Results from the MAC 1 Vertex Chamber * HARRY

The design, construction, and performance characteristics of a high precision gaseous drift chamber made of thin walled proportional tubes are described. The device achieved an average spatial resolution of 45 pm in use for physics analysis with the MAC detector. The B-lifetime result obtained with this chamber is discussed. Invited talk presented at Workshop on Vertex Detectors: State of the Art and Perspectives, Erice, Italy, September 21-26, 1986. * This work was supported in part by the Department of Energy under contract numbers DE-AC02-81ER40025 (CU), DE-AC03-76SF00515 (SLAC), and DEAC02-76ER00881 (UW); by the National Science Foundation under grant numbers NSF-PHY82-15133 (UH), NSF-PHY82-15413 and NSF-PHY82-15414 (NU), and NSF-PHY83-08135 (UU); and by the Istituto Nazionale di Fisica Nucleare. ., INTRODUCTION In the summer of 1983, the MAC and Mark-II collaborations gave evidence for bottom hadron lifetimes or l-2 psq based on data accumulated at the PEP e+estorage ring. at SLAC. The importance and fundamental nature of this measurement? prompted the MAC collaboration to propose, in autumn 1983, the addition of a high precision vertex chamber to the MAC detector. The design utilized thin walled cylindrical drift tubes made of aluminized mylar. The device was installed in autumn 1984, immediately provided useful data, and continued to operate until March, 1986, when MAC data taking ceased. It has achieved the highest spatial resolution -35 pmof any drift chamber used at colliders for physics results. In this paper, we describe the research and development of this chamber, its performance, and the B-lifetime result obtained with it. The success of this chamber has helped convince many collaborations, ineluding the Mark-III at SPEAR, CLEO at CESR, the upgraded TPC at PEP, AMY at TRISTAN, and JETSET at LEAR to build or propose similar devices. Indeed several features of this type of chamber are well matched to the probable conditions at the ELOISATRON or SSC: the small cell size is suited to the high rate and dense tracking environment anticipated; the simplicity and ruggedness makes a system of lo5 channels credible. It should be noted that the MAC vertex chamber has operated only 4.6 cm from colliding beams, and has suffered no degradation due to radiation damage; this augurs well for survival in the more extreme conditions of the ELOISATRON or SSC. Motivations and Strategy Our initial motivation for using the thin walled mylar tubes4tB ’ (often referred to as ‘straws’ because they are made in a spiral wrap, much like paper drinking straws) was simplicity and robustness. Problems of field wires and drift field mapping are eliminated. If a sense wire breaks, it is physically and electronically isolated from the other sense wires, so does not harm the rest of the chamber. However, there are other advantages to the use of straws. We have obtained 20 pm spatial resolution in beam tests with them, and 35 pm in our vertex chamber in use for physics results. Their small size is not only useful for the high rate environment, but aids pattern recognition in the dense regions of multitrack events. Their large, flat cathode makes straws less susceptible to some ills of radiation damage, including whisker growth and cathode coating. Our Monte Carlo studies’ -indicated that the spacing of ionization clusters, and not diffusion, dominates the spatial resolution for drift distances as short as those in straws. This is in contrast to jet type cells, and indicated that the best strategy for high spatial resolution is to trigger the timing discriminator at the least possible ionization, instead of using the centroid in time of ionization arriving at the sense wire. To achieve a low threshold, we decided to operate the chamber at the highest possible gas gain, even in the ‘self quenching streamer’ mode if possible. We also decided to operate the chamber at high pressure, to reduce the spacing between ionization clusters. Of course, a disadvantage of high gas gain is that the chamber might age quickly from beam-related radiation. We carefully designed our shielding and avoided this problem. Other disadvantages of straw chambers include: 1) the drift region close to the wire makes up a larger fraction of the active region than in jet type cells; this region can, but by no means must, have poor spatial