Multi Wire Proportional Chambers

The pion beta detector uses two cylindrical chambers MWPC1 and MWPC2 for tracking charged particles.

MWPC 1 has been exchanged on 17th-Nov-1999. It has a little different geometry, which can be found in New MWPC 1 properties.

The original chambers have the following properties:

Chamber                                  1 (inner)  2 (outer)


Active length [mm]                     350        540
All length [mm]                        580        730
Min. diameter [mm]                      90        206
Max. diameter [mm]                     152        255
Diameter anode [mm]                    120.3      240.2
Diameter inner cathode [mm]            115.2      235.2
Diameter outer cathode [mm]            125.0      245.2
Inner cathode - anode gap [mm]           2.55       2.5
Volume of chamber [cm^3]               660       2035
Total chamber thickness [mg/cm^2]       53.9       74.8
Total chamber thickness [rad.length]     1.4E-3     2.0E-3

Anode wires

Number of anode wires                  192        384
Anode wire spacing [mm]                  1.96       1.96
Anode wire tension (min-max) [N]    0.44-0.48  0.44-0.51
Resistence of anode wire [Ohm]         110        155
Resistance between anode-anode [Ohm]   560k       560k
Capacity between anode-ampl. [pF]     1000       1000

Cathode strips

Number of inner cathode strips         2 x 64     192
Angle of slope inner cathode [deg]      36.960     44.602
Number of outer cathode strips         2 x 64     192
Angle of slope outer cathode [deg]      33.650     42.339
Width of cathode strips [mm]             3.0        2.4
Gap between strips [mm]                  0.4        0.3
Resistence of strips (6u Al) [Ohm]      ~1.5       ~2.0

The chambers were fabricated in Dubna and sucessfully tested with radioactive sources and 32 channels read out at PSI end of 94. The following picture shows the efficencies and current of the chambers from those tests:

 The spacial resolution of the cathode strips was measured using the following technique:

 When a particle crosses the chamber and produces a signal, the charge is received by several (typically 5) cathode strips. Following picture gives an example of such an event:

The position of this hit is determined by fitting a gaussian to this hit pattern. While in the lateral (z) direction of the chamber all positions are possible, in the rotational (phi) direction only those positions near a wire can produce some charge, since the charge avalanche forms only around the anode wires. Therefore, by plotting measured phi-positions of arbitrary hits for a given z=const., one can see the individual wires:

 By measuring the width of those peaks, on can determine the resolution of the cathode strips in the z-direction, where no discretisation by the wires occur. The result of this measurement give a 0.2mm resolution (FWHM). The double pulse resolution which is critical for our experiment is not yet fully measured but is in the range of 1 to 2 mm.

November '95 tests

Some pictures about the chamber tests in November '95 can be found in aseparate document


Gas Consumption

The gas consumption was measured in te beamtime 1999 over 1 week. The Excel file can be downloaded. The gas pressure as a function of time is plotted here and can be downloaded as PS-file

Gas Consumption


 H.P. Wirtz, 13-Jul-1999