An 800-MeV/c kaon beam from the Low Energy
mass-Separated Beam (LESB--1) at the Alternating Gradient Synchrotron
(AGS) is slowed by 535 mm of BeO degrader and stopped in the scintillator
target. (BeO combines high density with low atomic number to minimize the
effects of multiple scattering.)
The intensity of the kaon beam incident on the detector has varied
up to 
per AGS spill (typically 1.6-s duration with
24-GeV protons striking the
89-mm-long Pt production target)
producing up to 
stopped in the target.
The incident ratio of 
: 
: p was
about 1 : 2 : 1. The beam counter system consists of a Cerenkov counter,
several scintillation
counters, and a 3-plane multi-wire proportional chamber, shown
schematically in Fig. 2.
The kaons and pions in the beam are independently tagged by the acrylic-radiator Cerenkov counter, using the scheme devised by Fitch [10], and shown schematically in Fig. 4.
Figure 4: Schematic showing the layout of the radiator, mirrors and
phototubes of the beam Cerenkov counter.
The light from kaons is refracted out of the downstream face of the 150-mm-diameter, 25.4-mm-thick radiator with 97% efficiency, since at the beam momentum of 800 MeV/c the Cerenkov angle is close to Brewster's angle, and the light is polarized in the plane containing the beam. A mirror which is a paraboloid of revolution then focuses this light onto a ring of ten 50-mm-diameter EMI 9954KB PMTs each equipped with a Winston cone [11] cast of black epoxy resin and aluminized on the reflecting surfaces. The light from pions, having a larger Cerenkov angle, is totally reflected by the flat surfaces of the radiator, and exits nearly normally from the conical edge. It is then reflected by a conical mirror into the Winston cones of a second ring of ten PMTs. Both mirrors are made of acrylic with the active surfaces aluminized and the other surfaces painted black.
Each ring of PMTs is connected to a LRS 4413 CAMAC discriminator whose
multiplicity output signal (100 mV per fired channel) is discriminated
to obtain the tagging signal. The efficiency of the kaon channel
is about 98% for a PMT multiplicity of seven. The pion channel
inefficiency is 
for a multiplicity of five PMTs.
The multiplicity outputs of the discriminators are also
fed to analog-digital converter (ADC), time-digital converter (TDC),
and transient digitizer (TD)
channels to be recorded with each event.
Upstream of the Cerenkov counter is a set of scintillator
counters designed to monitor the incident beam.
The B1,B2 counter assembly consists of two hodoscopes, both for
the horizontal profile. B1 has elements 40-mm high and 35, 30, 20, 20, 20,
55-mm wide going from left to right as seen by the beam. B2 is the same
with the order of elements reversed so that each counter intercepts
the gaps between fingers of the other. In the vertical direction
two counters, 12.7-mm high 
203-mm wide, are mounted 7.5 mm above and
below the centre of the hodoscope and intercept only the small tails
of the vertical beam distribution.
Two L-shaped counters are configured to detect any halo of beam particles
that miss the hodoscopes or the vertical-tail counters.
Each scintillator counter element
is viewed through an adiabatic ultra-violet-transmitting (UVT)
acrylic lightguide by
one or both halves of a 25.4-mm-square Hamamatsu R1548 dual PMT.
All counters are 3-mm thick.
Immediately downstream of the Cerenkov counter is the beam multi-wire
proportional chamber (BMPC) used to monitor the beam profile.
It consists of 3 planes 
of 12-
m-diameter Au-plated
tungsten anode sense
wires strung 1.27-mm apart and read out in pairs.
High voltage is applied separately for each layer
to the cathode plane consisting of a film of 25-
m-thick
aluminized Mylar 3.18-mm away from the anode plane.
The X-plane has 72 anode pairs
running vertically and the U- and V-planes each have 60 pairs at
to the vertical. Each pair has an on-board preamplifier which drives the
signal to a post-amplifier and discriminator circuit whose output
feeds a LRS 1879 Fastbus pipeline TDC channel. The chamber operates with
an 80 : 20 gas mixture of CF
and isobutane for fast response.
At a high voltage of 3500 V
each plane is typically >97% efficient.
A lead collimator downstream of the BMPC restricts the beam in the
horizontal direction. It is 660-mm long and has a gap of 63.5 mm
in the horizontal direction.
Two counters, B3 and B3S, are located at the upstream face of
the degrader, for beam tuning and diagnostic purposes. B3 is
76 mm 
64 mm and B3S is 51 mm 
38 mm.
Three layers of B4 counters are sandwiched between the degrader
and the upstream face of the target. The first two layers are 4-finger
hodoscopes (each finger 25.4 mm 
101.6 mm) B4Y and B4X.
The third layer, B4T is a single counter which has the same
hexagonal shape as the target.
It is 110-mm wide between parallel sides and extends 3 mm outside the target
on all sides.
The energy loss ( dE/dx) in any of the 6-mm-thick B4
scintillators is used as part of the identification of incoming kaons.
Dispersion-cancelling circuits on all beam scintillator PMT signals
are used to compensate for signal broadening due to the loss of high-frequency
components on the 
-m-long coaxial cables from the PMTs to the
counting room. There, the signals feed 
10 amplifiers which in turn
feed ADC and TD channels. A second output is discriminated for a TDC input.
Normally, only B4T and the kaon Cerenkov counter are included in the
beam information sent to the trigger.