2.1 Neutral current detector project at Sudbury Neutrino Observatory (SNO)

M.C. Browne, T.H. Burritt, P.J. Doe, C. Duba, S.R. Elliott, J.E. Franklin, J.V. Germani, K. Heeger, R. Meijer Drees, A.W. Myers, A.W.P. Poon, R.G.H. Robertson, T.D. Steiger, T. Van Wechel and J.F. Wilkerson

SNO will detect Cerenkov light emitted from electrons or positrons produced by charged-current neutrino interactions. These measurements will provide a measure of the flux of electron neutrinos from the sun. Neutrinos of any flavor, however, can produce free neutrons in the heavy water by neutral current interactions. Thus the measurement of the neutron production is a measurement of the total flux of neutrinos from the sun. Since solar burning produces only electron neutrinos, a comparison of the total neutrino flux to the electron neutrino flux could provide strong evidence for neutrino oscillations and therefore neutrino mass. The neutral current detectors (NCD's) are He-3 filled proportional counters designed to detect such neutrons.

This past year has seen the NCD project progress from the research-and-development stage, through a prototyping stage to imminent production. UW is playing a major role in the construction, testing, installation, and data analysis and acquisition segments of the project. The NCD project is a collaboration between UW, Los Alamos National Laboratory, Lawrence Berkeley National Laboratory, and Guelph University.

The final assembly of the NCD's and their initial testing will be done in the production facility that has been constructed at UW during the past year. The clean room was commissioned and is operating well. Major systems needed for construction such as the laser welder, the wire stringing apparatus, the vacuum bakeout, the gas handling system, and the acid etching system are all in place and are ready to go. We have nearly finalized the receiving and shipping logistics required to guarantee that the detectors will see a limited time above ground being activated by cosmic rays. We have identified a suitable Seattle-area underground storage site which can be used during any construction delays to safeguard counter parts from cosmic rays.

In addition to preparing the laboratory, we have also had a number of technical and production accomplishments during the year. A Ni tube has been produced by chemical vapor deposition on an anodized Al mandrel. Although the deposit did not meet our specifications with respect to thickness uniformity, the tube was leak tight. This is an improvement over the Teflon and the electroless-Ni coated mandrels previously used. Modifications to the process are being made to improve thickness uniformity.

All CVD Ni endcaps have been produced and are beginning to proceed through the production pipeline. They will be fabricated into parts with electronic and gas feedthroughs prior to being welded into detector tubes at the UW cleanroom. We have verified that the endcap production process produces parts which are radioactively clean, electronically stable at HV, and have sufficient glass-to-metal seal strength. All of the commercial service providers have been selected and are ready to proceed with production.

The signal readout cable must be buoyant and extremely low in radioactivity as it will reside in the heavy water; the most sensitive region in SNO to radioactive impurities. A prototype cable has been produced and tested and the final order has been placed. The delay line design is still being finalized.

The preamp design has been tested and finalized, and production has commenced. Construction of the NCD electronics and acquisition system has been delayed in order to concentrate the DAQ group's efforts on SNO data acquisition. As a result, the data acquisition system used during the initial stage of underground storage will have a limited number of channels.

Full production of the NCD's should begin in June 1996 and is expected to last until about December. The counters will be stored underground at SNO until installation into the heavy water.