade progresses, we wish to find a gas which is faster than 5.0 cm/..mu..sec since the time separation between collisions will at some point be less than drift time of 1..mu..sec for drift distance of 5 cm. 3 refs., 5 figs. « less

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