Schematic side and end views of the detector are shown in Fig. 2.
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Incoming kaons are stopped in the
target located at the center of
the detector enclosed in a solenoid magnet which produces a uniform 1.0-T
field along the beam direction.
Pions from 
decay are momentum analyzed as they
traverse the cylindrical drift chamber and are stopped in layers of
scintillators (range stack) in order to measure the range and kinetic energy.
The solid angle acceptance for 
detection is 
sr.
Surrounding the range stack and drift chamber is the photon
detection system covering nearly 
sr, comprised of a cylindrical barrel
section and two endcap sections of electromagnetic shower detectors.
The overall detector
function is illustrated by the 
event shown in Fig. 3 in which the
pion has been tracked out of the target, through the drift chamber,
and stopped in the range stack, and the two photons from
decay are detected in the range-stack and barrel-photon detectors.
Figure 3: Reconstructed 
event showing the detector elements hit
by the 
and the two 
s from the 
.
Particle identification is obtained by exploiting the
correlations among the range, momentum and kinetic
energy of charged decay products in order to distinguish pions from muons.
This kinematic technique is complemented by observing the characteristic
decay sequence of 
, followed by
(
)
in the range-stack photomultiplier signals from the pion stopping
counter using transient digitizers.
There are also possible backgrounds related to the beam, such as
beam pions scattering into the detector following 
stops, or
charge-exchange reactions (
) resulting in decays
such as 
, where the second charged particle
is missed.
In order to deal with these, the beam counters are capable of
efficient 
identification and detection of
secondary incident particles from the beam line. An active
highly-segmented target system allows tracking of
the stopping 
s and of the decay 
s near the decay vertex.
The individual subsystems are described in detail in the following sections.