Current Status

Updated: 8 Nov 1994

• The whole detector system has been designed, including the cosmic veto house and the air conditioning. The mechanical structure for the CsI calorimeter is currently being manufactured.

• More than 30 CsI modules for the calorimeter have been tested and meet our specifications. EMI photomultiplier tubes with UV-transparent windows have been selected and ordered for the readout of the pure CsI detectors. Specialized PMT voltage dividers have been designed, built and tested with the EMI tubes in beam, and have met our requirements regarding peak linearity and rate-dependent gain stability.

• Specialized trigger modules have been designed, built and tested in beam. The original triggering scheme has been demonstrated to satisfy our requirements. 90 % of the electronics modules are already at PSI and ready for assembly.

• An active target with 77 fibers 3x3 mm each has been built and successfully tested in-beam. A sensitivity of 30 - 40 ph-e/MeV was observed with a typical crosstalk of about 10%. The size of the pion beam spot on the active target is about one centimeter in diameter.

• The cylindrical wire chamber 1 is already at PSI and tested successfully, chamber 2 will be delivered shortly.

• A fast plastic veto counter surrounding the outer wire chamber consisting of 20 staves read out by 40 PMTs is under construction.

• The CsI response to monoenergetic positrons and gammas has been measured in several in-beam tests. The observed lineshape is in excellent agreement with the full Monte Carlo simulation using the CERN GEANT computer program. See 1993 Results

• Light uniformity of the CsI crystal response to ionizing radiation has been measured in beam as well as in a dedicated cosmic ray tomography setup. A wrapping method has been developed which brings the nonunifor- mity to about 3% over the full crystal length. We are working on a further reduction of the nonuniformity.

• In the 1994 beamtime, a trigger for pi -> e-nu events has been developed. An excellent separation of e-nu decay events from the Michel background was achieved on-line with this trigger.

Outlook

• During 1995, the detector will be assembled at PSI. The remaining CsI crystals will arrive during the year, and will be subsequently tested and integrated in the detector. The detector system is expected to be completed and assembled by the end of 1995.

• In 1996, the first data of the pion-beta decay will be collected. To get the statistics required in the first phase (0.5% uncertainty), the experiment will continue data taking during 1997 and 1998.