The identification of 
decays
is performed using a high-resolution magnetic spectrometer which consists of a
dipole magnet and a set of four drift chambers. The magnet has an
aperture of 2.45 
2.4m 
and produces a maximum magnetic field
of 3.7kGauss in the vertical direction. The transverse momentum kick at the
center of the
magnet is 256MeV/c. Two drift chambers are located upstream of the magnet for
the measurement of the decay vertex position whereas the other two, located
dowstream of the magnet,
are used for the bending angle determination of the tracks.
To reduce multiple scattering effects, the entire
spectrometer is contained in a large tank filled with helium
at atmospheric pressure. The total length of the spectrometer is about
22m.
The four drift chambers have an octagonal shape with a transverse dimension of
2.9m. Each chamber contains eight planes of grounded sense wires oriented
in four different directions orthogonal to the beam axis: 
(X,X 
), 
(Y,Y ), 
(U,U )
and 
(V,V ). The electric field is produced by
two field wire planes located on each side of the sense wire plane
at a distance of 3mm. The spacing between two wires, both on the field and
the sense
wire planes, is 1cm so that the maximum transverse drift distance in a cell
is 5mm. One view contains two staggered sense planes distant by 12mm along
the beam axis. It is isolated from the next view by two 22 
m thick
graphite coated mylar foils placed at a 
24mm distance from the center
plane of the view.
Figure 3: Drift chamber efficiency plateau.
The mylar foils are connected to high voltage and are used
to reduce the electric field intensity on the field wires. The sense wires
have a diameter of 20 m and are made of gold plated tungsten while the
field wires have a diameter of 120 m and are made of gold plated
copper-titanium. One sense wire plane contains 256 wires and the total
number of wires
in a chamber is 6160.
During the construction of the chambers, care was taken in order
to guarantee a precise positioning of the wires on the chamber frames.
The absolute transverse position of the wires is known to better
than 100 m/m while the 1cm gap between two wires is achieved to better than
10 m (rms) and the the parallellism of the wires is better than 25 m/m
(rms).
Additional details on the construction of the drift
chambers can be found elsewhere[3].
The drift chambers are operated with a standard Argon( 
-Ethane 
gas mixture with a small addition of water vapour ( 
1 
to slow down
aging processes. The high voltage applied to the field wires and the mylar
foils is respectively -2250V and -1405V. The gas gain obtained
with these operation conditions is about 6 10 
.
Each sense wire is connected to ground via a preamplifier circuit
which has a gain of about 30mV/ A and a rise time of 18ns. Signals
are discriminated and then digitized by a 40MHz TDC with a 25/16ns fine time
and a 204.8 s memory ring
buffer.
The detection
efficiency achieved with a discrimination threshold of 3 A is measured to
be better than
in each plane. Figure 3 shows the efficiency dependence on the applied
high voltage for two different current thresholds.
The space resolution obtained with these chambers is about 120 m in
each projection (X or Y) and is
almost insensitive to the beam intensity. The track
momentum resolution is given by
The thickness of a chamber corresponds to 4.3 10 
X 
while the total contribution from the helium gas contained in the tank
is about 4 10 X .