Multi-wire proportional chambers

Multi-Wire Proportional Chambers (MWPCs) provided information of the position of muon decay electron hits. The electronics diagram for the MWPC imaging system is given in Fig. 3.21. When an electron passed through the chamber, it ionized a gas mixture of argon, isobutane, freon, and methylal vapor, which filled the chamber. The ionized electrons drifted toward the closest anode wire(s), and near the anode where the electric field became very strong, electrons were increased from the consecutive series of ionization of the gas molecules, leading to a multiplication process. This process induced a current in the nearest cathode wires in each of two planes in orthogonal directions (y and z in our case). Each wire in a plane was connected to a delay line, and the signal reached the end of the line with a time delay (with respect to the electron hit, measured by EM1 and EM2) that was proportional to the distance of the hit position from the edge of the plane, or the end of the delay line. The delay time was measured at both ends of the delay line, and the difference between them was proportional to the distance from the center of the chamber.

  

Figure 3.21: Electronics diagram for the Multi-Wire Proportional Chamber imaging system. Only one of three identical chambers is shown. The time of the electron is measured by a pair of plastic scintillators, while its energy is given by the NaI crystal MINA.

For instance, for z wires, the distance from the center Z_WC is given by

where ZL and ZH are the delay times at the beam upstream and downstream ends of the delay line, respectively, and dispZ is a dispersion constant, determined from calibrations using an electron source, where the physical displacement of the source by a known distance was mapped to the shift in the delay time. The parameter, offsetZ was adjusted such that the center of the distribution of electron hits corresponded to zero in the zscale. The position in the y-dimension was measured in an identical manner. With three sets of MWPCs providing hit positions, the trajectories of muon decay electrons were determined from a least-squares fit to three points on a straight line. The fitted line was extrapolated back to a perpendicular plane bisecting the target, providing positional information of the muon at the time of muon decay.

The energy of electrons was measured by a large NaI crystal (MINA). The cuts in MINA energy improved position resolution of the imaging system, by favoring higher energy electrons which scatter through smaller angles in window materials. More details of the data treatment will be given in Sec. 7.1.