We have studied gas amplification properties of a proportional tube filled
with CO2/isobutane gas mixtures. Thr: gas gain was measured for X- and beta
-rays as a function of the anode-wire surface field and was used to estimat
e gain variation along 3.6 m-long stereo anode wires of our proposed centra
l drift chamber for JLC. During the gas gain study, we observed a strong sa
turation effect for point ionizations. (C) 2000 Elsevier Science B.V. All r
ights reserved.