Lifetime Tests for Mac Vertex Chamber*

A vertex chamber for MAC was proposed in fall 1983 to increase precision in the measurement of the B hadron and 7 lepton lifetimes. The chamber had to be placed within the existing central drift chamber, making access for repairs difficult and costly. We therefore used for detector elements thin-walled aluminized mylar drift tubes (“straws”) because of their simplicity and robustness. lp2 The diameter of the drift tubes was 6.9 mm. The radial extent of the proposed chamber was from 4.6 cm to 10 cm, the inner wall of the central drift. It was clear that radiation levels, from synchrotron x-rays and overfocussed electrons, were potentially high. Since the drift distance is short in the straws, we wished to operate them at the highest possible gas gain, to achieve the best spatial resolution. There was a likelihood of drawing large currents in the chamber and thus causing radiation damage. We therefore undertook a study of radiation hardness under the conditions of our proposed design. Tests were conducted in the vessel illustrated in Fig. 1. One face of the cubical vessel had a Lucite window, through which we could illuminate the straws within. A list of materials in the chamber appears in Table I. To correspond to final operating conditions, tests were conducted at 4 atma without gas circulation. Table II contains a list of gases used and operating conditions. We have observed that pulses from the 5.9 keV x-rays from Fe55 remain relatively saturated for pressurized, highly quenched Argon-Hydrocarbon mixtures. However, the pulses from single primary electrons, generated by the photoelectric