Search

This paper reports on the design, construction, and operational characteristics of a delay-line readout implemented on the cathode foils of a cylindrical drift chamber system. The readout was used to determine the position of an event along the length of the 1.74 m drift wires in the MEGA detectors used at the Los Alamos Meson Physics Facility. The performance of the system is interpreted by comparison to a PSPICE simulation, and to simple analytical models.

The trigger for the MEGA detector system is based on signals from single, high-energy photons interacting in one of the three MEGA pair spectrometers. The trigger is divided into a fast and a slow stage. The first stage produces a fast output if a specific pattern of detector hits is observed in the scintillators and high-speed wire chambers of a pair spectrometer. The second, slow-stage interrogates drift chamber hit patterns and provides a veto when the pattern fails a minimal requirement for reconstruction of the hits into a pair of circular orbits. The trigger interacts with the photon detector electronics by gating limited sections of the detector during the read-out of an event. This paper describes the two stage trigger system, the photon detector electronics, and the implementation of the trigger outputs to strobe the data acquisition system. The performance of the trigger is compared to Monte Carlo simulations of the photon detector response.

The construction techniques used in the development of a set of large pair spectrometers for the MEGA detector are discussed. These spectrometers consist of carbon-fiber-composit cylinders on which the conversion foils are mounted, and appropriately spaced wire chambers in each layer which track the conversion leptons. Close tolerances on all dimensions are maintained. The detector has been operating well in the stopped muon beam line at LAMPF.

The MEGA experiment is designed to search for the rare decay μ→eγ with a branching ratio sensitivity of ∼5×10−13. Production data have been taken during 1992 and 1993, and the detector is working as expected. Following a complete analysis, the present data set should represent an improvement of 12–15 in sensitivity over the previous limit of μ→eγ.