The main goal of the vertex beam pipe is to separate the ultra high vacuum (UHV) of the ring, around 1 x 10-9 Torr, from the detector which sits in air. This must be done with as little material as possible since one of the primary goals of the experiment is to study the decays of short lived particles produced in the collision. When these particles decay they produce charged particles to which the elements of the detector are sensitive These charged tracks can be extrapolated back to the collision point to see if they came from the collision itself or at a vertex from a particle that had a lifetime and decayed away from the collision point. The accuracy of this extrapolation depends, in part, on how much material the charged particles traverse before they get to the sensitive elements of the detector. The smaller the amount of material in the vertex beam pipe the more sensitive the experiment can be to particles with short lifetime.

      The plan for the upgrade of CESR will have 3 Amps of beam current circulating around the ring. According to our observations of the vertex beam pipe in CLEO now, we expect 15-20 Watts/cm of power will be deposited on the CLEO III vertex beam pipe by the beam. Just outside of the pipe is the most sensitive element of the CLEO III experiment, a detector consisting of large wafers of silicon. The charge deposited by tracks in this silicon is detected by sensitive integrated circuits located as close as possible to the silicon wafers. For the good operation of this detector and its electronics, temperature fluctuations in the region must be kept at a minimum, well below 5°C. Thus the CLEO III vertex beam pipe needs to be actively cooled along its entire length to ensure that the significant amount of heat generated by the beam is not deposited on the silicon detector.

      The vertex beam pipe has to be electrically conductive. An image current trails the beam as it goes around the storage ring. Any gap in the conductivity of the beam chamber will be a site of resistive heating and a source of high frequency electro-magnetic waves. Neither of these can be tolerated in the center of the CLEO III experiment due to their impact on the sensitive parts of the experiment.       The vertex beam pipe has to join with the rest of the CESR beam chamber. Its ends have to be of compatible materials and shapes. It must be machined to accurate tolerances as small misalignments can have consequences in construction and in performance as a shield between the beam and detector.

      Finally the vertex beam pipe will be inaccessible after it is installed in CLEO. Any failure in any way would be catastrophic for the CLEO experiment. The vertex beam pipe must be thoroughly tested for mechanical and vacuum reliability before it can be installed in CLEO III.

      Beryllium provides the ideal choice for the material of the vertex beam pipe. Due to its low atomic number and its high strength it can form a strong pipe able to withstand the pressure differential with very little material. It has good UHV properties and electrical conductivity. Drawbacks are the difficulty and expense in obtaining and machining pure beryllium. Also, it bears reminding, that in order to achieve the UHV inside the vertex beam pipe, strict cleanliness in fabrication and handling of all parts must be observed at all time.

Last modified: Wed Jan 17 16:58:30 EST 2001